LECTURE 9

Topic: “Instrumentation and automation of refrigeration machines”

Target: Study the structure and principle of operation of instrumentation and automation devices refrigeration machines carriages

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

2. Educational control program “Air conditioning in a passenger car.”

Lecture outline:

1. Automation principles refrigeration units.

2. Basic concepts about automatic control

automation devices.

4. Regulators for filling the evaporator with refrigerant.

Principles of refrigeration automation

Options environment- temperature, humidity, wind direction and strength, precipitation, solar radiation continuously change during the day, as well as due to the rapid movement of the car. The thermal 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 refrigerated room. Refrigeration units, fully or partially automated, are used on refrigerated rolling stock. The degree of automation of the refrigeration unit is selected depending on its design, size and operating conditions. In fully automated installations, start-up, shutdown of machines and regulation of refrigeration capacity are carried out automatically without the intervention of operating personnel. ARV and sections are equipped with such installations ZB-5. Full automation requires large initial costs and subsequent maintenance costs for complex machines and devices. However, the full automation of ARV refrigeration units made it possible to abandon the support of wagons along the route by service personnel and switch to periodic maintenance at specialized points (PTO ARV).

When operating partially automated refrigeration units, constant duty of maintenance personnel is required. The presence of personnel makes it possible 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. Safety automation in such machines must be provided in full, as for a fully automated installation.

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

Automated refrigeration units always operate in optimal mode. This allows you to reduce the time it takes to reach the required temperature in the cargo room, thereby increasing the time between repairs of equipment and reducing energy consumption. An automated refrigeration unit more accurately maintains the set temperature in the refrigerated room, which cannot be achieved with manual control. This allows you to maintain the quality of transported goods and reduce their losses during transportation. The automation system reliably protects the refrigeration unit from hazardous operating conditions, increasing its service life and ensuring safety for operating personnel. Automation improves production culture, improves and facilitates the working conditions of service personnel. In practice, the responsibilities of the train crew are reduced to periodic inspections and checks of the operating mode of the equipment and to the elimination of identified malfunctions. Naturally, automation systems are different. In relation to automation systems, air conditioning installations 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; according to the nature of the air treatment process: wet humidification and drying chambers with direct spraying and filtration189 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 cold water or brine (heated hot water or brine), or chambers with direct cooling air coolers, or chambers with solid or liquid desiccant absorbers - adsorbents; according to the air treatment scheme: direct-flow chambers (without the use of recirculation), or chambers with a constant or variable value of primary recirculation, or chambers with double recirculation, constant or variable. A special device for regulating humidity (special air drying is carried out by cooling it more deeply than is necessary to maintain the temperature regime with subsequent heating) is not used in carriage air conditioning units. In summer, when air drying is required, it is carried out simultaneously with the cooling process in the air cooler. In winter, when air humidification is necessary, it is carried out due to the moisture released by passengers. Thus, according to the first sign, 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 premises within specified limits. Wet chambers, solid and liquid adsorbents, and heat exchange using water or brine cooling are not used in passenger cars. It follows from this that in terms of the second characteristic, the automatic systems of carriage air conditioners are quite simple. Neither variable, nor even double recirculation, both constant and variable, is used in cars. The presence of recirculation with a constant ratio of outside and recirculated air only complicates the ventilation system, without making any changes to the automatic control system. Thus, according to the third characteristic, and therefore in general, the automation systems for air conditioning units in passenger cars, in comparison with the automation systems for other air conditioners, both comfortable and technological, are relatively simple. To maintain the temperature in the refrigerated room within a given range, it is necessary to adjust the refrigeration capacity of the installation, designed for the maximum need for cold. The regulation can be smooth or positional (stepwise).

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

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

The working time coefficient can also be defined as the ratio of heat inflows into the refrigerated room Q t to the cooling capacity of the installation Q 0, i.e. b = Qт/Q 0.

