What installation is used to obtain artificial cold. Heat pump - heat from cold

Compressor refrigeration units are the main consumers of electricity at enterprises for the processing and storage of perishable food products, which requires finding reserves to save energy resources. Since most of the territory of our country is characterized by long winters with low air temperatures, a very promising direction for saving energy resources is the widespread use of natural cold. Let's note some directions of use of natural cold.

The simplest and most common way is to directly supply cold air to the cooling or food storage chambers when the outside air temperature is equal to or lower than that required in the chambers. Holes are made in the outer walls for air to be taken in with a fan and released through a flap check valve (Fig. 94). Air is distributed in the chamber through an air duct with adjustable windows, which are automatically closed by dampers when the fan stops. The temperature in the chamber is maintained by a two-position temperature switch that turns on or off the fan. When placing unpackaged products in the chamber, it is necessary to install filters for cleaning the air from dust and microorganisms (for example, LAIK SP-6/15 or LAIK SP-6/15A) at the fan suction. It has been established that in areas with a relative air humidity of 85% and higher, in chambers with unpackaged products, outside air without humidification can be used. In other cases, an air humidification system is provided. Given the seasonal nature of the use of natural cold, it is advisable to combine equipment for natural and artificial cooling in the chambers. When working with artificial cooling in the summer, the openings in the fences are closed with heat-insulated hatches. For the main areas of mass cultivation of potatoes and vegetables, the storage period coincides with the period of stable standing. low temperatures outside air. In this regard, the method of storing products in bulk under conditions of active ventilation using natural cold is becoming widespread. Outside air is supplied by a fan to a variable-section air duct located under the perforated storage floor (Fig. 95). The supplied air is humidified, passes through the products from the bottom up and is removed from the storage through the deflector. The fan and humidifier are automatically switched on by a signal from sensors of differentiated temperature controllers at an outdoor air temperature of 2 ... 3 ° C below the temperature that the mass of the product has. Air humidification is carried out with water vapor or water spray. The optimal values ​​of air humidity before entering the product are 90% or more, and the specific air consumption per 1 ton of products is more than 100 m 3 / h.

In the dairy industry, cooling of the refrigerant using external heat exchangers or in cooling towers is also widespread. As heat exchangers, you can use standard air coolers with a high degree fins and powerful fans (for example, VOG-230) installed outdoors (on the roof of the compressor shop). Considering the limited operating time of heat exchangers using natural cold, general scheme coolant circulation (water, brines) must be mobile and have switching based on different operating modes: coolant cooling only by external heat exchangers; teamwork outdoor units and evaporators of the refrigeration unit; cooling of the refrigerant only in the evaporators of the refrigeration unit. AT winter time ice water can be obtained in cooling towers with complete or partial shutdown of refrigeration equipment. On fig. 96 shows the connection diagram of the cooling tower for cooling the coolant, operating in three modes: cold accumulation at night, coolant circulation circuit (cooling tower - tank - pump); cooling technological equipment accumulated cold and subcooling of the coolant in the cooling tower; coolant cooling in the evaporator. The parameter by which one or another cooling method is selected is the temperature of the coolant entering the technological apparatus.

Standard GPV type cooling towers are used to produce water with a temperature of 1…4°C at an outside air temperature of –5°C and below. The disadvantage of film cooling towers is the formation of ice on the structural elements, which leads to a sharp decrease in the amount of circulating air and. rise in chilled water temperature. This shortcoming is eliminated in the Y10-OU0 installation for natural cooling of circulating water in winter. It provides cooling of water from 10 to 5±1°C at an ambient temperature of -5°C and below. During the summer period, the installation performs the functions of a cooling tower in the circulating water supply system. A defrosting system is provided for periodic de-icing. The cooling tower is mounted in an open area with free drain from the sump to the accumulation block, while the difference in elevations between the drain pipe of the sump and the water level in the accumulation block is at least 1 m.
The method of accumulating winter cold by freezing ice piles deserves special attention, which allows a significant part of the summer time to do without machine cooling, which saves energy resources, lubricants, and increases the service life of equipment.
Another reserve for energy savings due to natural cold is the use of air condensers, which can be used as precondensers in combination with shell-and-tube and evaporative condensers. In winter, air precondensers can take over the entire heat load from the unit, while the condensing temperature can be arbitrarily low, which leads to savings in electricity for cold generation. The use of natural cold for cooling is an inexhaustible source of effective technical solutions, and a combination of two or more types of free cooling can achieve quite high technical and economic indicators.

