Question

A condenser is essentially a type of heat exchanger that is used to remove heat from a vapor and convert it to liquid. The picture below shows a condenser that is used in a commercial refrigerator with refrigerant-134a as the working fluid. Water enters the condenser at 18ºC at a rate of 0.25 kg/s and leaves at 26ºC. The refrigerant enters the condenser at 1.2 MPa and 50ºC and leaves at the same pressure but at T=41.3ºC. a) Find the rate at which heat is taken from the refrigerant. b) Find the mass flow rate of the refrigerant through the condenser.

Now consider the condenser as a part of a refrigeration cycle shown below. The compressor consumes 3.3 kW of power. c) Find the refrigeration load, Q L  . d) Find the COP for the refrigeration cycle.

Answer 1

heat duty of condenser= mass flow rate of water* specific heat of water* temperature difference of water = 0.25*4.184 Kj/kg.deg.c (26-18) =8.368 Kj/sec. This is also heat load of the condenser.

at the inlet conditions of 50 deg.c and 1.2Mpa, 1200 Kpa, from Refrigeration tables data, R-134a is super heated whose enthalpy = 423.822Kj/kg

at the exit conditions of 41.3 and 1.2 Mpa(same pressure, it is subcooled and enthalpy is = 258.28 Kj/kg

Enthalpy difference between inlet and outlet= 423.822-258.28 = 165.524 Kj/kg

Flow rate of Refrigerant = heat duty/specific enthalpy change =8.368/165.525 Kg/sec=0.050 kg/sec

load on the refrigerator = heat load+W = 8.368 Kw+3.30 Kw= 11.668 Kw

COP of refrigerator= Q/W= 11.668/3.3=3.535