A refrigerator that relies on a compressor to increase the pressure of a refrigerant to achieve a refrigeration cycle. Compression refrigerator is composed of compressor, condenser (condenser), refrigeration heat exchanger (evaporator), expander or throttle mechanism and some auxiliary equipment. The compressor is its core equipment.
Classification According to the types of refrigerants used, compression refrigerators are divided into gas compression refrigerators and vapor compression refrigerators. Vapor compression refrigerators include ammonia refrigerators and freon refrigerators. Gas compression refrigerators are divided into air refrigerators and helium refrigerators. According to different types of compressors used, compression refrigerators are divided into reciprocating refrigerators, centrifugal refrigerators and rotary refrigerators (screw refrigerators, rolling rotor refrigerators), etc. Vapor compression chillers are classified into single-stage, multi-stage (two-stage or three-stage) and cascade type according to their system composition.
The working principles of various refrigerators have their common points and different points.
Gas compression refrigerator uses gas as refrigerant, which is composed of compressor condenser, regenerator, expander and cold box, etc. (Figure 1 [Working Principle of Gas Compression Refrigerator]). The gas compressed by the compressor is first cooled in the condenser, and releases heat to the cooling water (or air), then flows through the regenerator to be further cooled by the recirculated gas, and enters the expander for adiabatic expansion. The pressure and temperature of the compressed gas At the same time decline. When the gas expands in the expander, it performs external work and becomes part of the compressor's input work. At the same time, the expanded gas enters the cold box and absorbs the heat of the object to be cooled, that is, the purpose of cooling is achieved. After that, the gas flows back through the regenerator, exchanges heat with the compressed gas, and then enters the compressor to be compressed. All gas refrigerators should use regenerators, which can not only improve the economic efficiency of the refrigerator but also reduce the temperature of the compressed gas in front of the expander, thus reducing the refrigeration temperature. Gas refrigerators can reach a wide range of refrigeration temperatures, from above 0 â„ƒ to below -100 â„ƒ; when the refrigeration temperature is high, the economic efficiency is poor, but when the refrigeration temperature is below -90 â„ƒ, the economic efficiency is higher than Steam refrigerator.
Vapor compression refrigerator is composed of compressor, condenser, evaporator, throttle mechanism and some auxiliary equipment. The refrigerant of this type of refrigerator can be liquefied at ordinary temperature and ordinary low temperature, and the refrigerant periodically condenses and evaporates during the operation of the refrigerator. Commonly used vapor compression refrigerators are single-stage, two-stage and cascade.
â‘ Single-stage vapor compression chiller: A vapor compression chiller that only undergoes one-stage compression of refrigerant from evaporating pressure to condensing pressure, referred to as single-stage refrigerator. The single-stage refrigerator is composed of a compressor, a condenser, a throttling mechanism and an evaporator, etc. (Figure 2 [Working principle of single-stage vapor compression refrigerator]). The high-pressure vapor discharged from the compressor releases heat through the condenser and condenses into a liquid. Next, the liquid refrigerant is throttled by a throttle valve (expansion valve), the pressure and temperature are reduced simultaneously, enter the evaporator, absorb the heat of the refrigerant (use it to re-cool the cooled object) and evaporate into vapor. Then, the vapor enters the compressor and continues to compress, so the cycle is endless. In order to improve economy, some single-stage refrigerators are also equipped with a supercooler and a regenerator after the condenser. The evaporation temperature of a single-stage refrigerator is usually between -30 and 5 Â° C.
â‘¡ Two-stage vapor compression refrigerator: the refrigerant from the evaporation pressure to the condensation pressure needs to undergo two-stage compression of the vapor refrigerator (Figure 3 [the working principle of the two-stage vapor compression refrigerator]). It has one more compressor, one intercooler and throttle valve than the single-stage refrigerator. The refrigerant vapor compressed by the high-pressure compressor is condensed into a liquid in the condenser and then divided into two paths: one way enters the intermediate condenser through the throttle valve A to cool the exhaust of the low-pressure compressor and the liquid in the coil. The refrigerant vapor evaporated in the intermediate condenser enters the high-pressure compressor together with the exhaust of the low-pressure compressor to continue compression; the other way is cooled in the coil and throttled to the evaporating pressure through the throttle valve B, and enters the evaporator to evaporate and cool. The vaporized vapor enters a low-pressure compressor and is compressed to an intermediate pressure, and enters an intermediate condenser. Compared with the single-stage refrigerator, the two-stage refrigerator can reach a lower evaporation temperature, usually between -30 ~ -70 â„ƒ.
