SIMPLE VAPOUR COMPRESSION CYCLE
The vapour compression cycle is used for refrigeration in preference to gas cycles; making use of the latent heat enables a far larger quantity of heat to be extracted for a given refrigerant mass flow rate. This makes the equipment as compact as possible.
A liquid boils and condenses — the change between the liquid and the gaseous states — at a temperature which depends on its pressure, within the limits of its freezing point and critical temperature (see Figure 2.2). In boiling it must obtain the latent heat of evaporation and in condensing the latent heat is given up.
Figure 2.2 Evaporation and condensation of a fluid
Heat is put into the fluid at the lower temperature and pressure thus providing the latent heat to make it vaporize. The vapour is then mechanically compressed to a higher pressure and a corresponding saturation temperature at which its latent heat can be rejected so that it changes back to a liquid. The cycle is shown in Figure 2.3 . The cooling effect is the heat transferred to the working fluid in the evaporation process, i. e. the change in enthalpy between the fluid entering and the vapour leaving the evaporator.
In order to study this process more closely, refrigeration engineers use a pressure-enthalpy or P-h diagram (Figure 2.4) . This diagram is a useful way of describing the liquid and gas phase of a substance. On the vertical axis is pressure,
Liquid at 35°C Condenser
Figure 2.3 Simple vapour compression cycle with pressure and enthalpy values for R134a
Figure 2.4 Pressure—enthalpy, P-h diagram, showing vapour compression cycle
P, and on the horizontal, h, enthalpy. The saturation curve defines the boundary of pure liquid and pure gas, or vapour. In the region marked vapour, the fluid is superheated vapour. In the region marked liquid, it is subcooled liquid. At pressures above the top of the curve, there is no distinction between liquid and vapour. Above this pressure the gas cannot be liquefied. This is called the critical pressure. In the region beneath the curve, there is a mixture of liquid and vapour.
The simple vapour compression cycle is superimposed on the P-h diagram in Figure 2.4. The evaporation process or vaporization of refrigerant is a
Constant pressure process and therefore it is represented by a horizontal line. In the compression process the energy used to compress the vapour turns into heat and increases its temperature and enthalpy, so that at the end of compression the vapour state is in the superheated part of the diagram and outside the saturation curve. A process in which the heat of compression raises the enthalpy of the gas is termed adiabatic compression. Before condensation can start, the vapour must be cooled. The final compression temperature is almost always above the condensation temperature as shown, and so some heat is rejected at a temperature above the condensation temperature. This represents a deviation from the ideal cycle. The actual condensation process is represented by the part of the horizontal line within the saturation curve.
When the simple vapour compression cycle is shown on the temperature- entropy diagram (Figure 2.5) , the deviations from the reversed Carnot cycle can be identified by shaded areas. The adiabatic compression process continues beyond the point where the condensing temperature is reached. The shaded triangle represents the extra work that could be avoided if the compression process changed to isothermal (i. e. at constant temperature) at this point, whereas it carries on until the condensing pressure is attained.
Figure 2.5 Temperature-entropy diagram for ideal vapour compression cycle
Expansion is a constant enthalpy process. It is drawn as a vertical line on the P-h diagram. No heat is absorbed or rejected during this expansion, the liquid just passes through a valve. Since the reduction in pressure at this valve must cause a corresponding drop in temperature, some of the fluid will flash off into vapour to remove the energy for this cooling. The volume of the working fluid therefore increases at the valve by this amount of flash gas, and gives rise to its name, the expansion valve. No attempt is made to recover energy from the expansion process, e. g. by use of a turbine. This is a second deviation from the ideal cycle. The work that could potentially be recovered is represented by the shaded rectangle in Figure 2.5.
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