The temperature in the refrigerated room of refrigerated cars is usually regulated by periodically turning the refrigeration unit on and off 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, the cycle duration and temperature fluctuation limits 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 refrigerated room reaches the lower limit, the thermostat sends an electrical impulse to turn off the unit. With an increase in heat flow into the car, the operating time 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 carries out a process in whole or in part without the intervention of maintenance personnel. A control object - a complex of technical elements that perform the main technological task - is characterized by the values ​​of certain quantities at its input and output. If we consider a refrigerated car as a control object, then the output value will be the temperature in the cargo room t vag , and the input value is the cooling capacity of the refrigeration machine Q 0. The output value that needs to be maintained in a certain interval is called an adjustable parameter and is designated X 0. The value at the input of an object is a parameter with the help of which the value of the output value is controlled. External influence on the control object, causing a deviation of the controlled parameter from the initial value X 0, called load. In this case, these will be heat inflows into the car Q n. Actual value of the controlled parameter X under load Q n deviates from the specified value X 0. This deviation is called mismatch:  X=X – X 0. Impact on the object that reduces the mismatch  X, is a regulatory influence. In our example, this will be the cooling capacity of the machine Q 0. If Q 0 = Qн, then  X = 0, a the adjustable parameter does not change: X 0 - const .

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

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

Rice. 1. Automatic control system

Regulation can be performed based on load and mismatch. In the first case, the regulator

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

Automation systems differ in their purpose: control, alarm, protection, regulation and combined. They differ from each other in the composition of elements and the connections between them. The block diagram of an automatic system determines what parts it consists of. For example, an automatic control system includes a control object and an automatic controller, consisting of several elements - a sensitive element, a master device, a comparison element, a regulator, etc. In Fig. Figure 2 shows a simple single-circuit automatic control system, widely used in the automation of refrigeration units. The operation of the object is characterized by the parameter X at the output through which regulation is carried out. An external load is applied to the object Q n. Management is carried out by regulatory influence Q 0. The automatic controller should change the value in this way Q 0 to value X. corresponded to the specified X 0. The system contains direct and feedback. The direct communication circuit serves to generate and transmit regulatory influence to the object Q 0; The feedback circuit provides information about the progress of the process. The direct communication circuit includes an amplifier (A), an actuator (AM) ) and the regulatory authority (RO). A sensitive element (SE) is included in the feedback circuit ).

Rice. 2. Block diagram of automatic control

Both circuits are closed by a comparison element (EC). The regulator may not use individual elements (amplifier, actuator). Some parts can perform the functions of several elements.

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

This converted value enters the comparison element, the other input of which receives a signal X 2, representing a task to the controller from the device 3. In the comparison element, a subtraction operation is performed, which results in a mismatch  X= X– X 0.

Signal  X makes the remaining elements of the circuit work. In the amplifier its power increases to X 3 and influences the actuator, which converts this signal into a convenient form of energy. X 4 and changes the position of the regulatory body. As a result, the flow of energy or matter supplied to the object changes, i.e. the regulatory influence changes.

Using a refrigerated car as an example, you can trace the interaction of the elements of the structural diagram (Fig. 1 and 2).

Temperature in the carriage X senses the temperature-sensitive system of the thermostat, converts it into pressure X 1 and acts on the thermostat spring ES, adjusted to a certain compression force by the screw of the setting device 3. When the temperature in the car rises t vag as a result of heat inflows Q n the mismatch increases  X.

At a certain value t vag the thermostat contacts close, including electrical system refrigeration machine control Uh, which receives energy E from an external source. Actuators THEM electrical systems include the chiller RO, which affects the magnitude Q n to the object. Block diagrams of others 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 direct communication circuit is broken and the signal X3 is supplied to the service personnel (bell, switching on of the signal lamp), who must make the adjustment. In the automatic protection system, instead of an actuator and a regulator, there is a control device that turns off the refrigeration unit. In alarm and protection systems, the signal X3 changes abruptly when the value X reaches the set value. Automatic regulators are classified by purpose: pressure, temperature, level regulators, etc. They differ in the design of the sensing element. Regulators come in direct and indirect action. If the power of the mismatch signal is sufficient to influence the regulator, the regulator is considered to be direct. Indirect-acting regulators use an external energy source to drive the regulator E(electric, pneumatic, hydraulic, combined), supplied through a power amplifier U.

Depending on the method of influencing the object, regulators of smooth and positional (relay) action are distinguished. In smooth-acting regulators, the regulator can take any position between maximum and minimum. For positional regulators, the regulating body can occupy two or more specific positions. Depending on the type of setting element, regulators can be stabilizing, program, tracking, or optimizing. Stabilizing regulators maintain the controlled variable at a constant set level. Software controllers change the controlled variable according to a predetermined program, monitoring controllers - depending on changes in some external parameter, Optimizing controllers, analyzing external parameters, ensure optimal operation of the process. In refrigeration units, stabilizing regulators are more often used.

The control system coordinates the characteristics individual elements machines when their cooling capacity changes.