â‘¢ Cascade refrigerator: two (or several) single-stage or two-stage compressed vapor compression refrigerators with different refrigerants as the working medium, and a composite refrigerator connected by a condensing evaporator. A condensing evaporator is a heat exchanger that uses the evaporation of high-temperature refrigerant to condense low-temperature refrigerant. Cascade refrigerator can reach very low evaporation temperature. Figure 4 [Working principle of cascade refrigerator] is the working principle of cascade refrigerator composed of two single-stage refrigerators. Its high temperature stage is composed of high temperature compressor, condenser, throttle valve and condensing evaporator; low temperature stage is composed of low temperature compressor, condensing evaporator, regenerator, throttle valve and evaporator. The high temperature class and the low temperature class are each a single-stage refrigerator. The condensing evaporator connects the high temperature level and the low temperature level: for the high temperature level, it is an evaporator; for the low temperature level, it is a condenser. The condensing evaporator transforms the heat output of the low temperature level into the cooling capacity of the high temperature level. In the low-temperature stage, usually use a refrigerant with a lower boiling point (such as R13). After the shutdown, the refrigerant will fully vaporize and cause excessive pressure increase. In order to prevent this phenomenon, a balance vessel is usually installed in the low-temperature system.
When cascaded with two single-stage refrigerators, the evaporation temperature of the low-temperature stage is generally -40 ï½ž -80 â„ƒ. When a single-stage refrigerator is cascaded with a two-stage refrigerator, the evaporation temperature can be as low as -110 â„ƒ; if ternary (such as R22, R13 and R14) is cascaded, the evaporation temperature can be as low as -140 â„ƒ.
Main equipment of vapor compression refrigerator The main equipment of vapor compression refrigerator includes compressor, condenser, evaporator, condensing evaporator and throttling mechanism.
Compressors Refrigeration compressors are available in reciprocating, centrifugal and screw types. The working principle and overall structure of the refrigeration compressor are basically the same as those of other compressors, but according to the requirements of the refrigerator, the structure has the following characteristics: â‘ High sealing requirements, and no leakage inward or outward is allowed. Therefore, large and medium-sized refrigeration compressors are equipped with shaft seals where the main shaft extends out of the body, and small refrigeration compressors are made semi-closed or fully enclosed. The semi-hermetic compressor usually integrates the machine body and the motor casing, or connects the two with a flange. The fully enclosed type is also limited to small reciprocating compressors and rolling rotor compressors. A sealed steel casing is used to seal the compressor and the motor, and it is generally not dismantled. â‘¡ Freon can be dissolved in lubricating oil, so it is often equipped with a heater in the oil tank of the crankcase. Some screw compressors and rolling rotor compressors use the oil injection method to achieve internal sealing and cooling. In addition to the oil injection device, a high-efficiency oil separator is also provided. â‘¢The compressor sucks saturated vapor. Ammonia gas is easy to carry liquid, so the reciprocating ammonia compressor is equipped with a safety cover to prevent liquid impact. â‘£When multi-stage compression, the flow rate of each stage is different, so most of the multi-stage centrifugal compressors and screw compressors are equipped with an intermediate air supplement system, which is equipped with a power saver to improve the economic operation of the refrigerator.
The condenser takes away the heat of the refrigerant by the surrounding medium, where the refrigerant is cooled and condensed into a liquid. There are two types of condensers: water-cooled and air-cooled. â‘ Water-cooled condenser is the most widely used type of condenser, used in medium and large-scale refrigerators. Water-cooled condensers are: shell and tube type, refrigerant condenses outside the tube (see tube and shell heat exchanger); sleeve type, refrigerant condenses in the tube cavity (see sleeve type heat exchanger); spray type , The refrigerant condenses in the tube (see coil heat exchanger); in the evaporative type, the refrigerant condenses in the tube, the water is sprayed outside the tube, and air blows over the surface of the tube. The shell-and-tube structure is relatively compact, with good heat transfer effect and wide application. Shell-and-tube condensers used in Freon refrigerators are often equipped with fins outside the tube to enhance heat transfer due to the low heat release coefficient of Freon condensation. â‘¡Air-cooled condensers are mostly used in small Freon refrigerators, and are divided into two types: forced air convection (fan blast) and natural convection. The former has better heat transfer effect. The air-cooled condensers are all made into a coiled tube type, the refrigerant is condensed in the tube, and the tube is provided with fins.
