Automation of refrigeration installations. Automation of refrigeration machines and installations Automation of refrigeration machines
Automation of production processes is the most important condition for the technical progress of any industry.
The goal of the automation of refrigeration units is the replacement of manual labor, accurate maintenance of the specified parameters, prevent accidents, an increase in the service life of the equipment, reducing costs, an increase in production culture.
The operation of automated refrigeration unit costs cheaper, as it disappears the need for part of the service personnel engaged in manual operations for starting, regulation and stopping refrigeration equipment, visual observation of the operation of machines and devices.
Automation devices can be performed as separate operations: control, alarm, switching on and off actuators, and a set of these operations: automatic protection and regulation.
Any operation carried out by a machinist of modern refrigerations can be automated. However, not all operations are advisable to automate.
Automation of regulation and protection processes are necessary in cases where these processes require manual labor costs and when the driver cannot provide accurate regulation and reliable protection. It is also very important to automate work in harmful and explosive rooms.
Absorption and steamer refrigeration machines due to the lack of moving mechanisms (except pumps) are easier to fully automate than large compression that require continuous observation and qualified service.
Large and medium refrigeration units are provided with partial automation, in which only part of the processes is automatically adjusted. More often, such refrigeration units operate on a semi-automatic mode, in which the stop of the machine occurs automatically, and the start manually.
The main parts of any automatic system are: a measuring (sensitive) element, or a sensor that perceives the change in the adjustable value; The regulator changing the measuring element in the measuring element feeding the substance or energy into the adjustable object, and the transfer device connecting the sensor with the actuator. The measuring element is usually equipped with a device for adjusting to a specified value of the adjustable value.
Automatic control devices Must include or disable compressors and pumps with load changes. Compressors are controlled using a temperature relay, stopping compressors with a decrease in the size of the brine or pressure in the evaporators below the specified limit and including them when increasing the temperature in the evaporator. Sometimes refrigeration machines include with a time relay, which is set to power on the compressor.
Automatic regulation devices Designed to maintain the specified parameters of the refrigeration operation: temperature, pressure, level. Due to the smooth control of the cooling capacity, you can maintain a predetermined temperature of the coolant when a thermal load decreases. It is achieved in the following ways:
installation of pressure regulators "to ourselves" supporting constant pressure in evaporators and throttling pairs before compressor;
Installation of pressure regulators "After themselves", which is constrained part of the vapor from the discharge line into the suction. Due to this, part of the vapor that could enroll in the compressor from the evaporator, it cuts off and the cooling capacity of the installation drops;
Connecting an additional harmful space in a piston compressor that reduces the freedom of refrigerant steam from the evaporator.
Adjusting the refrigerant supply to the evaporator pursues two objectives: ensuring the safe operation of the compressor, by protecting it from hydraulic impact and decrease or increase the cooling capacity of the installation.
Automatic alarm Notifying the changes in the mode that may entail the triggering of automatic protection elements, and notifies on the turn on and off of machines, magnetic valves, valves and appliances. An example of a signaling device is a remote indicator of the level of the BU, connected to the actuating mechanisms - solenoid valves or sound signaling devices - the revisers.
Automatic protection It makes it possible to avoid dangerous for the refrigeration machine consequences of excessive increase in pressure of the discharge, lowering the pressure and temperature of evaporation, disorders of the mode of operation of lubricants, etc.
To protect the settings from the emergency mode, automation schemes include devices that turn off refrigeration units with sharp disorders of the operation mode.
The removal of the secondary readings of control and measurement devices (thermometers, pressure gauges, flow meters, level pointers) to the central shield, where the control station is located, allows you to control the operation of the refrigeration unit centrally. Part of the measurements are written to the authentic devices (thermometers, pressure gauges).
The complex automation of the refrigeration unit is to equip it with automatic control, regulation and protection devices, as well as control and alarm tools that ensure the good operation of these devices.
Control questions
1. What gives the automation of refrigeration plants?
2. Name the main automation elements.
3. What elements is the automatic regulation system?
4. Tell us about the TRV device,
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5. Explain the design and principle of the solenoid valve.
6. How do membrane pneumatic valves work?
7. Name how to regulate the cooling capacity.
8. Tell us about the work of the pressure relay.
9. Tell us about the device of the hands.
10. What do you know about the water-regulating valve?
11. List ways to protect the compressor from the danger of hydraulic impact.
12. Explain the device and the principle of operation of the level remote indicator.
13. What types of automatic alarm do you know?
14. Trace the operation of automation devices in a two-stage refrigeration circuit.
15. Tell us about the features of the automation of refrigeration turbines.
16. Tell us about the automation schemes of individual nodes of ammonia refrigeration units.
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Automation system It is a sequential combination with the help of pipelines of all elements of the refrigeration unit, which ensures accurate maintenance of the predetermined cooling temperature, continuous control and protection of the machine from accidents, as well as the reliability of refrigeration equipment. The system should provide for easy temperature adjustment and cost-effective operation of the installation. The circuit of the automation system is chosen depending on the cooling capacity and installation assignment.
Apply refrigeration Automation Systems With performance control by pressing electromagnetic valves, as well as on and off refrigeration units. In transport, the most common automation systems arranged on the second principle.
The device of the automatic control system of the freon machine is due to the type of compressor, evaporator and a capacitor, a method for changing the cooling capacity, as well as the number of compression stages or cooling cascades.
