The process of lowering body temperature is called cooling. There are natural and artificial cooling.
Natural cooling allows the body to cool down to temperature environment. This cooling is provided by cold water or air.
For cooling to a temperature lower than the ambient temperature, it is used artificial cooling, which can be carried out using any physical process associated with heat removal.
Artificial refrigeration is used in absorption, crystallization, gas separation, freeze drying and air conditioning processes.
Quite low temperatures can be achieved using refrigeration mixtures. A mixture of ice and CaCl 2 (up to 30%) allows you to reach a temperature of -55 °C. However, cooling this way requires a lot of ice and salt, so its use is limited.
Modern refrigeration machines use the property of a number of low-boiling liquefied gases (ammonia, freons, carbon dioxide, etc.) to absorb a large amount of heat from the environment when evaporating.
Artificial refrigeration can be divided into moderate(up to a temperature of -100 °C) and deep(to a lower temperature).
In industry, deep cooling is used to liquefy separated steam-gas and gas mixtures. The gases obtained in this way are widely used in the chemical industry: nitrogen - for the production of chemical fertilizers, oxygen, methane and ethylene - for the production mineral acids and so on.
In artificial refrigeration units, the necessary reduction in the temperature of the working fluid is carried out. By aggregate state of the working fluid refrigeration units
They include gas, gas-liquid, vapor-liquid and adsorption (using the solid phase).
Artificial cooling in most cases is carried out by two methods:
Evaporation of low-boiling liquids;
Expansion of various pre-compressed gases using throttling or expansion.
When low-boiling liquids evaporate, the latter are cooled due to a decrease in internal energy.
Throttling is the process of expansion of gas as it passes through a constriction device, resulting in a decrease in gas pressure. The energy required for gas expansion during throttling, when there is no heat input from the outside, can only be obtained from the internal energy of the gas itself. Throttle effect(Joule-Thomson effect) is a change in gas temperature during throttling in the absence of heat exchange with the environment.
Expansion- this is the expansion of gas in an expansion machine - an expander. In its design, this unit is similar to a piston compressor or turbocharger. During expansion, the gas cools due to a decrease in internal energy and external work.
If the pressure is reduced to 0.007 bar, then water will begin to boil at a temperature of only 4 ° C - these are its properties. In this case, it would be enough to supply a coolant with a temperature of, for example, 10 ° C to the kettle, and with the help of this coolant the water in the kettle would boil as if from a flame gas burner, and this coolant would cool, for example, to a temperature of 7 °C, just as the products of gas combustion are cooled under a boiling kettle. Coolant cooled from 10 to 7 °C is called refrigerant, and it can be successfully used, for example, in air conditioning systems.
In the ABKhM evaporator exactly such processes occur. This machine does not use freons as a refrigerant, but, like in a kettle, ordinary water, which boils in an evaporator, the pressure inside of which is close to absolute vacuum.
At the same time, a refrigeration machine should still be somewhat more complex than a kettle. The vacuum from the evaporator will disappear as soon as steam begins to form from the water. To prevent this from happening, steam must be removed. In conventional compressor refrigeration machines, the vapor formed during the boiling of refrigerants is sucked off by a compressor. Theoretically, it would be possible to suck out water vapor with a compressor, but in practice this problem is difficult to solve, because the specific volume of water vapor at low pressure is very large, and an overly large compressor would be required. At this point, the idea of a water refrigeration machine could have gone into the realm of science fiction if such a substance as a solution of lithium bromide in water had not been discovered. A special feature of this solution is its ability to greedily absorb (in scientific terms, “absorb”) water vapor. If a concentrated solution of lithium bromide, called an absorbent, is sprayed into the same volume as the evaporator, then the vacuum in this volume will be maintained, since the steam will go into solution. True, the absorbent will very soon lose its ability to absorb, the heat will be transferred to the circulating water circulating through the absorber coil, and removed to the atmosphere through the cooling tower.
The weak solution from the absorber A is supplied by pump 3 to the generator G, through the tubes of which coolant from the heat source T circulates. Under the influence of this heat, the vapor from the weak solution will evaporate and rush through the blinds (shown by the arrow) into the space of the condenser K cooled by circulating water, on the tubes of which the steam will condense, the condensate will return to the evaporator AND, and the partially dehydrated (concentrated) lithium bromide solution will return to the absorber. The salt concentration in the solution decreases, and at the same time its absorption capacity deteriorates. To maintain the absorption capacity of the solution at a constant high level, you need to evaporate the excess steam from it. And for evaporation there is no more suitable energy than thermal energy.
The physical nature of heat and cold is the same, the difference is only in the speed of movement of the molecules and the atom. In a more heated 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. Thus, thermal energy is the internal energy of motion of molecules and atoms.
Cooling a body is the removal of heat from it, accompanied by a decrease in temperature. The simplest method of cooling is heat exchange between the cooled body and the environment - outside air, river sea water, soil. But in this way, even with the most perfect heat exchange, the temperature of the cooled body can only be reduced to ambient temperature. This type of cooling is called natural cooling. Cooling the body below ambient temperature is called artificial. It uses mainly latent heat absorbed by bodies when their state of aggregation changes.
The amount of heat or cold is measured in calories or kilocalories (kilocalories). A calorie is the amount of heat required to heat 1 g of water by 1 at normal atmospheric pressure, kilocalorie - for heating 1 kg of water by 1C under the same conditions.
There are several ways to obtain artificial cold. The simplest of them is cooling with ice or snow, the melting of which is accompanied by the absorption of quite large quantity 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 reduced to almost 0°C. In practice, in a room cooled by ice or snow, the air temperature can only be maintained at 5-8°C. For ice cooling, water ice or solid carbon dioxide (dry ice) is used.
