A heat exchanger is used to heat cold water at 15°C entering at a
A heat exchanger is used to heat cold water at 15°C entering at a rate of 2 kg/s by hot air at 85°C entering at a rate of 3 kg/s. The heat exchanger is not insulated and is losing heat at a rate of 25 kJ/s. If the exit temperature of hot air is 20°C, then which of the following is incorrect?
A. The exit temperature of cold water is closest to 38°C
B. The exit temperature of cold water is closest to 21°C
C. The exit temperature of cold water is closest to 28°C
D. The exit temperature of cold water is closest to 35°C
Please scroll down to see the correct answer and solution guide.
Right Answer is:
SOLUTION
Concept:
By steady flow energy equation
Q – W = ΔH
Calculation:
Given:
ṁair = 3, ṁwater = 2, Cpw = 4.2, CPa = 1.005
W = 0; Q = - 25 kJ/s (since heat is lost)
Now,
Q – W = ṁwaterCpw(Tho - Thi) + ṁairCpa(Tco - Tci)
- 25 = ṁairCPa × [20 - 85] + ṁwater × Cpw × [T – 15]
- 25 = 3 × 1.005 × (-65) + 2 × 4.2 × (T – 15)
∴ T = 35°C