The device and principle of operation of refrigeration equipment. The principle of operation of refrigeration units. Video

Industrial refrigeration equipment has become very widespread in various industries. The main area of ​​application of units and installations belonging to this class is the maintenance of certain temperature conditions, necessary for long-term storage variety of goods, materials and substances. They are used to cool liquids as well as food products, chemical raw materials, technological mixtures etc.

Main characteristics of industrial refrigeration equipment

Used in industry, it is able to create operating temperatures from -150 to + 10C. Units belonging to this class are adapted to work in rather harsh conditions and have a high degree of reliability of components.

Industrial refrigeration machines operate on the principle heat pump transferring energy from the heat sink to the heat sink. In the vast majority of cases, the role of the first is the environment, and the refrigerant is the receiving object. The latter belongs to the class of substances that are capable of boiling at a pressure of 1 atm and a temperature that differs significantly from that of the external environment.

Industrial refrigeration equipment consists of 8 main components:

• compressor;
• evaporator;
• flow regulator;
• fan;
• solenoid valve;
• reversing valve;

The condenser sucks in vapors of a substance that acts as a refrigerant, where its pressure and temperature are increased. After that, the refrigerant enters the compressor unit, the most important parameters of which are compression and displacement. The condenser cools the heated refrigerant vapor, due to which the thermal energy is transferred to environment. The evaporator is the component through which the medium to be cooled and the vaporized refrigerant pass.

Industrial refrigeration machines and installations are used to cool sufficiently large volumes that are used by warehouses, vegetable warehouses, freezing lines, freezing tunnels, as well as large and complex systems conditioning. In particular, such refrigeration equipment most often used for industrial needs in food processing shops (meat, poultry, fish, milk, etc.)

Classification of industrial installations

All industrial refrigeration units are divided into compression and absorption. In the first case, the refrigeration equipment is a vapor condensation machine that compresses the refrigerant through compressor or turbo compressor units. Such systems use freon or ammonia as the most effective substances in terms of temperature absorption.

Absorption plants condense a vaporous refrigerant with a solid or liquid absorbent, from which the working substance is evaporated when heated due to a higher partial pressure. These units are continuously and intermittently operating, and the first type of units is divided into pumping and diffusion.

Compressor-type refrigeration equipment differs according to the compressor type into open, semi-hermetic and hermetic units. Depending on the method of cooling the condensing unit, the machines are equipped with water or air cooling systems. Absorption units use a larger amount of water in the process of operation and have significant dimensions and weight. They have a number of advantages compared to compressor refrigeration units, in particular, simple design, higher reliability of components, as well as the ability to use inexpensive heat sources and quiet operation.

Depending on the power of the industrial refrigeration equipment the value of possible emissions of thermal energy is calculated. This heat can be used in 3 ways:
- to the environment. Heat transfer is carried out by means of an external compressor.
- V production room. In this case, allocated thermal energy allows you to save money needed for heating.
- energy recovery. The released heat is transferred to the place where it is most needed.

The main types of industrial refrigeration equipment

When choosing industrial refrigeration equipment, it is necessary to focus on the main technical specifications proposed models. Should be paid Special attention on the maximum amount of heat release, as well as its dynamics during the production shift. In addition, it is important to take into account the hydraulic resistance index of the units and components of the system. It is necessary to determine the direction of heat removal, as well as to decide on the possibility of duplicating the entire refrigeration system.

To date, the following types of refrigeration equipment are most often used in industry:

• . This type of units is used in meat, sausage, fish and bakery production.
• cabinets and chambers shock freezing. Equipment of this type is used in enterprises engaged in the production of fish, meat and vegetable products, as well as processing and storage of fruits, berries, etc.
• food chillers. This type of refrigeration machines is excellent for cooling various liquids and certain categories. food products;
• chillers for cooling plastics. Such units are used for cooling raw polymers and finished products.
• liquid separators and receivers and collectors;
• freezing tunnels. This type of equipment is used for freezing piece, packaged and packaged goods in large quantities.

Basic concepts related to the operation of the refrigeration machine

Cooling in air conditioners is carried out due to the absorption of heat during the boiling of the liquid. When we talk about a boiling liquid, we naturally think of it as hot. However, this is not entirely true.

First, the boiling point of a liquid depends on the ambient pressure. The higher the pressure, the higher the boiling point, and vice versa: the lower the pressure, the lower the boiling point. At normal atmospheric pressure, equal to 760 mm Hg. (1 atm), water boils at plus 100°C, but if the pressure is low, as, for example, in the mountains at an altitude of 7000-8000 m, water will begin to boil already at a temperature of plus 40-60°C.

Secondly, under the same conditions, different liquids have various temperatures boiling.

For example, freon R-22, widely used in refrigeration, has a boiling point of minus 4°.8°C at normal atmospheric pressure.

If liquid freon is in an open vessel, that is, at atmospheric pressure and ambient temperature, then it immediately boils, while absorbing a large amount of heat from the environment or any material with which it is in contact. In a refrigeration machine, freon does not boil in an open vessel, but in a special heat exchanger called an evaporator. At the same time, freon boiling in the evaporator tubes actively absorbs heat from the air flow washing the outer, as a rule, finned surface of the tubes.

Let's consider the process of liquid vapor condensation on the example of freon R-22. The condensation temperature of freon vapor, as well as the boiling point, depends on the ambient pressure. The higher the pressure, the higher the condensation temperature. So, for example, the condensation of R-22 freon vapor at a pressure of 23 atm begins already at a temperature of plus 55°C. The process of condensation of freon vapor, like any other liquid, is accompanied by the release a large number heat to the environment or, in the case of a refrigeration machine, by transferring this heat to a stream of air or liquid in a special heat exchanger called a condenser.