When cooled by water ice, a change in its state of aggregation occurs - melting (melting). Cooling capacity, or cooling capacity of a clean water ice, is called the specific heat of fusion. It is equal to 335 kJ/kg. The heat capacity of ice is 2.1 kJ/kg degrees.
Water ice is used for cooling and seasonal storage of food products, vegetables, and fruits in climatic zones with long cold periods, where it can be easily prepared under natural conditions in winter.
Water ice is used as a cooling agent in special glaciers and ice warehouses. Glaciers come with bottom loading of ice (glacier-cellar) and side loading - pocket type.
Ice cooling has significant disadvantages: the storage temperature is limited by the melting temperature of the ice (usually the air temperature in ice warehouses is 5-8°C), a quantity of ice must be placed in the glacier sufficient for the entire storage period, and added as necessary; significant labor costs for the preparation and storage of water ice; big sizes ice rooms approximately 3 times larger than the food room; significant labor costs to comply with the necessary storage requirements food products and drainage of melt water.
Ice-salt cooling is carried out using crushed water ice and salt. By adding salt, the rate of ice melting increases and the melting temperature of ice drops lower. This is explained by the fact that the addition of salt causes a weakening of molecular cohesion and destruction of the ice crystal lattices. The melting of the ice-salt mixture takes place with the removal of heat from the environment, as a result of which the surrounding air is cooled and its temperature decreases. As the salt content in the ice-salt mixture increases, its melting point decreases. The salt solution with the lowest melting point is called eutectic, and its melting point is called the cryohydrate point. The cryohydrate point for an ice-salt mixture with table salt is -21.2°C, with a salt concentration in the solution of 23.1% relative to total mass mixture, which is approximately equal to 30 kg of salt per 100 kg of ice. With further concentration of salt, there is not a decrease in the melting temperature of the ice-salt mixture, but an increase in the melting temperature (at a 25% concentration of salt in the solution to the total mass, the melting temperature rises to -8 ° C).
When an aqueous solution of table salt is frozen in a concentration corresponding to the cryohydrate point, a homogeneous mixture of ice crystals and salt is obtained, which is called a eutectic solid solution.
The melting point of the eutectic solid solution of table salt is -21.2°C, and the heat of fusion is 236 kJ/kg. The eutectic solution is used for zero-torque cooling. To do this, a eutectic solution of table salt is poured into zeros - tightly sealed forms - and frozen. Frozen zeros are used to cool counters, cabinets, refrigerated portable cooler bags, etc. In trade, ice-salt cooling was widely used before the mass production of equipment with a machine cooling method.
Cooling with dry ice is based on the property of solid carbon dioxide to sublimate, i.e., when absorbing heat, it passes from a solid state to a gaseous state, bypassing the liquid state. Physical properties dry ice has the following sublimation temperature at atmospheric pressure - 78.9°C, heat of sublimation 574.6 kJ/kg.
Dry ice has the following advantages over water ice:
It is possible to obtain a lower temperature;
The cooling effect of 1 kg of dry ice is almost 2 times greater than 1 kg of water ice:
When cooling, no dampness occurs; in addition, when dry ice sublimates, gaseous carbon dioxide is formed, which is a preservative that promotes better preservation of products.
Dry ice is used for transporting frozen foods, cooling packaged ice cream, frozen fruits and vegetables.
Artificial cooling can also be achieved by mixing ice or snow with dilute acids. For example, a mixture of 7 parts snow or ice and 4 parts dilute nitric acid has a temperature of -35 ° C. Low temperatures can also be obtained by dissolving salts in diluted 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.
Producing artificial cold using snow or ice, as well as using cooling mixtures, has significant disadvantages: the labor-intensive processes of collecting ice or snow, their delivery, the difficulty of automatic control, and limited temperature capabilities.
Recently, due to the energy crisis and environmental pollution, the problem of using non-traditional environmentally friendly methods of obtaining cold for refrigeration processing of food products has become increasingly urgent. The most promising of them is the cryogenic method based on liquid and gaseous nitrogen using a machineless flow-through refrigeration system, which involves the one-time use of a cryoagent.
The prospects of this method of cold supply are increasing due to the discovery in Russia of large reserves (340 billion m3) 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 nitrogen cooling systems have significant advantages: they are very reliable in operation and have a high freezing speed, ensuring almost complete preservation of quality and appearance product, as well as minimal loss of its weight due to shrinkage.
Particularly noteworthy is the ecological cleanliness of such systems (the Earth's atmosphere contains up to 78% nitrogen gas).
The most common and operationally convenient cooling method is machine cooling.
Machine refrigeration is a method of producing cold by changing the state of aggregation of the refrigerant, boiling it at low temperatures with removal from the cooled body or medium of the heat of vaporization necessary for this
For subsequent condensation of refrigerant vapors, a preliminary increase in their pressure and temperature is required.
The machine cooling method can also be based on adiabatic (without heat supply or removal) expansion of compressed gas. When a compressed gas expands, its temperature decreases significantly, since external work in this case is performed at the expense of the internal energy of the gas. The operation of air refrigeration machines is based on this principle.
Cooling by expansion of compressed gas, in particular air, is different from all cooling methods. In this case, 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 body being cooled).
The widespread use of machine refrigeration 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 high labor costs Maintenance, better sanitary and hygienic conditions for storing products, compactness and overall efficiency determine the feasibility of using machine cooling.
Wholesale and retail trade enterprises mainly use steam refrigeration machines, the operation of which is based on the boiling at low temperatures of special working substances - refrigerants. Steam refrigeration machines are divided into compression machines, in which refrigerant vapors are compressed in a compressor using mechanical energy, and absorption, in which which refrigerant vapors are absorbed by the absorbent.
