Automated control systems for electric drives of refrigeration units. Refrigeration automation systems. Temperature control in the refrigerated object

The main condition for the technical development of any industry is the automation of production processes, i.e. a set of technical measures that completely or partially exclude human participation in a certain stage of the production process.

The main goals refrigeration automation are:

mechanization of the production process;
accurate maintenance of the specified parameters of the equipment;
prevention of equipment breakdown;
increasing the service life of refrigeration equipment;
reduction of personnel and reduction of labor costs;
ensuring the safe work of personnel.

Any operation performed by the driver of modern refrigeration machines lends itself to automation, but this does not mean that it is necessary to automate all processes. Refrigeration equipment automation it is necessary only in cases where the performer's qualifications are not required at all to perform operations, or when the performer cannot achieve the required control accuracy. It is also imperative to automate all processes taking place in explosive and hazardous conditions for human health.

According to the degree of automation, refrigeration equipment can be conditionally divided into three groups:

1. Refrigeration equipment with manual control - all control functions and refrigeration system control performed by staff.
2. In partially automated refrigeration equipment, some processes are automated, but the equipment must work with the constant presence of personnel; in such machines, starting is most often done manually, and stopping is automated.
3. Fully automated refrigeration equipment does not require the constant presence of maintenance personnel, but does not cancel the need for periodic inspections and maintenance according to the established regulations. Basically, steam ejection and absorption refrigeration units are fully automated due to the absence of moving mechanisms in them.

Varieties of refrigeration automation systems

An automation system is a combination of an automation object and automatic devices, thanks to which it is possible to control the work refrigeration systems without the participation of service personnel.

Types of automation systems:

Open-loop systems - they are rarely used, they are divided into types:

an open-loop automation system with direct communication, in which tracking is carried out according to an indirect parameter (for example, in ventilation systems according to the outside air temperature);
open loop automation system with feedback, which performs only informational functions (measurement, signaling).

Closed systems, the principle of which is to determine the deviation of the actual value of the control parameter from the specified one. It is these automation systems that are used forcontrol work refrigeration unit... Types of closed automation systems:

automatic control systems, i.e. those that maintain the parameters at a given level;
automatic protection systems, i.e. those that automatically turn off the equipment when its normal operation is disrupted.

The main parts and devices of the automation system of the refrigeration plant

Main parts of the system refrigeration automation:

measuring (sensitive) element equipped with a device for refrigeration control settings parameters to a given value;
a sensor that registers a change in the controlled value;
refrigeration control panel, i.e. a regulating body that, upon a signal from a measuring element, changes the supply of a signal or energy to a regulated object;
transmission device that connects the sensor to the transmission mechanism.

Control panel for the refrigeration unit and refrigeration unit automation devices

The main element that controls the devices of the automation systems of the refrigeration plant is refrigeration unit control panel... The control panel contains automatic control, regulation and protection devices, as well as signaling devices, thanks to which the normal functioning of the refrigeration system is ensured.

Automatic control devices located on refrigeration unit control panel, regulate the operation of pumps and compressors when the load changes. When the refrigerant temperature drops, as well as when the pressure in the evaporators falls below the limit value, the compressors are automatically stopped; when the temperature in the evaporator rises, the compressors are automatically switched on. Sometimes for automatic control of compressors, a time relay is used, which is programmed for a certain time for turning on the units.

With the help of automatic control devices on the control panel, the key parameters of the refrigeration unit operation - temperature and pressure - are maintained at an optimal level. With a decrease in the heat load, the temperature of the coolant is maintained at a given level due to the smooth automatic regulation of the cooling capacity of the installation, which can be carried out in the following ways:

1) throttling of refrigerant vapors in front of the compressor, as a result of which the pressure decreases;
2) bypassing part of the vapors from the discharge line to the suction line;
3) an increase in the dead space in the reciprocating compressor, as a result of which the suction of refrigerant vapors from the evaporator decreases.

Automatic control devices that change the refrigerant flow to the evaporator also ensure safe compressor operation and protection against water hammer.

Automatic signaling is used to notify the operator of the refrigeration plant about a change in the operating mode of the equipment, which can trigger the automatic protection. Also, the automatic alarm notifies the operator with a sound signal about turning on and off equipment, fittings and devices.

Automatic protection of refrigeration equipment avoids the dangerous consequences of a violation of the normal operating parameters of refrigeration machines. With abrupt changes in operating parameters (a strong increase in the discharge pressure, a decrease in pressure and evaporation temperature, non-observance of the operating mode of the lubrication system, refrigeration system check and other situations), specially designed devices turn off refrigeration units, preventing their breakdown.

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MINISTRY OF EDUCATION AND SCIENCE OF THE REPUBLIC OF MARIY EL

STATE BUDGETARY PROFESSIONAL EDUCATIONAL INSTITUTION

REPUBLIC OF MARIY EL

"TRANSPORTATION AND ENERGY TECHNICUM".

Course work on the topic

Refrigeration automation

PM 01.02 Automation systems for agricultural organizations

Smirnov A.V.

Krasny Yar

Introduction

1.3 Refrigeration cycle diagram

2.1 Methodology for designing a circuit

Conclusion

Bibliography

Introduction

Automated control and regulation systems are an integral part of the technological equipment of modern production, contribute to the improvement of product quality and improve the economic performance of production by choosing and maintaining optimal technological modes.

Automation frees a person from the need to directly control mechanisms. In the automated production process, the role of a person is reduced to adjustment, adjustment, maintenance of automation equipment and monitoring their operation. If automation facilitates the physical work of a person, then automation has the goal of facilitating mental work as well. The operation of automation equipment requires highly qualified service personnel.

Compressor refrigeration units occupy one of the leading places among other industries in terms of automation level. Refrigeration plants are characterized by the continuity of the processes taking place in them. In this case, the production of cold at any time must correspond to the consumption (load). Almost all operations in refrigeration plants are mechanized, and transient processes in them develop relatively quickly. This explains the high development of automation in refrigeration technology.

Parameter automation offers significant benefits:

Provides a decrease in the number of working personnel, that is, an increase in the productivity of its labor,

Leads to a change in the nature of the work of the service personnel,

Increases the accuracy of maintaining the parameters of the produced cold,

Increases labor safety and reliability of equipment operation,

control devices

The purpose of automation of refrigeration machines and installations is to increase the economic efficiency of their work and ensure the safety of people (primarily service personnel).

The economic efficiency of the chiller is ensured by lower operating costs and lower equipment repair costs.

Manual equipment and partially automated machines operate with a constant presence of service personnel.

Fully automated equipment does not require the constant presence of maintenance personnel, but does not exclude the need for periodic inspections and checks according to the established regulations.

An automated refrigeration plant must contain one or more automation systems, each of which performs specific functions. In addition, there are devices that unite (synchronize) the operation of these systems.

An automation system is a combination of an automation object and automatic devices that allow you to control the operation of automation without the participation of maintenance personnel.

The object of the course project is a refrigeration unit in the complex, its individual elements.

The purpose of this course project is to describe the technological process of refrigeration equipment, the development of a functional diagram of this installation and the choice of technical means of automation.

1. Description of the technological process

1.1 Automation of refrigeration compressor stations

Artificial cold is widely used in the food industry, in particular for the preservation of perishable food. Cooling ensures high quality of stored and produced products.

Artificial cooling can be carried out periodically and continuously. Periodic cooling occurs when ice melts or when solid carbon dioxide (dry ice) is sublimated. This cooling method has a big disadvantage, since the refrigerant loses its cooling properties during melting and sublimation; during long-term storage of food, it is difficult to ensure a certain temperature and humidity in the refrigerator compartment.

In the food industry, continuous cooling with the use of refrigeration units is widespread, where the refrigerant - liquefied gas (ammonia, freon, etc.) - performs a circular process in which it restores its original state after the refrigeration effect.

The refrigerants used boil at a certain pressure, depending on the temperature. Consequently, by changing the pressure in the vessel, it is possible to change the temperature of the refrigerant, and, consequently, the temperature in the refrigerating chamber. The compressor sucks in freon from evaporator II, compresses them and through oil separator III pumps it into condenser IV. In the condenser, freon is condensed due to cooling water, and the liquid freon from the condenser, cooled in the linear receiver V, through the control valve VI enters the evaporator II, where, evaporating, it cools the intermediate coolant (brine, ice water), pumped to the cold consumers by the pump VII ...

Regulating valve VI serves for throttling of liquid freon, the temperature of which is reduced in this case. The automation system provides for automatic control of the compressor operation and emergency protection. The command for the automatic start of the compressor is an increase in the temperature of the brine (ice water) at the outlet of the evaporator. To control the temperature, a type temperature controller is used, the sensor of which is installed on the brine (ice water) outlet line from the evaporator.

When the compressor is operating in automatic mode, the following emergency protection functions are in place: against a decrease in the oil pressure difference in the lubrication system and the crankcase - a differential pressure sensor is used; from a decrease in the suction pressure and an increase in the discharge pressure - a pressure sensor is used; from an increase in the discharge temperature - a temperature sensor is used; from the lack of water flow through the cooling jackets - a flow switch is used; from an emergency increase in the level of liquid freon in the evaporator - a semiconductor level switch is used.

When the compressor is started in automatic mode, a valve with an electromagnetic drive is opened to supply water to the cooling jackets and the valve on the bypass is closed.

The brine pressure in the discharge pipeline is monitored by a pressure switch.

Remote control of the temperature of air, brine, water at the control points of the refrigeration unit is carried out by thermal converters.

The equipment for monitoring, control and signaling of the rest of the technological equipment is located in the control panel panels.