In our imagination, the very concept of "thermal physics" is usually associated with the production of heat, the efficiency of fuel combustion, and energy production. It is clear that for the inhabitants of Siberia, heat is more important than cold. Nevertheless, the production of cold is also one of the urgent tasks for scientists working in the field of thermal physics. And the most remarkable - for the production of cold, they propose to attract all the same heat!

Why it is necessary to produce cold, I think many of us understand. Cold is needed to store products, to create a favorable microclimate in the premises, for certain production processes. Each of us has a refrigerator in the house, all normal public buildings are equipped with air conditioners. Imagine a cafe, shop, hotel or business center without air conditioning, and you will understand that the cooling system is no less important than the heating system, even when it comes to Siberia. In winter, of course, we need warmth. In the summer? Summer in our area also sometimes breaks records for heat. And there is nothing to say about the southern countries.

In short, modern comfort parameters and the need for food storage somehow require the production of cold. And I must say that from year to year the need for artificial cold increases both in Russia and abroad.

How is cold produced? Today, there are two main types of refrigeration machines - vapor compression refrigeration machines and lithium bromide absorption machines. The first type is well known to us - this is how our household refrigerators, powered by the mains, are arranged. The operation of such machines is based on a change in the state of aggregation of the refrigerant - freon (freon) - under the influence of mechanical energy. For the transformation electrical energy in the mechanical one, as we know, compressors are used.

As for refrigerating machines of the second type, their operation is based on the chemical interaction of the substances of the working pair - the absorbent and refrigerant, and the change in the state of aggregation of the refrigerant under the influence of thermal energy. In other words, for their work, such machines use heat.

And here we are just approaching the most important point regarding refrigerating machines of the second type. So, if in the first case we need to spend electricity to produce cold, then in the second case we can quite use the “excess” heat, which under other circumstances very often flies out into the chimney (literally). Of course, ordinary energy resources - gas or fuel oil - can also serve as heating sources for such machines, but steam from boiler houses, intermediate extractions from thermal power plants, hot water, flue gases or waste steam from industries can also be used with might and main. In other words, the heat emitted into the atmosphere, thanks to absorption machines, is quite suitable for the production of cold. That is, in this case, there is no need to spend valuable energy resources - it is enough to use the "surplus" heat prudently, of which there is especially a lot just in the summer, when it makes sense to cool the premises.

Needless to say, economy is one of the key benefits absorption lithium bromide refrigerating machines before vapor compression. As we understand, in the conditions of constant growth of electricity tariffs, this becomes especially important.

Another important advantage is environmental friendliness associated with the absence of freons (freons), the use of which is limited in many countries in accordance with the Montreal and Kyoto protocols. Lithium bromide machines are not subject to such restrictions. Used here as an absorbent, an aqueous solution of lithium bromide is non-volatile and non-toxic, belonging to low-hazard substances.

Another advantage is related to the low noise level during the robot. You can also mention the ease of maintenance, long service life and fire and explosion safety.

Due to these advantages, such machines are able to find wide application both in everyday life and in economic activity. The range of their application is quite wide - from metallurgical enterprises, nuclear power plants, petrochemical plants - to greenhouses, apartment buildings, shopping centers and other public buildings where it is required to create a comfortable microclimate. And most importantly (we emphasize again), this comfort can be achieved with minimal cost electricity!