Naturally, in order for the process of boiling freon in the evaporator and cooling the air, as well as the process of condensation and heat removal in the condenser to be continuous, it is necessary to constantly “pour” liquid freon into the evaporator, and constantly supply freon vapor to the condenser. Such a continuous process (cycle) is carried out in a refrigeration machine.

The most extensive class of refrigeration machines is based on a compression refrigeration cycle, the main structural elements which are a compressor, an evaporator, a condenser and a flow regulator (capillary tube), connected by pipelines and representing a closed system in which the compressor circulates the refrigerant (freon). In addition to providing circulation, the compressor maintains a high pressure of about 20-23 atm in the condenser (on the discharge line).

Now that we have considered the basic concepts associated with the operation of the refrigeration machine, let's move on to a more detailed consideration of the compression cooling cycle diagram, the design and functionality of individual components and elements.

Rice. 1. Scheme of the compression refrigeration cycle

An air conditioner is the same refrigeration machine designed for heat and moisture treatment of the air flow. In addition, the air conditioner has significantly greater capabilities, more complex design and numerous additional options. Air processing involves giving it certain conditions, such as temperature and humidity, as well as the direction of movement and mobility (speed of movement). Let us dwell on the principle of operation and the physical processes occurring in the refrigeration machine (air conditioner). Cooling in the air conditioner is provided by continuous circulation, boiling and condensation of the refrigerant in a closed system. The refrigerant boils at low pressure and low temperature, and condenses at high pressure and high temperature. A schematic diagram of a compression refrigeration cycle is shown in fig. 1.

Let's start the consideration of the cycle operation from the evaporator outlet (section 1-1). Here the refrigerant is in a vapor state with low pressure and temperature.

The vaporous refrigerant is sucked in by the compressor, which raises its pressure to 15-25 atm and temperature to plus 70-90°C (section 2-2).

Further in the condenser, the hot vaporous refrigerant cools and condenses, that is, it passes into the liquid phase. The condenser can be either air-cooled or water-cooled depending on the type of refrigeration system.

At the condenser outlet (point 3), the refrigerant is in a liquid state at high pressure. The dimensions of the condenser are chosen so that the gas is completely condensed inside the condenser. Therefore, the temperature of the liquid at the outlet of the condenser is somewhat lower than the condensation temperature. Subcooling in air-cooled condensers is typically around plus 4-7°C.

In this case, the condensation temperature is approximately 10-20°C higher than the atmospheric air temperature.

Then the refrigerant in the liquid phase at high temperature and pressure enters the flow regulator, where the pressure of the mixture decreases sharply, while part of the liquid can evaporate, passing into the vapor phase. Thus, a mixture of vapor and liquid enters the evaporator (point 4).

The liquid boils in the evaporator, removing heat from the surrounding air, and again goes into a vapor state.

The dimensions of the evaporator are chosen so that the liquid completely evaporates inside the evaporator. Therefore, the temperature of the steam at the outlet of the evaporator is higher than the boiling point, so-called overheating of the refrigerant in the evaporator occurs. In this case, even the smallest droplets of refrigerant evaporate and no liquid enters the compressor. It should be noted that if liquid refrigerant enters the compressor, the so-called “water hammer”, damage and breakage of valves and other parts of the compressor are possible.

The superheated vapor exits the evaporator (point 1) and the cycle is restarted.

Thus, the refrigerant constantly circulates in a closed circuit, changing its state of aggregation from liquid to vapor and vice versa.

All refrigeration compression cycles include two specific pressure levels. The boundary between them passes through the discharge valve at the compressor outlet on one side and the outlet from the flow regulator (from the capillary tube) on the other side.

The compressor discharge valve and flow control outlet are the dividing points between the high and low pressure sides of the chiller.

on the side high pressure all elements operating at condensing pressure are located.

on the side low pressure all elements operating at evaporation pressure are found.

Although there are many types of compression chillers, circuit diagram cycles are almost the same.

Theoretical and real cooling cycle.

The refrigeration cycle can be represented graphically as an absolute pressure versus heat content (enthalpy) diagram. The diagram (Fig. 2) shows a characteristic curve showing the process of saturation of the refrigerant.

The left part of the curve corresponds to the state of saturated liquid, the right part corresponds to the state of saturated vapor. The two curves join in the center at the so-called “critical point”, where the refrigerant can be in both liquid and vapor states. The zones to the left and right of the curve correspond to supercooled liquid and superheated steam. A zone corresponding to the state of the mixture of liquid and vapor is placed inside the curved line.

Consider a diagram of a theoretical (ideal) refrigeration cycle in order to better understand the acting factors (Fig. 3).

Let us consider the most characteristic processes occurring in the compression cooling cycle.

Compressing steam in a compressor.

The cold vaporous saturated refrigerant enters the compressor (point C`). In the process of compression, its pressure and temperature increase (point D). The heat content also increases by an amount determined by the segment HC'-HD, that is, the projection of the line C'-D onto the horizontal axis.

Condensation.

At the end of the compression cycle (point D), hot vapor enters the condenser, where it begins to condense and transition from a hot vapor state to a hot liquid state. This transition to a new state occurs at constant pressure and temperature. It should be noted that although the temperature of the mixture remains almost unchanged, the heat content is reduced due to the removal of heat from the condenser and the transformation of vapor into liquid, so it is displayed on the diagram as a straight line parallel to the horizontal axis.

The process in the condenser occurs in three stages: removal of overheating (D-E), condensation itself (EA) and supercooling of the liquid (A-A`).