The characteristics represent the dependence of cooling capacity, energy consumption for compressor operation and condenser cooling on external conditions, i.e. on the ambient temperature. They allow you to establish a mutual relationship between the parameters of the compressor, evaporator and condenser. The characteristics are constructed using the heat balance equations of the “refrigerating machine - refrigerated room” system and energy relationships that describe the operation of the main elements of the machine, taking into account changes in the parameters of the refrigerant and the environment over time. In this case, balance and energy relationships are presented as a function of the temperature of the object being cooled (boiling point of the refrigerant) and the ambient temperature (condensation temperature of the refrigerant).

The process of regulating a machine to the required cooling mode or to a given temperature mode can theoretically be implemented quantitatively or in a quality way. The first involves changing the refrigerant flow 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 is self-set depending on the cooling capacity of the compressor, evaporator and condenser. Therefore, the control process determines not only the balance of the compressor’s cooling capacity Q ok and evaporator Q oi , but also the temperature level of heat removal or supply. Consequently, the regulation of a steam compressor machine is a combined process that combines quantitative and qualitative methods.

The executive body of the control system (cooling capacity regulator) is the throttle valve. The operating mode of the machine, which corresponds to the point where the characteristics of the compressor and evaporator intersect Q ok = Q oi , provided by changing the flow area of ​​the valve. The scheme for 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 characteristics Q ok = f(T 0) (T 0 - boiling point of the refrigerant) corresponds to the change in heat inflows of the refrigerated 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 its operating mode are shown with dashed lines. Dot A determines the operating point of the “machine - refrigerated room” system as an object of regulation when transitioning from one operating mode to another. At the same time, the point A′corresponds to the operating mode during the compressor control process, and the point A′′ - when the evaporator characteristic changes. Regulation of the cooling capacity of a machine with a piston compressor is carried out by smooth or stepwise (positional) regulation of its performance. In low- and medium-power machines, the following methods of smooth control using external or built-in devices have become widespread: structural devices: refrigerant bypass from the discharge side to the suction side (balancing), which is carried out by control valves controlled by a pressure or temperature sensor; suction throttling with switching the compressor to operate at reduced suction pressure; changing the volume of dead space by connecting additional external volume to it; change in compressor shaft rotation speed.

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

Step control in machines of small and medium refrigeration capacity is mainly carried out using the “start-stop” method with a maximum cycle frequency of up to 5-6 per hour; for multi-stage compressors, they effectively use the shutdown of individual cylinders by pressing the suction valves using mechanical pushers. The movement of the pushers is controlled by hydraulic, pneumatic or electromagnetic drives. An electronic capacity control system is being introduced that affects the suction valves with an electromagnetic field.

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

An example of proportional smooth control is the regulation of air temperature in a car in winter, when, with increasing heat loss from the car, the temperature of the water in the water heating boiler gradually increases. In this case, the impulse signaling the need to increase the water temperature in the boiler is a change in the outside air temperature. The most advanced, but also the most complex type of proportional control is isodromic control, based on the use of sensitive and flexible feedback, thanks to which the controlled parameter changes 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, which is where it got its name (in Greek, iso - constant, equal; dromos - running, speed). Currently it is used in most various processes, for example, for automatically driving sea ships along a given course.

Due to the complexity of the equipment, difficult operating conditions during vibration and shaking, and most importantly due to the lack of practical need for extremely precise control of air temperature, isodromic control is not used in air conditioning units for cars.

When choosing a control method, it is necessary to take into account the initial and operating costs, manufacturability and reliability of the design. 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 comparative effectiveness of the main methods for regulating the performance of piston compressors are shown in Fig. 4. For the start-stop methods (line 1) and squeezing the intake valves (line 2 ) are characterized by low energy losses and practical independence from the operating mode. When throttling at the suction (line 3 ) there is a sharp drop in efficiency with increasing boiling point of the refrigerant, so this method is used in compressors that operate in a narrow range of boiling pressures. Balancing (line 4 ) - least effective option regulation, since it is associated with energy losses of 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 in the cooling capacity of the machine.

In refrigeration machines with screw compressors, the following methods of regulating cooling capacity are used: suction throttling, balancing, changing the shaft speed, using a spool system.

Throttling is ensured by automatically closing the throttle valve installed at the compressor inlet. The effectiveness of this method is limited by a reduction in productivity to 70% of the nominal; With deeper throttling, efficiency decreases significantly.

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

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

The use of this method is usually limited to dry compression compressors.