Characteristic feature of ammonia refrigeration automation - Increased requirements for the safety of operation due to the high toxicity of ammonia, its explosiveness, as well as the risk of destruction of compressors from hydraulic shocks.
In the vehicles of refrigerated rolling stock, restaurant wagons, in passenger air conditioning cars for cooling cabinets and small chambers of the short-term storage of products apply the following automated Freon Refrigeration Units:
- compressor motor;
- condensing compressor;
- evaporator-adjusting station;
- condenser evaporator;
- compressor-condenser-evaporator.
Compressors of these aggregates are usually vertical or V-shaped, multi-cylinder block crankcase, with air-cooled cylinders. There are also sealed units in which the compressor together with the electric motor is placed in a hermetic casing. Such units include home refrigerators.
Fig. 1 - Scheme of the refrigerator "ZIL" Moscow
ZIL-MOSCOW refrigerator is equipped with a compressor (7) (Fig. 1) with an electric motor (5), a capacitor (1), an evaporator (2), a thermostat (5), a capillary tube (4), filter (5), start-up and Power relay. The compressor has a fitting (6) for charging chladone-12. The unit operation is adjusted using a thermostat that automatically maintains the specified temperature in the refrigeration cabinet. The inclusion of an electric motor is carried out by a start relay, in one case with which a thermal relay is mounted that protects the engine from overload.
Cars-restaurants are equipped with freon installations for FRU and FAB to cool refrigeration cabinets and cameras. The scheme of the freon rotational installation (FRU) is shown on (Fig. 2), and the settings with a piston compressor - in Figure 3.
Fig. 2 - scheme of freon rotational refrigeration unit: 1 - evaporator; 2 - thermostatic valve; 3 - Liquid line; 4 - fuses; 5 - suction line; 6 - pressure relay; 7 - reinforcing shield; 8 - switches; 9 - plug socket; 10 - magnetic starter; 11 - discharge valve; 12 - gas filter; 13 - rotary compressor; 14 - air capacitor; 15 - electric motor; 16 - suction nozzle; 17 - check valve; 18 - Filter for liquid; 19 - Receiver; 20 and 21 - Receiver valves
Fig. 3 - IF-50 Freon Refrigeration Machine Scheme: 1 - Evaporative Battery; 2 - thermostatic valve; 3 - magnetic starter; 4 - sensitive cartridge of thermostatic valve; 5 - heat exchanger; 6 - pressure relay; 7 - Compressor Condenser Unit
The refrigeration equipment of the all-metal wagon-restaurant consists of three automatic compressor capacitor aggregates of the TA-0,9VR type, equipped with a PNF-5 DC electric motors with a voltage of 50 V. Each unit cools two drawers or cabinets equipped with evaporating batteries and accumulatory plates. In the car there are three sub-cutting box for storing fish, meat and drinks. The transfer department has a closet for confectionery storage; Refrigerated cabinet, which is located in the kitchen, serves to store gastronomic products; Next to it is a closet for cold dishes.
In refrigeration plants of restaurants are used two cooling systems - with the direct boiling of refrigerant and accumulation. For cooling, tubular evaporators made of copper pipes with flat brass ribs, as well as evaporators from copper pipes with a thin brass ribbons with fins from a thin brass ribbon, are applied to cool. Battery stoves are installed in the dwelling box for drinks and closet for confectionery. They are welded stainless steel tanks, inside which tubular lamellar evaporators are placed. The interlock space inside the tanks is flooded with water, which freezes during the operation of the installation and accumulates cold.
All boxes and cabinets are equipped with thermostatic valves. The cyclicity of the refrigeration units provides the RD-1 pressure relay, which automatically affects the launcher of the electric motors.
Fig. 4 - schemes of automated piston refrigeration systems with several cooled objects: A - with two-position adjustments; b - when servicing two cameras; in - when adjusting the temperature using thermostators; 1 - compressor; 2 - receiver; 3 - condenser; 4 - evaporator; 5 - thermostatic valves; 6 - pressure relay; 7 - magnetic starter; 8 - electric motor; 9 - automatic pressure throttle; 10 - check valve; 11 - intermediate relay; 12 - solenoid valve; 13 - thermostat; 14 - watercutting valve
Typical automation schemes of compression piston refrigeration settings with multiple cooled objects can be performed in various versions. Automation scheme with two-position regulation In one or two evaporators with the same air cooling temperature of the chamber (Fig. 4, a), it provides for the use of the evaporator temperature relay, a camera or low pressure relay of the compressor. When servicing a single refrigeration machine of two chambers at different temperatures (Fig. 4, b), automatic pressure throttle (9) (ADD) is used. Temperature control circuit with thermostat is shown in Figure 4, in.
Modern refrigeration machines and installations cannot be submitted without automation tools. They provide stable operation, protect against unacceptable operating modes and extend the life of the entire system.
Refrigeration automation devices include thermostatic valves; Performance regulators, pressure and oil level; Pilot, Safety and check valves; pressure relay and temperature; flow switch. This also includes various electrical and electronic devices: controllers, frequency converters, rotational speed controllers, engine protection machines, timers, and so on. Unfortunately, quite often on this responsible part of the equipment are trying to save. Often it is also encountered also with ignorance of possibilities and specifics of the use of automation. In this article we will try to give a brief overview of the main mechanical devices and solved with their help.Automation devices
For the smooth filling of the evaporator in order to the most efficient use of its heat exchange surface, thermostatic valves are intended (TRV). The refilling indicator is the overheating of the refrigerant - the difference in its temperature at the inlet and at the exit of the evaporator. It is by this parameter and regulation occurs. It is the opinion that TRV maintains the temperature of the cooled medium or the boiling pressure, but this is fundamentally impossible due to the characteristics of the TRV design.
