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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 unit. The automation system layout is selected depending on the cooling capacity and purpose of the installation.
Apply refrigeration machine automation systems with capacity control by releasing the solenoid valves, as well as turning the refrigeration units on and off. 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 ammonia plant automation 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 it is mounted thermal relay, 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. To cool undercar boxes and cabinets, 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, are used. 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 chambers; 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 versions. Automation diagram for two-position control in one or two evaporators with the same chamber air cooling temperature (Fig. 4, a) provides for the use of an evaporator, chamber or relay temperature relay low pressure compressor. 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 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.
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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. They help maintain basic operating parameters at the required level - 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 malfunctions and dangerous situations as a result of unacceptable increases in pressure, temperature, and malfunction of certain devices. All kinds of sensors, thermometers, pressure gauges and much more are used here
Cold is used in the technologies of many processes of processing agricultural products. Thanks to refrigerators, losses during storage of products are significantly reduced. Chilled products can be transported over long distances.
Milk intended for processing or sale is usually pre-cooled. Before being sent to a dairy industry enterprise, milk may be stored for no more than 20 hours at a temperature not exceeding 10 °C.
In agriculture, meat is chilled mainly on farms and poultry farms. In this case, the following cooling methods are used: in air, cold water, in water with melting ice and irrigation with cold water. Freezing of poultry meat is done either with cold air or by immersion in cold brine. Air freezing is carried out at an air temperature in refrigeration chambers from -23 to -25 ° C and an air speed of 3...4 m/s. For freezing by immersion in brine, solutions of calcium chloride or propylene glycol with a temperature of -10 ° C and below are used.
Meat intended for long-term storage is frozen using the same methods as freezing. Freezing
with air is carried out at a temperature of cooled air from -30 to -40 °C; when freezing in brine, the temperature of the solution is -25...-28 °C.
Eggs are stored in refrigerators at a temperature of -1...-2 °C and a relative humidity of 85...88%. After cooling to 2...3 °C, they are placed in a storage chamber.
Fruits and vegetables are cooled in stationary storage facilities. Fruit and vegetable products are stored in refrigerated chambers with cooling batteries in which a cold agent or brine circulates.
In air-cooled systems, air is first cooled, which is then forced into storage chambers by fans. In mixed systems, products are cooled with cold air and from a battery.
In agriculture, cold is obtained both machine-free (glaciers, ice-salted cooling) and using special refrigeration machines. In machine refrigeration, heat from the cooled medium is removed to the external environment using low-boiling refrigerants (freon or ammonia).
Steam compressors and absorption refrigeration machines are widely used in agriculture.
The simplest way to obtain the temperature of the working fluid below the temperature environment consists in the fact that this working fluid (refrigerant) is compressed in a compressor, then cooled to ambient temperature and then subjected to adiabatic expansion. In this case, the working fluid does work due to its internal energy and its temperature decreases compared to the ambient temperature. Thus, the working fluid becomes a source of cold.
In principle, any steam or gas can be used as refrigerants. In the first mechanically driven refrigeration machines, air was used as a refrigerant, but already from the end of the 19th century. it was replaced by ammonia and carbon dioxide, since the air refrigeration machine is less economical and more cumbersome than the steam one, due to the high air flow due to its low heat capacity.
In modern refrigeration units, the working fluid is a vapor of liquids that, at pressures close to atmospheric, boil at low temperatures. Examples of such refrigerants include ammonia NH3, sulfur dioxide SO2, carbon dioxide C0 2 and freons - chlorofluorocarbon derivatives of the type C m H x F y Cl2. Boiling point of ammonia at atmospheric pressure is 33.5 °C, “Freona-12” -30 °C, “Freona-22” -42 °C.
Freons are widely used as refrigerants - halogen derivatives of saturated hydrocarbons (C m H n), obtained by replacing hydrogen atoms with chlorine and fluorine atoms. In technology, due to the wide variety of freons and their relatively complex names, a conventional numerical designation system has been established, according to which each such compound has its own number, depending on its chemical formula. The first digits in this number conventionally indicate the hydrocarbon of which this freon is a derivative: methane - 1, ethane - 11, propane - 21. If the compound contains unsubstituted hydrogen atoms, then their number is added to these numbers. Next, to the resulting amount or to the original number (if all the hydrogen atoms in the compound are replaced), a figure expressing the number of fluorine atoms is added in the form of the next sign. This is how the designations are obtained: R11 instead of monofluorotrichloromethane CFCI2, R12 instead of difluorodichloromethane CF 2 C1 2, etc.
In refrigeration units, R12 is usually used as a refrigerant, and in the future R22 and R142 will be widely used. The advantages of freons are relative harmlessness, chemical inertness, non-flammability and explosion safety; Disadvantages - low viscosity, which promotes leakage, and the ability to dissolve in oil.
