A vapour compression cycle based refrigeration plant is being developed to operate a cold storage facility on a farm.

A vapour compression cycle based refrigeration plant is being developed to operate a cold storage facility on a farm. Fresh fruits and vegetables will be stored in this cold storage room right after the harvest before they are shipped to the market. The working fluid that has been selected is the commonly used refrigerant R134a, tabulated properties of which may be found in Tables A11, A12 and A13 at the rear of the prescribed text.

The cold storage room is to be maintained at 4°C to preserve the fresh fruits and vegetables. The maximum allowed compressor delivery pressure is 1000kPa (absolute). The maximum fluid frictional pressure loss in the condenser is 10kPa and the refrigerant is allowed to be sub cooled by 3°C. The refrigerant is throttled to 280kPa (absolute) and the maximum fluid frictional pressure loss in the evaporator is 20kPa. The refrigerant is allowed to be superheated by 8°C at the exit of the evaporator. The refrigeration compressor has an isentropic efficiency of 75% x (1.0X) where X is the last digit of your student number e.g. student number 3087787, thus 1.0X becomes 1.07.

There is a food processing facility on the farm that makes jams and pickles and this facility needs 1m³/hr of hot water at (70 + 1.1 * X)°C (where X is the last digit of your student number) for processing. It is advised to develop a sustainable process water heating system that uses heat rejected by the condenser of the refrigeration plant. Ground water from a nearby bore is used as process water and the ground water is at (25+0.1*X) °C temperature. (Note: A water cooled shell and tube or plate heat exchanger can be used for the condenser.)

The various components of the refrigeration plant are to be sized such that the cold storage could be maintained at 4°C. The cold room is insulated with earth wool insulation and has an average heat gain from the surroundings of 14.X kW (where X is the last digit of your student number). 

1. Sketch a schematic of the refrigeration plant system showing each component and the connections including cold storage and food processing facility. (5 Marks) 2. For the ideal case where the compressor has 100% isentropic efficiency and there are no pressure losses in the condenser and evaporator, a. Determine the specific enthalpy of the refrigerant at the entry and exit of the compressor, and the entry and exit of the expansion valve; (10 Marks) b. Sketch and label the process paths of the cycle on a T-s diagram and P-h diagram and h-s diagram, also showing the saturated liquid and vapour lines on each diagram. (12 Marks) c. Determine, for unit mass flow rate of refrigerant circulated: i. the rate of heat absorbed at the evaporator (4 Marks) ii. the necessary mechanical power input for the compressor (4 Marks) iii. the rate of heat rejection at the condenser (4 Marks) 3. Considering now the real case, where the compressor is not isentropic and there are pressure losses in the evaporator and condenser as described above: a. Determine the specific enthalpy of the refrigerant at the entry and exit of the compressor, and the entry and exit of the expansion valve; (10 Marks) b. Sketch and label the ideal and actual process paths of the cycle on a T-s diagram, P-h diagram and h-s diagram, also showing the saturated liquid and vapour lines on each diagram. (12 Marks) c. Determine, for unit mass flow rate of refrigerant circulated, the quantities (i) to (iii) as listed above, briefly contrasting the values calculated here and those calculated in (2) (15 Marks) 4. Now for the design requirement of 14.X kW of heat gain by the cold storage, determine, for the real case: a. the necessary mass flow rate of refrigerant; (4 Marks) b. the necessary mechanical power input for the compressor (4 Marks) c. the rate of heat rejection at the condenser (4 Marks) d. the maximum achievable temperature of the process water using rejected heat. (4 Marks) e. If the exit temperature is not sufficient, calculate the additional rate of heating required to achieve desired temperature and advise ways to provide this heating. (4 Marks)