Results/Discussions

a) Following the instruction given in the Appendix, construct the cycles representing the system at the two operating conditions on the attached p-h diagram.

Figure 2: T-S graph

From the enthalpies of the state points in the refrigeration cycles:

b(i) Calculate the work done by the compressor and compare it with the electrical input into the compressor

From figure 3, it was observed that the power supplied to the compressor is always greater than the work done by compressor. The refrigerant was compressed from state 1 to 2 in an irreversible process as heat was lost to the surrounding, thus explaining for the loss of energy in the system to surrounding.

Figure 3: Excerpt from table 2 – energy balance at compressor

b(ii) Calculate the heat rejected by the refrigerant at the condenser and compare it with the heat gained by the condenser coolant.

Figure 4: Excerpt from table 2 – energy balance at condenser

From figure 4, we can see that the heat rejected by refrigerant at condenser is always greater than the heat gained by condenser cooling water. This can be explained as the heat transferring process was non-ideal, i.e. some heat was lost to the surrounding during the course of the experiment.

In both cases, explain the discrepancies:

Temperature fluctuations during the experiment result in inaccurate data readings and thus result in erroneous calculations. This can be explained by the fact that the thermistor is a highly sensitive device. The website below gives a detailed explanation of the thermistor’s sensitivity:
www.omega.com/temperature/Z/pdf/z036-040.pdf

c) Determine the coefficients of performance COPc, COPh, COPc,o and COPh,o for the two sets of readings. Discuss the differences in the coefficients of performance.

The calculated results are tabulated below.

Figure 5: Excerpt from table 2 – Coefficients of performance

Firstly, it was observed that COPc is always smaller than COPh. COPc is essentially the measurement of how much heat is extracted in by the refrigerant whereas COPh is the measurement of how much heat is extracted out of the refrigerant.
Secondly, COPc,o and COPh,o are smaller than COPc and COPh. The loss in heat dissipation in converting electrical energy supplied by the compressor motor to the shaft of the compressor explains for the differences

Lastly, the COPs for result 1 is greater than that of result 2. By partially blocking the evaporator outlet, condenser water flow rate was reduced. This means that lesser heat was transferred from the cooler reservoir. Hence, the thermal efficiency of the refrigerator is reduced.