Figure 8.15 shows the circuit diagram steam compressor refrigeration unit and her ideal cycle in the 75 diagram. In the compressor 1 the wet vapor of the refrigerant is compressed, resulting in (section a-b) the result is dry saturated or superheated steam. 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 stress and do not use oils
Rice. 8.15.
/ - compressor; 2 - refrigerated room; 3- throttle valve; 4 - special grade capacitor. Superheated steam from the compressor with parameters pi and 02 enters the cooler (condenser 2). In a condenser at constant pressure, superheated steam gives off superheating heat to the cooling water (process b-c) and its temperature becomes equal to the saturation temperature 0 n2. Subsequently releasing the heat of vaporization (the process c-d), saturated steam turns into boiling liquid (point d). This fluid flows to the throttle valve 3, after passing through which it turns into saturated steam with a slight degree of dryness (x 5 = 0.1...0.2).
It is known that the enthalpy of the working fluid before and after throttling is the same, and the pressure and temperature decrease. The 7s diagram shows a dashed line of constant enthalpy d-e, dot e which characterizes the state of steam after throttling.
Next, the wet steam enters a cooled container called a refrigerator 4. Here, at constant pressure and temperature, the steam expands (the process e-a), taking away a certain amount of heat. The degree of steam dryness increases (x| = 0.9...0.95). Pair with state parameters characterized by a point 1, is sucked into the compressor, and the operation of the installation is repeated.
In practice, the steam after the throttle valve does not enter the refrigerator, but into the evaporator, where it takes away heat from the brine, which, in turn, takes heat away 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. Aqueous solutions of sodium chloride and calcium chloride, which have fairly low freezing temperatures, are used as brines. The solutions are suitable for use only at temperatures above those at which they freeze as a homogeneous mixture, forming salt ice (the so-called cryohydrate point). The cryohydrate point for a NaCl solution with a mass concentration of 22.4% corresponds to a temperature of -21.2 °C, and for a CaCl 2 solution with a concentration of 29.9 - a temperature of -55 °C.
An indicator of the energy efficiency of refrigeration units is the refrigeration coefficient e, which is the ratio of the specific refrigeration capacity to the energy consumed.
The actual cycle of a vapor compressor refrigeration unit differs from the theoretical one in that, due to the presence of internal friction losses, compression in the compressor occurs not along an adiabatic path, but along a polytrope. As a result, energy consumption in the compressor is reduced and the refrigeration coefficient is reduced.
To obtain low temperatures (-40...70 °C) required in some technological processes, single-stage steam compressor units turn out to be either uneconomical or completely unsuitable due to a decrease in compressor efficiency caused by high temperatures working fluid at the end of the compression process. In such cases, either special refrigeration cycles are used, or in most cases, two-stage or multi-stage compression. For example, two-stage compression of ammonia vapor produces temperatures down to -50 °C, and three-stage compression - up to -70 °C.
Main advantage absorption refrigeration units Compared to compressor engines, they use not electrical, but thermal energy of low and medium potentials to produce cold. The latter can be obtained from water vapor taken, for example, from a turbine in thermal power plants.
Absorption is the phenomenon of vapor absorption liquid substance(absorbent). In this case, the temperature of the steam may be lower than the temperature of the absorbent that absorbs the steam. For the absorption process, it is necessary that the concentration of the absorbed vapor be equal to or greater than the equilibrium concentration of this vapor above the absorbent. Naturally, in absorption refrigeration units, liquid absorbents must absorb the refrigerant at a sufficient speed, and at the same pressures, their boiling point must be significantly higher than the boiling point of the refrigerant.
The most common are water-ammonia absorption plants, in which ammonia serves as a refrigerant and water as an absorbent. Ammonia is highly soluble in water. For example, at 0 °C, up to 1148 volumes of vaporous ammonia are dissolved in one volume of water, and heat of about 1220 kJ/kg is released.
The cold in the absorption unit is produced according to the scheme shown in Figure 8.16. This diagram shows approximate values of the parameters of the working fluid in the installation without taking into account pressure losses in pipelines and losses in temperature pressure in the condenser.
In the generator 1 evaporation of a saturated ammonia solution occurs when it is heated with water steam. As a result, the low-boiling component - ammonia steam with a slight admixture of water vapor - is distilled off. If you maintain the solution temperature at about 20 °C, then the saturation pressure of ammonia vapor will be approximately 0.88 MPa. To prevent the NH 3 content in the solution from decreasing, use a transfer pump 10 from the absorber to the generator a strong concentrated
Rice. 8.16.
/-generator; 2- capacitor; 3 - throttle valve; 4- evaporator; 5-pump; b-bypass valve; 7- refrigerated container; absorber; 9-coil; 10- pump
bath ammonia solution. Saturated ammonia steam (x = 1), produced in the generator, is sent to the condenser 2, where ammonia turns into liquid (x = 0). After throttle 3 ammonia enters the evaporator 4, in this case, its pressure decreases to 0.3 MPa (/n = -10 °C) and the degree of dryness becomes approximately 0.2.„0.3. In the evaporator, the ammonia solution is evaporated due to the heat supplied by the brine from the cooled container 7. In this case, the temperature of the brine decreases from -5 to -8 °C. With pump 5 it is distilled back into container 7, where it is again heated to -5 °C, taking heat from the room and maintaining a constant temperature in it, approximately -2 °C. Ammonia evaporated in the evaporator with a degree of dryness x = 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, to ensure continuity of the heat exchange process, the absorbent is removed with cooling water. The strong ammonia solution obtained in the absorber pump 10 pumped to the generator.
