Consider a vapor-compression refrigeration cycle using R-134a as the refrigerant

Question

Consider a vapor-compression refrigeration cycle using R-134a as the refrigerant. It is proposed to use a turbine in place of the throttling valve. The R-134a enters the compressor as a saturated vapor at 200kPa, and exits the condenser as a saturated liquid at 900 kPa. The cycle is to remove 15 kW of heat from the cold space through the evaporator. The isentropic efficiency of the compressor is 0.75. and the proposed turbine has an isentropicefficiencyof0.60. Determine the mass flow rate of R-134a needed and the coefficient of performance of the cycle as a refrigerator for the cycle using a throttling valve and the cycle using the proposed turbine in place of the throttling valve, Determine the maximum possible coefficient performance for the cycle.

Explanation / Answer

Mass flow rate = m = capacity of refregeration plant / h1 - h4

h1 at 200 kpa = 1443kj/kg from ph chart

h4 = 3660.2 kj /kg

Capacity of plant = 15 KW = 15 x60 Kj/min = 900 kj /min

capacity of plant in tons = 900 / 211 = 4.265 Tons of referegration

mass flow rate m = 900/ 1443.2 - 3660.2 = 0.46 kg /min

Coefficient of performance C.O.P = h1 -h4 / h2 - h1

From reeferegerant table h1 = 1786.4 kj /kg Delta h = 30.1 kj /kg

h2 = 1736.4 +30.1 = 1766 .4 kj /kg

COP = 1443.2-3660.2 / 1766.4-1443.2 =3.342

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