Heat Exchanger

Question 1
Saturated vapor at 200^{\circ}C condenses to saturated liquid at the rate of 150 kg/s on the shell side of a heat exchanger (enthalpy of condensation h_{fg} = 2400 kJ/kg). A fluid with C_p = 4 kJ\dot kg^{-1} \dot K^{-1} enters at 100^{\circ}C on the tube side. If the effectiveness of the heat exchanger is 0.9, then the mass flow rate of the fluid in the tube side is ____ kg/s (in integer).
A
825
B
1126
C
1000
D
1256
GATE ME 2022 SET-2   Heat Transfer
Question 1 Explanation: 


Hot fluid
\begin{aligned} T_{h1} &=T_{h2}=200^{\circ} C\\ \dot{m}_h &=150kg/s \\ &= h_{fg}&=2400kj/kg \end{aligned}
Cold fluid
\begin{aligned} T_{c1} &=100^{\circ} C\\ \in &=0.9\\ \dot{m}_c &= ? kg/sec \end{aligned}
If fluid is under phase change then that fluid have in finite heat capacity rate.
In this case hot fluid is under phase change so C_{max}=\infty
It means cold fluid have minimum heat capacity rate
\begin{aligned} c_{min}&=\dot{m}_c c_c\\ \in &=\frac{Q_{act}}{Q_{max}}\\ 0.9&=\frac{\dot{m}_c c_c(T_{c2}-T_{c1})}{c_{min}(T_{h1}-T_{c1})}\\ 0.9&=\frac{T_{c2}-100}{200-100}\\ T_{c2}&=190^{\circ}C\\ Q&=\dot{m}_hh_{fg}=\dot{m}_cc_c(T_{c2}-T_{c1})\\ 150 \times 2400&=\dot{m}_c \times 4 \times (190-100)\\ \dot{m}_c&=1000kg/sec \end{aligned}
Question 2
During open-heart surgery, a patient?s blood is cooled down to 25^{\circ}C from 37^{\circ}C using a concentric tube counter-flow heat exchanger. Water enters the heat exchanger at 4^{\circ}C and leaves at 18^{\circ}C . Blood flow rate during the surgery is 5 L/minute. Use the following fluid properties:
\begin{array}{|c|c|c|} \hline \text{Fluid}& \text{Density }(kg/m^3)&\text{Specific heat (J/kg-K)}\\ \hline \text{Blood}& 1050 & 3740\\ \hline \text{Water}& 1000 & 4200 \\ \hline \end{array}
Effectiveness of the heat exchanger is _________ (round off to 2 decimal places).
A
0.42
B
0.12
C
0.36
D
0.88
GATE ME 2022 SET-1   Heat Transfer
Question 2 Explanation: 


For preferred heat exchange between load and water.
\begin{aligned} Q_{blood}&=Q_{water}\\ \dot{m}_h c_h(T_{h_1}-T_{h_2})&=\dot{m}_c c_c(T_{c_2}-T_{c_1})\\ \dot{m}_h c_h(37-25)&=\dot{m}_c c_c(18-4)\\ 12\dot{m}_h c_h&=14\dot{m}_c c_c\\ \dot{m}_h c_h&=\frac{14}{12}\dot{m}_c c_c\\ \dot{m}_hc_h&=1.167\dot{m}_cc_c\\ \dot{m}_hc_h& \lt \dot{m}_cc_c\\ \dot{m}_cc_c&=c_{min}, \dot{m}_hc_h=c_{max}\\ \varepsilon &=\frac{Q_{act}}{Q_{max}}=\frac{\dot{m}_cc_c(T_{c_2}-T_{c_1})}{c_{min}(T_{h_1}-T_{c_1})}=\frac{T_{c_2}-T_{c_1}}{T_{h_1}-T_{c_1}}\\ \varepsilon &=\frac{T_{c_2}-T_{c_1}}{T_{h_1}-T_{c_1}}=\frac{18-4}{37-4}=0.424\\ \varepsilon &=0.42 \end{aligned}
Question 3
A shell and tube heat exchanger is used as a steam condenser. Coolant water enters the tube at 300 K at a rate of 100 kg/s. The overall heat transfer coefficient is 1500 W/m^2.K, and total heat transfer area is 400 m^2. Steam condenses at a saturation temperature of 350 K. Assume that the specific heat of coolant water is 4000 J/kg.K. The temperature of the coolant water coming out of the condenser is _______K (round off to the nearest integer).
A
254
B
339
C
254
D
654
GATE ME 2021 SET-2   Heat Transfer
Question 3 Explanation: 