Let's briefly consider each stage.

Removal of overheating (D-E).

This is the first phase to occur in the condenser, and during this phase the temperature of the refrigerated vapor is reduced to the saturation or condensing temperature. At this stage, only excess heat is removed and there is no change in the state of aggregation of the refrigerant.

Approximately 10-20% of the total heat removal in the condenser is removed in this section.

Condensation (E-A).

The condensation temperature of the cooled vapor and the resulting liquid remains constant throughout this phase. There is a change in the state of aggregation of the refrigerant with the transition of saturated vapor to the state of saturated liquid. In this section, 60-80% of heat removal is removed.

Subcooling of the liquid (A-A`).

During this phase, the refrigerant, which is in a liquid state, undergoes further cooling, as a result of which its temperature decreases. It turns out a supercooled liquid (in relation to the state of a saturated liquid) without changing the state of aggregation.

Subcooling the refrigerant provides significant energy benefits: in normal operation, a one degree decrease in refrigerant temperature corresponds to an increase in chiller capacity of approximately 1% for the same level of energy consumption.

The amount of heat generated in the condenser.

Plot D-A` corresponds to the change in the heat content of the refrigerant in the condenser and characterizes the amount of heat released in the condenser.

Flow regulator (A`-B).

The subcooled liquid with the parameters at point A` enters the flow regulator (capillary tube or thermostatic expansion valve), where a sharp decrease in pressure occurs. If the pressure downstream of the flow regulator becomes low enough, then the refrigerant may boil directly downstream of the regulator, reaching the parameters of point B.

Evaporation of the liquid in the evaporator (B-C).

The mixture of liquid and vapor (point B) enters the evaporator, where it absorbs heat from the environment (air flow) and passes completely into a vapor state (point C). The process proceeds at a constant temperature, but with an increase in heat content.

As mentioned above, the vapor refrigerant is somewhat superheated at the outlet of the evaporator. the main task superheat phases (С-С`) - ensuring the complete evaporation of the remaining liquid droplets so that only the vaporous refrigerant enters the compressor. This requires an increase in the area of ​​the heat exchange surface of the evaporator by 2-3% for every 0.5°C of overheating. Since overheating usually corresponds to 5-8°C, the increase in the surface area of ​​the evaporator can be about 20%, which is certainly justified, since it increases the cooling efficiency.

The amount of heat absorbed by the evaporator.

Plot HB-HC` corresponds to the change in the heat content of the refrigerant in the evaporator and characterizes the amount of heat absorbed by the evaporator.

Real refrigeration cycle.

In reality, as a result of pressure losses occurring in the suction and discharge lines, as well as in the compressor valves, the refrigeration cycle is shown in the diagram in a slightly different way (Fig. 4).

Due to pressure losses at the inlet (section C`-L), the compressor must draw in at a pressure below the evaporation pressure.

On the other hand, due to pressure losses at the outlet ( section M-D`), the compressor must compress the vapor refrigerant to pressures above the condensing pressure.

The need to compensate for losses increases the work of compression and reduces the efficiency of the cycle.

In addition to pressure losses in pipelines and valves, the deviation of the real cycle from the theoretical one is also affected by losses during the compression process.

First, the compression process in the compressor differs from the adiabatic one, so the actual work of compression is higher than the theoretical one, which also leads to energy losses.

Secondly, there are purely mechanical losses in the compressor, leading to an increase in the required power of the compressor motor and an increase in compression work.

Thirdly, due to the fact that the pressure in the compressor cylinder at the end of the suction cycle is always lower than the vapor pressure before the compressor (evaporation pressure), the compressor performance also decreases. In addition, the compressor always has a volume that is not involved in the compression process, for example, the volume under the cylinder head.

Refrigeration Cycle Efficiency Evaluation

The efficiency of the refrigeration cycle is usually estimated by the coefficient useful action or coefficient of thermal (thermodynamic) efficiency.

The efficiency factor can be calculated as the ratio of the change in the heat content of the refrigerant in the evaporator (HC-HB) to the change in the heat content of the refrigerant during the compression process (HD-HC).

In fact, it represents the ratio of refrigeration power and electrical power consumed by the compressor.

Moreover, it is not an indicator of the performance of the refrigeration machine, but is a comparative parameter in assessing the efficiency of the energy transfer process. So, for example, if a chiller has a thermal efficiency coefficient of 2.5, this means that for every unit of electricity consumed by the chiller, 2.5 units of cold are produced.

The principle of operation of the refrigeration unit

To obtain artificial cold, technology uses the property of a liquid to change its boiling point depending on pressure.

To turn a liquid into vapor, a certain amount of heat must be applied to it. Conversely, the transformation of vapor into liquid (the process of condensation) occurs when heat is removed from the vapor.

The refrigeration unit consists of four main parts: a compressor, a condenser, a control valve and an air cooler (evaporator), connected in series with each other by pipelines.

In this scheme, a refrigerant circulates in a closed circuit - a substance that can boil at low temperatures, depending on the vapor pressure in the air cooler. The lower this pressure, the lower the boiling point. The boiling process of the refrigerant is accompanied by the removal of heat from the environment in which the air cooler is located, as a result of which this environment is cooled.

The refrigerant vapor formed in the air cooler is sucked off by the compressor, compressed in it and forced into the condenser. During compression, the pressure and temperature of the refrigerant vapor rises. Thus, the compressor creates, on the one hand, a reduced pressure in the air cooler, which is necessary for the refrigerant to boil at low temperature, and, on the other hand, an increased discharge pressure, at which the refrigerant can pass from the compressor to the condenser.