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

Automation of the operation of the refrigeration machine allows high accuracy maintain the required level of cooling process parameters that correspond to the optimal technological regime, and also partially or completely eliminate the participation of service personnel in operation refrigeration equipment.

In steam compressor machines, the objects of automation are heat exchangers, in particular the degree of filling of 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 superheat

at the exit from it. Indeed, when part of the heat transfer surface of the evaporator provides superheating of the refrigerant vapor, 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 calculated level worsens the energy performance of the machine and the reliability of its operation. Supplying more refrigerant to the evaporator than the heat transfer process can handle is associated with overfilling the evaporator and reducing 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 water hammer.

Systems for automatic control of the degree of filling of the evaporator based on the overheating of refrigerant vapors are smooth and positional (usually two-stage). As automatic control in smooth systems, thermostatic valves (TRVs) are widely used, in which the amount of superheat of the refrigerant vapor is obtained in the form of the difference between the temperature of the steam leaving the evaporator and the boiling point of the refrigerant. Thermostatic valves, which ensure the process of throttling the refrigerant from condensation pressure to evaporation pressure, are installed on the line between the condenser and the evaporator.

Schematic diagram automatic control of the refrigerant level in the evaporator using a expansion valve, used in RPS refrigerant machines, is shown in Fig. 5. Sensitive element of the measuring head 1 thermostatic valve made in the form of a membrane 2 or bellows, is under the influence of the pressure difference between 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 formed in a thermal system consisting of a thermal cylinder 6 and capillary 3 , enters the space above the membrane; the space under the membrane is connected with an equalizing tube 4 with compressor suction line 5 . In this case, the equalizing tube is connected to the suction line at the installation site of the thermal cylinder. In some designs, a solid absorber is introduced into the thermal cylinder and the entire thermal system is filled with gas.

Rod movement 12 as a result of deformation of the sensing element when the superheat temperature changes, it ensures the opening or closing of the shut-off valve 11 regulating the flow of liquid refrigerant from the condenser to the evaporator through the line 10 . With adjusting screw 8 change the spring tension 9 and, therefore, the required value of the superheat temperature. In the process of automatic control, the expansion valve must ensure the optimal filling level of the evaporator and the stability of the system throughout the entire required range of changes in cooling capacity, which is especially important for refrigeration machines of refrigerated rolling stock. Almost stable operation of the expansion valve system begins when it overheats (3 6) K. To expand the control 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 expansion valves

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

Technically, it is provided by a system of blinds or rotary dampers, the use of fans with variable installation angles of guide blades, the use of two-speed electric motors, as well as periodic shutdown of the fans. Changing the speed or flow of 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 partially flooding the surface of the condenser with liquid

refrigerant.

Automatic control devices, in addition to monitoring the parameters of the evaporator and condenser, support set temperature air in the refrigerated room, ensure timely removal of frost (“snow coat”) from the surface of the evaporator, regulate the oil level in oil separators, etc. The operation of the regulation system is combined with automatic protection, which includes a set of measures to safe operation 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 prevents the machine from restarting without eliminating the reason that caused the protection to operate.

In compressor refrigeration machines, the sensors of the protection system monitor the level of 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, preventing its overload or short circuit. A light or sound alarm can be introduced into the automatic protection system, notifying that the limit value controlled value or approaching a dangerous operating mode of the machine.

3. Classification and main elements

automation devices

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

Automatic control devices ensure that the refrigeration unit and its individual devices are turned on or off, and also control work processes. In refrigeration units 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 range (pressure switches); ensure timely defrosting of frost from the evaporator (pressure switches, software relays, thermostats); open or stop the supply of liquid or vapor refrigerant (solenoid valves, check valves); limit the flow of refrigerant into the compressor from the evaporator (suction pressure regulators).

Automatic protection devices turn off the entire refrigeration unit or individual devices when dangerous operating conditions occur: when the maximum permissible discharge pressure is reached (pressure switches); with vacuum on the suction side (pressostats); when the oil pressure in the compressor lubrication system drops (pressure difference); at low oil temperature in the compressor crankcase (thermostats); at high temperatures of refrigerant vapors compressed in the compressor (temperature switch); in case of motor overload or short circuit (thermal relays, circuit breakers, fuses).