Thermostatic valve (Scheme 1) consists of a heat-sensitive system (1) separated from the membrane case; capillary tube connecting the thermo-sensitive system with thermobalone (2); valve housings with seat (3); Adjusting spring (4).
The work of the TRV depends on the three main parameters: the pressure in the thermobalon, acting on the upper surface of the membrane (P1), the boiling pressure acting on the lower surface of the membrane (P2), and the pressure of the adjusting spring, also acting on the lower surface of the membrane (P3).Regulation is carried out by maintaining equilibrium between pressure in the thermobalone and the amount of boiling pressure and spring. Spring provides overheating adjustment.
TRV is installed on the liquid refrigerant line between the condenser and the evaporator. It occurs in it is throtting the working substance from condensation pressure to boiling pressure. According to the constructive execution of TRV, they are divided into valves with external and internal pressure equalization; Collapsible and unintended. TRV with internal alignment are used, as a rule, on low performance evaporators with a small drop of refrigerant pressure, for example, in the shopping equipment.
The TRV of small performance is carried out intimidrate (with a replaceable or fixed throttling insert), and TRV of great performance - collapsible, which allows you to replace individual elements if necessary, and not the entire valve.
Condensation pressure regulators for air-cooled capacitors are designed to maintain the minimum necessary working condensation pressure while reducing the ambient temperature. They provide the so-called "winter regulation". Scheme 2 shows a variant of such a solution for the condenser and the receiver installed on the street.For water-cooled condensers, valves are used, changing water consumption depending on the refrigerant pressure. These valves allow maintaining condensation pressure with high accuracy.
The boiling pressure regulators are installed on the absorption line for the evaporator to maintain the predetermined boiling pressure in refrigeration systems. In systems with several evaporators, the regulator is set for the highest boiling pressure evaporator.
Pressure regulators in the crankcase make it possible to avoid starting and operating the compressor with too high suction pressure, on the line of which are installed immediately before the compressor.
Such regulators are often used in refrigeration plants with hermetic or semi-hermetic compressors designed to work at low temperatures.
Performance regulators that compensate for the reduction of heat load are used in systems with one compressor, not equipped with other means of adjustment (spin valve, frequency converter). Install on the bypass line between the suction and discharge of the compressor, allowing you to avoid reducing the suction pressure and frequent starts of the compressor stops. The advantages of such regulators include simplicity and low cost, but there are a number of restrictions on their application. So, due to the reduction of the refrigerant speed in the system, leading to problems with the return of oil into the compressor, compensate for the drop in the load is possible by no more than 50%. The hot gas crossing into the suction line of hermetic or semi-hermetic compressor can lead to overheating of the winding of the electric motor. In addition, the discharge temperature is also growing. To reduce the suction temperature, the injection of the liquid refrigerant may be required by the discharge, which requires a thorough selection and setting of the system to prevent the hydroedar in the compressor.
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| Collapsible TPB Danfoss TE12 |
Relay trigger pressure is usually adjusted. Some models are configured and a response differential. Compact relays without the ability to configure (cartridge presss) are used primarily by large manufacturers of compressor, compressor capacitor aggregates and monoblocks.
Pressure drop relay is widely used as the compressor protection against the drop in the oil pressure in the crankcase. These devices often include a timer that turns off the compressor, if for a specified time the oil pressure is kept below the minimum necessary - for normal lubrication of the moving parts of the compressor.
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| Inseminious TPB in context |
In refrigeration technology, two types of refueling of the sensitive thermostat element are used - steam and adsorption. Thermostats with steam filler are used in systems where the temperature change occurs slowly (for example, in large-volume refrigeration chambers). In such thermostats, the relay housing must be in a warmer room than the sensitive element. The relay with adsorption refill can be used to control where the temperature changes quickly.
Application of automatic
Consider the use of automation devices using the example of a cooling system of a small refrigeration chamber, performed by the specialists of the company "Termokuly" using the automation of Danfoss.
The filling of the evaporator by the refrigerant is regulated using the collapsible TRV of those 5-3 with the external equalization of pressure. The temperature in the chamber corresponds to the electronic controller (not shown in the diagram), controlling the EVR 10 electromagnetic valve.
Maintaining condensation pressure in the winter period is carried out using the KVR condensation pressure regulator, the NRD differential valve and the NRV check valve. A characteristic feature of this technical solution is to install the KVR regulator in front of the condenser. This leads to a certain increase in the cost of the system, since the larger regulator is required compared to the regulator on the liquid line behind the capacitor. At the same time, it avoids problems with the launch of the system after a long stop in the case when the condenser and the receiver are installed on the street or in the unheated room. To regulate the condensation pressure, when the installation is running, stepped control of the condenser fans using two high-pressure relays of CR 5 with automatic discharge.The compressor control is carried out using a two-block KP 17 W relay: low pressure relay turns on and disconnects the compressor in operating mode, high pressure relays - stops in case of exceeding the working value. As an additional protection against the high pressure stop on the unit, the PR 5 with manual discharge is installed.