Design and principle of operation of a compression refrigeration machine. A compression refrigeration machine (Fig. 3.1) consists of the following main components: evaporator, compressor, condenser, receiver, filter, thermostatic valve. The automatic operation of the machine is ensured by a thermostatic valve and a pressure regulator. Auxiliary devices that help improve the efficiency and reliability of the machine include: a receiver, a filter, a heat exchanger, and a dryer. The machine is driven by an electric motor.
An evaporator is a cooling battery that absorbs heat from the environment due to the refrigerant boiling in it at a low temperature. Depending on the type of cooled medium, evaporators for cooling liquid and air are distinguished.
Rice. Diagram of the compression refrigeration machine:
1 - compressor; 2 - capacitor; 3 - receiver; 4 - filter; 5 -
thermostatic valve; 6 - evaporator; 7 - cooled
camera; 8 - electric motor; 9 - magnetic starter; 10 -
push-button switch; 11 - pressure switch
The compressor is designed to suck refrigerant vapors from the evaporator, compress them and pump them in a superheated state into the condenser. In small refrigeration machines, piston and rotary compressors are used, with piston ones being the most widespread.
A condenser is a heat exchange device used to liquefy refrigerant vapors by cooling them. Depending on the type of cooling medium, capacitors are produced with water and air cooling. Condensers with forced air movement have vertically positioned flat coils made of copper or steel finned tubes. Natural air cooling is used only in refrigerating machines of household electric refrigerators. Water-cooled condensers are shell-and-tube and shell-and-tube.
Receiver - a reservoir used to collect liquid refrigerant in order to ensure its uniform flow to the thermostatic valve and to the evaporator. In small refrigerant machines, the receiver is designed to collect refrigerant during machine repairs.
The filter consists of copper or brass mesh and cloth pads. It serves to clean the system and coolant from mechanical contaminants resulting from insufficient cleaning during manufacturing, installation and repair. Filters come in liquid and steam types. The liquid filter is installed after the receiver in front of the thermostatic 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 cup made of brass mesh is inserted into the suction cavity of the compressor.
The thermostatic valve ensures a uniform flow of refrigerant into the evaporator, sprays liquid refrigerant, thereby reducing the condensation pressure to evaporation pressure.
The efficiency of the refrigeration machine largely depends on the correct adjustment of the thermostatic valve. An excess of liquid refrigerant in the evaporator due to wet running of the compressor can lead to water hammer. If the evaporator is not sufficiently filled with liquid, part of its surface is not used, which leads to disruption of the normal operation of the machine and a decrease in the evaporation temperature of the refrigerant.
The pressure regulator consists of a pressure switch (low pressure regulator) and a pressure controller (high pressure switch). To regulate the temperature within certain limits, it is necessary that the cooling capacity of the refrigeration machine always exceeds the heat flow to it. Therefore, under normal conditions there is no need for continuous operation of the refrigeration machine.
Periodic switching on of the refrigeration machine is carried out automatically by a pressure switch. The required temperature regime is achieved by regulating the duration of breaks in the operation of the refrigeration machine. The pressure controller serves to protect against excessive pressure build-up in the discharge line. When the pressure in the condenser increases above 10 atm. (the norm is 6-8 atm.) it opens the circuit of the magnetic starter coil, the power to the electric motor is turned off and the refrigeration machine stops.
The operation of the refrigeration machine occurs as follows. Easily evaporating liquid (freon-12) enters the evaporator through a thermostatic valve. When exposed to low pressure conditions, it boils, turning into steam, and at the same time takes heat from the air surrounding the evaporator.
From the evaporator, refrigerant vapors are sucked out by a compressor, liquefied and, in a state superheated from compression, are pumped into the condenser. In a water- or air-cooled condenser they turn into liquid. Liquid refrigerant flows through the condenser pipes and accumulates in the receiver, from where it passes under pressure through a filter, where mechanical impurities (sand, scale, etc.) are retained.
The refrigerant, cleared of impurities, passing through the narrow hole of the thermostatic valve, is throttled (crumpled), sprayed and, with a sharp decrease in pressure and temperature, enters the evaporator, after which the cycle is repeated.
The operating cycle of the refrigeration machine, taking into account the interaction of automation devices, is as follows. When the electric motor is turned off, the contacts of the pressure switch are open, the thermostatic valve does not allow liquid refrigerant to pass from the condenser to the evaporator, since the needle has fully entered the saddle and tightly closed the flow area. At this time, the boiling process of the liquid refrigerant remaining after turning off the machine continues in the evaporator. Due to the influx of external heat, the temperature of the evaporator gradually increases and, consequently, the pressure of the vapors accumulated in it increases. The pressure in the evaporator will increase until the pressure switch of the pressure switch closes the contacts and the machine starts working.
When the machine is turned on, superheated vapors are sucked out of the evaporator into the compressor. This entails an increase in temperature and pressure in the sensitive cartridge of the expansion valve, as a result of which the needle valve opens the passage hole. The liquid refrigerant, boiling intensely, rushes into the evaporator pipes. 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.
A decrease in ambient temperature reduces the amount of heat inflow, 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 the excess refrigerant entering it leads to a decrease in the temperature of the exiting vapors and to the automatic closing of the needle valve of the thermostatic valve. A few seconds after stopping the machine, the pressure in the thermal cylinder and the evaporator is 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 achieved. The most common of them are freon and ammonia.
When choosing a refrigerant, one is guided by its thermodynamic, thermophysical, physicochemical and physiological properties. Its cost and availability are also important. Refrigerants must not be toxic, must not cause suffocation or irritate the mucous membranes of the eyes, nose and respiratory tract of humans.
