A cold storage plant is used to cool 9000 litres of milk per hour from 27 to 4°C
ID: 2086498 • Letter: A
Question
A cold storage plant is used to cool 9000 litres of milk per hour from 27 to 4°C, and the heat leakage into the plant is estimated to be 60 kW. The refrigerant used is ammonia and the temperature required in the evaporator is -6°C. The compressor delivery pressure is 10.34 bar and the condenser liquid is undercooled to 24 °C before throttling. The plant has a brine circulating system and the rise in temperature of the brine is to be limited to 3 K. Assuming that the vapour is dry saturated on leaving the evaporator and that the compression process is isentropic, calculate: (i) the power input required taking the mechanical efficiency of the compressor as 90%; (ii) the swept volume of each cylinder of the twin-cylinder, single-acting compressor, 14.6 for which the volumetric efficiency can be taken as 85% at a rotational speed of 200 rev/min; (ii) the rate at which the brine must be circulated in litres per second. For milk: specific heat capacity, 3.77 kJ/kg K; density, 1030 kg/m3. For brine: specific heat capacity, 2.93 kJ/kg K; density, 1190 kg/m3. (42.7 kW; 0.0157 m3; 27.1 litres/s)Explanation / Answer
data given:- 9000 litres milk temp. drop 270C to 40C in one hour, heat leakage=60 kw dt=230C
total heat to be extracted=m.c.dt=1030*9*3.77*23=803801.7 kj
heat extracted per second=223.278 kw
total heat extracted per second=223.278+60=283.27825 kw
refrigerant used is ammonia
evaporator temp= -60C = 267 k
condenser temp =27+3= 300C(condenser temp has to be a little higher to excahnge heat with brine soln for circulation)
COP=(267/36)=7.41
ideal work input= heat extraction rate/cop= 283.27825/7.41= 38.229 kw
actual work requirement=38.229/0.9 =42.47 kw
for brine requirement
heat extraction rate= m*c*dt
v= 283.27825/(1190*2.93*3)
0.0271 m3/s or 27.1 litres
for swept vol data about refrigerant is not given. just get specific heat values and specific volume and use the formula for efficiency.