This configuration of automation allows, with a relatively small cost of components, to obtain a simple and reliable cooling control system that ensures stable maintaining the specified parameters.
The article is prepared by Sergey Magnin and Sergey Buchin. We thank the company "Termeokoul" (www.thermocool.ru) for informational support
The maintenance staff of the non-automatic refrigeration unit allows and stops the refrigeration machine, adjusts the supply of a liquid agent to the evaporator, regulates the temperature in refrigeration chambers and the cooling capacity of compressors, monitors the operation of the devices, mechanisms, etc.
With automatic regulation of refrigeration machines, these manual operations disappear. Operation of the automated installation is much cheaper than the operation of the installation with manual adjustment (reducing the cost of maintenance of the service personnel). The automated installation is more economical by energy costs, more accurately supports the set temperature modes. Automatics devices quickly react to all sorts of deviations from normal working conditions, and when the hazard occurs, the installation is turned off.
Apply various automatic devices - control, control, protection, alarm and control.
Automatic control devices include or turn off in a certain sequence of machine and mechanisms; include backup equipment during system overloads; Include auxiliary devices when thawing from the surface of the cooling batteries, oil production, air, etc.
Automatic control devices are maintained under certain limits the main parameters (temperature, pressure, fluid level), on which the normal operation of the refrigeration unit depends, or regulate them in accordance with the specified program.
Automatic protection devices when hazardous conditions occur (excessive increase in the pressure of the discharge, overflow by liquid ammonia separators, damage to the lubrication system), turn off the refrigeration unit or part of it.
Automatic alarm devices fill light or sound signals when a controlled value reaches the specified or maximum valid values.
N. D. Kochetkov
322 Automation of refrigeration units
Automatic control devices (self-inspection devices) Regient-powered parameters of the machine (temperature at different points, pressure, number of circulating agent, etc.).
Comprehensive automation provides for the equipment of the cooling equipment of the automatic control, control and protection devices. Control and alarm means are needed only to monitor the correct action of these devices.
Currently, the installation is small and much of the average performance settings are automated completely; Large installations in most cases are automated partially (semi-automatic installations).
Automatic refrigeration control
Installations
The automatic control devices used are distinguished by a variety of functions performed and action principles.
Each automatic regulator consists of a sensitive element that perceives the change in the adjustable parameter; controller; Intermediate bond connecting the sensitive element and regulating body. Consider ways to regulate the main parameters and the most characteristic devices.
Regulating the temperature of refrigeration chambers. In refrigeration chambers, it is necessary to maintain constant temperatures, even if the thermal load on the cooling batteries is changing.
The constant temperature is maintained by adjusting the cold-performance batteries. Simple and distributed is a two-position regulation system. With this system, an individual temperature relay is installed in each chamber, for example, the type of TDDa is a two-position remote thermaller (Fig. 193), or other types. On the pipeline of the liquid refrigerant or brine before entering the battery, a solenoid valve is installed (Fig. 194). With an increase in the temperature of the air to the upper specified limit, the temperature controller automatically closes the electrical circuit of the solenoid valve. The valve is fully opened, and the coolant enters the battery; Cameras are cooled. When the air temperature drops to the lower specified limit, the temperature controller, on the contrary, opens the valve chain, stopping the flow of cold fluid in the battery.
Termobalon 1
(sensitive cartridge) Temperature controller TDDA (see Fig. 193), partially filled with liquid phone-12,
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placed in the refrigeration chamber, the temperature of which is required to be adjusted. Freon pressure in the thermobalone depends on its temperature, which is equal to the air temperature of the camera. With an increase in this temperature, the pressure in the thermobalone increases. Increased pressure through the capillary tube 2 is transmitted to the chamber 3, in which the bellows 4, representing
a corrugated tube. The siltphone is compressed and moves in the axial direction to the needle 5, which turns the angular lever 6 (see also the scheme to the right) around the axis 7 counterclockwise, overcoming the resistance of the spring 22. Lever 6 peashes on itself a plate-in-one with an 8-hinged PRA Which when moving the lever counterclockwise moves to the left. The finger 10 is bonded with a finger 10, moving in the slots of the contact plate 12. At some point, the finger comes into contact with the lever 9 and turns this lever, as well as the contact plate 12 (which is associated with the spring arm 11) around the axis 13 (in this case counterclockwise ). In it
324 Automation of refrigeration units
the bottom end of the contact plate is approaching a constant horseshoe magnet 18 and is rapidly attracted by it. The main 17 and the sparkling 26 contacts are closed. The control circuit of the solenoid valve mounted on the liquid line is closed, the valve opens, the liquid enters the battery.
With a decrease in the air temperature, the pressure in the thermobalone and in the chamber 3, where the bellows is located, decreases and the angular lever 6 under the action of the spring 22 is rotated clockwise. Finger 10 moves from the lever 9 to the end of the slot in the contact plate 12 (free stroke), presses on the plate and, overcoming the magnet attraction, turns sharply clockwise. At this point, the electrical contacts are blocked, the solenoid valve is closed and the flow of fluid in the battery stops.
Automatic regulation of refrigeration settings 325
The temperature of the chamber at which electrical con-clocks is blocked, is mounted depending on the springs tension 22. To adjust the device to a certain point of opening, the carriage 21 with a pointer 20 to the corresponding division of the temperature scale 19, which is achieved when rotating the screw 23 handle 24.
