Vapour Compression Systems

The vapor compression system consists of the following main stages (Fig. 2.1 and 2.2):

1. Step 1-2-3: Absorption of heat by the liquid refrigerant and conversion to gas in the evaporator. The refrigerant is a substance of low boiling on the required operating pressure. The surface of the evaporator in contact with air or water or any other liquid or substance that may be cooling. (From 1 to 2, the liquid refrigerant absorbs the latent heat energy at constant pressure and temperature, becoming saturated vapour without increasing the temperature from 2 to 3, it absorbs heat at constant pressure to become superheated steam with some increase in temperature. Overheating is necessary to prevent ingress of liquid refrigerant in the compressor).

2. Step 3-4: Compression of low temperature, low pressure refrigerant gas from the evaporator in the conditions of high temperature and gas pressure at the compressor inlet. (From 3 to 4, the reduction in the vapour volume during compression with pressure increase and Joule equivalent work done during the compression, the temperature of steam at a level higher than the ambient temperature).

3. Step 4-5-6-7: Denial of heat in the condenser, resulting in condensation of gaseous refrigerant in the liquid at high pressure. Condenser surface is cooled by moving air or water. (4 to 5, steam temperature drops when the overheating is removed and saturated vapour; from 5 to 6, saturated steam is condensed back into a liquid phase at constant temperature and pressure, from 6-7 supercooled liquid below the saturation temperature).

4. Step 7-1: the Expansion of the liquid refrigerant from the condenser high pressure, low pressure in the evaporator via a throttle valve control device is one of the expansion valve. Outlet of expansion valve control to turn on power regulation. (No heat, alteration or destruction during this process, but some flash gas is formed as a very small portion of the liquid refrigerant evaporates between the expansion valve and the evaporator. To reduce the flash gas condensers sometimes give more heat transfer area for sub-cooling of charge or for a very small amount of refrigerant separately decomposed in sub-cooler for cooling of liquid refrigerant before expansion valve.)

The intensity of heat transfer in the evaporator affects the refrigerant temperature of the suction and pressure. Environmental conditions and the intensity of heat transfer in the condenser affect discharge temperature and pressure. Suction and discharge pressure, that is, the degree of compression largely decides the extent of the work of the compressor and therefore the power consumption of the compressor.

As the compressor discharge and suction pressures are regulated heat transfer in the evaporator and condenser, the heat transfer, the effectiveness of these heat exchangers and control of refrigerant mass flow through the system through the expansion valve and performancer compressor on partial loads plays an important role in determining the overall operating efficiency of the system of vapour compression...