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

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

The regulatory body acts on a signal from the sensing element. In two-position devices (relays), the working element can occupy only two positions. For example, the electrical contacts of a pressure switch (pressostat) or a temperature switch (thermostat) can be closed or open, the solenoid valve valve can be closed or open. In devices of smooth (proportional) action, each change in the controlled variable corresponds to the movement of the regulating body (for example, smooth movement of the control valve when the thermal load on the evaporator changes). The device for setting the device sets the set value of the controlled or controlled quantity. The deviation of the controlled quantity, which does not cause movement of the control element, is called the dead zone, or differential of the device. Sensitive elements of pressure devices are made in the form of bellows and membranes. The bellows is a thin-walled corrugated tube. Bellows are made from brass, bronze, and 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 when heated, temperature is converted into mechanical movement (dilatometric elements). The movement occurs due to unequal linear expansion coefficients for different metals. In Fig. 3.6 a, b elements with two metal parts 1 And 2 from different materials, in Fig. 3.6 c, d - sensitive element made of bimetal, i.e. made of two layers of metals welded together.

In elements with thermal expansion of liquids, the dependence of the change in liquid volume on temperature is used. Sensors filled with mercury (Fig. 3.7, a, b), are used to convert temperature into an electrical signal without intermediate mechanical system. 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 not reliable enough, as breaks appeared due to vibrations and shocks while moving. mercury and the electrical circuit was disrupted. In addition, mercury contact sensors are designed for low electrical signal power.

Rice. 3.6. Dilatometric sensors

Rice. 3.7. Liquid

heat sensitive

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Automation system is a sequential connection through pipelines of all elements of a refrigeration unit, ensuring precise maintenance of a given refrigeration temperature, continuous monitoring and protection of the machine from accidents, as well as reliable operation of refrigeration equipment. The system must be capable of easy temperature control and economical operation of the installation. The automation system layout is selected depending on the cooling capacity and purpose of the installation.

Apply refrigeration machine automation systems with performance control by pressing solenoid valves, as well as turning on and off refrigeration units. In transport, the most common automation systems are those based on the second principle.

The design of the automatic control system of a freon machine is determined by the type of compressor, evaporator and condenser, the method of changing the cooling capacity, as well as the number of compression stages or cooling cascades.

A characteristic feature of automation of ammonia refrigeration units- increased requirements for operational safety due to the high toxicity of ammonia, its explosion hazard, as well as the danger of destruction of compressors from water hammer.

In refrigerated rolling stock cars, restaurant cars, and passenger cars with air conditioning, the following are used to cool cabinets and small chambers for short-term storage of products: automated freon refrigeration units:

• compressor-engine;
• compressor-condenser;
• evaporator-regulating station;
• evaporator-condenser;
• compressor-condenser-evaporator.

The compressors of these units are usually vertical or V-shaped, multi-cylinder crankcase, with air-cooled cylinders. There are also hermetically sealed units, in which the compressor and electric motor are placed in a sealed casing. Such units include installations of home refrigerators.

Rice. 1 - Diagram of the ZIL refrigerator Moscow

The ZIL-Moscow refrigerator is equipped with a compressor (7) (Fig. 1) with an electric motor (5), a condenser (1), an evaporator (2), a thermostat (5), a capillary tube (4), a filter (5), a starting and power relay. The compressor has a fitting (6) for charging freon-12. The operation of the unit is regulated by a thermostat, which automatically maintains the set temperature in the refrigerator. The electric motor is turned on by a starting relay, in the same housing with which a thermal relay is mounted, protecting the motor from overload.

Dining cars are equipped with FRU and FAK freon units for cooling refrigerated cabinets and chambers. The diagram of a freon rotary unit (FRU) is shown in (Fig. 2), and installations with a piston compressor are shown in Fig. 3.

Rice. 2 - Diagram of a freon rotary refrigeration unit: 1 - evaporator; 2 - thermostatic valve; 3 - liquid line; 4 - fuses; 5 - suction line; 6 - pressure switch; 7 - reinforcement panel; 8 - switches; 9 - plug socket; 10 - magnetic starter; 11 - discharge valve; 12 - gas filter; 13 - rotary compressor; 14 - air condenser; 15 - electric motor; 16 - suction pipe; 17 - check valve; 18 - liquid filter; 19 - receiver; 20 and 21 - receiver shut-off valves

Rice. 3 - Diagram of the freon refrigeration machine IF-50: 1 - evaporative battery; 2 - thermostatic valve; 3 - magnetic starter; 4 - sensitive thermostatic valve cartridge; 5 - heat exchanger; 6 - pressure switch; 7 - compressor-condensing unit

The refrigeration equipment of the all-metal dining car consists of three automatic compressor-condensing units of the FAK-0.9VR type, driven by direct current electric motors PNF-5 with a voltage of 50 V. Each unit cools two boxes or cabinets equipped with evaporative batteries and storage plates. The carriage has three undercar compartments for storing fish, meat and drinks. The dispensing department has a cabinet for storing confectionery products; a refrigerated cabinet, which is located in the kitchen, serves to store gastronomic products; Next to it is a cabinet for cold dishes.

