The following schematic shows a standard refrigeration
system with typical property values. Notice that the same system
can be used either for a refrigerator or air conditioner, in which
the heat absorbed in the evaporator (qevap)
is the desired output, or for a heat pump, in which the heat rejected
in the condenser (qcond) is the desired
output.
In this example we wish to evaluate the following:
As with the Steam Power Plant, we find that we can solve each component of this system separately and independently of all the other components, always using the same approach and the same basic equations. We first use the information given in the above schematic to plot the four processes 1-2-3-4-1 on the P-h diagram. Notice that the fluid entering and exiting the condenser (State 2 to State 3) is at the high pressure 1 MPa. The fluid enters the evaporator at State 4 as a saturated mixture at -20°C and exits the evaporator at State 1 as a saturated vapor. State 2 is given by the intersection of 1 MPa and 70°C in the superheated region. State 3 is seen to be in the subcooled liquid region at 30°C, since the saturation temperature at 1 MPa is about 40°C. The process 3-4 is a vertical line (h3 = h4) as is discussed below.
We will cover in class the methods of evaluating
the solution of this exercise using the refrigerant R134a tables.
Notice that the refrigerant tables do not include the subcooled
region, however since the constant temperature line in this region
is essentially vertical, we use the saturated liquid value of
enthalpy at that temperature.
The usefulness of plotting the system processes on the P-h diagram is that we can then estimate the performance of this system by inspection to a high degree of accuracy, thus allowing a convenient check of our calculations. The approach follows:
Thus the P-h diagram is a widely used and very useful tool for doing an approximate evaluation of a refrigerator or heat pump system. In fact, in the official Reference Handbook supplied by the NCEES to be used in the Fundamentals of Engineering exam, only the P-h diagram is presented for R134a. You are expected to answer all the questions on this subject based on plotting the cycle on this diagram as shown above.