Freon-12 (R-12) has the chemical formula CHF 2 C1 2 (difluorodichloromethane). It is a gaseous colorless substance with a weak specific odor, which begins to be felt when the volumetric content of its vapor in the air is over 20%. Freon-12 has good thermodynamic properties
Freon-22 (R-22), or difluoromonochloromethane (CHF 2 C1), like freon-12, has good thermodynamic and performance properties. It is distinguished by 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 (NH 3) is a colorless gas with a suffocating, strong characteristic odor. Ammonia has a fairly high volumetric refrigeration capacity. Its production is based mainly on the method of combining hydrogen with nitrogen at high blood pressure with the presence of a catalyst. Ammonia is also used to obtain low temperatures (down to -70°C) in high vacuum. The heat of vaporization, heat capacity and thermal conductivity coefficient of ammonia is higher, and the viscosity of the liquid is lower than that of freons. Therefore, it has a high heat transfer coefficient. The cost of ammonia is low compared to other refrigerants
As is known, some refrigerants have ozone-depleting properties, which cannot but worry the international community
The ability of chlorine-containing refrigerants to cause this process is called ozone depletion potential - ODP (Figure 3 2)
Rice. Ozone Depletion Potential
The ability of various substances to cause global warming processes is called global warming potential - GWP (Figure 3.3)
Rice. Global Warming Potential
The lifespan of refrigerants in the atmosphere is also a very important factor. This is an indicator 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 (Figure 3 4)
Rice. Lifespan of refrigerants in the atmosphere
In 1985, the Convention for the Protection of the Ozone Layer was adopted in Vienna. It was joined by 127 states, including Russia and CIS countries.
In 1989, the Montreal Protocol came into force on the gradual reduction, and then the complete cessation in 2030, of the production of ozone-depleting refrigerants. TO dangerous groups freons R-11, R-12, R-113, R-114, R-115, R-12B1, R-13B1, R-114B2 were classified. In the 90s The text of the protocol was tightened by introducing restrictions not only on the production, but also on the trade, export and import of any refrigeration equipment containing ozone-depleting substances.
Russian Federation accepted the obligations arising from the Montreal Protocol for the Protection of the Ozone Layer. According to the decisions taken, R-502 has been banned for production since January 1, 1996. For R-22, more distant dates have been established - a reduction in production and use since 2005 and a complete ban starting in 2020.
To replace R-502 and R-22, the world's major chemical manufacturers have developed and are producing transitional (containing chlorofluorocarbons) and ozone-safe (consisting only of fluorocarbons) mixtures of refrigerants.
Transition refrigerants include R-402, R-403B and R-408A, which can be used in existing equipment. Most of these new working substances have appeared on the Russian market today.
Ozone-safe refrigerants R-507, R-404A, R-134A can be recommended both for use in new equipment and for the reconstruction of low-temperature refrigeration systems. They are designed to replace R-22 in existing and currently produced equipment.
It is becoming increasingly difficult for manufacturers to rationally select a refrigerant for a specific facility. Therefore, the problem of using natural substances, and primarily ammonia, as refrigerants is now most pressing for manufacturers of refrigeration equipment.
Ammonia refrigeration units have been in operation for about 120 years. In Russia, the overwhelming majority of the need for cold for stationary refrigerators is provided by ammonia refrigeration units.
In the 90s and in Western Europe The use of ammonia has increased significantly because it:
Does not destroy the ozone layer,
Has no direct impact on the global thermal effect;
Has excellent thermodynamic properties;
Has a high heat transfer coefficient during boiling and condensation;
Has high energy efficiency in the refrigeration cycle;
It has a low cost, its production is affordable, the problems of its flammability and toxicity are now solvable, which makes it attractive for manufacturers of refrigeration equipment.
Refrigerating machines and units. A refrigeration machine is a set of mechanisms, apparatus and devices connected in series into a system for producing artificial cold. Compact, structural combinations of individual or all elements of a refrigeration machine are called a refrigeration unit.
Based on the type of refrigerant used, ammonia and freon refrigeration units are distinguished. Based on the design features of compressors, units are divided into open and sealed, and condensers - air-cooled and water-cooled.
Depending on the composition of the elements included in them, refrigeration units can be compressor, compressor-condenser, evaporation-control, evaporation-condenser and complex units. Trade enterprises use compressor-condenser units and, when cooling with a coolant, evaporative control units.
A compressor-condensing unit consists of a compressor, a condenser (air or water cooled), an electric motor, automation devices and auxiliary devices (receivers, dryers, heat exchangers, etc.). An evaporation-control unit is a structural connection of an evaporator, auxiliary equipment, a control station and automation devices. Complete units include all elements of the refrigeration machine.
Refrigerators are supplied separately and as a set with commercial refrigeration equipment. The equipment package includes a built-in evaporative battery and a built-in or separately packaged refrigeration unit. If the unit is intended to be installed outside of equipment, it must include mounting copper tubing.
To cool prefabricated chambers, cabinets, counters and display cases, freon refrigeration units with a cooling capacity of up to 3 thousand kcal/h are used. These are mainly compressor-condensing units that melt on freon-12 and freon-22. Depending on the location of the electric motor and the method of transmitting mechanical energy, there are open and sealed units.
In open-type units, the electric motor is mounted separately from the compressor, and the transmission of mechanical energy is carried out by a pulley-belt mechanism.
Sealed refrigeration units are the most promising. The tightness of the system is achieved through the use of a welded casing, reducing the number of detachable connections and using a thermostat instead of a pressure switch. Compared to open-type units, sealed units have significant advantages.
By combining the electric motor and compressor into a unit with a single eccentric shaft, the need for a transmission mechanism was eliminated. This made it possible to reduce the weight and size of the compressor and unit, and increase the shaft rotation speed to 3 thousand rpm.
In a sealed unit, due to the reduction in the number of detachable connections and the absence of seals, the leakage of refrigerant was reduced, which made it possible to reduce its working reserve in the system. The operating consumption of refrigerant has also decreased, since there is no need to periodically refuel the machines.