The refrigeration units of dining cars use two cooling systems- with direct boiling of the refrigerant and storage. Tubular evaporators made of copper pipes with flat brass fins, as well as evaporators made of copper pipes with a cross-section of 12×1 mm with fins made of thin brass tape. Accumulation plates are installed in the undercar drawer for drinks and the cabinet for confectionery. They are welded stainless steel tanks, inside of which tubular plate evaporators are placed. The interpipe space inside the tanks is filled with water, which freezes during operation of the installation and accumulates cold.

All drawers and cabinets are equipped with thermostatic valves. The cyclic operation of refrigeration units is ensured by the RD-1 pressure switch, which automatically acts on the starting equipment of electric motors.

Rice. 4 - Schemes of automated piston refrigeration units with several cooled objects: a - with two-position regulation; b - when servicing two cameras; c - when regulating temperature using thermostats; 1 - compressor; 2 - receiver; 3 - capacitor; 4 - evaporator; 5 - thermostatic valves; 6 - pressure switch; 7 - magnetic starter; 8 - electric motor; 9 - automatic pressure throttle; 10 - check valve; 11 - intermediate relay; 12 - solenoid valve; 13 - thermostat; 14 - water control valve

Typical automation schemes for compression piston refrigeration units with several cooled objects can be implemented in various options. Automation diagram for two-position control in one or two evaporators with same temperature cooling the chamber air (Fig. 4, a) involves the use of an evaporator, chamber temperature relay or a compressor low pressure relay. When servicing two chambers with different temperatures with one refrigeration machine (Fig. 4, b), an automatic pressure throttle (9) (APD) is used. The temperature control circuit using thermostats is shown in Figure 4, c.

Automation of refrigeration units facilitates work, makes it safe, improves and simplifies technological processes. This is the most important condition for technical progress. Automation is carried out to reduce the share manual labor, maintaining stable parameters of temperature, humidity, pressure, as well as preventing emergency situations and increasing service life. Since fewer maintenance personnel are required, automated units are cheaper to operate.

Automation of refrigeration units affects the management of individual operations - alarms, control, starting and shutting down certain mechanisms. In general, comprehensive management is carried out - regulation and protection. Almost any process can be automated, but this is not always advisable. Steam ejector and absorption units are the easiest to automate, since apart from pumps they have no unnecessary moving mechanisms. With large compression models, things are more complicated. They require constant monitoring and maintenance by qualified personnel, so only partial automation is used. The main elements of the system are a measuring sensor, a regulating body and a transmission device. They are all interconnected.

5 reasons to purchase refrigeration units from AkvilonStroyMontazh Company

1. Widest model range

1. Possibility of manufacturing non-standard refrigeration units

1. Flexible pricing policy

1. Innovative solutions in the control of refrigeration units

1. Energy-saving technological principles

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Types of automation devices There are several automation methods that significantly simplify production processes. Both individual options and their complex are used.

Control. Special technical automation solutions are responsible for independently turning on and off compressors and pumps in accordance with the designated mode or during load fluctuations. Temperature and time relays are installed that respond to changes or monitor a specific schedule. Regulation. Help support the right level main operating parameters - temperature, pressure, humidity. Smooth performance control allows you to maintain a specific coolant temperature when the heat load decreases. Control of the refrigerant supply to the evaporator is also used. This is necessary to ensure the safe operation of the compressor, increase or decrease productivity. Alarm. Notifies about dangerous changes in operating parameters, modes, and malfunctions in the functioning of the system. Protection. Helps eliminate the possibility of operational failures, dangerous situations as a result of an unacceptable increase in pressure, temperature, or malfunction of certain devices. All kinds of sensors, thermometers, pressure gauges and much more are used here.

Full automation of refrigeration units implies their equipping with all the listed control, monitoring, protection, and alarm means. Through their use, it is possible to obtain more advanced equipment that increases the productivity of the organization. The AkvilonStroyMontazh company offers installations of all types, equipped with modern automation equipment. At your request, our engineers will automate an existing refrigeration system or develop fully automated installations for you.

Introduction………………………………………………………………………………………..