1.2 Analysis of disturbing effects of the automation object

This scheme provides for monitoring, regulation, control and signaling of the parameters of the technological process.

Control of the upper and lower levels of liquid freon in the linear receiver, in which the level is controlled, on which the filling of the receiver depends.

Also, the temperature of the air in the refrigeration unit is subject to control, on which the cooling and the amount of produced cold depend. refrigeration automation compressor air cooler

Controlling the pressure of the cold brine in the discharge pipeline, which depends on the discharge by the pump, the pump, acting on the cold brine, changes its supply.

Also, the temperature of cold water coming from the pool to the condenser is controlled, which is necessary for the condensation (cooling) of freon vapors.

At the outlet of the condenser, the temperature of the liquid freon is controlled, which enters the linear receiver.

The control valve VI installed on the pipeline serves to throttle the liquid freon, due to which the temperature is reduced at the same time.

An increase in the temperature of the brine (ice water) at the outlet of the evaporator controls the operation of the compressor and serves as a command to automatically start the compressor.

A valve with an electromagnetic drive is installed on the pipeline from the receiver, acting on which the supply of liquid freon to the evaporator is regulated.

If there is no water flow through the cooling jackets or the water pressure is below the set limit, the compressor is turned off.

On the water supply to the cooling jackets, a valve with an electromagnetic drive is installed on the pipeline, acting on which, when the compressor is started in an automatic mode, changes its position to the open state, and at the same time the valve closes.

From an emergency rise in the level of liquid ammonia, temperature sensors are installed in the evaporator to monitor the upper level. Through the valve installed in the pipeline from the receiver, the level of liquid freon in the evaporator is regulated.

1.3 Refrigeration cycle diagram

The refrigeration cycle is basically identical with other normal technologies. The most important difference is the additional piping from the liquid line to the pulse injection valve on the compressor. To allow access to boiling free liquid, piping should be installed on a horizontal section of the liquid line and, above all, directed downward. A filter must be installed to protect the pulse injection valve and compressor; the sight glass allows a visual inspection of the fluid supply. Dimensions of liquid line to injection pulse valve: 10 mm (3/8 ”). The design and control of the cycle has an important influence on the injection cycle and therefore on the overall performance of the product. The suction gas superheat and the difference between condensing and suction pressure should be kept as small as possible (minimum superheat must be set).

Good insulation of the suction line / short pipe runs;

Refusal of heat exchangers (when possible);

Low pressure drop in pipes and components;

Small temperature difference between evaporator and condenser;

Condensing pressure control.

2. Development of a functional diagram of a refrigeration unit

2.1 Methodology for designing a circuit

Automation schemes are the main technical document that defines the functional block structure of individual nodes for automatic monitoring, control and regulation of the technological process and equipping the control object with instruments and automation equipment (including telemechanics and computer equipment).

The object of control in automation systems of technological processes is a set of main and auxiliary equipment, together with shut-off and regulating bodies built into it, as well as energy, raw materials and other materials determined by the characteristics of the technology used.

Automation tasks are solved most effectively when they are worked out in the process of developing a technological process.

During this period, the need to change technological schemes is often revealed in order to adapt them to the automation requirements established on the basis of technical and economic analysis.

The creation of effective automation systems predetermines the need for a deep study of the technological process not only by designers, but also by specialists of installation, commissioning and operating organizations. When developing automation schemes for technological processes, it is necessary to solve the following:

Obtaining primary information about the state of the equipment technological process;

Direct impact on the technological process for management;

Stabilization of technological parameters of the process;

Control and registration of technological parameters of processes and state

technological equipment;

These tasks are solved on the basis of an analysis of the operating conditions of technological equipment, the identified laws and criteria for object control, as well as the requirements for the accuracy of stabilization, control and registration of technological parameters, for the quality of regulation and reliability.

Automation tasks, as a rule, are implemented with the help of technical means, including: selective devices, means for obtaining primary information, means for converting and processing information, means for presenting and issuing information to service personnel, combined, complete and auxiliary devices. The result of drawing up automation schemes are:

1 Selection of methods for measuring technological parameters;

2 Selection of the main technical means of automation that most fully meet the requirements and conditions for the operation of the object being automated;

3 Determination of drives of executive mechanisms of regulating and shut-off bodies of technological equipment, controlled automatically or remotely;

4 Placement of automation equipment on boards, consoles, technological equipment and pipelines, etc., and determination of methods for presenting information about the state of the technological process and equipment.

The modern development of all industries is characterized by a wide variety of technological processes used in them.

Technological equipment and communications in the development of automation schemes should be depicted, as a rule, in a simplified manner, without specifying individual technological devices and auxiliary pipelines. However, the process diagram depicted in this way should give a clear idea of the principle of its operation and interaction with automation tools.

All devices and automation equipment shown on automation diagrams are assigned reference designations (positions), which are saved in all project materials.

Designations on electrical equipment automation diagrams at the stage of working documentation or in one-stage design must correspond to the designations adopted in electrical circuit diagrams.

When determining the boundaries of each functional group, the following circumstance should be taken into account: if any device or regulator is connected with several sensors or receives additional influences under another parameter (for example, a correction signal), then all circuit elements performing additional functions belong to that functional group that they influence.

The ratio regulator, in particular, is a part of the functional group, which is influenced by an independent parameter.

The automation scheme is carried out in the form of a drawing, which schematically shows in schematic conventional images: technological equipment, communications, controls and automation tools, indicating the connections between technological equipment and automation tools, as well as connections between individual functional blocks and automation elements.

Automation schemes can be developed with a greater or lesser degree of detail. However, the amount of information presented in the diagram should provide a complete picture of the main decisions taken on the automation of this technological process and the possibility of drawing up, at the project stage, application lists of instruments and automation equipment, pipeline fittings, panels and consoles, basic installation materials and products, and at the stage working project - the whole complex of design materials provided for in the project.

The automation scheme is performed, as a rule, on one sheet, which depicts the automation means and equipment of all monitoring, regulation, control and signaling systems related to a given technological installation. Auxiliary devices such as gearboxes and air filters, power supplies, relays, circuit breakers, switches and fuses in power circuits, junction boxes and other devices and mounting elements are not shown on automation diagrams.

Automation schemes can be performed in two ways: with a conventional image of boards and control panels in the form of rectangles (as a rule, in the lower part of the drawing), which show the automation equipment installed on them; with the image of automation equipment on technological schemes near selection and receiving devices, without constructing rectangles, conventionally depicting shields, consoles, control and management points.

When the diagrams are executed according to the first method, they show all the devices and automation equipment that are part of the functional block or group, and the place of their installation. The advantage of this method is great clarity, which greatly facilitates reading the diagram and working with design materials.

When constructing diagrams according to the second method, although it gives only a general idea of the decisions made to automate the facility, a reduction in the amount of documentation is achieved. Reading automation schemes made in this way is difficult, they do not reflect the organization of control and management points of the object.

In the expanded image, the diagrams show: selective devices, sensors, converters, secondary devices, actuators, regulating and shut-off bodies, control and signaling equipment, complete devices (centralized control machines, telemechanical devices), etc.

With a simplified image, the diagrams show: selection devices, measuring and regulating devices, actuators and regulating bodies. To depict intermediate devices (secondary devices, converters, control and signaling equipment, etc.), general designations are used in accordance with the current standards for designations in automation schemes.

The composite image assumes that automation tools are mostly expanded, but some nodes are shown in a simplified way.

Automation devices and means built into technological equipment and communications or mechanically connected with them are shown in the drawing in the immediate vicinity of them. Such means of automation include: selective devices for pressure, level, composition of matter, sensors that perceive the effect of measured and regulating quantities (measuring orifice devices, rotameters, counters, expansion thermometers, etc.), actuators, regulating and shut-off bodies.

2.2 Functional diagram of the automation of the refrigeration module

An automated refrigeration unit consists of two compressors (KM) equipped with automatic protection devices, two oil separators (MO), an oil collector (MC), a pre-condenser (FKD), a condenser (KD) with fans, a linear receiver (RL) with two level sensors, two air coolers (VO) installed in the chamber and equipped with fans, filling regulators and solenoid valves (SV), a liquid separator (coolant) with two level sensors, a drain receiver (RD) with a low level sensor and CB, two water pumps.

2.3 Operation of the nodes of the functional diagram of the refrigeration module automation

The main controlled variable in this scheme is the air temperature in the refrigerating chamber. It is regulated by turning the KM on and off, and in winter it can be maintained by turning on and off the electric heaters VO No. 1 and VO No. 2

To control each KM, a small-sized automatic control panel of the PAK type was designed. KM are equipped with standard devices for automatic protection against emergency operating modes

BO filling is automatically controlled by steam superheat BO defrosting is carried out with hot ammonia steam in time

The following blocking is provided: Switching on the KM is possible only after switching on the water pump and the KD fan; After switching off KM No. 1 (No. 2), the SV on the liquid supply line to VO No. 1 (No. 2) must be closed

According to the level of liquid freon in the coolant, an emergency shutdown of the CM is carried out in the RD, the lower level of the liquid is monitored and signaled, and in the RL, the lower and upper levels

2.3.1 Compressor automatic protection unit

As already noted, a standard PAK-type control panel has been designed for each KM. This console provides automatic control and protection of the CM from emergency operation modes. On the front of the control panel there are a key for selecting the KM mode, buttons, a lamp (multi-digital) alarm. The control panel is connected to the contacts of the chamber thermostat, as well as the contacts of the protection devices: the lubrication system control relay (RCCS) 4a (13a); two-block pressure switch (DRD) 5a (14a); discharge temperature control relay (RT) 3a (12a) - it is planned to use the ERT developed at the Agroholod Institute; water flow switch (RP) 6a (15a); level switch (RU) 25b, 26b for coolant - development "Agroholod".