In the condenser, hot vapors of the refrigerant condense, i.e., they turn into a liquid. Condensation of vapors is carried out as a result of the removal of heat from them by the air cooling the condenser.

To obtain cold, it is necessary that the boiling (evaporation) temperature of the refrigerant be lower than the temperature of the cooled medium.

The AR-3 refrigeration unit is a single unit mounted on a frame with a heat-insulating wall separating the evaporative part (air cooler) from the rest of the equipment. The evaporative part enters into an opening made in the front wall of the cargo space. Outside air is sucked through the condenser by the axial fan inwards engine room.

On the same shaft with the condenser fan, there is an air cooler fan that circulates air in the cargo space.

Thus, in the AR-3 refrigeration unit there are two independent air systems:
— cooled air circulation system in the cargo space (air from the floor of the cargo space is sucked in by the axial fan into the air cooler through the guide air duct, cooled and thrown out under the ceiling of the cargo space);
— condenser cooling system.

An axial fan located inside the engine room sucks in air from the environment through the blinds of the front body panel, enters the condenser, cools it and is thrown out through the blinds installed on the side doors of the engine room.

To cool the carburetor engine, air is taken in through special window in the front wall of the body and> is thrown into the engine room. The heated air from the engine room exits through the side door shutters.

The control panel and all automation devices, as well as measuring devices, are located on the left (along the vehicle) side of the refrigeration unit and have free access.

Fuel is supplied to the carburetor engine from a tank fixed at the top of the unit.

The refrigeration plant is a closed hermetic system consisting of four main parts: an air cooler, a freon compressor, a condenser and a thermostatic expansion valve, connected in series by pipelines. This system is filled with freon-12 refrigerant, which continuously circulates in it, passing1 from one part to another.

The compressor sucks in the freon vapor formed during boiling from the air cooler 8, compresses them to the condensation pressure. Simultaneously with an increase in the pressure of the steam, their temperature also rises to 70-80 ° C. The heated freon vapor from the compressor is pumped through the pipeline into the condenser. Freon vapor condenses in the condenser, i.e., they turn into a liquid. Condensation of vapors is carried out as a result of deprivation from them. heat from the air blowing outer surface capacitor.

Liquid freon from the condenser enters the receiver (reserve tank). From the receiver, liquid freon is sent to the heat exchanger, where, passing through the coils, it is supercooled due to heat exchange with cold freon vapor moving towards it from the air cooler. Then the liquid freon enters the filter-drier, where it is cleaned of moisture and contaminants with a moisture-absorbing substance - silica gel.

Rice. 2. Refrigeration
1 - control panel; 2 - instrument panel; 3 - block of fans; 4 - condenser; 5 - filter-drier; 9- heat exchanger; 10- heat-insulating wall; 1st engine UD-2; 15 - relay-regulator RR24-G; 16 - thermostatic pressor FV-6; 19 - electric motor A-51-2;

From the filter-drier, liquid freon is directed to a thermostatic expansion valve, which serves to regulate the amount of freon entering the air cooler (evaporator).

In the thermostatic valve, passing through a hole of small diameter, freon is throttled, that is, it sharply lowers its pressure. In this case, its pressure decreases from the condensation pressure to the evaporation pressure.

A decrease in pressure leads to a decrease in the temperature of freon. Freon in the form of a vapor-liquid mixture enters the air cooler through the liquid distributor, and the cycle is repeated.

Freon, flowing through the air cooler tubes at low pressure, boils intensively and, evaporating, passes from a liquid state to a vapor state.

The heat required for evaporation (latent heat of vaporization) is perceived by freon through the walls of the air cooler from the air of the cargo space blown by the fan through the ribbed surface of the air cooler.

Rice. 3. Scheme of air flows in the refrigeration unit: A - air flow for cooling the condenser; B - air flow for cooling the carburetor engine

Under these conditions, the air temperature of the cargo space decreases and the products in the cargo space cool down by transferring their heat to the colder air.

The thermostatic valve divides the freon system into two parts: the high pressure line (discharge or condensation pressure) - from the compressor discharge cavity to the thermostatic valve and the low pressure line (suction or evaporation pressure) - from the thermostatic valve to the compressor suction cavity.

From the air cooler, freon vapors are sucked off by the compressor through the suction pipeline and fed to the heat exchanger, where they, passing through the annulus, are overheated by liquid freon passing through the coil. Then the freon vapor enters the compressor, and the process of freon circulation in the refrigeration unit described below occurs in a closed cycle.

In the condenser, freon, turning from vapor into liquid, gives off heat to the blown air from the surrounding atmosphere, and in the air cooler, turning from liquid into vapor, absorbs the heat of the air of the cargo space, thereby lowering the temperature in the cargo space.

Thus, in the refrigeration unit, the refrigerant is circulated - freon-12, which itself is not consumed, and only the mechanical energy of the compressor driven by a carburetor or electric motor is expended to produce cold.

The power of a refrigeration unit is determined by the refrigeration capacity per hour of operation and is measured by the amount of heat (kilocalories per hour) that the refrigeration unit can take within an hour from the refrigerated medium, in this case from the refrigerator cargo space.

The compressor of the refrigeration unit is driven through a V-belt transmission by a carburetor engine, and when operating from an electrical network, by an electric motor.

From the compressor pulley, the movement is also transmitted by a V-belt to the generator direct current and a shaft of fans that create air flows through the condenser and air cooler.

The temperature (from -15° to +4°С) in the cargo area of ​​the body is maintained automatically by means of a two-position thermostat TDDA.

When it is required to maintain a positive temperature in the cargo area of ​​the body, the cooling capacity of the unit can be drastically reduced by means of a control valve on the suction line. In this case, the valve spool must be turned all the way clockwise.

