Alternatively, the work rate required per ton, in Btu/h, could be calculated as:

Pressure-Enthalpy Chart

Figure 33-3 illustrates a mechanically driven refrigeration cycle on a pressure versus enthalpy (p-h) chart. The p-h chart is divided into three general areas by the saturated liquid line and the saturated vapor line. The area to the left of the saturated liquid line is called the subcooled region, the area to the right of the saturated vapor line is called the superheated region, and the area between the saturated liquid and saturated vapor lines is called the liquid vapor mixture, or wet, region. Due to the shape of this region, it is also sometimes called the vapor dome. If, for example, refrigerant at point A on the saturated liquid line absorbs heat with no change in pressure, evaporation will take place and its enthalpy will increase. Evaporation would be complete at point B on the saturated vapor line. Any additional heat absorbed at constant pressure would move the refrigerant into the superheat region, as shown by point C.

In the figure, arrows indicate the four parts of the refrigeration cycle described below:

• Compression occurs from F to H. Notice that the heat content of the vapor increases and that the mechanical work expended on the refrigerant vapor to increase its pressure also superheats it. In the ideal cycle, compression occurs at constant entropy.

• Heat rejection occurs in the condenser from H to J. First, superheat is removed and then condensing occurs. In practice, there will be some pressure drop between H and J and the heat rejection process extends further to the left than point J into the sub-cooled region as some subcooling occurs in the condenser.

• Expansion, or throttling, occurs from J to D as refrigerant pressure is reduced from condenser pressure to evaporator pressure. The vertical line indicates that this occurs at constant enthalpy.

• In the evaporation process, the refrigerant is shown to enter the evaporator as a liquid-vapor mixture (inside the vapor dome). As the refrigerant flows through the evaporator, it absorbs heat and is completely vaporized at point E. In practice, some superheating takes place in the evaporator and some as the refrigerant flows to the compressor. This superheating is indicated from E to F.

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