The evaporator absorbs the heat of the object to be cooled directly or indirectly (through the refrigerant) by evaporation of the refrigerant. The evaporator can be divided into: an evaporator for cooling liquid for indirect cooling; an evaporator for cooling air for direct cooling. The liquid cooling evaporator is used to cool the refrigerant. Commonly used are: shell and tube type, refrigerant evaporates outside the tube; immersion type, refrigerant evaporates inside the tube; dry evaporator, etc. Shell-and-tube evaporators used in Freon refrigerators are generally equipped with fins outside the tubes to enhance heat transfer. The structure of the dry evaporator is similar to the tube and shell type, but the refrigerant evaporates in the tube, and a smooth tube or an inner fin tube can be used, and the heat transfer effect is good. The evaporator for cooling air is often made into a snake tube type, the tube is covered with fins, the air flows forcibly outside the tube, and the refrigerant evaporates inside the tube. This combination of evaporator and blower is called an air cooler. In addition, the free-air convection coil evaporator, that is, the cooling exhaust pipe, is often used in refrigeration equipment (see refrigeration equipment).
The condensing evaporator is only used in cascade refrigerators. It relies on the evaporation of the refrigerant in the high temperature stage to condense the refrigerant vapor in the low temperature stage. It is both a high temperature stage evaporator and a low temperature stage condenser. The condensing evaporator has a tube type, a tube winding type and a tube shell type, etc., all of which adopt the method of evaporation inside the tube and condensation outside the tube.
Throttling mechanism A control mechanism that reduces refrigerant liquid from condensing pressure to evaporating pressure. It can control the amount of liquid supply at the same time. The throttle mechanism includes a throttle valve, a floating ball regulating valve, a thermal expansion valve, a capillary tube and a throttle orifice plate.
â‘ Throttle valve: It is a manually operated control valve. Turning the hand wheel can change the channel section of the valve and control the amount of liquid supply. It is often used in larger refrigerators.
â‘¡ Float valve: an automatic control valve. It relies on the change of the refrigerant liquid level in the evaporator or intercooler, and changes the channel cross section of the valve through the action of the floating ball and the transmission mechanism. It is only used in ammonia refrigerators.
â‘¢ Thermal expansion valve: an automatic control valve that changes the cross-section of the channel depending on the superheat of the refrigerant vapor at the outlet of the evaporator (Figure 5 [Structure of thermal expansion valve]). The thermal expansion valve is installed at the inlet of the evaporator, and the temperature sensor is placed on the outlet pipe of the evaporator. The temperature sensitive bag is filled with temperature sensitive working fluid (refrigerant or other gas, liquid). When the liquid supply of the evaporator is too small, the superheat of the vapor at the outlet of the evaporator increases, and the temperature and pressure of the temperature-sensitive working fluid increase. Increase; on the contrary, when the liquid supply volume is too large, the steam superheat at the outlet of the evaporator becomes smaller, the valve channel automatically becomes smaller, and the liquid supply volume decreases accordingly. Water pushes the adjusting lever under the valve to adjust the superheat of steam. Most thermal expansion valves are used in Freon refrigerators.
â‘£ Capillary tube: It is also called throttling tube. It is usually an elongated copper tube with a length of 0.1 to 2 meters and an inner diameter of 1 to 3 mm. It is used in small refrigerators and air conditioning equipment.
â‘¤ Throttling orifice plate: used in some centrifugal refrigerators, divided into single orifice plate and perforated plate (several orifice plates work in series), etc. The circulation section of the orifice plate cannot be adjusted, and is only applicable to refrigerators with stable working conditions.
Performance and applications The main performance indicators of compression chillers are working temperature (condensation temperature and evaporation temperature), cooling capacity, power and cooling coefficient. The refrigerating capacity and power of the compression refrigerator mainly depend on the structural size and speed of the compressor, and also vary with the operating temperature. Table [Characteristics and applications of compression refrigerators] are the characteristics and applications of various compression refrigerators.
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