Refrigeration machines and installations designed to artificially reduce and maintain a low temperature below the ambient temperature from 10 °C to -153 °C in a given cooled object. Machines and installations for creating lower temperatures are called cryogenic. Removal and transfer of heat is carried out due to the energy consumed in this case. The refrigeration unit is carried out according to the project, depending on the design task that defines the cooled object, the required cooling temperature range, energy sources and types of cooling medium (liquid or gaseous).

The refrigeration plant may consist of one or more refrigeration machines equipped with auxiliary equipment: power and water supply systems, instrumentation, regulation and control devices, as well as a heat exchange system with the object being cooled. The refrigeration unit can be installed indoors, on outdoors, in transport and in various devices in which it is necessary to maintain a given low temperature and remove excess air moisture.

The system of heat exchange with the cooled object can be with direct cooling by a refrigerant, in a closed system, in an open system, as in dry ice cooling, or air in an air chiller. closed system may also be with an intermediate refrigerant that transfers cold from the refrigeration unit to the object being cooled.

The beginning of the development of refrigeration engineering on a large scale can be considered the creation by Karl Linde in 1874 of the first ammonia vapor-compressor refrigeration machine. Since then, many varieties of refrigeration machines have appeared, which can be grouped according to the principle of operation as follows: vapor-compression, simply called compressor, usually with an electric drive; heat-using refrigerating machines: absorption refrigerating machines and steam jet; air-expansion, which at temperatures below -90 °C are more economical than compressor ones, and thermoelectric, which are built into devices.

Each type of refrigeration units and machines has its own characteristics, according to which their field of application is selected. Currently, refrigeration machines and installations are used in many areas of the national economy and in everyday life.

2. Thermodynamic cycles of refrigeration units

The transfer of heat from a less heated to a more heated source becomes possible if some compensating process is organized. In this regard, the cycles of refrigeration plants are always implemented as a result of energy costs.

In order for the heat removed from the "cold" source to be given to the "hot" source (usually to the surrounding air), it is necessary to raise the temperature of the working fluid above the ambient temperature. This is achieved by rapid (adiabatic) compression of the working fluid with the expenditure of work or the supply of heat to it from the outside.

In reverse cycles, the amount of heat removed from the working fluid is always greater than the amount of heat supplied, and the total work of compression is greater than the total work of expansion. Due to this, installations operating on such cycles are energy consumers. Such ideal thermodynamic cycles of refrigeration plants have already been discussed above in paragraph 10 of topic 3. Refrigeration plants differ in the working fluid used and the principle of operation. The transfer of heat from a "cold" source to a "hot" one can be carried out at the expense of work or heat.

2.1. Air chillers

In air refrigeration units, air is used as a working fluid, and heat is transferred from a “cold” source to a “hot” one at the expense of mechanical energy. The decrease in air temperature necessary for cooling the refrigerating chamber is achieved in these installations as a result of its rapid expansion, in which the time for heat exchange is limited, and the work is mainly done due to internal energy, in connection with which the temperature of the working fluid drops. The scheme of the air refrigeration unit is shown in Figure 7.14

Rice. 14. : HC - cooling chamber; K - compressor; TO - heat exchanger; D - expansion cylinder (expander)

The temperature of the air entering from the refrigeration chamber XK into the compressor cylinder K rises as a result of adiabatic compression (process 1 - 2) above the ambient temperature T3. When air flows through the tubes of the TO heat exchanger, its temperature at a constant pressure decreases - theoretically to the ambient temperature Tz. In this case, the air gives off heat q (J/kg) to the environment. As a result, the specific volume of air reaches the minimum value v3, and the air flows into the cylinder of the expansion cylinder - expander D. In the expander, due to adiabatic expansion (process 3-4) with useful work equivalent to the shaded area 3-5-6-4-3 , the air temperature drops below the temperature of the objects cooled in the refrigerator compartment. The air cooled in this way enters the refrigerating chamber. As a result of heat exchange with cooled objects, the air temperature at constant pressure (isobar 4-1) rises to its original value (point 1). In this case, heat q2 (J/kg) is supplied to the air from the cooled objects. The value q 2, called the cooling capacity, is the amount of heat received by 1 kg of the working fluid from the cooled objects.

2.2. Vapor-compressor refrigeration units

In vapor-compressor refrigeration units (VCRs), low-boiling liquids are used as a working fluid (Table 1), which makes it possible to implement the processes of heat supply and removal according to isotherms. For this, the processes of boiling and condensation of the working fluid (refrigerant) are used at constant pressures.

Table 1.

In the 20th century, various freons based on fluorochlorocarbons were widely used as refrigerants. They caused active destruction of the ozone layer, and therefore their use is currently limited, and ethane-based refrigerant K-134A (discovered in 1992) is used as the main refrigerant. Its thermodynamic properties are close to those of Freon K-12. Both refrigerants do not differ significantly molecular weights, heat of vaporization and boiling point, but, unlike K-12, K-134A refrigerant is not aggressive towards the Earth's ozone layer.

The scheme of the PCKhU and the cycle in T-s-coordinates are shown in fig. 15 and 16. In PKHU, the pressure and temperature are reduced by throttling the refrigerant as it flows through the pressure reducing valve RV, the flow area of ​​which can vary.

The refrigerant from the refrigerating chamber XK enters the compressor K, in which it is compressed adiabatically in the process 1 -2. The resulting dry saturated steam enters the pressure vessel, where it condenses at constant pressure and temperature in process 2-3. The released heat q1 is transferred to the “hot” source, which in most cases is the ambient air. The resulting condensate is throttled in the pressure reducing valve РВ with a variable flow area, which allows you to change the pressure of the wet steam coming out of it (process 3-4).