The actuation of any of the listed automatic protection devices turns off the CM and at the same time the signal lamp turns on, in which the corresponding figure is displayed, which shows for what reason the CM is turned off. Since the XM operates in automatic mode, a warning lamp on the watchman's shield turns on in the event of an emergency stop of the CM. On this signal, the watchman calls the driver, who eliminates the cause of the accident and turns on the CM.

Automatic protection devices work in this way. The RCCS is triggered in the event of a decrease in the oil pressure drop on the oil pump discharge line and in the KM crankcase below the set value.

When the water flow through the KM jacket decreases, or when it completely disappears, the water flow switch is activated.

If the discharge temperature exceeds the set one, then the discharge PT is triggered.

The DRP monitors agent suction pressures and discharge pressures. This relay has two measuring units (two bellows), which, through a lever system, affect the same pair of contacts. If the suction pressure becomes below the allowable value, due to which air may be sucked into the system, which will lead to foaming of the oil, or the discharge pressure becomes higher than the allowable one (this can lead to the destruction of the CM), then this relay turns off the CM motor.

In the coolant, the upper and lower alarm levels of ammonia are monitored. The contacts of both sensors are connected to both PAK consoles because the coolant is a common vessel for both CMs. Duplication of control of the level in the coolant is necessary in order to avoid water hammer and thereby prevent the failure of the CM. If during operation the level in the coolant reaches the upper value, then the sensor 25b will work and turn off the CM. Note that connecting the RD to the coolant significantly reduces the possibility of increasing the level in the coolant to the upper value.

2.3.2 Unit for automatic activation of the reserve water pump

The technological scheme provides for two pumps (one is working, the other is backup). The automation circuitry provides automatic activation of the standby water pump in this way. An electrical contact pressure gauge 29 a is installed on the common discharge line of the water pumps. If at this point the water and water discharge pressure drops below the allowable pressure when the main pump is running, the electrocontact pressure gauge reacts to this and gives a command to automatically turn on the backup water pump.

2.3.3 Unit for defrosting air coolers

The defrosting is carried out on a time basis. For this, in the automation scheme, two MCP motor time relays were designed with a maximum delay of 24 hours.

The defrosting of the VO is carried out in turn with a frequency of once a day. Defrosting lasts from 20 to 30 minutes.

During the start-up period, the defrosting of the VO is carried out manually, and in the storage mode - automatically. Thawing is carried out with hot ammonia vapor, which is supplied to the VO from the KM injection line.

In the process of defrosting VO # 1, KM # 2 works, and when VO # 2 is defrosting, KM # 1 works. In this case, with the help of 13 SWs, they make up the corresponding paths of movement of the agent. The corresponding positions of the CB during the manual and automatic defrosting of the VO are the same. Consider defrosting VOs # 1 and # 2 manually in starting mode. For example, the defrosting of VO # 1 is carried out in this way. KM 31 and fan No. 1 are turned off. KM No. 2, fan No. 2 are operating in the starting mode, the water pump and fan No. 3 KD are also operating. Using the universal switch, which belongs to VO No. 1, close SV A3 (on the liquid line) and A2 (on the steam line), A9 ... A12, and open A1 and A4. CB VO No. 2 A7 and A6 are open, and A5 and a8 are closed. Open SV A13.

Automatic defrosting of VOs No. 1 and No. 2 is carried out by time. The peculiarity of defrosting in automatic mode is that after defrosting (lasts 20 - 30 minutes), for example, VO # 1, this VO is not turned on during the day, but VO # 2 works. After a day, VO No. 2 is thawed, which then does not work for a day. During these days, VO No. 1 works, and so on. So, in the storage mode, only one VO and one CM are always in operation.

3. The choice of technical means of the refrigeration unit

3.1 Selection and justification of the choice of instruments and automation equipment

The compressor is equipped with a pressure difference sensor-relay of the RKS-OM5 type (1) designed to control the alarm and two-position regulation of the pressure difference in the lubrication systems of refrigeration units in mobile and stationary installations and to automate technological processes. Controlled media: freons, air, water, oil; ammonia for the RKS-OM5A sensor. The devices are produced with a dead zone directed towards the increase in the pressure difference relative to the set point. The response limit is set on the scale using the adjustment screw. The output device has one changeover contact. Breaking capacity of contacts at a voltage of 220 V is not more than 300 V -A for alternating current and 60 W for direct current.

Devices of the specified type are designed to operate at an ambient temperature of --50 to +65 ° С, and the RKS-OM5A sensor at temperatures from --30 to +65 ° С and relative humidity up to 98%.

Overall dimensions 66x104x268 mm. weight no more than 1.6 kg.

Execution is ordinary, export tropical.

The pressure of the brine in the discharge pipeline is controlled by a pressure switch D220A (11), from a decrease in the suction pressure and an increase in the discharge pressure - a pressure switch D220A (2) is used

Dual pressure switches, type D220 (2, 11) have a low pressure sensor (LPD) and a high pressure sensor (HPD), acting with a system of levers on one common switching contact device. The technical characteristics of the drills are given by DND provides switching of contacts when the controlled pressure drops to the set value and returns to the initial position when the controlled pressure rises (taking into account the dead zone). The DVD switches contacts when the controlled pressure rises to the set value and returns to its original position when the controlled pressure drops (taking into account the dead zone). Structurally, each sensor includes a sensitive element - a bellows and a setpoint adjustment unit. The DND also provides a node for setting the dead zone. The spread of operations does not exceed 0.01 MPa for DND and 0.02 MPa for DVD. D220A-12 Maximum allowable pressure of the medium, 2.2 MPa. Limits of the setting of operation, (- 0.09) - (+ 0.15) MPa. Basic error of operation, 0.02 MPa. Dead zone, 0.03-0.1 MPa. The controlled environment is ammonia in refrigeration units at stationary (modification A) and non-stationary (modification AR) objects). Overall dimensions, 200X155X85mm.

The signal from the temperature sensor goes to the temperature sensor-relay of the TR-OM5 type (3) is intended for use in control systems and two-position regulation of the temperature of liquid and gaseous media in refrigeration and other installations. Sensors ТР-ОМ5-00 - ТР-ОМ5-04 are produced with a dead zone directed towards increasing the temperature of the monitored medium relative to the response setting, and the rest of the devices - towards lowering the temperature. The contact device has one changeover contact. The switched power of contacts is not more than 300 V -A at 220 V AC and 60 W at 220 V DC. The sensors are designed to operate at ambient temperatures from --40 to +50 ° С and relative humidity up to 98%. The limits of the setting of operation (- 60) - (- 30) ° С. The basic error is ± 1.0 ° C. Dead zone adjustable 4 - 6 ° C. Capillary length 1.5; 2.5; 4.0; ten.

Overall dimensions 160x104x68 mm, weight no more than 2.2 kg. Execution ordinary, export, tropical.

Bellows flow switch RPS (4) is designed to control the presence of a water flow with a temperature of up to 70 ° C in automation systems for various technological processes. The relay must be installed horizontally. The response limit is adjusted using a special screw on the scale. Before installing the relay, a hole is drilled in the bushing located between the two bellows, the diameter of which is determined according to the graph of the dependence of the flow rate on the pressure at the inlet to the relay. The schedule is given in the instruction manual. The output device has one make contact. The response error does not exceed 10% of the nominal flow rate.

The relay is designed to operate at ambient temperatures from 5 to 50 ° C and relative humidity up to 95%. Nominal bore diameter, 20 mm. Maximum allowable pressure of the medium, 0.1 MPa. Trip setpoint limits, 0-100 l / min. The permissible current of the contact device is 2 A at a voltage of 220 V AC. Overall dimensions are 135x115x18 mm, weight is not more than 2.5 kg. Execution ordinary, export, tropical.

Semiconductor level switches of the PRU-5M and PRU-5MI types (7b, 8b, 9b, 12b, 13b) are designed to control the level of ammonia, freon, water, diesel fuel, oil and other liquids with a density of at least 0.52 g / cm3 in stationary and ship installations. The devices consist of a primary (PP) and a transmitting (PRP) transducer. In the primary converter, the movement of the float is converted into an AC signal by means of coils included in the bridge circuit. The change in voltage across the coils occurs as a result of a change in their inductance due to the movement of the float from the magnetic material. The signal from the PP goes to the differential amplifier PRP with an output electromagnetic relay. Depending on the position of the level of the monitored liquid, the output relay is triggered, the contacts of which can be used in external monitoring and control circuits of actuators.

The primary converter of the PRU-5MI relay is designed for operation in hazardous areas of premises and outdoor installations, the transmitting converter is used outside hazardous areas.

Material of PP parts in contact with the controlled environment - steel 12X18H10T and steel 08 KP; the float, depending on the aggressiveness of the controlled environment, has a corresponding protective coating.

Power supply of the relay with alternating current voltage of 220 or 380 V and frequency of 50 or 60 Hz. Power consumption no more than 10 VA. Overall dimensions: PP 90x135x180 mm; PRP 152x90x X295 mm; weight: PP no more than 2.5 kg; PRP not more than 2.7kg. Execution is ordinary, tropical.

