Heat and Mass Transfer - Heat Exchanger
Exam Duration: 45 Mins Total Questions : 24
Which one of the following heat exchangers gives parallel straight line pattern of temperature distribution for both cold and hot fluid?
- (a)
Parallel flow with unequal heat capacities
- (b)
Counter flow with equal heat capacities
- (c)
Parallel flow with equal heat capacities
- (d)
Counter flow with unequal heat capacities
Match List I (Heat exchanger process) with List II (Temperature area diagram) and select the correct answer using the codes given below the lists.
List I | List II |
P. Counter flow sensible heating | 1. |
Q. Parallel flow sensible heating | 2. |
R. Evaporating | 3. |
S. Condensing | 4. |
5. |
- (a)
P Q R S 3 4 1 2 - (b)
P Q R S 3 2 5 1 - (c)
P Q R S 4 3 2 5 - (d)
P Q R S 4 2 1 5
In a counter flow heat exchanger, the product of specific heat and mass flow rate is same for the hot and cold fluids. If NTU is equal to 0.5, then the effectiveness of the heat exchanger is
- (a)
1.0
- (b)
0.5
- (c)
0.33
- (d)
0.2
Which one of the following diagrams correctly shows the temperature distribution for a gas to gas counter flow heat exchanger?
- (a)
- (b)
- (c)
- (d)
A condenser is designed to condense 0.76 kg/min of steam with cooling water entering at 20 oC and leaving at 65 oC. Overall heat transfer coefficient = 3400 W/m2 -K. The surface area required for this heat exchanger is (saturation temperature of steam = 95.6oC, hfgsteam = 2270 kJ/kg)
- (a)
0.17 cm2
- (b)
0.27m2
- (c)
0.17m2
- (d)
0.15m2
In a parallel flow double pipe heat exchanger water flows through the inner pipe and is heated from 20°C to 70°C. Oil flowing through the annulus is cooled from 200°C to 100 °C. It is desired to cool the a il to a lower exit temperature by increasing the length of the heat exchanger. The minimum temperature to which the oil may be cooled is
- (a)
60°C
- (b)
80°C
- (c)
70°C
- (d)
90°C
In a counter flow heat exchanger, hot fluid enters at 60°C and cold fluid leaves at 30°C. Mass flow rate of the hot fluid is 1kg/s and that the cold fluid is 2 kg/s.Specific heat of the hot fluid is 10 kJ/kg-K and that of the cold fluid is 5 kJ/kg-K. The Log Mean Temperature Difference (LMTD) for the heat exchanger in °C is
- (a)
15
- (b)
30
- (c)
35
- (d)
45
AMTD (Arithmetic Mean Temperature Difference) will be 5% higher than LMTD (Log Mean Temperature Difference) when \({\triangle T_1\over \triangle T_2}\) is equal to
- (a)
3.2
- (b)
2.2
- (c)
2.0
- (d)
3.0
For a given counter flow heat exchanger, given m1 = m2 = 1 kg/s.
Th1 = 420oC, TC1 = 20oC, C1 = 1 kJ/kg-K, C2 = 4 kJ/kg-K
If effectiveness of heat exchanger is 75%, then heat transfer rate is (in kJ)
- (a)
325
- (b)
300
- (c)
350
- (d)
270
For a given counter flow heat exchanger, given m1 = m2 = 1 kg/s.
Th1 = 420oC, TC1 = 20oC, C1 = 1 kJ/kg-K, C2 = 4 kJ/kg-K
Exit temperature To2 of fluid 2 is equal to
- (a)
85°C
- (b)
95°C
- (c)
190°C
- (d)
105°C
An oil flows into a double pipe counter flow heat exchanger at the rate of 0.4 kg/s. The inner tube has internal diameter of 2.0 cm and the inner diameter of the outer tube is 3.0 cm. The water flows in the inner tube at the rate of 0.6 kg/s with its inlet and exit temperatures as 200C and 50oC respectively. The convective heat transfer coefficient on water side is 8000 W/m2-K and on oil side of tube is 80 W/m2-K. The oil temperature at inlet is 160°C. Neglecting the thickness of both the inner and outer tubes, then
Overall heat transfer coefficient, if Cpo = 2.25 kJ/kg-K
- (a)
81.07 W/m2-K
- (b)
91.73W/m2-K
- (c)
85.89W/m2-K
- (d)
79.208W/m2-K
An oil flows into a double pipe counter flow heat exchanger at the rate of 0.4 kg/s. The inner tube has internal diameter of 2.0 cm and the inner diameter of the outer tube is 3.0 cm. The water flows in the inner tube at the rate of 0.6 kg/s with its inlet and exit temperatures as 200C and 50oC respectively. The convective heat transfer coefficient on water side is 8000 W/m2-K and on oil side of tube is 80 W/m2-K. The oil temperature at inlet is 160°C. Neglecting the thickness of both the inner and outer tubes, then
Heat transfer rate
- (a)
77.36 kW
- (b)
80.09 kW
- (c)
75.36 kW
- (d)
70.73 kW
An oil flows into a double pipe counter flow heat exchanger at the rate of 0.4 kg/s. The inner tube has internal diameter of 2.0 cm and the inner diameter of the outer tube is 3.0 cm. The water flows in the inner tube at the rate of 0.6 kg/s with its inlet and exit temperatures as 200C and 50oC respectively. The convective heat transfer coefficient on water side is 8000 W/m2-K and on oil side of tube is 80 W/m2-K. The oil temperature at inlet is 160°C. Neglecting the thickness of both the inner and outer tubes, then
Exit temperature of oil Th2 is
- (a)
70oC
- (b)
81oC
- (c)
76.26oC
- (d)
93oC
If, Th1 = 500oC, Th2 = 300oC, TC1 = 50oC, TC2 = 140oC,di = 5 cm,do = 6 cm, hi = 250 W/m2-K,ho = 400 W/m2-K.Assume thermal conductivity of tube is very large.