Rice. 15. Schematic diagram (a) and cycle in T-s-coordinates (b) of a vapor compressor refrigeration unit: KD - capacitor; K - compressor; HK - refrigerator; RV - pressure reducing valve

Since the throttling process occurring at a constant enthalpy value (h3 - h) is irreversible, it is depicted by a dotted line. The moist saturated steam of a small degree of dryness obtained as a result of the process enters the heat exchanger of the refrigerating chamber, where, at constant pressure and temperature, it evaporates due to the heat q2b taken from the objects in the chamber (process 4-1).

Rice. 16. : 1 - refrigerator; 2 - thermal insulation; 3 - compressor; 4 - compressed hot steam; 5 - heat exchanger; 6 - cooling air or cooling water; 7 - liquid refrigerant; 8 - throttle valve (expander); 9 - expanded, cooled and partially evaporated liquid; 10 - cooler (evaporator); 11 - evaporated coolant

As a result of "drying", the degree of dryness of the refrigerant increases. The amount of heat taken from the objects cooled in the refrigerating chamber, in T-B-coordinates, is determined by the area of ​​the rectangle under the 4-1 isotherm.

The use of low-boiling liquids as a working fluid in PCCU makes it possible to approach the reverse Carnot cycle.

Instead of a throttling valve, an expansion cylinder - an expander can also be used to lower the temperature (see Fig. 14). In this case, the installation will work according to the reverse Carnot cycle (12-3-5-1). Then the heat taken from the cooled objects will be greater - it will be determined by the area under the 5-4-1 isotherm. Despite the partial compensation of energy costs for the compressor drive by the positive work obtained during the expansion of the refrigerant in the expansion cylinder, such installations are not used due to their design complexity and large overall dimensions. In addition, in installations with a variable cross-section throttle, it is much easier to regulate the temperature in the refrigerator compartment.

Figure 17.

To do this, it is enough just to change the flow area of ​​the throttling valve, which leads to a change in pressure and the corresponding temperature of the saturated refrigerant vapor at the outlet of the valve.

Currently, instead of reciprocating compressors, blade compressors are mainly used (Fig. 18). On the greater cost-effectiveness of PKHU in comparison with air installations is also evidenced by the fact that the ratio of the coefficients of performance of PCCL and the reverse Carnot cycle

In real steam compressor installations, not wet, but dry or even superheated steam enters the compressor from the heat exchanger-evaporator of the refrigeration chamber (Fig. 17). This increases the removed heat q2, reduces the intensity of heat exchange between the refrigerant and the cylinder walls, and improves the conditions for lubricating the piston group of the compressor. In such a cycle, some supercooling of the working fluid occurs in the condenser (section of the isobar 4-5).

Rice. 18.

2.3. Steam jet refrigeration units

The cycle of a steam-jet refrigeration plant (Fig. 19 and 20) is also carried out at the expense of thermal rather than mechanical energy.

Rice. 19.: HK - refrigerator; E - ejector; KD - capacitor; РВ - pressure reducing valve; H - pump; KA - boiler unit

Rice. 20.

In this case, the spontaneous transfer of heat from a more heated body to a less heated body is compensatory. The vapor of any liquid can be used as a working fluid. However, the cheapest and most available refrigerant is usually used - water vapor at low pressures and temperatures.

From the boiler plant, steam enters the nozzle of the ejector E. When steam flows out at high speed, a vacuum is created in the mixing chamber behind the nozzle, under the action of which the refrigerant from the cooling chamber is sucked into the mixing chamber. In the diffuser of the ejector, the mixture velocity decreases, while the pressure and temperature increase. Then the steam mixture enters the HP condenser, where it turns into a liquid as a result of the removal of heat q1 to the environment. Due to the repeated decrease in the specific volume during the condensation process, the pressure drops to a value at which the saturation temperature is approximately equal to 20 °C. One part of the condensate is pumped by the pump H to the boiler unit KA, and the other part is subjected to throttling in the valve PB, as a result of which, when the pressure and temperature decrease, moist steam with a small degree of dryness is formed. In the HK evaporator heat exchanger, this steam is dried at a constant temperature, removing heat q2 from the cooled objects, and then re-enters the steam ejector.

Since the cost of mechanical energy for pumping liquid phase in absorption and steam ejector refrigeration units are extremely small, they are neglected, and the efficiency of such units is estimated by the heat use coefficient, which is the ratio of the heat taken from the cooled objects to the heat used to implement the cycles.

To obtain low temperatures as a result of heat transfer to a "hot" source, other principles can in principle be used. For example, the temperature can be lowered as a result of water evaporation. This principle is used in hot and dry climates in evaporative air conditioners.

3. Household and industrial refrigerators

Refrigerator - a device that maintains a low temperature in a heat-insulated chamber. Usually they are used to store food and other items that require storage in a cold place.

On fig. 21 shows a diagram of the operation of a single-chamber refrigerator, and in fig. 22 - the purpose of the main parts of the refrigerator.

Rice. 21.

Rice. 22.

The operation of the refrigerator is based on the use of a heat pump that transfers heat from the working chamber of the refrigerator to the outside, where it is given to the external environment. In industrial refrigerators, the volume of the working chamber can reach tens and hundreds of m3.

Refrigerators can be of two types: medium temperature food storage chambers and low temperature freezers. However, in Lately most widespread two-chamber refrigerators that includes both components.

Refrigerators are of four types: 1 - compression; 2 - absorption; 3 - thermoelectric; 4 - with vortex coolers.