The device is regulated by a certain difference in closure temperature and opening of electrical contacts. This difference depends on the magnitude of the free move 10 in the slot of the contact plate. The free move changes when moving the top end of the lever 9 along the slot, which is achieved when the cam is turned around around the axis 13. The more the cam radius in the touch of the lever 9, the greater the free move and the greater the difference in the temperature of the closure and opening of contacts.
The TDDA temperature controller ensures that the solenoid valve is turned off within the temperature scale from -25 to 0 ° C. Possible error of ± 1 ° C. The minimum differential of the device is 2 ° C, the maximum is at least 8 ° C. Mass of the device is 3.5 kg , Capillary length 3 m.
For large refrigerators, a multipoint centralized system of automatic temperature control in the chambers is developed - Amur machine. Such machines are manufactured by Pa 40, 60 and 80 adjustment points. They can be used not only to regulate the air temperature, but also the boiling point of the refrigerant, the temperature of the brine, etc. The machine has devices for measuring temperature at the control points.
Solenoid (electromagnetic) valves (see Fig. 194) work as follows. When the voltage is applied to the electromag coil, an electric field occurs, which retracts the core; The unloading valve associated with it is lifted by opening the saddle of the small diameter. After that, the fluid from the discharge side, i.e., from the cavity above the valve (in the VAT) or above the membrane (in the VVM ventilator) through the through holes n small saddle enters the cavity under the valve. The valve is unloaded from the pressure, which pressed it to the saddle, and opens for fluid duct under pressure from the injection pipeline. After turning off the solenoid coil, on the contrary, the header with the discharge valve is lowered down, overlapping the saddle of the small diameter. The pressure on top to the main valve increases, and it is under the action of its own weight and spring is lowered onto its saddle, overlapping the fluid flow.
Solenoid valves are among the most common devices for ammonia and freon refrigeration
326 Refrigeration Automation
novok. For liquid and gaseous freon and ammonia, brine and water, solenoid valves are produced with a diameter of a conditional pass from 6 to 70 mm. Previously used predominantly piston solenoid valves of type SPE; Recently, membrane valves of the TSM type of improved design are used. The temperature of the working environment may vary from -40 to + 50 ° C. The solenoid valve (with a filter in front of it) is installed on a horizontal section of the pipeline in a vertical position.
The adjustment of the air temperature is also possible by changing the temperature or consumption of the refrigeration agent (when cooled cooling) in batteries using proportional-nyline PRT temperature regulators. Such regulators are rarely used.
To automatically control the temperature of the air with the use of small freon settings with one cooled object-volume, the compressor is turned on and off. To turn on and off, use devices that react to the temperature or boiling pressure in the evaporator, or directly at the air temperature of the camera.
Regulation of the cooling capacity of compressors. The heat-loading of refrigeration chambers can vary widely depending on the number and temperature of the incoming products, ambient temperature and other factors. The cooling and dity of the installed compressors is chosen with the calculation of maintaining the required temperatures under the most difficult conditions.
In small freon installations of direct evaporation, the performance of compressors is adjusted simultaneously with adjusting the temperature of the cooled object by the start and stop method at the corresponding values \u200b\u200bof one of the adjustable parameters.
In brine cooling machines, the most convenient parameter for regulating the performance of the compressor is the pickle of the brine when leaving the evaporator. In the case of a decrease in the thermal load, the toll temperature in the evaporator is quickly reduced to the lower specified limit and the temperature controller (for example, the type of TDDA), the erosion chain of the magnetic starter coil, remains-lifts the compressor electric motor. With increasing temperature to the top specified limit, the temperature controller includes a new compressor to work. The greater the thermal load on the evaporator (cooling batteries), the longer the compressor works. Changing the working time coefficient achieved the necessary Automatic regulation of refrigeration settings 327
the average performance of the compressor.
In medium and large installations, the system contains a large number of batteries intended for cooling many rooms. When the specified temperatures are reached in separate rooms, part of the cooling batteries must be turned off with the cooling capacity of the compressors, respectively, is reduced.
The most acceptable in this case is a multi-position (stepwise) regulation by changing the working volume, described by pistons of compressors. In settings with several compressions, the multi-position control is carried out on and off the individual compressors controlled by temperature controls with offset settings. The presence of two identical compressors allows three stages of cold-productivity: 100- 50-0%. Two AV-100 and AU-200 compressors give four stages of cooling capacity: 100-67-33-0%. The stepwise regulation of multi-cylinder nonmediate computers may be turned off from the work of individual cylinders by pressing the suction valves with a special mechanism controlled by low pressure relay.
Much less often use smooth regulation of the manufacturer of the compressor-throttling of the suction steam, the change in the magnitude of the dead volume of the compressor, etc. These methods of energy-ki are disadvantageous. Comparatively promising is the method of regulating the cooling capacity by changing the number of revolutions of the compressor (the use of multi-speed electric motors).
Regulation of refrigerant supply to the evaporator. Regardless of the magnitude of the heat load, automatic control devices must ensure proper filling of the evaporator with a refrigerant. Excess fluid in the evaporator can not be allowed, as it leads to a decrease in the cost-effectiveness of work and to the occurrence of hydraulic impact ("wet stroke").
In the event of a lack of fluid, some of the surface is not used, which also worsens the operation of the operation due to the decrease in evaporation rate.
