Problem 4.12 - Home Air Conditioner and Hot Water System with an Internal Heat Exchanger

We wish to do a preliminary thermodynamic evaluation of the following proposed heat pump system designed for summertime hot water heating and space cooling.


Notice that this system uses a heat exchanger to subcool the refrigerant at the outlet of the hot water heater while heating the refrigerant at the outlet of the evaporator. This is intended to serve the dual purpose of increasing the both the hot water heating and air cooling capacity of the system

.Using the conditions shown on the diagram

• a) Neatly sketch the complete air conditioning cycle above on the P-h diagram provided, showing clearly all six processes (1) - (2) - (3) - (4) - (5) - (6) - (1).

• b) Using the R134a property tables determine the enthalpies at all six stations and verify and indicate their values on the P-h diagram. Note that in order to determine the enthalpy at stations (4) and (5) you will need to consider the heat transferred in the heat exchanger as well as the energy equation applied to an adiabatic throttle. [h ₅ = h ₄ = 107.3 kJ/kg]

• c) Determine the mass flow rate of the refrigerant R134a. [0.023 kg/s]

• d) Determine the power absorbed by the hot water heater [4.41 kW] and the Coefficient of Performance of the hot water heater [COP _HW = 4.41] (defined as the heat absorbed by the hot water divided by the work done on the compressor). Determine the time taken for 120 liters of water at 30°C to reach the required temperature of 50°C. [38 minutes]

• e) Determine the heat transferred from the air blowing through the insulated cooling duct to the evaporator [3.41 kW] , and the Coefficient of Performance [COP _AC = 3.41] of the air conditioning system.

• f) Consider the air passing through the insulated duct of the evaporator section. Notice that the temperature of the air passing through the duct is decreased by 10°C. Neglecting the work done by the fan determine the mass flow rate of the air through the duct [20.4 kg/min] .

• g) Discuss the advantages of the above heat pump system over other means of space cooling and water heating.