# (Detailed solution) GATE ME | Heat Transfer | Heat Exchangers questions

## GATE ME questions on Heat Exchangers

Question #1

Steam in the condenser of a thermal power plant is to be condensed at a temperature of 30 oC with cooling water which enters the tubes of the condenser at 14 oC and exits at 22 oC. The total surface area of the tubes is 50 m 2, and the overall heat transfer coefficient is 2000 W/m K. The heat transfer (in MW) to the condenser is ______ (correct to two decimal places)

[GATE 2018 S2, 2 marks]

Question #2

Saturated steam at 100°C condenses on the outside of a tube. Cold fluid enters the tube at 20° C and exists at 50°C. The value of the Log Mean Temperature Difference (LMTD) is ________°C.

[GATE 2017 S1, 1 mark]

Question #3

In a counter-flow heat exchanger, water is heated at the rate of 1.5kg/s from 40°C to 80°C by an oil entering at 120°C and leaving at 60°C. The specific heats of water and oil are 4.2 kJ/kgK and 2 kJ/kgK respectively. The overall heat transfer coefficient is 400 W/m K. The required heat transfer surface area (in m 2) is
(A) 0.104
(B) 0.022
(C) 10.4
(D) 21.84

[GATE 2017 S2, 2 marks]

Question #4

For a heat exchanger, ΔTmax is the maximum temperature difference and $\Delta T_{min}$ is the minimum
temperature difference between the two fluids. LMTD is the log mean temperature difference. $C_{min}$ and $C_{max}$ are the minimum and the maximum heat capacity rates. The maximum possible heat
transfer ($Q_{max}$) between the two fluids is

(A)$C_{min}LMTD$

(B)$C_{min}\Delta T_{max}$

(C)$C_{max}\Delta T_{min}$

[GATE 2016 S3, 1 mark]

Question #5

Consider a parallel-flow heat exchanger with area 𝐴p and a counter-flow heat exchanger with area $A_c$ . In both the heat exchangers, the hot stream flowing at 1 kg/s cools from 80 oC to 50 oC. For the
cold stream in both the heat exchangers, the flow rate and the inlet temperature are 2 kg/s and
10 oC, respectively. The hot and cold streams in both the heat exchangers are of the same fluid.
Also, both the heat exchangers have the same overall heat transfer coefficient. The ratio $A_c/A_p$
is _________

[GATE 2016 S2, 2 marks]

Question #6

A balanced counterflow heat exchanger has a surface area of 20 m2 and overall heat transfer coefficient of 20 W/m2-K. Air (Cp=1000 J/kg-K) entering at 0.4 kg/s and 280 K is to be preheated by the air leaving the system at 0.4 kg/s and 300 K. The outlet temperature (in K) of the preheated air is
(A) 290
(B) 300
(C) 320
(D) 350

[GATE 2015 S2, 2 marks]

Question #7

Saturated vapor is condensed to saturated liquid in a condenser. The heat capacity ratio is $C_r\frac{C_{min}}{C_{max}}$. The effectiveness $\epsilon$ of the condenser is

(a) $\frac{1-exp[-NTU(1+C_r)]}{1+C_r}$

(b) $\frac{1-exp[-NTU(1-C_r)]}{1-C_rexp[-NTU(1-C_r)]}$

(c) $\frac{NTU}{1+NTU}$

(d)$1-exp(-NTU)$

[GATE 2015 S3, 1 mark]