Are such machines being developed in our country? Yes, they are developing! They even produce. Just such a sample, developed by specialists from the Institute of Thermal Physics of the Siberian Branch of the Russian Academy of Sciences, is being produced in the Kemerovo region. Moreover, it is important to note that domestic cars have some advantages in comparison with foreign ones. For example, they, as they say, “adjust” to a specific consumer. Our experts use a flexible design system and assemble on site. Moreover, they can offer customers a machine of very high power - up to 5.3 MW. In addition, given the complex realities, the developers have provided - especially for emergency cases - duplication automatic system control of the manual system (using the "buttons").

However, this individual approach also revealed its weaknesses. This is about market competition with foreign serial samples (coming mainly from China). So, foreign manufacturers, "stamping" such machines on the conveyor, are able to resort to dumping. And if we talk about the Chinese, then they can generally count on state support, carrying out the conquest Russian market. The state is not going to help our producers (and will not).

So for the time being, we are not talking about mass production of domestic cars. This, of course, is only in the plans. Therefore, at present (which is very important), the specialists of the IT SB RAS are perfecting their brainchild, adapting to the needs of each consumer as much as possible. Perhaps there is a plus in this individual approach. It is possible that such a “manual assembly” will someday become an indicator High Quality and will be highly appreciated in the market.

-> 04/13/2011 - Methods of obtaining cold and characteristics of cooling sources

Getting cold is reduced to a decrease in the heat content in a solid, liquid or gas. Cooling is the process of taking away heat, leading to a decrease in temperature or a change in the state of aggregation of a physical body. Distinguish natural and artificial cooling.

Natural cooling is the removal of heat from the cooled body to the environment. With this method, the temperature of the cooled body can only be reduced to the ambient temperature. This is the easiest way to cool without wasting energy.

artificial cooling is the cooling of the body below the ambient temperature. Refrigeration machines or refrigeration units are used for artificial cooling. This method of cooling requires energy.

There are several ways to obtain artificial cold. The simplest is cooling with ice or snow. Ice cooling has a significant drawback - the cooling temperature is limited by the melting temperature of the ice. As coolers, water ice, ice-salt mixtures, dry ice and liquid refrigerants (freons and ammonia) are used.

Ice-salt cooling is carried out using crushed water ice and salt. Due to the addition of salt, the rate of ice melting is increased and the melting temperature of ice is lowered. Dry ice cooling is based on the action of solid carbon dioxide - when heat is absorbed, dry ice changes from a solid to a gaseous state. With dry ice, you can get a lower temperature than with water ice: the cooling effect of 1 kg of dry ice is almost 2 times greater than that of 1 kg of water ice, no dampness occurs during cooling, the carbon dioxide gas released has preservative properties, contributes to a better preservation of products. Dry ice is used for transportation of frozen products, cooling of packaged ice cream, storage

The most common and convenient in operation is machine cooling. Compared to other types of refrigeration, machine refrigeration has the following advantages:

• the possibility of creating a low temperature over a wide range;
• automation of the cooling process;
• availability of operation and maintenance, etc.

Machine cooling has become the most widely used in trade due to a number of advantages:

• automatic maintenance of a constant storage temperature depending on the type of products;
• rational use of useful capacity for cooling products, ease of maintenance;
• high profitability and the possibility of creating the necessary sanitary and hygienic conditions for storing products.

Machine cooling is based on the property of some substances to boil at a low temperature, while absorbing a large amount of heat from the environment. Such substances are called refrigerants (refrigerants).

Refrigerants- These are the working substances of steam refrigeration machines, with the help of which low temperatures are obtained. Refrigerants must have a high heat of vaporization, low boiling point, high thermal conductivity. At the same time, refrigerants should not be explosive, flammable, or poisonous. The cost of refrigerants is important. Freon 12, freon 22 and ammonia are the most suitable for these requirements. Freon enters trade enterprises in metal cylinders, painted in aluminum color and having conditional marking R12 or .

Operation of a steam compression refrigeration machine. Standard cycle.

Steam Compression Chiller Cycle is a thermodynamic process in which a liquid refrigerant vaporizes, compresses and condenses in a continuous cycle to cool a chamber or space.

Thermodynamic cycle- these are two or more connected processes that ultimately return the working fluid to its initial state. The cycle of connected processes of a machine refrigeration system is called the cycle of a vapor compression refrigeration machine. A simple steam compression chiller cycle.

