Problem 4.10 - A Warehouse Chiller and Office Heater

We wish to do a preliminary thermodynamic evaluation of the following proposed heat pump system designed for chilling a large warehouse to around 4°C while heating the office space to 20°C.. The basic idea for this system was inspired by an article in the ammonia21.com website on Sobey's refrigerated warehouse in eastern Canada which was honored with the ASHRAE 2010 Technology Award for having built a similar system using ammonia as the refrigerant.

Notice that our proposed system using refrigerant R134a includes an internal heat exchanger to subcool the refrigerant at the outlet of the office space heater while superheating the refrigerant at the outlet of the evaporator. This is intended to serve the dual purpose of increasing both the heating and chilling capacity of the system.
Using the conditions shown on the diagram

• a) Neatly sketch the complete chiller/heat pump 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.

• c) Determine the mass flow rate of the refrigerant R134a. [0.23 kg/s]

• d) Determine the power absorbed by office space heater [kW] and the Coefficient of Performance of the office space heater (COPHP) (defined as the heat rejected by the condenser to the office air divided by the work done on the compressor). [49 kW, COPHP = 4.9]

• e) Determine the heat transferred from the air blowing through the insulated chilling duct to the evaporator [kW], and the Coefficient of Performance (COPAC) of the air chilling system. [39 kW, COPAC = 3.9]

• f) Consider the air passing through the insulated duct of the space chiller section. Assuming that the temperature of the air passing through the duct is decreased by 10°C, and neglecting the work done by the fan determine the mass flow rate of the air through the duct. [233 kg/min]

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

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