Cooling the electric motor winding with the flow of sucked-in refrigerant vapors made it possible to increase the load on the electric motor and reduce its parameters, power, dimensions and weight. For example, with equal cooling capacity, the rated power of the electric motor of a sealed unit is 40% less than that of an open type unit. In this regard, energy consumption is significantly reduced.
Sealed units have an important quality for stores, especially trading floors - a relatively low level of noise emitted. Reducing the size of units allows for more rational use of warehouse retail space, as well as the capacity of commercial refrigeration equipment.
The purpose and principle of operation of individual elements of sealed machines is somewhat different from open type machines. Automatic control of the operation of a sealed refrigeration machine is carried out not by a pressure switch, but by a thermostat (temperature switch). Protection of the electric motor from overheating and the condenser from excess pressure is provided by the compressor thermal relay.
Below is a description of the main components of refrigeration machines.
Refrigeration units ACL 88TN (Fig. 35) and ACP 12TN, made on the basis of licensed Electrolux compressors, are small in size and have a low noise level. They are intended for installation in commercial refrigeration equipment of both domestic and imported origin.
Rice. Refrigeration unit ACL 88TN
Refrigeration units BC 4000 (2) and VN 2000 (2) are units with scroll compressors from Copeland (Fig. 36).
The use of a scroll-type compressor has significantly increased the reliability of the product compared to both hermetic piston and open-type compressors. The scroll compressor has no valves and, when used correctly, cannot jam.
The VN 2000 (2) refrigeration unit is used in low-temperature chambers with a volume of 12-14 m2, where it can provide temperatures down to -18 C.
Refrigeration unit BC 4000 (2) is designed for cooling medium-temperature chambers with a volume of 24-30 m 3 Specifications refrigeration units are given in table.
Rice. Refrigeration unit BC 4000 (2)
Compressor-condensing units of the SM MX series with a hermetic and semi-hermetic compressor (Fig. 3 7), which has internal motor protection, an electrical control panel protected from the external environment, can be installed outdoors.
Technical characteristics of refrigeration units
| VN 2000(2) | BC 4000(2) | ACI 88 IN | ACP12TN | |
| Refrigerant | R22 | R22 | R22 | R22 |
| Refrigerant boiling temperature range, °C | -45 -15 | -25 -5 | -25 -5 | -25 -5 |
| Ambient temperature, °C | +5 +45 | +5 +45 | +5 +45 | +5 +45 |
| Cooling capacity at a refrigerant boiling point of 15°C (for VN 2000(2) at -35°C) and an ambient air temperature of 20°C, W | 2010 | 4360 | 600 | 800 |
| Compressor | ZF09K4E Coreland | ZS21K4E Coreland | L88TN Electrolux | P12TN Electrolux |
| Electric motor voltage, V. rotation speed rpm | 380 3000 | 380 3000 | 220 3000 | 220 3000 |
| Dimensions, mm | 860x560x610 | 860x560x610 | 440x380x255 | 440x380x255 |
| Macca, kg | 90 | 90 | 30 | 30 |
Mounted in a soundproof casing made of galvanized steel. The SM and MX series equipment creates and maintains temperatures from 5 to -30 C.
The units operate effectively in refrigeration chambers at retail establishments, and are also widely used for cooling warehouses.
The monoblock (Figure 3-8) is a single unit that includes a hermetic compressor, an air condenser, an air cooler and an electronic control panel. The monoblock is installed on prefabricated refrigeration chambers with a wall thickness of no more than 120 mm, mounting it into the hole in the chamber panel on the wall or ceiling.
Rice. Compressor-condensing unit
Fig 3 8. Monoblock
A split system (Fig. 3.9) is a fully equipped refrigeration equipment consisting of two separate parts. It is used for cooling stationary refrigeration chambers.
Rice. Split system
The automation system ensures maintenance of the required temperature in the refrigeration chamber, protection against emergency conditions and periodic defrosting of the air cooler.
All equipment is supplied with protection monitors that monitor the mains voltage.
Operates from a mains voltage of 220 or 380 V, retains cold at ambient temperatures up to 45°C,
The world's largest manufacturer of compressors with refrigeration capacity from 1 to 173 kW for commercial refrigeration equipment, air conditioning, and heat pumps is the Copeland company.
Copland hermetic reciprocating compressors are manufactured to specifications to ensure their use in any climatic zone globe, which is achieved thanks to a wide range of operating voltages of electric motors. These compressors are manufactured to operate on certified refrigerants and high-quality lubricating oils from well-known global companies at high temperatures (above 0°C), medium temperatures (from 0°C to -15°C) and low temperatures (from -15°C to -20°C) modes.
With the introduction of hermetic compressors, a new range of air-cooled condensing units appeared. This new range, with many attractive features, both standard and customized, is designed to work with environmentally friendly refrigerants R-22 and R-134A. It has a wide performance range and high energy efficiency. All units [have silent and smooth operation.
Two main ranges of units are offered. The HAN range with a normal capacitor size is used to ensure:
Standard storage mode, when the temperature of the product being stored is no more than 10°C higher than the temperature set in the storage;
Compact and low cost;
Operation under conditions normal temperature environment.
The HAL range with a more powerful capacitor is used when:
The amount of load on the unit often and sharply changes over time (when periodically loading large quantities of product at the same time or the need to quickly cool products, for example, milk);
It is necessary to achieve high energy efficiency, which ensures low operating costs;
The work will be done in conditions of elevated ambient temperature.
Copland sealless refrigeration compressors combine the latest design developments with the advantages of the latest refrigerants. Sealless compressors have high performance, long service life and a wide range of applications (high, medium and low temperature operating conditions).
Models DLH, D6C, Discus, as well as two-stage compressors, have devices for connecting a differential mechanical oil pressure switch or an electronic sensor for the Sentronic oil pressure protection system.