A simple steam compression chiller cycle consists of four basic processes: expansion, evaporation, compression and condensation. In these processes, the pressure, temperature and state of the refrigerant change. In each individual process, the properties of the refrigerant change. But at the end of the last process, the refrigerant returns to its initial state with the same qualities that it had at the beginning of the first process, and a cycle is formed. The components for performing these processes are presented in the previous section.

In order to understand the steam compression chiller cycle, it is necessary to first consider each process separately. By understanding individual processes, one can analyze them in relation to other processes that make up the cycle. It is necessary to understand the interrelationship of processes, since changes in one process cause corresponding changes in others that make up the cycle of a steam compression chiller.

The refrigerant in the receiver is in liquid and gaseous states at high temperature and pressure. During the cycle, the liquid refrigerant passes into the liquid line and then into the refrigerant flow controller.

The refrigerant at the flow regulator inlet is in a liquid state at high temperature and pressure. As the refrigerant passes through the small hole in the valve or capillary tube, its pressure is reduced to that of the evaporator. Reducing the pressure of the refrigerant produces a corresponding decrease in the saturation temperature of the liquid refrigerant. As a result, part of the refrigerant boils and lowers the temperature of the rest of the liquid. The vapor-liquid mixture exits the refrigerant flow controller and enters the evaporator.

Refrigerant at the evaporator inlet It is a cool vapor-liquid mixture with low temperature and pressure. The rest of the liquid evaporates at saturation temperature, corresponding to the pressure in the evaporator. The evaporating liquid absorbs the latent heat in the chamber. The vapor leaving the evaporator is slightly superheated to prevent liquid from entering the compressor.

Refrigerant at compressor inlet is superheated steam at a lower temperature and pressure. The compressor causes the movement of the refrigerant due to the low pressure zone in the cylinders during suction. Since the pressure in the cylinder is lower than the vapor pressure in the evaporator, the refrigerant flows through the suction line to the compressor due to the pressure difference. In the suction pipe, the steam absorbs heat from the environment, which further increases its superheat. During compression, the temperature and pressure of the steam increase, and the heated steam under pressure is ejected into the discharge pipeline.

Refrigerant at the condenser inlet It is superheated steam at high temperature and pressure. Since the ambient temperature of the condenser is lower than the saturation temperature of the vapor, the refrigerant condenses. Thus, the latent heat of vaporization absorbed in the evaporator is transferred to the outside of the chamber. By the time the refrigerant reaches the bottom of the condenser, it has given off enough dry and latent heat to condense and become slightly colder. The liquid leaves the condenser and enters the receiver in the same state in which it left it. The cycle ends.1

Complete 5 tasks

The physical nature of heat and cold is the same, the difference is only in the speed of movement of molecules and atoms. In a hotter body, the speed of movement is greater than in a less heated one. When heat is supplied to the body, the movement increases, when heat is removed, it decreases. Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, thermal energy is the internal energy of the motion of molecules and atoms.

Body cooling - is the removal of heat from it, accompanied by a decrease in temperature. The simplest way of cooling is heat exchange between the cooled body and the environment - outside air, river and sea water, soil. But in this way, even with the most perfect heat exchange, the temperature of the cooled body can only be lowered to the ambient temperature. Such cooling is called natural. Cooling the body below ambient temperature is called artificial. It is worth saying that for it they use mainly latent heat absorbed by bodies when their state of aggregation changes.

The amount of heat or cold is measured in calories or kilogram-calories (kilocalorie). A calorie is the amount of heat essential to heat 1 g of water by 1°C at normal atmospheric pressure, a kilocalorie is to heat 1 kg of water by 1°C under the same conditions.

There are several ways to obtain artificial cold. The simplest one is cooling with ice or snow, the melting of which is accompanied by

swallowed pretty a large number heat. If heat inflows from the outside are small, and the heat transfer surface of ice or snow is relatively large, then the temperature in the room can be lowered to almost 0°C. In practice, in a room cooled by ice or snow, the air temperature can only be maintained at a level of 5-8 "C. With ice cooling, water ice or solid carbon dioxide (dry ice) is used.