Sealless diaphragm valves with an unloading spool 15kch888r SVM (5,6, 9v) are controlled by an electromagnetic drive in a waterproof design. The tightness of the shut-off element is ensured with a pressure drop across the spool of at least 0.1 MPa. Ambient temperature for water and air up to 50 ° С, for brine and froen from --50 to +50 ° С. Nominal bore diameter 25, 40, 50, 65. Face-to-face length 160, 170, 230, 290. Working medium brine (-40) - (+45), with oil (-30) - (+45). Conditional pressure 1.6 MPa. The type of current and voltage is variable 127, 220, 380; constant 110, 220. Weight 6.2; 7.8. Manufacturer or supplier "Semenovskiy Valve Plant".

The sensing element TCM (14-18, 19a) is a frameless copper wire winding covered with a fluoroplastic film and placed in a thin-walled metal sleeve with ceramic powder. Sensing element - copper type ECHM - 070 - diameter 5 mm and length 20, 50 or 80 mm. The measurement limits of copper sensitive elements are from - 50 to + 200 ° С, the inertia is 15 and 25 s for the nominal static characteristics of 50M and 100M, respectively.

The signal from the TCM goes to the eight-channel device UKT38-V. UKT38-V (19b) An eight-channel temperature control device with a built-in spark protection barrier

UKT38-V is designed to control the temperature in several zones simultaneously (up to 8) and alarm about the exit of any of the monitored parameters outside the specified limits, as well as for their registration on a computer.

It is used to connect sensors located in hazardous areas in technological equipment in the food, medical and oil refining industries. The device has an intrinsically safe level circuit, which ensures its explosion protection.

UKT38-V is an eight-channel comparison device with eight inputs for connecting sensors, a spark protection unit, a microprocessor-based data processing unit that generates an "Alarm" signal, and one output relay. The registration of the monitored parameters on the computer is carried out through the OWEN AC2 network adapter via the RS-232 interface.

Instrument inputs

UKT38-B has 8 inputs for connecting measuring sensors.

Inputs UKT38-B can only be of the same type and are performed in one of the following modifications:

01 for connecting resistance thermocouples of the TSM 50M or TSP 50P type;

03 for connecting resistance thermocouples such as ТСМ 100М or ТСП 100П;

04 for connecting thermocouples of type TXK (L) or TXA (K);

The data processing unit is designed to process input signals, indicate monitored values and generate an alarm.

The UKT38-B data processing unit includes 8 comparison devices.

Output devices

UKT38-V has one output relay "Emergency" for turning on the alarm or emergency shutdown of the unit.

To control the temperature, a temperature controller of the RT-2 type (106) is used, the sensor of which 10a is installed on the brine (ice water) outlet from the evaporator.

Temperature controllers of the RT-2 (10b) type are designed for two-position RT2, three-position RTZ and proportional RT-P temperature control in automation systems for ventilation, air conditioning and other technological processes. Regulators operate in a set with resistance thermocouples ТСМ and ТСП with nominal static characteristics 1 \ w Gr. 23 and 100P, respectively.

Two-position controllers have an adjustable return zone of 0.5-10 ° C; three-position regulators - adjustable dead zone 0.5-10 ° C. Proportional controllers work in combination with an actuator with a feedback rheostat with a resistance of 120 or 185 ohms. The minimum value of the proportional band is not more than 1 ° C, the maximum is not less than 5 ° C, the sensitivity is no more than 10% of the proportional band. The basic permissible error is no more than 1 ° C at a scale of up to 40 ° C and no more than 2 ° C at a scale of over 40 ° C.

Output contacts switch AC circuits up to 2.5 A and DC circuits up to 0.2 A at voltages up to 220 V.

Power supply of the regulators with 220 V AC, 50 or 60 Hz. Power consumption up to 8 VA.

Regulators are designed to operate at ambient temperatures from 5 to 50 ° C and relative humidity up to 80%.

Overall dimensions 90x150x215 mm, weight no more than 2.5 kg.

Execution ordinary, export, tropical.

Conclusion

Today, the technology of manufacturing refrigeration units is at a very high level. The development of new models of refrigeration units today has even affected the field of microelectronics. Also, the technology of production of refrigeration machines and digital computer technologies were not spared.

The use of computer-controlled refrigeration units in everyday life significantly adds convenience to their operation, creates time savings, and computer control over the state of the unit's components maintains its more reliable and safe operation for many years.

The use of computer-controlled refrigeration units in production increases production efficiency, ensures reliable temperature control, thereby reliably preserving raw materials, and ensures minimal losses.

Perhaps the main disadvantage of such installations is the complexity and high cost of repairing electronic parts of computer control. In addition, electronic components require special operating conditions. Another disadvantage is that computer-controlled refrigerators are quite expensive, but the savings on minimal losses of raw materials during storage in production fully justify the cost of the units.

Another not unimportant problem is the lack of specialists in servicing such equipment. But most enterprises invite specialists from abroad to service imported refrigeration units, since most of the digitally controlled refrigerators are supplied from abroad.

Bibliography

1. Krylov N.V. , Grishin LM Economics of the refrigeration industry. M., Agropromizdat, 1987, 272 p .;

2. Refrigeration technology. 1986, no. 11, p. 2-4;

3. Assessment and improvement of the conditions for refrigerated storage of vegetables. Yankovsky et al., Collection of works of LTIHP. Refrigeration and storage of food products. L., 1974, no. 2, p. 125-132;

4. Uzhansky VS Automation of refrigeration machines and installations. M., Food Industry, 1973, 296 p.

5. Design of automation systems for technological processes. Reference manual, ed. A.S. Klyuev 2nd edition, revised and enlarged Moscow Energoatomizdat 1990.

6. Technological measurements and instrumentation in the food industry Moscow VO "Agropromizdat" 1990.

7. Kolesov L.V. Fundamentals of automation - M .: Kolos, 1984

8. Kirsanov V.V. Mechanization and automation of animal husbandry. - M .: Publishing center "Academy"; 2004.

9. Shishmarev V.Yu. Automation of technological processes.- M .: Publishing center "Academy"; 2007.

10. Shepovalov V.D. Means of automation of industrial animal husbandry. - M .: Kolos, 1981.

11. Gerasimovich L.S., Kalinin L.A. Electrical equipment and automation of agricultural units and installations. - M .: Kolos, 1981.

12. Kudryavtsev I.F., Kalinin L.A. Electrical equipment and automation of agricultural units and installations. - M .: Agropromizdat, 1988.

13. Daineko V.A. Electrical equipment of agricultural enterprises.-M.: Minsa: New edition, 2008.

14. Kaganov I.L. Course and diploma design. - M .: Agropromizdat, 1990.

15. Akimtsev Yu.I., Veyalis B.S. Power supply for agriculture.-M .: Kolos, 1994.

16. Sibikin Yu.D. Power supply for industrial and civil buildings. - M .: Academy, 2006.

17. Sokolova E.M. Electrical and electromechanical equipment. General industrial machinery and household appliances.- M .: Masterstvo, 2001.

Posted on Allbest.ru

Appointment

Plants for propane cooling of natural gas are designed to simultaneously ensure the required parameters of the dew point for water and hydrocarbons through the condensation of water and hydrocarbon fractions (HC) at low temperatures (up to minus 30 0 С). The cold source is an external propane refrigeration cycle.

The main advantage of such plants is the low pressure loss of the feed stream (throttling of the natural gas flow is not required) and the ability to extract the C3 + product fraction.

To prevent hydrate formation, an inhibitor injection is used: ethylene glycol (for temperatures not lower than minus 35 0 С) and methanol (for temperatures up to minus 60 0 С).

Main advantages

Reliability

A continuous process based on the condensation of water and HC fractions in the presence of an inhibitor of hydrate formation.
No cyclical fluctuations.
Shell and tube heat exchanger gas-gas with low temperature head.
The service factor of the refrigeration compressor motor is 110%.
Automatic system for maintaining pressure in the receiver when operating in cold climates.
Electric heating of the inhibitor collector in a three-phase separator.

Efficiency

Cold separator with efficient coalescing packing and long residence time.
Gas-propane heat exchanger (chiller) with submerged tube bundle.

Possible options

Refrigeration cycle economizer (standard for systems over 150 kW and evaporating temperatures below minus 10 0 С).
Inlet separator.
Gas-liquid heat exchanger (allows to reduce the power consumption of the compressor).

Technology system

The moisture-saturated natural gas stream is fed to the inlet separator (1), in which free water and HC fractions are removed from the stream. The gas fraction is sent to the gas-gas heat exchanger (2) for pre-cooling with a dry stripped gas stream from the cold separator. To prevent hydrate formation in the heat exchanger, nozzles are provided for the injection of an inhibitor (methanol or ethylene glycol).

Rice. 3 Schematic diagram of a propane refrigeration unit

After pre-cooling in the gas-gas heat exchanger, the stream is fed to the gas-propane heat exchanger (chiller) (4), in which the temperature of the stream is reduced to a predetermined value by means of heat exchange with a stream of boiling propane. The feed stream is in the tube bundle, which in turn is immersed in the refrigerant volume.

The vapor-liquid mixture formed as a result of cooling enters the separation into a low-temperature three-phase separator (5), where it is divided into streams of stripped gas, condensate, and a hydrate formation inhibitor saturated with water.

Dry stripped gas (DSG) is fed in countercurrent to the gas-gas heat exchanger (2) and then removed outside the unit.

Liquid fractions are diverted by independent automatic level controllers to the appropriate lines.

Gas processing made easy

One of our main tasks is to combat the myth that gas processing is difficult, time-consuming and expensive. Surprisingly, projects that are implemented in the United States in 10 months take up to three years in the CIS. Plants occupying 5,000 m2 in the USA can hardly fit on 20,000 m2 in the CIS. Projects that pay off in the United States in 3-5 years, even with a significantly lower cost of product realization, never pay off in Russia and Kazakhstan.

