Question

In a vapor-compression refrigeration cycle, ammonia exits the evaporator as saturated vapor at -22°C. The refrigerant enters the condenser at 16 bar and 160°C, and saturated liquid exits at 16 bar. There is no significant heat transfer between the compressor and its surroundings, and the refrigerant passes through the evaporator with a negligible change in pressure. If the refrigerating capacity is 150 kW, determine: (a) the mass flow rate of the refrigerant, in kg/s. (b) the power input to the compressor, in kW. (c) the coefficient of performance. (d) the isentropic compressor efficiency, in percent. (e) the rate of entropy production, in kW/K, for the compressor.

Answer 1

Sol: Given Tq = -22°¢ P2 = 16 bar = P3 53 = 160°Ć Refrigerating capacity, RC = 150kw gi a pn h Process 1-2. Tsentropic compression • Process 2-3 : Constant pressure heat rejetin 1. process 3-4 : Iso-enthalpic expansion • process 4-1: Constant pressure heat absorption (a) the may flow rate of the refrigerant, in ! + Kg/sec. > Refrigerating capacity, RC= Refrigerating effect (RE) X mars flow rate (m) i mass flove rate (m)= RC = 150 kofsec 6 - RE RE. R.E. = ha-ha

From Ammonia R77 table. at temperature, T1=-225, 62 = 1578.1 kule from graph, hq=hz at Pz = 16 bar, h2=538:74kttle =h4 at P₂ = 16 bar, b₂ = 1633.3 kilg : from egn. @ mass flowrrate (n) = 150 kit sec = 150 kotsec (1578.1 - 538.74) kotieg rate (my ha bt I see = 150 10]9.36 Lim = 0.144 kg/sec) (b) Power input Pin = work done x in = (by-hi) x m _=(1633.3-1573-1) X0.144 = 7.94 kJ fec. (c) cop = RC 150 kr (see Pin 794 lofsee ..mer inform 218.89. (e) The rate of entropy production, in kolk, for the compressor.

) A Rate of entropy production in kolk, , for compressor - Vapour entropy (kJ/kg =) 'x may flow rate (m) = 5.6159 X 0.144 At P₂ = 16 bar ] = 0.8086 kW/K Vapour entropying kilik = 5.6159

Inorder to solve problem, the specific enthalpy values have been taken from Ammonia R717 table.

The T-s and P-h diagram of Vapour compression refrigeration system have been provided for better understanding.