All sealless compressors are capable of direct starting. It is also possible to equip it with electric motors with switching the electrical circuit from “star” to “delta” at start-up or using part of the winding to reduce the starting current. For optimal starting conditions without load, a special device can be installed on all Discus models, as well as on DLH models.
Each compressor is equipped with a motor protection device. Single-phase electric motors are equipped with a thermal overload protection relay. In three-phase electric motors, thermistors are built into the motor winding.
The refrigeration capacity control equipment can be used for all single stage 3, 4, 6 and 8 cylinder compressors. For D3D compressors, the Moduload load changing system has been developed with particularly low energy consumption.
For ultra-low temperature applications, Discus models are equipped with Demand Cooling, which allows the compressor discharge temperature to be controlled by injecting small amounts of liquid refrigerant into the compressor. Thanks to the Demand Cooling system, a single-stage compressor becomes a good alternative to a two-stage one. In cases where the boiling point of the refrigerant must vary significantly depending on demand (for example, from -50°C to -20°C), the Demand Cooling system becomes more cost-effective.
The Copland company also produces twin (TWIN) compressors. Twin compressors are available on all 2-stage and Discus models except those equipped with Demand Cooling. The main advantages of twin compressors: double cooling capacity, reduced 50% modulation of cooling capacity and high efficiency even at partial load.
Air-cooled compressor-condensing units are produced on the basis of most models of sealless compressors. They are supplied filled with oil, fully equipped with automation equipment and ready to work. Additionally, at the customer's request, they can be equipped with: crankcase heaters, a fan speed controller (to regulate the condensation temperature), a protective casing for outdoor installation, and various modifications of the receiver according to the capacity.
The Copeland Matched Scroll compressor is one of the most advanced hermetic compressors used in air conditioning, medium temperature refrigeration and heat pump applications. The operating boiling temperature range of a scroll compressor is from positive to -20°C.
Compared to sealed or sealless piston compressors, scroll compressors have such significant advantages as:
High reliability and extended service life due to fewer parts required to compress the refrigerant;
Overload resistance;
Low noise level due to the absence of valves and reciprocating movement of parts, as well as a high degree of consistency in the movement of parts thanks to the patented “Compliance” principle;
Higher refrigerant supply coefficient due to the absence of “dead” space;
Low vibration levels due to smooth, continuous compression;
Increased performance, stability of the compressor when mechanical impurities, wear products or liquid refrigerant enter the compression zone;
Low starting torque and starting currents (starting without load), for single-phase models there is no need for starting equipment;
Compact and light weight.
The refrigeration coefficient of a scroll compressor when operating in standard European air conditioning mode reaches a value of 3.37 W versus 2.75-2.95 W for a sealed piston analogue.
A low-temperature Glacier-type scroll compressor has appeared on the equipment market, operating efficiently and reliably at large pressure drops. It can operate on R-22, R-404A, R-507, R-134A refrigerants at boiling temperatures down to -45°C.
All types of Copeland compressors are supplied filled with mineral oil for R-22 operation or polyester oil for operation with ozone-safe refrigerants or R-22.
Scroll compressors (Fig. 3.10) are intended for use in air conditioners in industrial, commercial and administrative buildings.
Maneurop products are in particular demand on the climate control equipment market. Its Performer brand compressors, thanks to low level noise and a high degree of reliability, satisfy all operating requirements and consumer requests.
By installing compressors in pairs, three or four in a row, you can achieve a cooling system performance of up to 180 kW.
A distinctive feature of Performer compressors is the presence of a moving contact between the scrolls, which, using two patented floating seals, ensures perfect axial tightness and reduces stress and deformation.
High precision and modern machine processing technologies prove that a simple film of oil is what is needed to accurately seal the ends of the spiral, reduce contact between moving parts, minimize friction between them, increase volumetric productivity and reduce vibration, which guarantees high performance compressor and extends its service life.
Rice. Maneurop Performer scroll compressor
The advantages of scroll compressors of the Regformer brand are:
Higher efficiency. Controlled rotating parts with floating seals and improved spiral geometry;
Minimum noise level. An effective system for balancing the compressor and protecting it from vibration;
Increased reliability. Extended service life due to the absence of friction between the spirals and engine cooling with absorbed refrigerant;
Easy to install. Most models use hard solder fittings or union nut fittings as standard connection options. Reverse rotation protection devices, as well as protection for the motor itself, are integral part designs. No additional devices are required when installing the compressor;
Large oil reserve and larger volume of refrigerant charge than most other compressors, longer service life.
The corporate color of the compressors is blue.
Danfoss Maneurop is working on expanding the power range from 3.5 to 25 hp. and introduces new refrigerants. In addition to developments in the use of ozone-friendly refrigerants R-407C and R-134A and in order to fight for a cleaner environment, Danfoss Maneurop began using R-410A refrigerant in compressors with power from 3.5 to 6.5 hp.
Remote and centralized refrigeration supply
The traditional refrigeration supply scheme for commercial enterprises is carried out on the basis of separate blocks, i.e., a separate refrigeration unit operates for each consumer.
But equipping stores with refrigeration equipment with built-in compressors results in additional costs for installing air conditioners to remove heat from the built-in units to the trading floors.
Heat inflows into sales areas from refrigeration units built into the equipment lead to a decrease in turnover and an increase in unforeseen expenses.
Uncomfortable conditions for the buyer (high temperature in the sales area and high noise levels, unpleasant extraneous odors) lead to the fact that he is in a hurry to leave the store, which leads to a decrease in turnover;
Uncomfortable conditions for sellers and service personnel lead to a decrease in the quality of service, therefore, the image of the enterprise decreases and turnover decreases;
The service life of built-in units is 2-3 times lower than when using remote refrigeration supply systems and 4-6 times lower than when using central units, as a result of which production costs for maintenance and replacement of equipment increase;
Operation of compressor equipment in extreme conditions with extreme high temperature and condensation pressure causes frequent equipment failures, and this leads to losses from product spoilage;
Additional costs for air conditioning increase the company's overall energy consumption costs by 20-30%.