Cold is used in the technologies of many processes for processing agricultural products. Thanks to refrigerators, losses during storage of products are significantly reduced. Chilled food can be transported over long distances.

Milk intended for processing or sale is usually pre-cooled. Before being sent to a dairy enterprise, milk may be stored for no more than 20 hours at a temperature not exceeding 10 "C.

In agriculture, meat is cooled mainly on farms and poultry farms. In this case, the following cooling methods are used: in air, in cold water, in water with melting ice and irrigation with cold water. Freezing of poultry meat is done either with cold air or by immersion in cold brine. Air freezing is carried out at an air temperature in refrigerating chambers from -23 to -25 ° C and an air speed of 3 ... 4 m / s. For freezing by immersion in brine, solutions of calcium chloride or propylene glycol with a temperature of -10 ° C and below are used.

Meat intended for long-term storage is frozen in the same way as freezing. Freezing

air is carried out at a temperature of cooled air from -30 to -40 ° C, when freezing in brine, the temperature of the solution is -25 ...- 28 ° C.

Eggs are stored in refrigerators at a temperature of -1 ...- 2 ° C and a relative humidity of 85 ... 88%. After cooling to 2 ... 3 ° C, they are placed in a storage chamber.

Fruits and vegetables are chilled in stationary storage facilities. Fruit and vegetable products are stored in refrigerating chambers with cooling batteries in which a cold agent or brine circulates.

In air-cooled systems, air is first cooled, which is then blown into the storage chambers by fans. In mixed systems, food is cooled with cold air and battery.

In agriculture, cold is obtained both without a machine (glaciers, ice-salted cooling) and with the help of special refrigerators. In machine cooling, the heat from the medium to be cooled is removed to the external environment using low-boiling refrigerants (freon or ammonia).

In agriculture, steam compressors and absorption refrigeration machines are widely used.

The simplest way to obtain a working fluid temperature below ambient temperature is that this working fluid (refrigerant) is compressed in a compressor, then cooled to ambient temperature and then subjected to adiabatic expansion. In this case, the working fluid performs work due to its internal energy and its temperature decreases in comparison with the ambient temperature. Thus, the working fluid becomes a source of cold.

In principle, any vapor or gas can be used as refrigerants. In the first refrigeration machines with a mechanical drive, air was used as a refrigerant, but already from the end of the 19th century. it was replaced by ammonia and carbon dioxide, since an air refrigeration machine is less economical and more cumbersome than a steam one, due to the high air flow rate due to its low heat capacity.

In modern refrigeration plants, the working fluid is a vapor of liquids, which, at pressures close to atmospheric, boil at low temperatures. Examples of such refrigerants are ammonia NH3, sulphurous anhydride SO2, carbon dioxide CO 2, and freons - fluorochlorine derivatives of hydrocarbons such as C m H x F y Cl2. The boiling point of ammonia at atmospheric pressure is 33.5 ° C, "Freon-12" -30 ° C, "Freon-22" -42 ° C.

Freons are widely used as refrigerants - halogen derivatives of saturated hydrocarbons (C m H n) obtained by replacing hydrogen atoms with chlorine and fluorine atoms. In technology, due to the wide variety of freons and their relatively complex names, a conditional numerical designation system has been established, according to which each such compound, depending on the chemical formula, has its own number. The first digits in this number conventionally denote the hydrocarbon, the derivative of which is this freon: methane - 1, ethane - 11, propane - 21. If unsubstituted hydrogen atoms are present in the compound, then their number is added to these numbers. Further, to the resulting sum or to the initial number (if all hydrogen atoms in the compound are substituted), add a number expressing the number of fluorine atoms in the form of the next sign. This is how the designations are obtained: R11 instead of monofluorotrichloromethane CFCI2, R12 instead of difluorodichloromethane CF 2 C1 2, etc.

In refrigeration plants, R12 is usually used as a refrigerant, and in the future, R22 and R142 will be widely used. The advantages of freons are relative harmlessness, chemical inertness, incombustibility and explosion safety; disadvantages - low viscosity, conducive to leakage, and the ability to dissolve in oil.

Figure 8.15 shows a schematic diagram steam compressor refrigeration unit and its ideal cycle in a 75-diagram. In the compressor 1 wet refrigerant vapor is compressed, resulting in (section a-b) dry saturated or superheated steam is obtained. Usually the degree of overheating does not exceed

130 ... 140 "C, so as not to complicate the operation of the compressor due to increased mechanical stresses and do not use oil

Rice. 8.15.

/ - compressor; 2 - refrigerated room; 3- throttle valve; 4 - capacitor of special grades. Superheated steam from the compressor with parameters pi and 02 enters the cooler (condenser 2). In the condenser at a constant pressure, superheated steam gives off the superheated heat to the cooling water (process B-c) and its temperature becomes equal to the saturation temperature of 0 H2. Subsequently giving up the heat of vaporization (process c-d), saturated steam turns into a boiling liquid (point d). This fluid flows to the throttle valve 3, after passing through which it turns into saturated steam of a slight degree of dryness (x 5 = 0.1 ... 0.2).

It is known that the enthalpy of the working fluid before and after throttling is the same, and the pressure and temperature decrease. The 7s diagram shows the dashed line of constant enthalpy d-e, point e which characterizes the state of the steam after throttling.

Then the wet steam enters a refrigerated container called a refrigerator. 4. Here, at constant pressure and temperature, steam expands (process e-a), taking away a certain amount of heat. At the same time, the degree of steam dryness increases (x | = 0.9 ... 0.95). Steam with state parameters characterized by a point 1, sucked into the compressor, and the operation of the installation is repeated.

In practice, the steam after the throttle valve enters not into the refrigerator, but into the evaporator, where it takes away heat from the brine, which, in turn, takes away heat from the refrigerator. This is due to the fact that in most cases the refrigeration unit serves a number of cold consumers, and then the non-freezing brine serves as an intermediate coolant, continuously circulating between the evaporator, where it is cooled, and special air coolers in refrigerators. As brines, aqueous solutions of sodium chloride and calcium chloride are used, which have sufficiently low freezing points. The solutions are suitable for use only at temperatures above those at which they freeze as a homogeneous mixture, forming salty ice (the so-called cryohydrate point). The cryohydrate point for a NaCl solution with a mass concentration of 22.4% corresponds to a temperature of -21.2 "C, and for a CaCl 2 solution with a concentration of 29.9, a temperature of -55 ° C.

The indicator of the energy efficiency of refrigeration units is the refrigeration coefficient e, which is the ratio of the specific refrigeration capacity to the consumed energy.

The actual cycle of a vapor compressor refrigeration unit differs from the theoretical one in that, due to the presence of internal frictional losses, compression in the compressor occurs not along the adiabat, but along the polytrope. As a result, the energy consumption in the compressor is reduced and the coefficient of performance is reduced.

To obtain low temperatures (-40 ... 70 ° C) required in some technological processes, single-stage steam compressor plants turn out to be either uneconomical or completely unsuitable due to a decrease in compressor efficiency due to high temperatures of the working fluid at the end of the compression process. In such cases, either special refrigeration cycles or, in most cases, two-stage or multi-stage compression are used. For example, two-stage compression of ammonia vapors obtain temperatures up to -50 ° C, and three-stage compression - up to -70 ° C.

Main advantage absorption refrigeration units in comparison with compressor stations - the use of heat energy of low and medium potentials for generating cold is not electrical, but thermal energy. The latter can be obtained from water vapor taken, for example, from a turbine in combined heat and power plants.

Absorption is the phenomenon of vapor absorption by a liquid substance (absorbent). In this case, the temperature of the steam can be lower than the temperature of the absorbent that absorbs the steam. For the absorption process, it is necessary that the concentration of the absorbed vapor be equal to or greater than the equilibrium concentration of this vapor above the absorbent. Naturally, in absorption refrigeration units, liquid absorbents must absorb the refrigerant at a sufficient rate, and at the same pressures, their boiling point must be significantly higher than the boiling point of the refrigerant.

The most common water-ammonia absorption plants, in which ammonia serves as a refrigerant and water as an absorbent. Ammonia is highly soluble in water. For example, at 0 ° C, up to 1148 volumes of vaporous ammonia dissolve in one volume of water, and a heat of about 1220 kJ / kg is released.

Cold in the absorption unit is generated according to the scheme shown in Figure 8.16. This diagram shows the approximate values of the parameters of the working fluid in the installation without taking into account the pressure loss in the pipelines and the loss of the temperature head in the condenser.

In the generator 1 the saturated ammonia solution is evaporated when it is heated with water vapor. As a result, a low-boiling component is distilled off - ammonia vapor with an insignificant admixture of water vapor. If the temperature of the solution is maintained at about 20 ° C, then the saturation pressure of ammonia vapor will be approximately 0.88 MPa. So that the NH 3 content in the solution does not decrease, using a transfer pump 10 strong concentrate is continuously fed from the absorber to the generator.

Rice. 8.16.

