Refrigeration & Equipment Engineering

Unit 9 - Heat load calculation

9.5.1.Example - 1

1. 20 kg of water at 25ºC is raised to 80 ºC Calculate the heat required. Take the specific heat of water = 4.19KJ/kgok
Solution:

Weight of the water m = 20 kg
Initial temperature T1 = 25ºC
Final temperature T2 = 80ºC
specific heat of water c = 4.19KJ/kgok
Q or H = m c ΔT
= m c (T₂-T₁)
= 20 x 4.19 x (80-25)
= 4609 KJ

2. At a flow rate of 0.2m³/sec. If the water is cooled from 39^oC to 2^oC. Calculate the heat energy removed.
Solution:
Mass flow rate of water = 0.2x1000=200 kg/sec.
ΔT=T2-T1 = 2-39 = -37^oC
Q or H = m c ΔT
= 200x4.19x(-37)
= -31006 KJ/Sec.

Here T₂ is less than T₁. The answer obtained is negative indicating the heat is rejected.

3. 10kg of water at 0^oC is frozen into ice at 0^oC Calculate the heat removed. Take latent heat of fusion of water under atmospheric pressure is 335KJ/kg.
QL = m x hf
= 10 x 335
= 3350 kj
(Latent heat of fusion of water under atmospheric pressure is 335kj/kg)

4. Calculate the quantity of heat required to melt 12lb of ice at 32^oF into water at 32^oF. The latent heat of fusion of water under atmospheric pressure is 144 Btu per pound
QL = m x hf
= 12 x 144
= 1728 Btu

Note:Since 12lb of ice absorb 1728 Btu in melting into water,it follows that 12lb of water at 32^oF will reject 1728 Btu in returning to the solid state.

5.A refrigerating machine preserve food at -7ºC temperature at the room temperature of 23^oC find C.O.P.
T₁ = -7+273 = 266^oK
T₂ = 23 + 273 = 296^oK
T₁
(C.O.P.)R = -------------
T₂ – T₁
266
= --------------- = 8.86
296 – 266

6. A machine working on a carnot cycle operates between 305K and 260K. Determine C.O.P. when it is operated as
1. a refrigerating machine
2. Heat pump
3. heat engine
T₁ = 260^oK
T₂ = 305^oK

1. C.O.P. of a refrigerating machine
T₁
(C.O.P)R = --------------- = 5.78
T₂ – T₁

2. C.O.P. of a heat pump
T₂
(C.O.P)P = --------------- = 6.78
T₂ – T₁

3. C.O.P. of a heat engine
T₂ – T₁
(C.O.P)E = ---------------- = 0.147
T₂