Much more efficient than the system cold supply systems serving several consumers - remote and centralized cold supply.
Remote refrigeration supply is a refrigeration supply system based on autonomous compressor-condensing units located in the engine room, isolated from the retail premises. Moreover, each unit can provide cold to several consumers.
Centralized refrigeration supply (central) is a type of remote refrigeration supply system. It is a multi-compressor unit with a single microprocessor control, usually based on semi-hermetic piston or scroll compressors. For medium-temperature and low-temperature consumers, two separate circuits are used.
Currently, such installations are most widespread in large grocery stores and supermarkets.
| Hypermarket ( shopping mall) | Supermarket | Minimarket | |
| Total cooling capacity, kW | |||
| including: for medium-temperature consumers (-10 °C) | |||
| for low temperature consumers (-15 °C) | |||
| Central | Central / compressor and condensing units | Compressor and condensing units |
|
| Average service life, years |
When using a centralized refrigeration system, not only operating costs, but often also capital costs are significantly reduced. And the more cold consumers there are, the more profitable it is to use centralized cooling supply.
The installation of a central cold supply allows you to use the heat of condensation for heating needs and heating process water.
Depending on the cooling capacity and the requirements for its regulation, it has from 2 to 6 compressors connected in parallel and having common discharge and suction systems. Such a compression unit, isolated from retail and auxiliary premises, provides cold to 20-25 end consumers connected to it by refrigeration lines.
In addition, such systems are designed with the necessary power reserve, which allows for scheduled maintenance and emergency repairs of any refrigeration unit without loss of cooling supply to the equipment. Previously, such systems were produced mainly by manufacturers of prestigious and expensive brands. Currently, central cold is available to a wider range of consumers.
There are medium-temperature and low-temperature installations of centralized systems with a total cooling capacity of up to 80 kW. These systems will make it possible to create a “seamless” line of display cases and reduce the noise level in the sales area to a minimum.
For a diagram of centralized refrigeration supply, see Fig.
Invention: in refrigeration technology. The essence of the invention: cold is obtained by compression and expansion of a fluorine-based refrigerant. Inorganic hexafluorides or mixtures thereof are used as a refrigerant.
The invention relates to refrigeration technology and can be used in gas energy refrigeration machines and heat pumps containing a compressor and expander, mainly of a turbine or centrifugal type. From the prior art there is a method for producing cold in a refrigeration unit by compressing a gaseous refrigerant in a turbocompressor and expansion in a turboexpander (see St. USSR author N 169543, class F 25 B 11/00, 1965; St. USSR author N 183773, class F 25 B 9/00, 1966; motor vehicle code of the USSR N 1433193, class F 25 B 9/00, 1990, motor vehicle code of the USSR N 1778468, class F 25 B 9/00, 1992 or patent Great Britain N 2174792, class F 4 H 1986). In this case, air, nitrogen, hydrogen, helium, xenon, freons or mixtures of gases are used as a gaseous refrigerant, as, for example, in cars. USSR N 565052, class. F 25 B 9/00, 1977; auto sv. USSR N 802348, class. F 25 B 9/00,1981. The closest known to the invention is a method for producing cold in a refrigeration unit by compressing a gaseous refrigerant in a turbocompressor and expanding in a turboexpander (see Aut. St. USSR N 473740, class F 25 B 11/00, 1975), in which as The refrigerant uses a mixture of gases based on fluorine compounds, containing octafluorocyclobutane FS-318 (C 4 F 8) and difluorochloromethane F-22 (CHClF 2). However, this refrigerant has an environmentally unfavorable effect on the ozone layer. The invention is aimed at expanding the choice of gaseous refrigerants for energy refrigeration machines or heat pumps with high heat capacity, providing increased cooling capacity and a reduction in the weight and size parameters of turbomachines used as a compressor and expander (expander) to primarily produce cold. The solution to this problem is ensured by the fact that in the method of producing cold in a refrigeration unit by compressing a gaseous refrigerant based on fluorine compounds in a turbocompressor and expanding in a turboexpander, inorganic hexofluorides or a mixture thereof are used as a refrigerant. The use of heavy inorganic hexofluorides as a refrigerant, while expanding the choice of working medium to provide various operating parameters of a refrigeration machine or heat pump, makes it possible to increase the cooling capacity due to their high heat capacity and density and reduce the dimensions of the turbocharger and turboexpander for the ability to operate at low rotor speeds. When implementing the method, regardless of the circuit design of a specific refrigeration unit or heat pump, inorganic hexofluoride, for example XeF 6 , circulates in a gaseous state along the operating circuit; WF 6; MoF 6; UF 6, or a mixture of these gases. The specific type of hexofluoride compound is determined depending on the optimal combination of the required operating parameters with the thermophysical properties of the refrigerant by calculation.
Claim
A method for producing cold in a refrigeration unit by compressing a gaseous refrigerant based on fluorine compounds and its expansion, characterized in that inorganic hexafluorides XeF 6, WF 6, MoF 6, UF 6 or mixtures thereof are used as a refrigerant.
Similar patents:
The invention relates to the field of refrigeration technology, in particular, to turbocompressor units and can be used for cooling or freezing various products, both in stationary conditions and on vehicles, for example, on ships
The invention relates to gas compression technology, and more specifically to compressors for compressing refrigerant vapors and gases, with high final temperatures at the end of compression, and intended for operation as part of industrial compressor shops in all areas of the use of artificial refrigeration and compression of air and other gases
The invention relates to refrigeration equipment, and more specifically to cooling methods (installations for their implementation and distribution manifolds of such installations), in which various products or products located in a closed volume are cooled using a cryogenic liquid supplied to this volume, heated, evaporated and forming with the gaseous medium located in this volume various circulation circuits that wash the products or products placed in the volume
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 more heated 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. Thus, thermal energy is the internal energy of motion of molecules and atoms.