1 Description technological process …………………………………………......

1.1 Automation of refrigeration compressor stations………………………….

1.2 Analysis of the disturbing influences of the automation object…………………...

1.3 Refrigeration cycle diagram………………………………………………………..

2 Development functional diagram refrigeration unit……………………….

2.1 Methodology for developing a circuit………………………………………………………

2.2 Functional diagram of automation of the refrigeration module……………….. .

2.3 Operation of the functional diagram components of the refrigeration module automation….

2.3.1 Automatic protection unit for compressors…………………………………..

2.3.2 Node automatic switching on backup water pump………………

2.3.3 Air cooler defrosting unit……………………………………………………………..

3 Selection of technical means of a refrigeration unit……………….........

3.1 Selection and justification for the choice of instruments and automation equipment……………..

Conclusion……………………………………………………………………………

References………………………………………………………………………………………

INTRODUCTION

Automated control and regulation systems are an integral part of the technological equipment of modern production, help improve product quality and improve the economic performance of production by selecting and maintaining optimal technological conditions.

Automation frees people from the need to directly control mechanisms. In an automated production process, the role of a person is reduced to setting up, adjusting, servicing automation equipment and monitoring their operation. If automation facilitates human physical labor, then automation aims to facilitate mental labor as well. The operation of automation equipment requires highly qualified technical personnel.

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

Automating parameters provides significant benefits:

Ensures a reduction in the number of working personnel, i.e., an increase in their labor productivity,

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

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

Increases labor safety and reliability of equipment operation,

control devices

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

The economic efficiency of the refrigeration machine is ensured by reduced operating costs and reduced equipment repair costs.

Automation reduces the number of maintenance personnel and ensures the machine operates in optimal mode.

The safety of refrigeration equipment is ensured by the use of automatic devices that protect the equipment from hazardous operating conditions.

According to the degree of automation, refrigerating machines and installations are divided into 3 groups:

1 Manually operated refrigeration equipment.

2 Partially automated refrigeration equipment.

3 Fully automated refrigeration equipment.

Manually operated equipment and partially automated machines operate with the constant presence of maintenance personnel.

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

An automated refrigeration system must contain one or more automation systems, each of which performs specific functions. In addition, there are devices that combine (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 automation operation without the participation of maintenance personnel.

The object of the course project is the refrigeration unit as a whole and its individual elements.

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

1 DESCRIPTION OF THE TECHNOLOGICAL PROCESS

1.1 Automation of refrigeration compressor stations

Artificial cold finds wide application V food industry, in particular when canning perishable foods. Cooling ensures high quality of stored and released products.

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

In the food industry, continuous cooling using refrigeration units is widespread, where the refrigerant - liquefied gas (ammonia, freon, etc.) - undergoes a circular process in which, after the refrigeration effect has been achieved, it restores its original state.

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

Control valve VI serves to throttle liquid ammonia, the temperature of which decreases. The automation system provides automatic control of the compressor operation and emergency protection. The command to automatically start 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 pipeline.

from the evaporator.

When the compressor operates in automatic mode, the following emergency protection functions: against a decrease in the difference in oil pressure in the lubrication system and the crankcase - a pressure difference sensor-relay is used; from a decrease in suction pressure and an increase in discharge pressure - a pressure sensor-relay is used; from an increase in discharge temperature - a temperature sensor-relay 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 ammonia in the evaporator - a semiconductor level relay is used.

When the compressor starts in automatic mode, the valve with an electromagnetic drive opens to supply water to the cooling jackets and the valve on the bypass closes.

Automatic control of the level of liquid ammonia in the evaporator is carried out by semiconductor level relays, a control valve with an electromagnetic drive installed on the supply of liquid ammonia to the evaporator.

The upper and lower levels of liquid ammonia in the linear receiver are controlled by semiconductor level relays.

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

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

The monitoring, control and signaling equipment for the remaining process equipment is located in the control panel panels.

1.2 Analysis of the disturbing effects of the automation object

This scheme provides for monitoring, regulation, management and signaling of process parameters.

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

Also subject to control is the air temperature in the refrigeration unit, which determines the cooling and the amount of cold produced.

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

The temperature of the cold water coming from the pool to the condenser, which is necessary for condensing (cooling) the ammonia vapor, is also controlled.

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

Control valve VI installed on the pipeline serves to throttle liquid ammonia, thereby reducing the temperature.

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.

Automation of production processes is the most important condition for technical progress in any industry.