At water ice cooling there is a change in its state of aggregation - melting (melting). The cooling capacity, or the cooling capacity of pure water ice, is commonly referred to as the specific heat of fusion. It is equal to 335 kJ/kᴦ. The heat capacity of ice is 2.1 kJ/kg degree.

Water ice is used for cooling and seasonal storage of food products, vegetables, fruits in climatic zones with a long cold period, where it can be easily prepared under natural conditions in winter.

Water ice is used as a coolant in special glaciers and in ice warehouses. Glaciers come with bottom ice loading (glacier-cellar) and with side - pocket type.

Ice cooling has significant drawbacks: the storage temperature is limited by the melting temperature of ice (usually the air temperature in ice warehouses is 5-8 "C), it is extremely important to put enough ice into the glacier, sufficient for the entire period of storage, and add it as it is extremely important; significant labor costs for the preparation and storage of water ice; large dimensions of the ice room, approximately 3 times the size of the food room; significant labor costs to comply with necessary requirements required for the storage of food products and the removal of melt water.

Ice-salt cooling made using crushed water ice and salt. Thanks to the addition of salt

the rate of ice melting increases, and the temperature of melt-jjjfH ice falls lower. This is because the addition of salt causes a weakening of the molecular cohesion and the destruction of the crystal lattices of ice. The melting of the ice-salt mixture proceeds with the removal of heat from the environment, as a result of which the surrounding air is cooled and its temperature decreases. With an increase in the salt content in the ice-salt mixture, its melting point decreases. The salt solution with the lowest melting point is called the eutectic, and its melting point is called the cryohydrate point. The cryo-hydration point for an ice-salt mixture with table salt is -21.2 "C at a salt concentration in the solution of 23.1% relative to the total mass of the mixture, which is approximately equal to 30 kg of salt per 100 kg of ice. With a further salt concentration, there is no decrease the melting temperature of the ice-salt mixture, and an increase in the melting temperature (at a 25% salt concentration in the solution to the total mass, the melting temperature rises to -8°C).

Freezing aqueous solution salt at a concentration corresponding to the cryohydrate point, a homogeneous mixture of ice crystals and salt is obtained, which is commonly called eutectic solid solution.

The melting point of the eutectic solid solution of sodium chloride is -21.2°C, and the heat of fusion is 236 kJ/kᴦ. The eutectic solution is used for zero-torus cooling. To do this, a eutectic solution of table salt is poured into zeros - tightly sealed forms - and they are frozen. Frozen grains are used to cool counters, cabinets, coolers, etc. In the Le trade, pre-salt cooling was widely used before the mass production of machine-cooled equipment.

Dry ice cooling based on the property of solid carbon dioxide to sublimate, i.e., upon absorption of heat

to pass from the solid state to the gaseous state, bypassing the liquid state. The physical properties of dry ice are as follows: sublimation temperature at atmospheric pressure - 78.9 "C; sublimation heat 574.6 kJ / kᴦ.

Dry ice has the following advantages over water ice:

* you can get a lower temperature;

* the cooling effect of 1 kg of dry ice is almost 2 times greater than that of 1 kg of water ice;

* during cooling, dampness does not occur, in addition, during the sublimation of dry ice, gaseous carbonic acid is formed, which is a preservative agent that contributes to a better preservation of products.

Dry ice is used for transportation of frozen products, cooling of packaged ice cream, frozen fruits and vegetables.

Artificial cooling can also be achieved by mixing ice or snow with diluted acids. For example, a mixture of 7 parts of snow or ice and 4 parts of diluted nitric acid has a temperature of -35°C. A low temperature can also be obtained by dissolving salts in dilute acids. So, if 5 parts of ammonium nitrate and 6 parts of sodium sulfate are dissolved in 4 parts of diluted nitric acid, then the mixture will have a temperature of -40 ° C.