\begin{aligned} \mathrm{NTU}&=\frac{U A}{\left(\dot{m} c_{p}\right)_{\text {small }}}=\frac{1500 \times 400}{100 \times 4000}=1.5\\ \epsilon_{H E}&=1-e^{-N T U}=\frac{T_{c e}-T_{c i}}{T_{h i}-T_{c i}}=\frac{T_{C e}-300}{350-300}=0.7768\\ T_{c e}&=338.84 \mathrm{~K} \simeq 339 \mathrm{~K} \end{aligned}
Question 4
In a concentric tube counter-flow heat exchanger, hot oil enters at 102^{\circ}C and leaves at 65^{\circ}C. Cold water enters at 25^{\circ}C and leaves at 42^{\circ}C. The log mean temperature difference (LMTD) is _______^{\circ}C (round off to one decimal place)
A
40
B
43.25
C
56.5
D
49.3
GATE ME 2020 SET-1   Heat Transfer
Question 4 Explanation: 
\begin{aligned} T_{h i}&= 102^{\circ} \mathrm{C}, \quad T_{h e}=65^{\circ} \mathrm{C} \\ T_{c, i}&=25^{\circ} \mathrm{C}, \quad T_{c e}=42^{\circ} \mathrm{C} \\ \mathrm{LMTD}&=\frac{\Delta T_{1}-\Delta T_{2}}{\ln \left(\frac{\Delta T_{1}}{\Delta T_{2}}\right)} \\ &\Delta T_{1}=102^{\circ} \mathrm{C}-42^{\circ} \mathrm{C}=60^{\circ} \mathrm{C} ; \\ & \Delta T_{2}=65^{\circ} \mathrm{C}-25^{\circ} \mathrm{C}=40^{\circ} \mathrm{C}_{7} \\ \mathrm{LMTD}&=\frac{60-40}{\ln \left(\frac{60}{40}\right)}=\frac{20}{0.4054}=49.3^{\circ} \mathrm{C} \end{aligned}
Question 5
Hot and cold fluids enter a parallel flow double tube heat exchanger at 100^{\circ}C \; and \; 15^{\circ}C, respectively. The heat capacity rates of hot and cold fluids are C_h=2000W/K and C_c=1200W/K, respectively. If the outlet temperature of the cold fluid is 45^{\circ}C, the log mean temperature difference (LMTD) of the heat exchanger is _________K (round off to two decimal places).
A
21.45
B
45.25
C
57.71
D
98.65
GATE ME 2019 SET-2   Heat Transfer
Question 5 Explanation: 


Given data:
C_{h} = 2000 W/K Energy balance:
Energy released by hot fluid = Energy received by cold fluid
\begin{aligned} \mathrm{C}_{\mathrm{h}}\left(\mathrm{T}_{\mathrm{hi}}-\mathrm{T}_{\mathrm{he}}\right) &=\mathrm{C}_{\mathrm{c}}\left(\mathrm{T}_{\mathrm{cc}}-\mathrm{T}_{\mathrm{ci}}\right) \\ 2000\left(100-\mathrm{T}_{\mathrm{hc}}\right)&=1200(45-15) \\ 2000 \times\left(100-\mathrm{T}_{\mathrm{he}}\right)& =1200 \times 30 \mathrm{T}_{\mathrm{he}}=82^{\circ} \mathrm{C} \\ \mathrm{LMTD}&=\frac{\theta_{1}-\theta_{2}}{\ln \left(\frac{\theta_{1}}{\theta_{2}}\right)} \\ &=\frac{85-37}{\ln \left(\frac{85}{37}\right)}=\frac{48}{\ln \left(\frac{85}{37}\right)} \\ \mathrm{LMTD}&=57.71^{\circ} \mathrm{C} \text{ or } \mathrm{K}\end{aligned}
This is logarithmic mean temperature difference either we take in ^{o}C or we take in K the final answer will be 57.71 K (or)^{o}C because it is a difference.
Question 6
Steam in the condenser of a thermal power plant is to be condensed at a temperature of 30^{\circ}C with cooling water which enters the tubes of the condenser at 14^{\circ}C and exits at 22^{\circ}C. The total surface area of the tubes is 50 m^{2}, and the overall heat transfer coefficient is 2000 W/m^{2} K. The heat transfer (in MW) to the condenser is ______ (correct to two decimal places).
A
0.5
B
1.16
C
2.25
D
3.45
GATE ME 2018 SET-2   Heat Transfer
Question 6 Explanation: 
Since steam is condensing, the temperature of hot fluid remains constant.
\therefore Temperature profiles of hot and cold fluids are