/-generator; 2- capacitor; 3 - throttle valve; 4- evaporator; 5-pump; b-bypass valve; 7- cooled container; absorber; 9-coil; 10- pump

bath ammonia solution. Saturated ammonia vapor (x = 1), obtained in the generator, is sent to the condenser 2, where ammonia turns to liquid (x = 0). After choke 3 ammonia enters the evaporator 4, in this case, its pressure decreases to 0.3 MPa (/ n = -10 ° C) and the degree of dryness becomes approximately 0.2. „0.3. In the evaporator, the ammonia solution is evaporated due to the heat supplied by the brine from the cooled tank 7. In this case, the temperature of the brine decreases from -5 to -8 ° C. By pump 5 it is distilled back into tank 7, where it is again heated to -5 ° C, taking heat from the room and maintaining a constant temperature in it, about -2 ° C. The ammonia evaporated in the evaporator with a dryness degree x = 1 enters the absorber 8, where absorbed by a weak solution supplied through the bypass valve 6 from the generator. Since absorption is an exothermic reaction, in order to ensure the continuity of the heat exchange process, the absorbent is removed with cooling water. The strong ammonia solution obtained in the absorber pump 10 pumps into the generator.

Thus, in the considered installation, there are two devices (generator and evaporator), where heat is supplied to the working medium from the outside, and two devices (condenser and absorber), in which heat is removed from the working medium. Comparing the schematic diagrams of the steam compressor and absorption units, it can be noted that the generator in the absorption unit replaces the discharge part, and the absorber replaces the suction part of the reciprocating compressor. Compression of the refrigerant occurs without the expense of mechanical energy, apart from the small cost of pumping the strong solution from the absorber to the generator.

In practical calculations, the cooling coefficient e, which is the ratio of the amount of heat q 2 perceived by the working fluid in the evaporator to the amount of heat q u spent in the generator. The coefficient of performance calculated in this way is always lower than the coefficient of performance of the steam compressor plant. However, a comparative assessment of the energy efficiency of the considered methods of obtaining cold as a result of direct comparison of the methods of only the refrigerating coefficients of the absorption and steam compressor installations is incorrect, since it is determined not only by the amount, but also by the type of energy expended. The two methods of obtaining cold should be compared according to the value of the reduced coefficient of performance, which is the ratio of the refrigerating capacity q 2 to fuel heat consumption q it i.e.? pr = Yag I- It turns out that at evaporation temperatures from -15 to -20 ° C (used by the bulk of consumers), e pr absorption plants are higher than steam compressor plants, as a result of which, in a number of cases, absorption plants are more profitable not only when supplying them with steam taken from turbines, but also when supplying them with steam directly from steam boilers.

LECTURE 9

Topic "Instrumentation and automation of the refrigeration machine"

Target: To study the device and principle of operation of instrumentation and automation devices for refrigeration machines of cars

1. Chillers and air conditioning units. Pigarev V.E., Arkhipov P.E. M., Route, 2003.

2. Educational control program "Air conditioning in a passenger carriage".

Lecture plan:

1. Principles of automation of refrigeration units.

2. Basic concepts of automatic regulation

automation devices.

4. Regulators for filling the evaporator with refrigerant.

Refrigeration automation principles

Environmental parameters - temperature, humidity, wind direction and strength, precipitation, solar radiation - continuously change during the day, as well as due to the rapid movement of the carriage. The heat load on the car also changes accordingly. In order to maintain stable air parameters inside the car under these conditions, it is necessary to continuously change the performance of the cooling system (in summer) or heating (in winter), and if necessary, the performance of the ventilation system. Consequently, no matter how perfect the ventilation, heating, cooling and power supply systems themselves are and no matter how well their parameters are coordinated with each other and with the thermal loads on the car, the air conditioning installation will not be able to provide comfortable conditions in the car if its control will not be automated, and the refrigeration machine will provide the required heat treatment of perishable cargo and maintain the specified temperature regime of the refrigerated space. Refrigerated rolling stock uses refrigeration units, fully or partially automated. The degree of automation of the refrigeration unit is selected depending on its design, dimensions and operating conditions. In fully automated systems, start-up, shutdown of machines and regulation of cooling capacity are carried out automatically without the intervention of service personnel. Such installations are equipped with ARV and sections ZB-5. Full automation requires high initial and subsequent maintenance costs for complex apparatus and instruments. However, full automation of ARV refrigeration units made it possible to abandon the maintenance of cars along the route by service personnel and switch to their periodic maintenance at specialized points (PTO ARV).

When operating partially automated refrigeration units, a constant watch of service personnel is required. The presence of personnel allows you to abandon the automation of turning on and off the refrigeration machine, the process of defrosting the air cooler, etc. As a result, a significant reduction in initial costs is achieved. Protective automation in such machines should be provided in full, as well as for a fully automated installation.

From partially automated installations, semi-automated installations are conventionally distinguished, in which the switching on and off of the equipment is performed manually by a mechanic, and the maintenance of the established operating mode is carried out by automation devices. Semi-automated refrigeration units include units of a 5-car section of BMZ.

Automated refrigeration units always operate at optimum performance. This makes it possible to reduce the time required to reach the required temperature in the cargo space, thereby increase the turnaround time of the equipment and reduce energy consumption. An automated refrigeration unit more accurately maintains a given temperature regime in a refrigerated room, which cannot be achieved with manual control. This allows you to preserve the quality of the transported goods and reduce their losses during transportation. The automation system reliably protects the refrigeration unit from hazardous operating modes, increasing its service life and ensuring safety for the operating personnel. Automation raises the culture of production, improves and facilitates the working conditions of the service personnel. In practice, the duties of the train crew are reduced to periodic inspections and checks of the equipment operation mode and to the elimination of identified malfunctions. Naturally, the automation systems are different. With regard to automation systems, air conditioning units can be classified according to three criteria: according to controlled air parameters: temperature or humidity, or both of these parameters, i.e. by heat content; by the nature of the air treatment process: wet humidification and drying chambers with direct spraying and filtration of the steam-air mixture, or chambers with surface wetting and also direct heat and mass transfer, or chambers using heat exchange through a cold (or hot) wall cooled with cold water or brine (heated hot water or brine), or chambers with direct expansion air coolers, or chambers with solid or liquid desiccants - adsorbents; according to the air treatment scheme: direct-flow chambers (without the use of recirculation), or chambers with a constant or variable primary recirculation, or chambers with double constant or variable recirculation. A special device for regulating humidity (special air drying is carried out by its deeper cooling than is necessary to maintain the temperature regime with subsequent heating) is not used in carriage air conditioning installations. In summer, when air dehumidification is required, it is performed simultaneously with the process of cooling it in the air cooler. In winter, when it is necessary to humidify the air, it is carried out due to the moisture release of passengers. Thus, according to the first feature, the process of automatic regulation of the operation of car air conditioning units is the simplest and comes down to maintaining the temperature in the car's premises within the specified limits. Wet chambers, solid and liquid adsorbents, heat exchange using water or brine cooling are not used in passenger cars. It follows from this that, according to the second feature, the automation systems of carriage air conditioners are quite simple. Neither variable, let alone double recirculation, both constant and variable, is not used in wagons. The presence of recirculation with a constant ratio of outdoor and recirculated air only complicates the ventilation system, without making any changes to the automatic control system. Thus, according to the third criterion, and therefore, in general, the automation systems of air conditioning units for passenger cars are relatively simple compared to the automation systems of other air conditioners, both comfortable and technological. To maintain the temperature in the cooled room within the specified interval, it is necessary to regulate the cooling capacity of the installation, calculated for the maximum demand for cold. Regulation can be smooth or positional (stepwise).

Smooth regulation can be done: by smoothly changing the compressor shaft rotation frequency; bypassing (bayling) steam from the discharge line to the suction line; a change in the working volume of the compressor (in screw compressors); by opening the suction valve on a part of the piston stroke, etc. Many of the above methods are rarely used because of the complexity of their structural implementation or because of significant energy losses.

Positional control can be performed by changing the coefficient of working time, i.e. change in the duration of the refrigeration unit per cycle. This method is widely used in systems with high thermal storage capacity. Positional control is also performed by stepping the compressor crankshaft speed using multi-speed electric motors. The frequency of rotation of the motor shaft is changed by switching the stator poles. On refrigerated rolling stock, refrigeration capacity is regulated by changing the working time coefficient. The cyclical operation of the refrigeration unit is achieved by periodic switching on and off. The ratio of the operating time of the refrigeration unit р to the total cycle time  is called the working time coefficient: b =p /  .

The working time coefficient can also be defined as the ratio of heat gains into the cooled room Q t to the refrigerating capacity of the installation Q 0, i.e. b = Qt/Q 0.

The temperature in the refrigerated room of refrigerated cars is usually controlled by periodic switching on and off of the refrigeration unit using a two-position automatic device - a thermostat (temperature switch). During cyclic operation, the temperature in the refrigerated room does not remain constant, but varies within certain limits, which depend on the setting of the thermostat differential. As the differential increases, cycle times and temperature fluctuations increase. When the temperature in the refrigerated room reaches the upper set limit, the thermostat will turn on the refrigeration unit. After the temperature in the cooled room reaches the lower limit, the thermostat gives an electrical impulse to shut down the unit. With an increase in heat inflows into the car, the duration of the installation increases.

2. Basic concepts

about automatic regulation

An automatic control system is a combination of a control object and a control device that carry out a process in whole or in part without the intervention of the operating personnel. A control object - a complex of technical elements that perform the main technological task - is characterized by the values of some quantities at its input and output. If we consider a refrigerated wagon as a control object, then the output value will be the temperature in the cargo space t vag , and the input value is the refrigerating capacity of the refrigerating machine Q 0. The output value, which is required to be maintained within a certain interval, is called the controlled parameter and is denoted X 0. An input quantity to an object is a parameter that controls the output quantity. External influence on the control object, causing a deviation of the controlled parameter from the initial value NS 0, called the load. In this case, these will be heat flows into the car. Q n. Actual value of the controlled parameter X under load Q n deviates from the set value X 0. This deviation is called mismatch:  X = X - X 0. Impact on the object, which reduces the misalignment  NS, is a regulatory effect. In our example, this will be the cooling capacity of the machine Q 0. If Q 0 = Qн, then  X = 0, and the adjustable parameter does not change: NS 0 - const .