Obtaining artificial cold with the help of snow or ice, as well as with the help of cooling mixtures, has significant disadvantages: the complexity of the processes of harvesting ice or snow, their delivery, the difficulty of automatic control, limited temperature capabilities.

AT recent times In connection with the energy crisis, environmental pollution, the problem of using non-traditional environmentally friendly methods of obtaining cold for the refrigeration processing of food products becomes more and more urgent. most promising

of which is cryogenic method based on liquid and gaseous nitrogen with the use of a machineless flow-through cold supply system, which provides for a one-time use of a cryoagent.

perspective this method refrigeration supply increases due to the discovery in Russia of large reserves (340 billion ml) of underground high-nitrogen gases. The cost of purified nitrogen is an order of magnitude lower than nitrogen obtained using the air separation method.

Machineless flow-through nitrogen cooling systems have significant advantages: they are very reliable in operation and have a high freezing rate, which ensures almost complete preservation of quality and appearance product, as well as minimal loss of its mass due to shrinkage.

Of particular note is the ecological cleanliness of such systems (the Earth's atmosphere contains up to 78% of gaseous nitrogen).

The most common and operationally convenient method of cooling is machine cooling.

Machine Cooling- a method of obtaining cold by changing the state of aggregation of the refrigerant, its boiling at low temperatures with the removal of the heat of vaporization, which is extremely important for this, from the cooled body or medium.

For the subsequent condensation of refrigerant vapors, a preliminary increase in their pressure and temperature is required.

The machine cooling method can also be based on the adiabatic (without supply and removal of heat) expansion of compressed gas. When the compressed gas expands, its temperature drops significantly, since the external Work in this case is performed at the expense of the internal energy of the gas. This principle is based on the operation of air coolers.

Cooling by expansion of a compressed gas, in particular air, is different from all cooling methods. At the same time, the air does not change its state of aggregation, like ice, mixtures and freon, it only heats up, perceiving the heat of the environment (from the cooled body).

The widespread use of machine cooling in trade is explained by a number of its operational properties and economic advantages. Stable and easily adjustable temperature conditions, automatic operation of the refrigeration machine without much labor for maintenance, better hygienic storage conditions for products, compactness and overall economy make it advisable to use machine refrigeration.

At wholesale and retail mainly steam refrigeration machines are used, the action of which is based on boiling at low temperatures of special working substances - refrigerants. Steam refrigeration machines are divided into compression, in which the refrigerant vapor is compressed in the compressor with the expenditure of mechanical energy, and absorption, in which the refrigerant vapor is absorbed by the absorbent.

The device and principle of operation of a compression refrigeration machine. Compression refrigeration machine (Fig. 3.1) consists of the following basic units: evaporator, compressor, condenser, receiver, filter, thermostatic valve. The automatic operation of the machine is ensured by a thermostatic expansion valve and a pressure regulator. Auxiliary devices that contribute to improving the efficiency and reliability of the machine include: receiver, filter, heat exchanger, dryer. The machine is powered by an electric motor.

Evaporator - cooling battery, which absorbs the heat of the environment due to the boiling in it

at low refrigerant temperature. Given the dependence on the type of cooled medium, evaporators are distinguished for cooling liquid and air.

The compressor is designed to suck refrigerant vapors from the evaporator, compress and inject them in an overheated state into the condenser. In small refrigeration machines, piston and rotary compressors are used, with piston compressors being the most widely used.

Capacitor - a heat exchanger used to reduce refrigerant vapors by cooling them. By type

Cooling medium condensers are produced with water and air cooling. Forced air condensers have vertically arranged flat coils made of copper or steel ribbed tubes. natural air cooling It is applied only in refrigerating machines of household electric refrigerators. Water-cooled condensers are shell-cooled and shell-and-tube.

Receiver - a reservoir used to collect liquid refrigerant in order to ensure its uniform flow to the thermostatic expansion valve and to the evaporator. In small refrigerant machines, the receiver is designed to collect refrigerant during machine repairs.

Filter consists of copper or brass nets and cloth pads. It serves to clean the system and refrigerant from mechanical impurities resulting from their insufficient cleaning during manufacture, installation and repair. Filters are liquid and steam. The liquid filter is installed after the receiver in front of the expansion valve, the steam filter is installed on the suction line of the compressor.

