Question

(d) A counter flow heat exchanger is used to heat air entering at 400°C with a flow rate 07
of 6 kg/s by exhaust gas entering at 800°C with a flow rate of 4 kg/s. The overall
heat transfer coefficient is 100 W/mʻK and outlet temperature of the air is 551.5°C.
The specific heat at constant pressure for both air and exhaust gas can be taken as
1100 J/kgK. Calculate:
(i) Heat transfer area needed,
(ii) Number of transfer units.​

Answer 1

A counterflow heat exchanger is used to heat air entering at 400° C with a flow rate of 6kg/s by exhaust gases entering at 800°C at a rate of 4 kg/s. The heat transfer area required is 48.056 m^2. The Number of transfer Units is 1.092

1) For a counterflow heat exchanger,

(800 - T) * 4 = (551.6 - 400) * 6

So, we get T = 572.6°C.

2) Now, we calculate logarithmic mean temperature θm = 208.20 °C

3) Now, we know Q = m Cp dT = U A θm

So,

100 * A * 208.2 = 4 * 1100 * (800 - 572.6)

So, A = 48.056 m^2

4) For a heat exchanger we know,

NTU = UA/Cmin

So, NTU = 100 * 48.056 / ( 4 * 1100)

Therefore NTU = 1.092