Rice. 23.: 1 - capacitor; 2 - capillary; 3 - evaporator; 4 - compressor

Rice. 24.

The main components of the refrigerator are:

1 - a compressor that receives energy from the electrical network;

2 - a condenser located outside the refrigerator;

3 - evaporator located inside the refrigerator;

4 - thermostatic expansion valve (TRV), which is a throttling device;

5 - refrigerant (a substance circulating in the system with certain physical characteristics- usually it is freon).

3.1. The principle of operation of a compression refrigerator

The theoretical basis on which the principle of operation of refrigerators is built, the scheme of which is shown in fig. 23 is the second law of thermodynamics. The refrigerant gas in refrigerators makes a so-called reverse carnot cycle. In this case, the main heat transfer is based not on the Carnot cycle, but on phase transitions - evaporation and condensation. In principle, it is possible to create a refrigerator using only the Carnot cycle, but in this case, to achieve high performance, either a compressor that creates a very high pressure or a very large area of ​​\u200b\u200bthe cooling and heating heat exchanger is required.

The refrigerant enters the evaporator under pressure through a throttling hole (capillary or expansion valve), where, due to a sharp decrease in pressure, evaporation liquid and turning it into vapour. In this case, the refrigerant takes away heat from the inner walls of the evaporator, due to which the interior of the refrigerator is cooled. The compressor sucks in the refrigerant in the form of vapor from the evaporator, compresses it, due to which the temperature of the refrigerant rises and pushes it into the condenser. In the condenser, the refrigerant heated as a result of compression cools down, giving off heat to the external environment, and condenses, i.e. turns into a liquid. The process is repeated again. Thus, in the condenser, the refrigerant (usually freon) condenses under the influence of high pressure and turns into a liquid state, releasing heat, and in the evaporator, under the influence of low pressure, the refrigerant boils and turns into a gaseous state, absorbing heat.

A thermostatic expansion valve (TRV) is needed to create the necessary pressure difference between the condenser and evaporator at which the heat transfer cycle occurs. It allows you to correctly (most fully) fill the internal volume of the evaporator with boiled refrigerant. The flow area of ​​the expansion valve changes as the heat load on the evaporator decreases, and as the temperature in the chamber decreases, the amount of circulating refrigerant decreases. A capillary is an analog of a TRV. It does not change its cross section, but throttles a certain amount of refrigerant, depending on the pressure at the inlet and outlet of the capillary, its diameter and type of refrigerant.

When the required temperature is reached, the temperature sensor opens electrical circuit and the compressor stops. As the temperature rises (due to external factors) the sensor turns on the compressor again.

3.2. The principle of operation of the absorption refrigerator

The absorption water-ammonia refrigerator uses the property of one of the widespread refrigerants - ammonia - to dissolve well in water (up to 1000 volumes of ammonia per 1 volume of water). The principle of operation of the absorption refrigeration unit is shown in fig. 26, and its schematic diagram is in fig. 27.

Rice. 26.

Rice. 27.: GP - steam generator; KD - capacitor; РВ1, РВ2 - pressure reducing valves; HK - refrigerator; Ab - absorber; H - pump

In this case, the removal of gaseous refrigerant from the evaporator coil, required for any evaporative refrigerator, is carried out by absorbing it with water, the ammonia solution in which is then pumped into a special container (desorber / generator) and there it is decomposed into ammonia and water by heating. Vapors of ammonia and water from it under pressure enter the separation device ( distillation column), where the ammonia vapor separates from the water. Further, almost pure ammonia enters the condenser, where, cooling, it condenses and again enters the evaporator through the throttle for evaporation. Such a heat engine can use a variety of devices, including jet pumps, to pump the refrigerant solution, and not have moving mechanical parts. In addition to ammonia and water, other pairs of substances can be used - for example, a solution of lithium bromide, acetylene and acetone. The advantages of absorption refrigerators are noiseless operation, the absence of moving mechanical parts, the ability to work from heating by direct combustion of fuel, the disadvantage is low cooling capacity per unit volume.

3.3. Working principle of thermoelectric refrigerator

There are devices based on the Peltier effect, which consists in the absorption of heat by one of the junctions of thermocouples (dissimilar conductors) when it is released at the other junction in the case of passing current through them. This principle is used, in particular, in cooler bags. Both lowering and increasing the temperature are possible with the help of vortex tubes proposed by the French engineer Rank, in which the temperature changes significantly along the radius of the swirling vortex air flow moving in them.

The thermoelectric refrigerator is based on Peltier elements. It is silent, but not widely used due to the high cost of cooling thermoelectric elements. However, small car refrigerators and drinking water coolers are often produced with cooling from Peltier elements.

3.4. The principle of operation of the refrigerator on vortex coolers

Cooling is carried out by expanding the air pre-compressed by the compressor in blocks of special vortex coolers. They are not widely used due to the high noise level, the need to supply compressed (up to 1.0-2.0 MPa) air and its very high consumption, low efficiency. Advantages - greater safety (no electricity is used, no moving parts and no hazardous chemical compounds), durability and reliability.

4. Examples of refrigeration units

Some diagrams and descriptions of refrigeration units for various purposes, as well as their photographs are shown in Fig. 27-34.

Rice. 27.

Rice. 28.

Rice. 29.

Figure 32.

Rice. 33.

For example, compressor-condenser refrigeration units (AKK type) or compressor-receiver units (AKR type), shown in fig. 34 are designed for operation with temperature maintenance from +15 °С to -40 °С in chambers with volume from 12 to 2500 m3.