The devices that regulate the supply of fluid into the evaporator are the thermoregulating valves of the TRV and float control valves PRV. In the same devices, the process of throttle fluid is carried out.
The main type of productable thermal control valves -Mambrane, in a metal case. The inclusion scheme of TRV is shown in Fig. 195. The effect of the device depends on the overheating of the feather coming out of evapo-
328 Automation of refrigeration installations
tel. The absence of overheating indicates excess fluid in the evaporator and the possibility of entering it into the suction line and into the compressor. In this case, TRV automatically stops supplying the fluid to the evaporator. A large overheating of the chest of refrigerant during suction is, on the contrary, a sign of a shortage of it in the evaporator. At the same time, the TRV condition enhances fluid supply.
In the ammonium valve of the TRV, the thermobalon (sensitive element of the device) is filled with freon-22, close to the working pressures to Ammon Aku. The thermobalon is tightly attached to the suction pipeline; It has ammonia vapor temperature overlooking the evaporator.
Automatic refrigeration adjustment 329
When the temperature changes, the pressure in the thermobalone changes. The valve of the valve is mechanically connected to the membrane on which the pressure of the thermobalone pair is applied on top of the capillary tube, and the pressure from the evaporator along the equalization tube (through the fitting 7). From the difference in these pressures, proportional to the outlet of the vapor at the outlet of the evaporator depends on the displacement of the membrane, and at the same time the opening of the valve controlling the flow of fluid into the evaporator. Ammonia enters the TRP through the fitting 10. The throttling is performed and the valve opening and partially in the throttle tube 8, which provides a more relaxed and equal measuring agent through the valve.
During the operation of the machine, the Trp supports constant steam overheating; The overheating value corresponding to the adjustment can be changed from 2 to 10 ° C. The setting is carried out using the screw 4 and the adjusting gear associated with it. When rotating the screw, the tension of the spring 3 changes, opposing the valve opening.
The TRP allows you to reliably adjust the supply of ammonia into evaporators of different types at boiling temperatures from 0 to -30 ° C. The power supply of co-gas-tube evaporators for cooling the brine is adjusted for small overheating (from 2 to 4 ° C). Different models of the TRP are produced, designed for cooling capacity from 6 to 230 kW (~ 5-200 μAL / h).
TRV for 12-190 kW 10-160 mcal / h) For freon installations in the design close to the valves of the TRP type. In small freon machines, membrane TRVs are used without equalizing lines.
The adjustment of the ammonia supply to evaporators and vessels with a free level of fluid is possible with the help of float controls of the Low PRV (Fig. 196).
PRV is set at the level, which is desirable to support the evaporator (or other vessel). The body of the device is connected to the evaporator with equalizing lines (liquid and steam). The change in the level of fluid in the evaporator leads to a change in the level in the PRV housing. At the same time, the position of the float inside the case is changing, which causes the valve to move and change the cross-sectional area for fluid flow from the condenser to the evaporator.
In the float valves of the indispensable type, the refrigeration agent Pos-le throttling in the valve opening comes directly into the evaporator, bypassing the float chamber. In the valves of the passage of the refrigerant, after throttling enters the float chamber, and from it is given to the evaporator.
330 Refrigeration Automation
Automatic refrigeration control 331
liquid level in evaporators and vessels. Unlike low pressure valves, PR-1 can be installed at different levels relative to the evaporator and condenser.
The fitting, connecting the valve with the bottom of the capacitor, is welded to the valve body. Inside the housing is a float associated with a lever with a needle valve. Ammonia through the hole in the valve seat, the channel and the throttle passes to the output
stacker and through it into the pipeline to the evaporator. Inside the body of the valve there is a capillary tube. The upper end is open, and the lower with the help of the channels is connected to the throttle tube. Pressure in the valve is set slightly lower than in the condenser; The liquid from it enters the valve body. Under the action of fluid float floats. The larger the fluid enters the housing of the pop shop, the more the valve opens to pass it into the evaporator. When using a valve type PR-1, the condenser is free from liquid. Therefore, the amount of ammonia in the system should be such that with the full flow of ammonia into the evaporator, the fluid level in it was not higher than between the first and second above the rows of the evaporator pipes. With this fill
332 Refrigeration Automation
the risk of fluid ammonia into the suction line is eliminated and favorable conditions for intensive heat exchange in the evaporator are created.
For positional control of the level of fluid in the refrigeration unit, indirect level control regulators are often used from a level indicator (for example,
DU-4, RU-4, PRU-2) and the solenoid valve controlled. These devices are included in the circuit (Fig. 198) so that in the case of an excessive increase in the level of fluid in the device, the remote pointer is once-in the electrical circuit of the control of the solenoid valve and it closes, stopping the supply of the refrigerant in the evaporator.
If the level of fluid in the evaporator drops compared to op-tyrimal, then the remote pointer will again be closed by the electrical chain of the solenoid valve; Fluid supply will be resumed.
Regulation of the cooling water supply to the condenser.
The water on the condenser is supplied through the water regulating valve
(Fig. 199) supporting approximately constant pressure and temperature of condensation at different loads. Condensation pressure perceives the valve membrane or a bellows, changing the spindle position and the section for water passage. In installations with cooling towers, water-adjusting valves do not apply.
Automatic Protection and Alarm 333
The cold is used in the technologies of many processing processes of agricultural products. Thanks to refrigerators, losses are significantly reduced during product storage. Chilled products can be transported over long distances.
