Heat rejected at the condenser
All the heat removed at the evaporator together with all the energy provided by the compressor must be rejected from the system. This heat is rejected at the condenser and is expressed by
Qc = (h2 — h3) (9.8)
Heat rejection is possible, in accordance with the second law of thermodynamics, by arranging that the temperature of the condensing refrigerant is greater than the temperature of the air or water used for cooling purposes. For the examples considered earlier, the condensing temperature was 35°C and heat could conveniently be rejected to cooling water or air at a temperature of about 25°C to 30°C.
The higher the temperature (and hence pressure) chosen for condensing, the greater will be the work done in compression and hence the greater the compressor power. Referring to Figures 9.2 and 9.3 it is seen that choosing a higher condensing pressure will move the state 2 along a line of constant entropy further into the superheated zone and will increase its enthalpy.
Calculate the heat rejected, and the rate of heat rejection, at the condenser used for examples 9.2 and 9.3.
See Figure 9.3. From examples 9.2 and 9.3, /z3 is 248.94 kJ kg’1 and h2 is 421.64 kJ kg-1. Hence, by equation (9.8), the heat rejected at the condenser is:
Qc = (421.64 — 248.94) = 172.7 kJ kg“1
Alternatively, the energy used for compression could be added to the refrigerating effect:
Qc = 22.96 + 149.74 = 172.7 kJ kg"1
Fig. 9.3 Pressure-enthalpy diagram for examples 9.3 and 9.4.
The rate of heat rejection at the condenser is determined by
2c = me (9-9)
Hence, in this example,
Qc = 2.351 x 172.7 = 406 kW Alternatively, the compressor power could be added to the refrigeration duty:
Qc = 53.98 + 352 = 405.98 = 406 kW
Posted in Engineering Fifth Edition