To prevent rust and mechanical particles from entering the cylinders of small freon refrigeration machines, a filter in the form of a brass mesh cup is inserted into the compressor suction cavity.

expansion valve provides a uniform flow of freon into the evaporator, atomizes the liquid refrigerant, thereby lowering the condensing pressure to the evaporation pressure.

The efficiency of the refrigeration machine largely depends on the correct adjustment of the expansion valve. Excess liquid refrigerant in the evaporator due to wet running of the compressor can lead to water hammer. In case of insufficient filling of the evaporator with liquid, part of its surface is not used, which leads to a violation of the normal mode of operation.

you machine and lowering the evaporation temperature of the refrigerant.

pressure regulator consists of a pressure switch (low pressure regulator) and a manocontroller (high pressure switch). To regulate the temperature regime within certain limits, it is extremely important that the cooling capacity of the chiller always exceeds the heat input to it. For this reason, under normal conditions, there is no need for continuous operation of the chiller.

Periodic activation of the refrigeration machine is carried out automatically by the pressure switch. The required temperature regime is achieved by regulating the duration of breaks in the operation of the refrigeration machine. The manocontroller serves to protect against excessive pressure increase in the discharge line. When the pressure in the condenser rises above 10 atm (the norm is 6-8 atm), it opens the magnetic starter coil circuit, the electric motor is turned off and the refrigeration machine stops.

The operation of the refrigeration machine is as follows. Easily evaporating liquid (HFC-12) enters through the expansion valve into the evaporator. Once in low pressure conditions, it boils, turning into steam, and at the same time takes away heat from the air surrounding the evaporator.

From the evaporator, freon vapors are sucked off by the compressor, liquefied and, in a state superheated from compression, are injected into the condenser. In a water- or air-cooled condenser, they turn into a liquid. Liquid freon flows down the pipes of the condenser and accumulates in the receiver, from where it passes through the filter under pressure, where mechanical impurities (sand, scale * etc.) are retained.

The refrigerant purified from impurities, passing through the narrow opening of the thermostatic valve, is throttled (wrinkled), sprayed, and with a sharp decrease in pressure and

temperature enters the evaporator, after which the cycle repeats.

The operating cycle of the refrigeration machine, taking into account the interaction of automation devices, is as follows. When the electric motor is off, the contacts of the pressure switch are open, the thermostatic valve does not allow liquid freon to pass from the condenser to the evaporator, since the needle has completely entered the saddle and tightly closed the flow area. In the evaporator, the process of boiling the liquid refrigerant remaining after turning off the machine continues. From the influx of external heat, the temperature of the evaporator gradually rises and, consequently, the pressure of the vapors accumulated in it increases. The pressure in the evaporator will increase until the pressure switch closes the contacts and the machine starts to work.

With the inclusion of the machine in operation, the suction of superheated vapors from the evaporator to the compressor begins. This entails an increase in temperature and pressure in the sensing cartridge of the thermostatic expansion valve, as a result of which the needle valve opens the orifice. The liquid refrigerant, boiling rapidly, rushes into the evaporator tubes. Boiling is accompanied by a significant decrease in the temperature of the vapor-liquid mixture, as a result of which the walls of the evaporator, the surrounding air and perishable products are cooled.

Lowering the ambient temperature reduces the amount of heat gain. Boiling becomes less intense, the amount of steam is reduced, the pressure in the evaporator drops to the limit at which the pressure switch opens the contacts and the machine stops. By the time the machine is turned off, the supply of liquid refrigerant to the evaporator is reduced, since an excess of refrigerant entering it leads to a decrease in the temperature of the escaping vapors and to automatic cover of the needle valve of the expansion valve. A few seconds after stopping the machine, the pressure in the bulb and evaporator

finally compared and the needle valve closes.

Refrigerants. Refrigerants are the working substances of steam refrigeration machines, with the help of which low temperatures are obtained. The most common of them are freon and ammonia.