The goal of automation of refrigeration units is to replace manual labor, accurately maintain given parameters, preventing accidents, increasing the service life of equipment, reducing costs, improving production standards.

The operation of automated refrigeration units is cheaper, since there is no need for some of the service personnel involved in manual operations to start, regulate and stop refrigeration equipment, and visually monitor the operation of machines and devices.

Automation devices can perform both individual operations: control, signaling, turning on and off actuators, and a combination of these operations: automatic protection and regulation.

Any operation performed by the operator of modern refrigeration units can be automated. However, it is not advisable to automate all operations.

Automation of regulation and protection processes is necessary in cases where these processes require manual labor and when the driver cannot provide accurate regulation and reliable protection. It is also very important to automate work in hazardous and explosive areas.

Absorption and steam ejector refrigeration machines, due to the lack of moving mechanisms (except pumps), are easier to fully automate than large compression machines, which require continuous monitoring and qualified maintenance.

Large and medium-sized refrigeration units are equipped with partial automation, in which only part of the processes is automatically regulated. More often, such refrigeration units operate in a semi-automatic mode, in which the machine stops automatically and starts manually.

The main parts of any automatic system are: a measuring (sensitive) element, or sensor, which senses a change in the controlled variable; a regulatory body that changes, based on a signal from the measuring element, the supply of substance or energy to the regulated object, and a transmission device that connects the sensor to the actuator. The measuring element is usually equipped with a device for adjusting to a given value of the controlled variable.

Automatic control devices must turn compressors and pumps on or off when load changes. The compressors are controlled by temperature switches that stop the compressors when the brine temperature or evaporator pressure drops below a preset limit and turn them on when the evaporator temperature rises. Sometimes refrigeration machines are turned on using a time relay, which is set to turn on the compressor.

Automatic control devices are designed to maintain the specified operating parameters of the refrigeration unit: temperature, pressure, level. Thanks to the smooth regulation of the cooling capacity, it is possible to maintain the set coolant temperature when the heat load decreases. This is achieved in the following ways:
installation of pressure regulators “upstream”, maintaining constant pressure in the evaporators and throttling the vapors in front of the compressor;
by installing pressure regulators “after themselves”, which transfer part of the vapor from the discharge line to the suction line. Due to this, part of the vapor that could enter the compressor from the evaporator is cut off and the cooling capacity of the installation drops;
connecting additional harmful space in the piston compressor, which reduces the suction of refrigerant vapors from the evaporator.

Regulating the supply of refrigerant to the evaporator has two goals: ensuring safe operation of the compressor by protecting it from water hammer and reducing or increasing the cooling capacity of the installation.

Automatic alarm notifies about changes in mode that may result in the activation of automatic protection elements, and notifies about turning on and off machines, magnetic valves, gate valves and devices. An example of a signaling device is a remote control level indicator connected to actuators - solenoid valves or sound signaling devices - bellowers.

Automatic protection allows you to avoid the dangerous consequences for the refrigeration machine of an excessive increase in discharge pressure, a decrease in pressure and evaporation temperature, disturbances in the operation of lubrication devices, etc.

To protect installations from emergency operation, automation schemes include devices that turn off refrigeration units in the event of sudden disturbances in operating mode.

The transfer of secondary readings of control and measurement devices (thermometers, pressure gauges, flow meters, level indicators) to the central panel, where the control station is located, allows you to control the operation of the refrigeration unit centrally. Some measurements are recorded by recording instruments (thermometers, pressure gauges).

Integrated automation of a refrigeration unit consists of equipping it with automatic control, regulation and protection devices, as well as monitoring and signaling devices that ensure the proper operation of these devices.

Security questions
1. What benefits does automation of refrigeration units provide?

2. Name the main elements of automation.

3. What elements does the automatic control system consist of?

4. Tell us about the device of the expansion valve,
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5. Explain the design and working principle of solenoid valve.

6. How do diaphragm pneumatic valves work?

7. Name the methods for regulating cooling capacity.

8. Tell us about the operation of the pressure switch.

9. Tell us about the structure of the RUCC.

10. What do you know about the water control valve?

11. List ways to protect a compressor from the danger of water hammer.

12. Explain the design and principle of operation of a remote level indicator.

13. What types of automatic alarms do you know?

14. Monitor the operation of automation devices in the diagram of a two-stage refrigeration unit.

15. Tell us about the features of automation of refrigeration turbine units.

16. Tell us about the automation schemes for individual components of ammonia refrigeration units.