A device that senses the AX mismatch and acts on the object to reduce the mismatch is called an automatic regulator, or simply a regulator.

The object and the regulator form an automatic control system (Fig. 1).

Rice. 1. Automatic control system

Regulation can be carried out on load and mismatch. In the first case, the regulator

perceives a change in the load and changes the regulatory influence by the same amount, maintaining equality Q 0 = Qн... However, it is easier to monitor the deviation of the controlled parameter. NS 0, those. change the regulatory action Q 0 depending on the value  NS.

Automation systems differ in their purpose: control, signaling, protection, regulation and combined. Between themselves, they differ in the composition of the elements and the connections between them. The structural diagram of an automatic system determines which links it consists of. For example, an automatic control system includes a control object and an automatic regulator, consisting of several elements - a sensitive element, a driver, a comparison element, a regulating body, etc. In fig. 2 shows a simple single-loop automatic control system widely used in refrigeration automation. The operation of the object is characterized by the parameter X at the outlet for which regulation is carried out. The object is affected by an external load Q n. Management is carried out by regulatory action Q 0. The automatic regulator should change the value Q 0 so that the value X. corresponded to the given NS 0. The system has direct and feedback circuits. The direct communication chain serves to form and transmit to the object of the regulatory influence Q 0; the feedback loop provides information on the progress of the process. The direct communication circuit includes an amplifier (U), an actuator (IM ) and the regulatory body (RO). A sensing element (SE ).

Rice. 2. Block diagram of automatic regulation

Both circuits are closed by a comparison element (ES). Individual elements (amplifier, actuator) may not be used in the regulator. Some parts can serve as multiple elements.

The system works as follows. The regulator perceives the regulated parameter as a sensitive element X and converts it to a value NS 1, convenient for further transmission.

This converted value enters the comparison element, to the other input of which the signal NS 2, representing the task to the regulator from the device 3. In the comparison element, a subtraction operation is performed, as a result of which the mismatch is obtained  NS= X– NS 0.

Signal  NS makes the rest of the circuitry work. In the amplifier, its power rises to NS 3 and acts on the actuator, which converts this signal into a form of energy convenient for use. X 4 and changes the position of the regulator. As a result, the flow of energy or matter supplied to the object changes, i.e. the regulatory influence changes.

Taking a refrigerated car as an example, it is possible to trace the interaction of the elements of the structural diagram (Fig. 1 and 2).

Temperature in the car X perceives the thermostat's temperature-sensitive system, converts it into pressure NS 1 and acts on the thermostat spring ES, adjusted to a specific compression force by the driver screw 3. When the temperature in the carriage rises t vag as a result of heat influx Q m mismatch increases  X.

At a certain value t wag closes the thermostat contacts, including the electrical control system of the refrigeration machine U, which receives energy E from an external source. Actuators THEM electrical system include the refrigeration machine RO, which affects the value Q n to the object. Block diagrams of other automatic devices can be obtained from the considered diagram. The signaling system differs from the control system in that it does not have an actuator. The feed-forward chain is broken and the signal X3 served to the service personnel (call, turning on the warning lamp), which must make the regulation. In the automatic protection system, instead of an actuator and a regulating body, there is a control device that turns off the refrigeration unit. In alarm and protection systems, the signal X3 changes abruptly when the quantity X reaches the set value. Automatic regulators are classified according to their purpose: regulators of pressure, temperature, level, etc. They differ in the design of the sensing element. Regulators are of direct and indirect action. If the power of the error signal is sufficient to influence the regulator, the regulator is considered direct acting. Indirect-acting regulators use an external power source to drive the regulator E(electric, pneumatic, hydraulic, combined) supplied through a power amplifier W.

Depending on the method of influence on the object, regulators of smooth and positional (relay) action are distinguished. In modulating regulators, the regulator can take any position between the maximum and minimum. In positional controllers, the regulator can have two or more specific positions. By the type of master element, regulators are stabilizing, software, tracking, optimizing. Stabilizing regulators maintain the regulated value at a constant set level. Software controllers change the controlled value according to a pre-planned program, follow-up ones - depending on changes in some external parameter, Optimizing controllers, analyzing external parameters, provide optimal process control. In refrigeration plants, stabilizing regulators are more often used.

The control system adjusts the characteristics of the individual elements of the machine with changes in their cooling capacity.

The characteristics represent the dependences of cooling capacity, energy consumption for compressor operation and condenser cooling on external conditions, i.e. from the ambient temperature. They make it possible to establish the interconnection of the parameters of the compressor, evaporator and condenser. The construction of characteristics is carried out according to the equations of the heat balance of the system "refrigeration machine - refrigerated room" and energy ratios describing the operation of the main elements of the machine, taking into account the time variation of the parameters of the refrigerant and the environment. In this case, the balance and energy ratios are represented as a function of the temperature of the cooled object (boiling point of the refrigerant) and the ambient temperature (condensation temperature of the refrigerant).

The process of regulating the machine to the required cooling mode or to a given temperature regime can theoretically be implemented quantitatively or qualitatively. The first involves changing the flow rate of the refrigerant through the evaporator, the second - changing its parameters. However, the temperature of the object being cooled is determined by the boiling point of the refrigerant, which self-adjusts depending on the refrigerating capacity of the compressor, evaporator and condenser. Therefore, the control process determines not only the balance of the compressor cooling capacity Q ok and evaporator Q oi , but also the temperature level of heat removal or supply. Therefore, the regulation of a steam compressor machine is a combined process, combining quantitative and qualitative methods.

The throttling valve serves as the executive body of the regulation system (cooling capacity regulator). The operating mode of the machine, which corresponds to the intersection point of the compressor and evaporator characteristics Q ok = Q oi , provide a change in the flow area of the valve. The diagram of matching the characteristics of the main elements of the machine at a certain constant value of the ambient temperature is shown in Fig. 3.

Evaporator characteristic Q ok = f(T 0) (T 0 - boiling point of the refrigerant) corresponds to a change in the heat flow of the cooled room, compressor characteristic Q ok = f(T 0) - regulation of its performance, flow characteristic of the throttle valve Q dv = f(T 0) sets the degree of its closing or opening. The characteristics of the listed elements of the machine when changing the mode of its operation are shown by dashed lines. Point A defines the operating point of the "machine - refrigerated room" system as an object of regulation during the transition from one operating mode to another. In this case, the point A′  corresponds to the operating mode in the process of compressor regulation, and the point A′′ - when changing the characteristics of the evaporator. Cooling capacity of a machine with a piston compressor is controlled by smooth or step (positional) regulation of its capacity. In machines of small and medium power, the following methods of smooth regulation with the help of external or built-in structural devices have become widespread: refrigerant bypass from the discharge side to the suction (balancing), which is carried out by control valves controlled from a pressure or temperature sensor; throttling at suction with the transfer of the compressor to work at a reduced suction pressure; changing the volume of dead space by connecting an additional external volume to it; change of the compressor shaft rotation frequency.

Rice. 3. Characteristics of the main elements of the refrigeration machine

Step-by-step regulation in machines of low and medium refrigerating capacity is mainly carried out by the "start-stop" method with a maximum cycle frequency of up to 5-6 per hour; for multistage compressors, the shutdown of individual cylinders is effectively used by depressing the suction valves with the help of mechanical pushers. The movement of the pushers is controlled by hydraulic, pneumatic or electromagnetic drives. An electronic performance control system is introduced with the effect of an electromagnetic field on the suction valves.

An example of stepwise proportional control is the regulation of the air temperature in the car in the summer, when, with an increase in the heat flow into the car, the refrigeration capacity of the refrigeration unit increases (the rotational speed of the compressor shaft increases or more of its cylinders are turned on). In this case, an impulse signaling the need to increase the refrigeration capacity is a further increase in the air temperature in the car.

An example of proportional modulating control is the regulation of the air temperature in the car in winter, when, with an increase in the heat loss of the car, the water temperature in the hot water heating boiler smoothly increases. In this case, an impulse signaling the need to increase the water temperature in the boiler is a change in the outside air temperature. The most perfect, but also the most complex type of proportional control is isodromic control, based on the use of sensitive and flexible feedback, due to which the controlled parameter varies within very narrow limits or even remains at an almost constant level. Initially, isodromic regulation was used to ensure a constant speed of rotation of machine parts, from which it got its name (in Greek, iso - constant, equal; dromos - run, speed). Currently, it is used in a wide variety of processes, for example, to automatically steer sea ships along a given course.

Due to the complexity of the equipment, the difficult conditions of its operation with vibration and shaking, and most importantly, due to the absence of a practical need for extremely precise air temperature control, isodromic regulation is not used in air conditioning installations for cars.

When choosing a control method, it is necessary to take into account the initial and operating costs, manufacturability and reliability of the structure. To assess the energy efficiency of the control system, the ratio of the compressor cooling capacity at a given degree of control to the nominal one is used:  = qop / qon = f (T 0). Indicators of the comparative efficiency of the main methods for regulating the performance of reciprocating compressors are shown in Fig. 4. For start-stop methods (line 1) and depressing the inlet valves (line 2 ) are characterized by low energy losses and practical independence from the operating mode. Suction throttling (line 3 ) there is a sharp drop in efficiency with an increase in the boiling point of the refrigerant, therefore this method is used in compressors that operate in a narrow range of boiling pressures. Balancing (line 4 ) - the least effective control option, since it is associated with the loss of energy of the compressed steam during its bypass, an increase in the suction temperature of the refrigerant, and, consequently, the discharge temperature; energy losses with this method correspond to the degree of reduction of the cooling capacity of the machine.