Milk intended for processing or implementation is usually pre-cooled. Before sending to the enterprise of the dairy industry, milk is allowed to be stored no more than 20 hours at a temperature not higher than 10 "C.
In agriculture, meat is cooled mainly on farms and poultry farms. At the same time, the following methods of cooling are used: in air, cold water, in water with melting ice and irrigation with cold water. The poultry meat rejuvenation is made by either cold air or a dive into a cold brine. Air focus is carried out at air temperature in refrigeration chambers from -23 to -25 ° C and air movement speed 3 ... 4 m / s. To rejuvenate with immersion in the brine, solutions of calcium chloride or propylene glycol with a temperature of -10 ° C and below are used.
Meat, intended for long-term storage, freeze the same methods as the rejection. 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 temperatures -1 ...- 2 ° C and relative humidity 85 ... 88%. After cooling to 2 ... 3 ° C, they are placed in the storage chamber.
Fruits and vegetables are cooled in stationary storage facilities. Fruit and vegetable products are stored in refrigeration chambers with cooling batteries in which the cold agent or brine circulates.
In air-cooled systems, the air is first cooled, which is then injected into the storage chambers. In mixed systems, products are cooled with cold air and from the battery.
In agriculture, the cold is obtained as a flammable way (glaciers, ice-salted cooling) and with special refrigeration machines. With engine cooling of the heat from the cooled medium, it is assigned to the outer surrounding space with low-boiling refrigerators (freon or ammonia).
In agriculture, steam compressors and absorption refrigeration machines are widely used.
The simplest way to produce the temperature of the working fluid below the ambient temperature is that this working fluid (refrigerator agent) is compressed in the compressor, then cooled to ambient temperature and then subjected to adiabatic expansion. At the same time, the working fluid makes work due to its internal energy and its temperature decreases compared to the ambient temperature. Thus, the working fluid becomes the source of getting cold.
In principle, any pairs or gas can be used as refrigerators. In the first refrigeration machines with a mechanical drive, air was used as a refrigerant, but from the end of the XIX century. It was replaced by ammonia and carbon dioxide, since the air refrigerator is less economical and more cumbersome than the vapor, due to the large air consumption due to its low heat capacity.
In modern refrigeration installations, the working fluids are pairs of liquids, which at pressures close to atmospheric boil at low temperatures. Examples of such refrigerators can serve as ammonia NH3, sulphide anhydride SO2, carbon dioxide C0 2 and freons - fluorochloro-derived hydrocarbons like 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 - halogen derivatives of saturated hydrocarbons (C M H n), obtained by replacing hydrogen atoms of chlorine and fluorine atoms. In the technique, due to the large variety of freon and relatively complex item, a conditional numerical designation system is established, according to which each such compound, depending on the chemical formula, has its own number. The first digits in this respect are conventionally denoted by a hydrocarbon derivative of which this freon: methane - 1, ethane - 11, propane - 21. If there are unsubstituted hydrogen atoms in the compound, their number is added to these numbers. Further to the amount obtained or to the original number (if all hydrogen atoms in the connection are substituted) add the number in the form of the next sign, expressing the number of fluorine atoms. So the notation is obtained: R11 instead of monofluorichloromethane CFCI2, R12 instead of difluorodichloromethane CF 2 C1 2, etc.
In refrigeration installations, R12 is usually used as a refrigerant, and in the future, R22 and R142 will be widely used. The advantages of freon are relative harmlessness, chemical inertness, non-combustibility and explosion safety; Disadvantages - low viscosity, promoting leakage, and the ability to dissolve in oil.
Figure 8.15 shows a schematic diagram parokompressor refrigeration unit And its ideal cycle in a 75-chart. In compressor 1 compressed wet pairs of the refrigerant, resulting in a plot a-b) It turns out dry saturated or superheated pairs. 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 not apply oils
Fig. 8.15.
/ - compressor; 2 - cooled room; 3- throttle valve; 4 - Condenser special varieties. From the compressor overheated steam with parameters pI and 02 enters the cooler (condenser 2). In the condenser at constant pressure, overheated pairs gives cooling water overheating heat (process B-s) And its temperature becomes equal to the saturation temperature of 0 H2. Returning the heat of vaporization in the future (process c-D) Saturated steam turns into a boiling liquid (point d). This liquid comes to the throttle valve. 3, After passing through which it turns into a saturated pairs of a small degree of dryness (x 5 \u003d 0.1 ... 0.2).
It is known that the enthalpy of the working body before and after throttling is the same, and the pressure and temperature decrease. The 7S diagram shows a barrier of constant enthalpy d-E, point e. which characterizes the condition of the steam after throttling.
Next, the wet pairs enters the cooled container called the refrigerator 4. Here, with unchanged pressure and temperature, steam expands (process e - a) Utvable certain amount of heat. The degree of dryness of the steam increases (x | \u003d 0.9 ... 0.95). Couples with status parameters characterized by point 1, Supported into the compressor, and the installation of the installation is repeated.
In practice, steam after the throttle valve does not enter the refrigerator, but in the evaporator, where hesitates heat from the brine, which, in turn, takes the heat from the refrigerator. This is explained by 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 brins, aqueous solutions of sodium chloride and calcium chloride are used, having sufficiently low freezing temperatures. The solutions are suitable for use only at temperatures exceeding those in which they freeze as a homogeneous mixture, forming salted ice (the so-called cry hydrate point). A clouded point for a solution of NaCl with a mass concentration of 22.4% corresponds to the temperature -21,2 "C, and for the SAS1 2 solution with a concentration of 29.9 - the temperature of -55 ° C.