\therefore (\mathrm{LMTD}) of H E=\frac{\Delta T_{i}-\Delta T_{e}}{\ln \frac{\Delta T_{i}}{\Delta T_{e}}}=\frac{16-8}{\ln \left(\frac{16}{8}\right)}=11.54^{\circ} \mathrm{C}
Total heat transfer rate between steam
\begin{aligned} \mathrm{C.W.} &=Q=U A \mathrm{LMTD} \mathrm{W} \\ &=2000 \times 50 \times 11.54 \\ &=11.54 \times 10^{5} \mathrm{W} \\ &=1.154 \mathrm{MW} \end{aligned}
Question 7
In a counter-flow heat exchanger, water is heated at the rate of 1.5kg/s from 40\,^{\circ} C to 80\,^{\circ} C by an oil entering at 120\,^{\circ} C and leaving at 60\,^{\circ} C. The specific heat of water and oil are 4.2\:kJ/kg \! \cdot\!K and 2\:kJ/kg \! \cdot\!K, respectively. The overall heat transfer coefficient is 400\, W/m^{2} \cdot K The required heat transfer surface area (in m^{2} ) is
A
0.104
B
0.022
C
10.4
D
21.84
GATE ME 2017 SET-2   Heat Transfer
Question 7 Explanation: 


\begin{aligned} \Delta T_{i} &=120-80=40^{\circ} \mathrm{C} \\ \Delta T_{e} &=60-40=20^{\circ} \mathrm{C} \\ (L M T D) \text { Counterflow }&=\frac{\Delta T_{i}-\Delta T_{e}}{\ln \frac{\Delta T_{i}}{\Delta T_{e}}} =28.86^{\circ} \mathrm{C} \\ &=\frac{40-20}{\ln \frac{40}{20}} = 28.86^{o}\mathrm{C}\\ Q &=\dot{m}_{C} c_{p_{c}}\left(T_{c e}-T_{c i}\right) \\ &=1.5 \times 4.2 \times 10^{3}(80-40) \mathrm{watt} \\ Q &=252000 \mathrm{W} \\ Q &=U A \Delta T_{m} \\ A &=\frac{Q}{U \Delta T_{m}}=\frac{1.5 \times 4.2 \times 10^{3} \times 40}{400 \times 28.86} \\ &=21.83 \mathrm{m}^{2} \end{aligned}
Question 8
Saturated steam at 100^{\circ}C condenses on the outside of a tube.cold fluid enters the tube at 20^{\circ}C and exits at 50^{\circ}C the value of the log mean Temperature Difference (LMTD) is _________^{\circ}C.
A
30
B
80
C
64
D
84
GATE ME 2017 SET-1   Heat Transfer
Question 8 Explanation: 
Temperature profiles are

\begin{aligned} \Delta T_{i}=100-20=80^{\circ} \mathrm{C} \\ \Delta T_{e}=100-50=50^{\circ} \mathrm{C} \end{aligned} (LMTD) (parallel or counter HE)
\begin{aligned} =\frac{\Delta T_{i}-\Delta T_{e}}{\ln \frac{\Delta T_{i}}{\Delta T_{e}}}=\frac{80-50}{\ln \frac{80}{50}} \\ =63.82^{\circ} \mathrm{C} \end{aligned}
Question 9
For a heat exchanger, \Delta T_{max} 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} andC_{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_{max}
D
C_{max} \Delta T_{min}
GATE ME 2016 SET-3   Heat Transfer
Question 9 Explanation: 
Maximum heat transfer possible
=C_{\min } \times(\Delta T)_{\max }
Question 10
Consider a parallel-flow heat exchanger with area A_{p} 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^{\circ}C to 50^{\circ}C . For the cold stream in both the heat exchangers, the flow rate and the inlet temperature are 2 kg/s and 10^{\circ}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 _________
A
0.258
B
0.684
C
0.489
D
0.927
GATE ME 2016 SET-2   Heat Transfer
Question 10 Explanation: 
Parallel flow

A_{p} = Area of parallel flow heat exchanger
Counter flow

A_{c}= Area of counter flow heat exchanger
Heat lost by hot stream in both heat exchanger
\begin{aligned} &=m c\left[T_{h i}-T_{h o}\right] \\ &=1 \times c \times\left[T_{h i}-T_{h o}\right] \\ &=1 \times c \times[80-50]=30 \mathrm{ckW} \end{aligned}
Heat gained by cold stream in both exchange
\begin{aligned} &=m c\left[T_{co}-T_{c i}\right] \\ &=2 \times c\left[T_{c o}-10\right] \end{aligned}
Heat lost by hot stream = Heat gain by cool stream
\begin{aligned} 30c &=2c(T_\infty -10) \\ T_\infty &= \frac{30}{2}+10\\ &=15+10=25^{\circ} C\\ A_P, LMTD&= \frac{\Delta T_i-\Delta T_e}{ln(\Delta T_i/\Delta T_e)}\\ &= \frac{70-25}{ln(70/25)}=43.705^{\circ} C\\ Q_H&=UA_P 43.705 \\ A_C, LMTD&= \frac{\Delta T_i-\Delta T_e}{ln(\Delta T_i/\Delta T_e)}\\ &= \frac{55-40}{ln(55/40)}=47.103^{\circ} C\\ Q_H&=UA_C 47.103 \\ \therefore \; UA_P 43.705 &=UA_C 47.103\\ \frac{43.705}{47.103}&=\frac{A_C}{A_P}=0.927 \end{aligned}
There are 10 questions to complete.

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