The composition of the refrigeration unit includes: 1 - compressor-condenser or compressor-receiver unit; 2 - air cooler; 3 - thermostatic valve (TRV); 4 - solenoid valve; 5 - control panel.

Cooling is divided into natural and artificial. The first energy is not wasted. Moreover, the temperature of the object tends to the temperature of the surrounding air. Artificial cooling is the reduction of the temperature of an object to a level below the same indicator of the environment. For such cooling, refrigeration machines or devices are needed. They are usually used in industry to achieve the desired storage conditions, flow chemical reactions, security. Thermal and refrigeration machines are very widely used in everyday life. Their principle of operation is based on the phenomena of sublimation and condensation.

Ice cooling

This is the most affordable and simplest form of cooling. It is especially convenient in areas where there is a possibility of accumulation of natural ice.

As a means of cooling, ice is used in the process of harvesting and storing fish, in the short-term storage of vegetable products, and in the transportation of chilled food products. Ice is used in cellars and glaciers. In such equipment, thermal insulation is very important. In stationary glaciers, the walls are hydro- and thermally insulated. They are designed for the temperature range +5...+8°C.

Ice-salt cooling

The ice-salt cooling method allows reaching even lower temperature conditions in the volume to be cooled. The combined use of ice and salt makes it possible to lower the temperature at which ice melts. That is the principle. The principle of the refrigeration machine.

For this purpose, ice and sodium chloride are mixed. Depending on the salt concentration, the ice temperature ranges from -1.8 to -21.2°C.

The melting point reaches a minimum if the salts in the mixture are 23%. In this case, the ice does not melt at the minimum value.

Dry ice is used to maintain low temperatures during the storage of fruits, ice cream, vegetables, semi-finished products. This is the name given to the solid state of carbon dioxide. At atmospheric pressure and heating, it changes from solid to gaseous, passing the liquid phase. The refrigeration capacity of dry ice is twice that of water ice. When dry ice sublimes, carbon dioxide is produced, which, among other things, performs a preservative function, contributing to the preservation of products.

Cooling methods using ice also have a number of disadvantages that limit their application. In this regard, the main method of generating cold is machine cooling.

artificial cooling

Mechanical refrigeration is the production of cold, which is produced by refrigeration machines and installations. This method has several advantages:

• in automatic mode, a constant temperature level is maintained, which is different for different groups of products;
• the cooled space is optimally used;
• it is convenient to operate the refrigerated premises;
• low maintenance costs.

How does it work

The principle of operation of the refrigeration machine is as follows. Of course, a person who only uses a refrigeration machine or is looking for one does not need to have a deep and comprehensive understanding of the operation of refrigeration machines. At the same time, knowledge of the fundamental principles of operation of such installations will not be superfluous at all. This information can help in conscious choice equipment and will facilitate the conversation with professionals when choosing refrigeration equipment.

It is also important to understand how the refrigeration machine works. In situations where refrigeration equipment fails and a specialist call is required, it makes sense to delve into the principle of operation of such machines. After all, understanding the explanations of a specialist that a replacement or repair of any part of the refrigeration machine is needed will allow you not to lose extra money.

The main principle of operation of the refrigeration machine is the removal of heat from the object subjected to cooling, and its transfer to another object. It is important to understand that heating or contracting an object is accompanied by the transfer of energy to it, while cooling and expansion takes away energy. This is the basis for heat transfer.

To transfer heat, refrigeration machines use refrigerants - special substances that take away heat from the object of cooling during boiling and expansion at a constant temperature. Subsequently, after compression, the energy is transferred to the cooling medium through condensation.

Assignment of individual nodes

The compressor of the refrigeration machine ensures the circulation of the refrigerant in the system, its boiling in the evaporator with injection into the condenser unit.

It is designed to suck out the freon refrigerant in the gaseous state from the evaporators, and, compressing it, pump it into the condenser, where it turns into a liquid. Then freon in a liquid state accumulates in the receiver. This unit is equipped with inlet and outlet shut-off valves. The further path of the refrigerant is from the receiver to the filter drier. Here, the remaining moisture and impurities are removed and enter the evaporator.

In the evaporator, the refrigerant reaches a boil, which removes heat from the cooled object. Further, the refrigerant, already in a gaseous state, enters the compressor from the evaporator, being cleaned from contaminants through a filter. Further, the working cycle of the unit is repeated, this is the principle. The principle of the refrigeration machine.

Refrigeration unit

Combining a set of parts and assemblies of a refrigeration machine on a single frame is commonly called a refrigeration unit. The combination of the chiller components by the manufacturer makes installation easier and faster.

The cooling capacity of such units is a parameter representing the amount of heat taken from the medium subjected to cooling in one hour. Under different operating modes, the cooling performance varies over a wide range. As the condensing temperature rises and the evaporation rate falls, the capacity decreases.

Refrigerants

Refrigerators used in commercial organizations use freon or freon as refrigerants, and ammonia for freezing on an industrial scale.

Freon is a heavy, colorless gas with a faint odor that is noticeable only when its concentration in the air reaches 20%. The gas is not flammable and will not explode. Lubricating oils are highly soluble in freon. At high temperatures, they form a homogeneous mixture with it. Freon does not affect taste qualities, aroma and color of products.

In refrigeration units with freon, there should not be more than 0.006% of the mass of moisture. Otherwise, it freezes in thin tubes, preventing the operation of the refrigeration machine. Due to the high fluidity of the gas, good sealing of the units is required.

Ammonia is a colorless, pungent gas that is dangerous to the human body. Its permissible content in the air is 0.02 mg/l. When the concentration reaches 16%, an explosion is possible. With a gas content of more than 11% and an open flame nearby, combustion begins.