Cooling a body is the removal of heat from it, accompanied by a decrease in temperature. The simplest method of cooling is heat exchange between the cooled body and the environment - outside air, water, soil. But in this way, even with the most perfect heat exchange, the temperature of the cooled body can only be reduced to ambient temperature. This type of cooling is called natural cooling. Cooling the body below ambient temperature is called artificial. It uses latent heat absorbed by bodies when their state of aggregation changes.
There are several ways to obtain artificial cold. The simplest of them is cooling with ice, the melting of which is accompanied by the absorption of a fairly large amount of heat. If heat inflows from the outside are small, and the heat transfer surface of the ice is relatively large, then the temperature in the room can be reduced to almost 0˚C. In practice, in a room cooled by ice, the air temperature can only be maintained at a level of 5 -8 ˚С.
When cooled by water ice, a change in its state of aggregation occurs - melting. The refrigeration capacity, or cooling ability, of pure water ice is called the specific heat of fusion. It is equal to 335 kJ/kg·degree.
Water ice is used for cooling and seasonal storage of food products, vegetables, and fruits in climatic zones with long cold periods, where it can be easily prepared under natural conditions in winter.
Water ice is used as a cooling agent in special glaciers and ice warehouses. Glaciers come with bottom loading of ice (glacier - cellar) and side loading - pocket type.
Ice cooling has significant disadvantages: the storage temperature is limited by the melting temperature of the ice (usually the air temperature in ice warehouses is 5-8 ºС), a sufficient amount of ice must be placed in the glacier for the entire storage period and added as necessary, significant labor costs for the preparation and storage of water ice; large dimensions of the ice room, approximately 3 times larger than the size of the food room; significant labor costs to comply with the necessary requirements for food storage and melt water drainage.
Ice-salt cooling is produced using crushed water ice and salt. By adding salt, the rate of ice melting increases and the melting temperature of ice drops lower. This is explained by the fact that the addition of salt causes a weakening of molecular cohesion and destruction of the ice crystal lattices. The melting of the ice-salt mixture takes place with the removal of heat from the environment, as a result of which the surrounding air is cooled and its temperature decreases. As the salt content in the ice-salt mixture increases, its melting point decreases. The salt solution with the lowest melting point is called eutectic, and its melting temperature is called the cryohydrate point. The cryohydrate point for an ice-salt mixture with table salt (H 2 O - NaCl) is 21.2 ºC with a salt concentration in the solution of 23.1% relative to the total weight of the mixture, which is approximately equal to 30 kg of salt per 100 kg of ice. With a further increase in salt concentration, there is not a decrease, but an increase in the melting temperature of the ice-salt mixture (Fig. 5.1).
Figure 5.1 - Dependence of the solidification temperature of the solution on the salt concentration in water.
The eutectic solution is used for zero-torque cooling. To do this, a eutectic solution of table salt is poured into zerotors - tightly sealed molds - and frozen. Frozen zerotors are used for cooling counters, cabinets, refrigerated portable bags - refrigerators, etc.
Cooling with dry ice is based on the property of solid carbon dioxide to sublimate, i.e. when absorbing heat, transition from a solid state to a gaseous state, bypassing the liquid state. The physical properties of dry ice are as follows: sublimation temperature at atmospheric pressure – 78.9ºС, heat of sublimation 574.6 kJ/kg.
Dry ice has the following advantages over water ice:
It is possible to obtain a lower temperature;
The cooling effect of 1 kg of dry ice is almost twice that of 1 kg of water ice;
When cooling, no dampness occurs; in addition, when dry ice sublimates, gaseous carbon dioxide is formed, which is a preservative that promotes better preservation of products.
Dry ice is used for transporting frozen foods, cooling packaged ice cream, frozen fruits and vegetables. Dry ice is produced artificially at carbon dioxide factories and stored in special containers with enhanced thermal insulation.
Producing artificial cold using ice, as well as using cooling mixtures, has significant disadvantages: the labor-intensive processes of preparing ice and its delivery, the difficulty of automatic control, and limited temperature capabilities.
Thermoelectric cooling is based on the Peltier effect (discovered by Jean Peltier in 1834), the essence of which is that, under the influence of passing electric current along a chain of 2 different conductors or semiconductors, different temperatures appear at the junctions (Fig. 5.2). If the temperature of the cold junction is lower than the ambient temperature, then it can be used as a cooler. A significant temperature difference at the junctions is provided by pairs composed of semiconductors made from compounds of bismuth, antimony, selenium with the addition of a small amount of additives.
Figure 5.2 - Schematic diagram of thermoelectric cooling.
The advantage of thermoelectric cooling is the absence of moving parts, working fluid, noiselessness, reliability and durability of operation, the disadvantage is high energy consumption. Thermoelectric cooling devices are used in some types of refrigerators and refrigerated bars.
Considering the disadvantages of all the above cooling methods, the most common and operationally convenient cooling method is machine cooling.
Machine refrigeration is a method of producing cold by changing the state of aggregation of the refrigerant, boiling it at low temperatures, and removing the heat of vaporization necessary for this from the cooled body or medium. For subsequent condensation of refrigerant vapors, a preliminary increase in their pressure and temperature is required.
The widespread use of mechanical cooling in trade is explained by a number of its operational properties and economic advantages: automatic maintenance constant temperature storage depending on the type of product, high proportion of use of useful containers for cooling, low costs of operation, maintenance and repair, ease of use and sanitary processing.
The complex of mechanisms and devices that carry out the refrigeration cycle is called a refrigeration machine. Trade enterprises use compression refrigeration machines, in which refrigerant vapors are compressed in a compressor using mechanical energy.