Thus, in the considered installation there are two devices (generator and evaporator), where heat is supplied to the working fluid from the outside, and two devices (condenser and absorber), in which heat is removed from the working fluid. Comparing the schematic diagrams of steam compressor and absorption plants, it can be noted that the generator in the absorption plant replaces the discharge part, and the absorber replaces the suction part of the piston compressor. Compression of the refrigerant occurs without the expenditure of mechanical energy, except for the small costs of pumping a strong solution from the absorber to the generator.
In practical calculations, the refrigeration coefficient e, which is the ratio of the amount of heat q 2 perceived by the working fluid in the evaporator to the amount of heat q u spent in the generator. The refrigeration coefficient calculated in this way is always less than the refrigeration coefficient of the steam compressor unit. However, a comparative assessment of the energy efficiency of the considered methods for producing cold as a result of a direct comparison of the methods of only the refrigeration coefficients of absorption and steam compressor units is incorrect, since it is determined not only by the quantity, but also by the type of energy expended. The two methods of obtaining cold should be compared based on the value of the reduced coefficient of performance, which is the ratio of the cooling capacity q 2 to fuel heat consumption q it i.e. ? pr = Yag Ya- It turns out that at evaporation temperatures from -15 to -20 °C (used by the majority of consumers), the e-efficiency of absorption units is higher than that of steam compressor units, as a result of which, in some cases, absorption units are more profitable not only when supplying them with steam taken from turbines, but also when supplied with steam directly from steam boilers.
To provide non-autonomous air conditioners with cold, refrigeration stations of various cooling capacities are used. Refrigeration stations are usually equipped with two or more refrigeration units operating with an intermediate coolant, usually water.
Let's consider the automation of individual elements of refrigeration units and the refrigeration station as a whole. The compressor is protected from high pressure on discharge and low pressure on suction using a pressure switch (Fig. 8.10, A). The operation of the system is controlled by a lubrication control relay. High-capacity compressors are water-cooled. To protect them from overheating in the event of a loss of cooling water supply, a flow switch is installed. If any of the parameters deviates, the corresponding protection relay is activated and the compressor stops. When the compressor motor stops, the solenoid valve of the cooling water pipeline interlocked with it closes.
Refrigeration unit evaporator protection (Fig. 8.10, b) is provided to avoid freezing of water in the evaporator pipes. A position thermostat sensor is installed on the pipeline of water leaving the evaporator, set to 1-3 °C. When the water temperature is below the set one, the regulator contacts open and the compressor motor stops. If the flow of water through the evaporator suddenly stops, the regulator may not operate due to the inertia of the system even if the evaporator freezes. To avoid this, install
Rice. 8.10.
- 1 - lubrication control relay; 2, 3 - relay low and high pressure;
- 4 - flow regulator; 5 - solenoid valve; 6 - flow switch;
- 7 - thermostat
a flow switch, which, when the water flow decreases to a critical value, is activated and stops the compressor motor.
The automation diagram of the refrigeration station is shown in Fig. 8.11. For simplicity, the diagram shows one refrigeration machine. From the tank 1 pumps supply water to the evaporators of refrigeration machines, the cooled water is drained into the tank 2 and is supplied by pumps to air conditioners, and then drained back into the tank 1. Water is supplied from the cooling tower to cool the condensers.
The compressor is protected using a relay 3 , 4 , 5, and the evaporator - relays b and 7. If any parameter deviates from the set value, the corresponding relay will operate, the compressor will stop, and after a short period of time the circulating water supply pumps will also stop. On the automation panel, the signal lamp of the unit in which the accident occurred will turn on and start sound signal 9.
Rice. 8.11.
refrigeration station
Tank water temperature 2 regulated by thermostat 10, set to maximum and minimum temperature(for example, 8 and 6 °C). At a water temperature of 8 °C, successively after a certain period of time using a command device 11 refrigeration units are turned on, and the compressor of the refrigeration unit is turned on only if the pumps supplying water to the evaporator and condenser are running, and if all parameters controlled by the safety devices are within normal limits. When the temperature drops cold water up to 6 °C, refrigeration units are switched off in the same sequence. To maintain constant water pressure supplied to the air conditioners, a direct-acting pressure regulator is installed 8. To save money tap water To cool the condensers of refrigeration machines, recycling water supply systems are used, in which heated water is cooled in cooling towers. The automation scheme for such cooling systems is discussed in Section. 7.5 (see Fig. 7.14).