An indicator of the energy efficiency of refrigeration installations is the refrigeration coefficient E, which is the ratio of specific cooling capacity to the energy spent.
The actual cycle of the park compressor refrigeration is different from the theoretical in that due to the presence of internal threads for friction, the compression in the compressor does not occur by adiabat, but by polytropy. As a result, energy costs in the compressor decreases and the refrigeration coefficient is reduced.
To produce low temperatures (-40 ... 70 ° C) required in some technological processes, single-stage parocompressors are or uneconomical, or completely unsuitable due to the decrease in the KPD of the compressor, due to the high temperatures of the working fluid at the end of the compression process. In such cases, or special refrigeration cycles are used, or in most cases two-stage or multistage compression. For example, a two-stage compression of ammonia vapors receive temperatures up to -50 ° C, and a three-step - to -70 ° C.
The main advantage absorption refrigeration unitscompared to compressor - the use for the production of cold is not electrical, but thermal energy of low and medium potentials. The latter can be obtained from the water vapor, selected, for example, from a turbine on the thermal power plane.
Absorbation is the phenomenon of the absorption of a pair of liquid substance (absorbent). In this case, the pair temperature may be lower than the temperature of the absorbent, absorbing steam. For the absorption process, it is necessary that the concentration of the absorbed pair is equal to or more equilibrium concentration of this steam above the absorbent. Naturally, in absorption refrigeration systems, liquid absorbents must absorb the refrigeration agent with sufficient speed, and at the same pressures, the temperature of their boiling should be significantly higher than the boiling point of the refrigerant.
The most common water-ammonia absorption settings in which ammonia serves as a refrigerant, and water is absorbent. Ammonia is well soluble in water. For example, at 0 ° C in one volume of water, up to 1148 of the vapor-shaped ammonia dissolves, and the heat is distinguished about 1220 kJ / kg.
The cold in the absorption unit is generated according to the scheme shown in Figure 8.16. This scheme causes exemplary values \u200b\u200bof the working bodies in the installation without taking into account the pressure losses in pipelines and the temperature pressure losses in the condenser.
In the generator 1 There is evaporation of a saturated ammonium solution when heated by its water vapor. As a result, the low-quality component is distilled off - ammonia couples with a slight admixture of water vapor. If you maintain a solution temperature of about 20 "C, then the saturation pressure of ammonia vapors will be approximately 0.88 MPa. So that the NH 3 content in the solution has not diminished using the pumping pump 10 From the absorber to the generator continuously fastened with strong concentration
Fig. 8.16.
/-generator; 2- condenser; 3 - throttle valve; 4- evaporator; 5-pump; B-overhead valve; 7- cooled capacity; absorber; 9-serpent; 10- pump
ammonia solution. Saturated ammonia couple (x \u003d 1), obtained in the generator, is sent to the condenser 2, where ammonia turns into liquid (x \u003d 0). After throttle 3 Ammonia enters the evaporator 4, In this case, it is reduced to 0.3 MPa (/ H \u003d -10 ° C) and the degree of dryness becomes equal to about 0.2. "0.3. In the evaporator, the ammonia solution is evaporated due to heat supplied with brine from the cooled tank 7. The temperature of the brine is reduced from -5 to -8 ° C. Using a pump 5 It turns into a capacity of 7, where it is heated to -5 ° C, selecting heat from the room and maintaining a constant temperature in it, approximately -2 ° C. Separated in the evaporator of ammonia with the degree of dryness x \u003d 1 enters the absorber 8, where it is absorbed by a weak solution supplied through the bypass valve 6 From the generator. Since absorption is an exothermic reaction, then to ensure the continuity of the process of heat exchange, the absorption is removed by cooling water. A strong ammonia solution obtained in the absorber 10 pumps into the generator.
Thus, in the considered installation there are two devices (generator and evaporator), where heat is summed up to the working fluid from the outside, and two devices (condenser and absorber), in which heat is removed from the working fluid. Comparing the fundamental schemes of parocompressor and absorption settings, it can be noted that the generator in the absorption unit replaces the injection, and the absorber is the suction part of the piston compressor. Compression of the refrigerant occurs without the costs of mechanical energy, except for small costs for pumping a strong solution from the absorber into the generator.
In practical calculations, refrigeration coefficient E is also taken as an energy indicator of the absorption unit, 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. Thus, the refrigeration coefficient is always less than the refrigeration coefficient of the park compressor installation. However, the comparative assessment of the energy efficiency of the considered methods of obtaining cold as a result of the immediate comparison of the methods of only the refrigeration coefficients of the absorption and parokompressor installations is incorrect, as it is determined not only by the number, but also the type of energy spent. Two methods of obtaining cold should be compared by the value of the reduced refrigeration factor, which represents the ratio of cooling capacity q 2. to fuel heat consumption q IT. i.e.? pr \u003d. Yag It turns out that at the temperatures of evaporation from -15 to -20 ° C (used by the bulk of consumers) E PR absorption sets higher than the parokompressors, as a result of which, in some cases, absorption installations are more profitable not only when they supply them to the ferry, selected from turbines, but also When supplying them to the ferry directly from steam boilers.
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