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REFRIGERATION PRINCIPLES OF COLD PRODUCTION BASIC KNOWLEDGE PRINCIPLES OF COLD PRODUCTION Different types of cold production ABSORPTION REFRIGERATION SYSTEM illustrate e w e r e H basic different s of principle tion. uc cold prod Compression refrigeration system Cold gas refrigeration system (Stirling principle) Absorption refrigeration system Thermoelectric cold production (Peltier element) In an absorption system ammonia (NH3) is used as refrigerant. As in a compression refrigeration system, liquid ammonia evaporates in an evaporator and thereby absorbs heat. The vaporous ammonia is then dissolved in water in an absorber. The ammonia solution is heated releasing vaporous ammonia. The ammonia vapour is condensed into liquid ammonia again in a condenser. Vortex cold generator To ensure a temperature difference between evaporation and condensation, a pressure difference must be present. This difference is created by a pump for the solution or a so-called auxiliary gas (hydrogen H2). When using the auxiliary gas, a pump can be omitted and the refrigerating machine does not have any moving parts. The supply of the drive energy is purely thermal via a boiler. The boiler can be heated electrically or by gas. The use of waste heat or solar energy is also possible. COMPRESSION REFRIGERATION SYSTEM The compression refrigeration system is used most frequently for cooling. It uses the effect that during evaporation of a liquid a lot of heat is absorbed. When condensing the vapour this heat is discharged again. 1 3 4 With different pressures during evaporation and condensing the temperature levels can be adjusted in such a way that heat is transported from the cold to the hot side. Because the refrigerant circulates in a closed circuit, this is called the refrigeration cycle. Instead of the piston compressor shown, screw, scroll, turbo or steam jet compressors can also be used. As refrigerant, liquids with a low boiling point, such as fluorinated hydrocarbons (FC), propane, CO2 or NH3, are used. 2 In the most simple case a compression refrigeration system consists of an evaporator 2 in the room to be cooled 1, a compressor 3, a condenser 4 and an expansion element 5. low concentration ammonia solution hydrogen and ammonia vapour THERMOELECTRIC REFRIGERATION SYSTEM and heat, are easy to control and do not have any moving parts or liquid fillings. Col Very low temperatures can be achieved, but the efficiency reduces greatly with increasing temperature difference. COLD GAS REFRIGERATION SYSTEM The cold gas refrigeration system corresponds to the familiar Stirling motor, but the direction of rotation is reversed. This reverses the cycle flow and the power generator becomes a work machine operating as a heat pump. liquid ammonia hydrogen Thermoelectric refrigeration systems operate using the Peltier effect. The principle of function corresponds to the reversal of the function of the thermocouple known from temperature measurements. A special semiconductor material is used in the Peltier element. 5 The cold gas refrigeration system based on the Stirling principle is used to generate very low temperatures. Areas of application are the liquefaction of air or the cooling of infrared cameras. ammonia vapour high concentration ammonia solution 6 1 Ho ot sid de Peltier elements are operated with direct current; the direction of the current determines the direction of the heat transport. Therefore, Peltier elements can cool 7 2 8 3 5 4 The cold gas refrigeration system consists of an operating cylinder 6 and a displacement cylinder 7. In the operating cylinder the operating gas (usually helium) is alternately compressed and expanded. The compressed hot gas discharges its heat in the heat exchanger 4 . During the expansion the gas cools down and absorbs on the cold side 2 of the displacement cylinder heat from the room to be cooled 1. Displacement pistons 8 and operating pistons 6 are moved via a crank drive 5 with offset phases. The recuperator 3 increases the efficiency. VORTEX COLD GENERATOR The vortex cold generator is an extraordinary refrigerating machine. Here a cold and a hot air flow is generated from a compressed air flow at room temperature. The compressed air enters a vortex chamber tangentially and is moved into fast rotation. At the centre of the vortex a cold air flow forms, while the outer layer of the vortex heats up. The cold air is extracted at the centre and can be used for cooling. The benefit of this refrigerating machine is that it is of a very simple design, does not have any moving parts, no toxic working materials and does not require a power supply. The low efficiency is a disadvantage. Cold air Hot air Compressed air 11