When choosing a refrigerant, they are guided by its thermodynamic, thermophysical, physicochemical and physiological properties. Its cost and availability are also important. Refrigerants should not be poisonous, should not cause suffocation and irritation of the mucous membranes of the eyes, nose and respiratory tract person.

Freon-12 (R-12) It has chemical formula CHF 2 C1 2 (difluorodichloromethane). It is a gaseous colorless substance with a slight specific odor, which begins to be felt when the volume content of its vapors in the air is over 20%. Freon-12 has good thermodynamic properties.

Freon-22 (R-22), or difluoromonochloromethane (CHF 2 C1), as well as freon-12, has good thermodynamic and operational properties. It has a lower boiling point and a higher heat of vaporization. The volumetric cooling capacity of freon-22 is approximately 1.6 times greater than that of freon-12.

Ammonia (NH3) It is a colorless gas with a suffocating strong characteristic odour. Ammonia has a fairly high volumetric cooling capacity. Its production is based mainly on the method of combining hydrogen with nitrogen in high pressure with a catalyst. Ammonia is also used to obtain low temperatures (up to -70°C) under high vacuum. The heat of vaporization, heat capacity and thermal conductivity of ammonia is higher, and the viscosity of the liquid is lower than that of freons. For this reason, it has a high heat transfer coefficient. One hundred-

The value of ammonia is low compared to other coolants~

agents.

As you know, some refrigerants have ozone-depleting potential, which cannot but disturb the international community.

The ability of chlorinated refrigerants to cause this process called ozone depletion potential- RRP (Fig. 3.2).

R~403BR^t04A R-22 R-134A R-12 R-502

Rice. 3.3. Global warming potential

Rice. 3.2. Ozone depletion potential

The lifetime of refrigerants in the atmosphere is also a very important factor. This is a measure of the time during which various substances remain in the atmosphere and can affect the environment. In other words, the longer a chemical or freon remains in the atmosphere, the less environmentally friendly it is (Fig. 3.4).

80-60- 40-20-0

This is the removal of heat from the cooled body to the environment. With this method, the temperature of the cooled body can only be reduced to the ambient temperature. This is the easiest cooling method without wasting energy.

artificial cooling is the cooling of the body below the ambient temperature. Refrigeration machines or refrigeration units are used for artificial cooling. Wherein cooling method energy needs to be expended.

There are several ways to get artificial cold. The simplest is cooling with ice or snow. Ice cooling has a significant drawback - the cooling temperature is limited by the temperature of ice melting. As coolers, water ice, ice-salt mixtures, dry ice and liquid refrigerants (freons and ammonia) are used.

Ice-salt cooling made using crushed water ice and salt. Due to the addition of salt, the rate of ice melting is increased and the melting temperature of ice is lowered. Dry ice cooling is based on the action of solid carbon dioxide - when heat is absorbed, dry ice changes from a solid to a gaseous state. With dry ice, you can get a lower temperature than with water ice: the cooling effect of 1 kg of dry ice is almost 2 times greater than that of 1 kg of water ice, no dampness occurs during cooling, the carbon dioxide gas released has preservative properties, contributes to a better preservation of products. Dry ice is used in the transportation of frozen products, the cooling of packaged ice cream, the storage of frozen fruits and vegetables.

The most common and convenient in operation is machine cooling. Compared to others, machine cooling has the following advantages:

• the possibility of creating a low temperature over a wide range;
• automation of the cooling process;
• availability of operation and maintenance, etc.

The basis machine cooling the property of certain substances to boil at a low temperature is supposed, while absorbing a large amount of heat from the environment. Such substances are called refrigerants ().

Refrigerants- these are the working substances of steam refrigeration machines, with the help of which low temperatures are obtained. Refrigerants must have a high heat of vaporization, low boiling point, high thermal conductivity. At the same time, refrigerants should not be explosive, flammable, or poisonous. The cost of refrigerants is important. Freon 12, freon 22 and ammonia are the most suitable for these requirements. Freon enters trade enterprises in metal cylinders, painted in aluminum color and labeled R12 or R22.