## 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 ^{o}C with cooling water which enters the tubes of the condenser at 14 ^{o}C and exits at 22 ^{o}C. 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 ^{o}C to 50 ^{o}C. For the

cold stream in both the heat exchangers, the flow rate and the inlet temperature are 2 kg/s and

10 ^{o}C, 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 m^{2} and overall heat transfer coefficient of 20 W/m^{2}-K. Air (C_{p}=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]**