In refrigeration machines with screw compressors, the following methods of regulating the refrigerating capacity are used: throttling at the suction, balancing, changing the shaft rotation speed, and a slide valve system.

Throttling is provided by automatic closing of the throttle valve installed at the compressor inlet. The effectiveness of this method is limited by a decrease in productivity up to 70% of the nominal; with deeper throttling, efficiency is significantly reduced.

Rice. 4. Energy efficiency of the main methods of regulating the performance of reciprocating compressors

Balancing is carried out by bypassing part of the refrigerant through the safety valve from the discharge side to the suction side.

Application of this method is usually limited to dry compressors.

The most economical regulation by switching off part of the volume of the working cavities during the compression process is provided by the spool system. Despite the complication of the compressor design, such a system opens up additional circuit possibilities for improving steam refrigeration machines.

Automation of the refrigeration machine allows high accuracy to maintain the required level of the parameters of the cooling process, corresponding to the optimal technological mode, as well as partially or completely exclude the participation of maintenance personnel in the operation of refrigeration equipment.

In steam compressor machines, the objects of automation are heat exchangers, in particular the degree of filling the evaporator with liquid refrigerant and the pressure of the condensation process. The objective and technically most convenient indicator reflecting the degree of filling of the evaporator is steam overheating

at the exit from it. Indeed, when a part of the heat transfer surface of the evaporator provides superheating of the refrigerant vapors, a decrease in its supply leads to a decrease in the degree of filling, and, consequently, to an increase in superheat. At the same time, an increase in the overheating temperature above the design level worsens the energy performance of the machine and the reliability of its operation. The amount of refrigerant supplied to the evaporator in excess of the heat transfer capability is associated with overflowing the evaporator and decreasing superheat. The latter leads to a decrease in the cooling capacity of the machine, and in some cases to the operation of the compressor on wet steam, which can lead to a water hammer.

Systems for automatic control of the degree of filling of the evaporator by superheating of refrigerant vapors are smooth and positional (usually two-stage). Thermostatic expansion valves (TRV) are widely used as automatic control in floating systems, in which the superheat of the refrigerant vapor is obtained as the difference between the temperature of the vapor leaving the evaporator and the boiling point of the refrigerant. Thermostatic expansion valves, ensuring the process of throttling the refrigerant from condensing pressure to evaporating pressure, are installed on the line between the condenser and the evaporator.

A schematic diagram of automatic control of the refrigerant level in the evaporator using the expansion valve, used in RPS freon machines, is shown in Fig. 5. Sensing element of the measuring head 1 thermostatic expansion valve made in the form of a membrane 2 or bellows, is influenced by the pressure difference between the superheated steam, corresponding to the superheat temperature, and the refrigerant at the outlet of the evaporator 7 corresponding to the boiling point. Superheated steam that is generated in a thermosystem consisting of a thermocylinder 6 and capillary 3 , enters the space above the membrane; the space under the membrane is connected with an equalizing pipe 4 with compressor suction line 5 ... In this case, the equalizing pipe is connected to the suction line at the place where the bulb is installed. In some designs, a solid absorber is introduced into the thermal balloon and the entire thermal system is filled with gas.

Moving the stem 12 as a result of deformation of the sensing element when the temperature of overheating changes, it opens or closes the shut-off valve 11 regulating the flow of liquid refrigerant from the condenser to the evaporator through the line 10 ... With the adjusting screw 8 change the tightening force of the spring 9 and, therefore, the required overheating temperature. In the process of automatic control, the expansion valve must ensure the optimal level of filling of the evaporator and the stability of the system in the entire required range of changes in cooling capacity, which is especially important for refrigeration machines of refrigerated rolling stock. Practically stable operation of the expansion valve system begins when overheating (3 6) K. To expand the regulation range and increase its stability, several expansion valves can be used in the system.

Rice. 5. Scheme of automatic control of the refrigerant level in the evaporator using the expansion valve

The process of automatic regulation of the refrigerant condensation pressure in machines with air-cooled condensers is carried out by changing the speed or flow rate of the cooling air.

Technically, it is provided with a shutter or butterfly valve system, the use of fans with a variable angle of installation of the guide blades, the use of two-speed electric motors, as well as periodic switching off of the fans. A change in the speed or flow rate of the cooling air leads to a change in the heat transfer coefficient of the condenser, and therefore to

changes in temperature and pressure of the condensation process.

In some cases, an increase in the condensation temperature is achieved by partial flooding of the condenser surface with liquid

refrigerant.

Automatic control devices, in addition to monitoring the parameters of the evaporator and condenser, maintain the set air temperature in the cooled room, ensure the timely removal of frost ("snow coat") from the evaporator surface, regulate the oil level in oil separators, etc. The operation of the control system is combined with automatic protection, which includes a set of measures for the safe operation of refrigeration machines and prevents emergency modes by turning off the machine.

The automatic protection system includes appropriate sensors (protection relays and devices for converting pulses from these relays into a stop signal). In some cases, the protection system is supplemented with an interlock, which excludes restarting the machine without eliminating the cause that triggered the protection.

In compressor chillers, protection system sensors monitor the level of the maximum pressure and temperature of the refrigerant at the compressor discharge, the minimum pressure at the suction, the pressure and temperature of the oil in the lubrication system, and the operation of the electric motor, which prevents its overload or short circuit. A light or sound alarm can be introduced into the system of automatic protection, notifying about reaching the limit value of the monitored value or approaching a dangerous mode of operation of the machine.

3. Classification and basic elements

automation devices

By purpose, automation devices can be divided into four main groups: regulation, protection, control, alarm.

Automatic control devices provide switching on or off of the refrigeration unit and its individual devices, and also control the operation processes. In refrigeration plants of rolling stock, control devices perform the following functions: correctly fill the evaporator with refrigerant (thermostatic valves, etc.); maintain the temperature in refrigerated rooms at specified intervals (thermostats, duostats); regulate the pressure in the condenser in a given interval (pressure switches); ensure timely thawing of frost from the evaporator (pressure switches, program relays, thermostats); open or stop the supply of liquid or vapor refrigerant (solenoid valves, check valves); limit the flow of refrigerant to the compressor from the evaporator (suction pressure regulators).

Automatic protection devices turn off the entire refrigeration unit or individual devices when dangerous operating modes occur: when the maximum allowable discharge pressure is reached (pressure switches); with vacuum on the suction side (pressure switches); in case of a drop in oil pressure in the compressor lubrication system (pressure difference); at low oil temperature in the compressor crankcase (thermostats); at a high temperature of the refrigerant vapor compressed in the compressor (temperature switch); when the motor is overloaded or short-circuited (thermal relays, circuit breakers, fuses).

Automatic control devices measure and, in some cases, record certain parameters of the refrigeration unit, for example, temperature in a refrigerated room (thermograph), electricity consumption (electricity meter), equipment operating time (hour meters), etc. Automatic alarm devices include light or sound signals when the set value of the controlled value is reached or when approaching a dangerous mode of operation of the machine.

Automation devices consist of the following main parts: a sensitive element (sensor), a transmitting mechanism, a regulating (working) body, a device for adjusting (setter). The sensing element perceives the controlled value (temperature, pressure, liquid level, etc.) and converts it into a convenient form of energy for remote transmission. The transmission mechanism connects the sensitive element with the regulating (working) body.

The regulator acts on the signal of the sensing element. In devices of on-off action (relay), the working body can occupy only two positions. For example, the electrical contacts of the pressure switch (pressure switch) or the temperature switch (thermostat) can be closed or open, the solenoid valve can be closed or open. In devices of smooth (proportional) action, each change in the controlled value corresponds to the movement of the regulating element (for example, smooth movement of the control valve valve when the heat load on the evaporator changes). The device for adjusting the device sets the setpoint of the controlled or monitored value. The deviation of the controlled value, which does not cause movement of the regulator, is called the dead band, or the differential of the device. Sensitive elements of pressure devices are made in the form of bellows and diaphragms. The bellows is a thin-walled corrugated tube. Bellows are made of brass, bronze, stainless steel. When the pressure in the bellows changes, its length can change significantly. Membranes are made in the form of round elastic plates fixed around the perimeter. Membranes can be elastic (metal) and soft (rubber, plastic, rubberized fabrics).

204 Temperature sensitive elements are made in the form of bimetallic plates and temperature-sensitive systems with various fillers. In elements based on the expansion of solids upon heating, temperature is converted into mechanical movement (dilatometric elements). The movement occurs due to the unequal coefficients of linear expansion for different metals. In fig. 3.6 a, b showing elements with two metal parts 1 and 2 made of different materials, in Fig. 3.6 c, d - bimetal sensing element, i.e. made of two layers of metals welded together.

In elements with thermal expansion of liquids, the dependence of the change in the volume of liquid on temperature is used. Mercury-filled sensors (fig. 3.7, a, b), are used to convert temperature into an electrical signal without an intermediate mechanical system. The sensor in Fig. 3.7, a has a relay characteristic, in fig. 3.7, b - smooth. The mercury contact temperature sensors previously used on refrigerated trains turned out to be insufficiently reliable, since due to vibrations and shocks on the move, ruptures of the mercury column appeared and the electrical circuit was disrupted. In addition, mercury-contact sensors are designed for low electrical signal power.

Rice. 3.6. Dilatometric Sensing Elements

Rice. 3.7. Liquid

thermosensitive