Question

1. (a) The air conditioning unit of a car is operating on a summer day when the temperature is 37°C. It cools the air to 6°C using 3kW of power output. The A/C has a COP of 4. Find the mass flow rate of the cold air that the A/C unit can provide on that particular day. (15 points)

2. (b) A building freezer that is used to freeze vegetables requires 40 tons of refrigeration to keep the food frozen. The temperature of the freezer is -35°C and the ambient temperature outside the building is 30°C. If the performance of the freezer is 20% of the theoretical best possible cycle, what is the power required to run the freezer. 1 ton of refrigeration is equivalent to 210 kJ/min. (15 points

Answer 1

Part (a):

COP of refrigerator is defined as ratio of Net cooling effect produced to Work Supplied to System.

To determine mass flow rate of Air

m_f= mass flow rate of water

C = Specific heat capacity of water at constant pressure = 1.00 kJ/kg

Mass flow rate = 2 Kg Per second.

(b) Refrigeration capacity required = 40 tons of refrigeration

We know 1 ton of refrigeration = 3.5 KJ/s

therefore 40 tons of refrigeration = 40*3.5 = 140 KJ/s

Best Thoretical COP will be, if Refrigeration system works on reverse carnot cycle

T_L = -35 C = 238 K

T_H = 30 C = 303 K

Working COP = 0.2* COP_theoretical

Working COP = 0.7323

Therefore