An ammonia refrigerating plant operates between 100 psia and

An ammonia refrigerating plant operates between 100 psia and 30 psia. The plant is required to freeze 500 pounds of water from 50F to 20F in 24 hours. Determine:

a) The rating of the plant in tons

b) Mass flow rate of the refrigerant

c) Power

d) Coefficient of Performance

Solution

Take sp heat of water as 1 BTU/lbF

and ice 0.49 BTU/lbF

Latent heat is 144 BTU/lb

add up all these for 500 lb and get heat extracted: 500*144 + 500*1*28 + 500*.49*12 in BTU

total is 88940 BTU

Per hour gives 3705.8 BTU

I ton means 12000 BTU./Hr , so the plant is rated as 0.3 ton by this defn.

b) mass flow of the ammonia refrigerant:

from tables sp enthalpy of ammonia at 100 psia and 50 F is 246.68 KJ/kg, that at 20F and 30psia is 1467.08 KJ/kg

diff is 1220 KJ/kg

Heat reqd to be removed is 3705.8 BTU./hr= 3909.8 KJ/hr

hence flow of coolant reqd is 3.2 Kg/hour or approx 7 lb per hour

Power:I KJ/hr = .2778 watt, so power of 3909.8 KJ/hr = 1086 watt = 1 KW approx.