Overall heat transfer coefficient based on outer surface area is
- (a)
141 W/m2-K
- (b)
136.99 W/m2-K
- (c)
132 W/m2-K
- (d)
129 W/m2-K
If, Th1 = 500oC, Th2 = 300oC, TC1 = 50oC, TC2 = 140oC,di = 5 cm,do = 6 cm, hi = 250 W/m2-K,ho = 400 W/m2-K.Assume thermal conductivity of tube is very large.
Length of tube, if Q = 100 kW
- (a)
130 m
- (b)
145 m
- (c)
138 m
- (d)
149 m
If, Th1 = 500oC, Th2 = 300oC, TC1 = 50oC, TC2 = 140oC,di = 5 cm,do = 6 cm, hi = 250 W/m2-K,ho = 400 W/m2-K.Assume thermal conductivity of tube is very large.
Number of tubes, if tube length is 3 m is
- (a)
50
- (b)
45
- (c)
47
- (d)
42
In a counter flow heat exchanger, a hot fluid enters at 80°C having heat capacity Ch = 2 kJ/kg-K. Cold water enters at 20oC having heat capacity Cc = 4 kJ/kg-K. If NTU = 0.25, then exit temperature of hot fluid is
- (a)
\(T_{h_2}\cong 58^oC\)
- (b)
\(T_{h_2}\cong 50^oC\)
- (c)
\(T_{h_2}\cong 64^oC\)
- (d)
\(T_{h_2}\cong 68^oC\)
The temperature distribution Curve for a heat exchanger as shown in the figure below (with usual notations) refers to which one of the following?
- (a)
Tubular parallel flow heat exchanger
- (b)
Tube in tube counter flow heat exchanger
- (c)
Boiler
- (d)
Condenser
In a double pipe counter flow heat exchanger, if Ch = Cc, then temperature profiles of two fluids along the length will be
- (a)
- (b)
- (c)
- (d)
A liquid having specific heat of 3.3 kJ/kg-K flowing at the rate of 20000 kg/h enters a parallel flow heat exchanger at 100oC. The flow rate of cooling water is 50000 kg/h with an inlet temperature of 30oC.The heat transfer area is 10m2 and the overall heat transfer coefficient is 1050 W/m2-K. Take for water, specific heat = 4.186 kJ/kg-K.
Effectiveness of the heat exchanger will be
- (a)
0.2
- (b)
0.3
- (c)
0.4
- (d)
0.6
A liquid having specific heat of 3.3 kJ/kg-K flowing at the rate of 20000 kg/h enters a parallel flow heat exchanger at 100oC. The flow rate of cooling water is 50000 kg/h with an inlet temperature of 30oC.The heat transfer area is 10m2 and the overall heat transfer coefficient is 1050 W/m2-K. Take for water, specific heat = 4.186 kJ/kg-K.
The outlet temperature of chemical will be
- (a)
72oC
- (b)
76oC
- (c)
80oC
- (d)
82oC
A one ton window air-conditioner removes 3.5 kJ/s from a room and in the process rejects 4.2 kJ/s in the air-cooled condenser. The ambient temperature is 30oC whereas condensing temperature of the refrigerant is 45oC. For the condenser, the product of overall heat transfer coefficient and corresponding area is 350 W/K. The temperature rise of the air as it flows over the condenser tubes is
- (a)
35oC
- (b)
5oC
- (c)
40oC
- (d)
None of these
If Ch = Cc and NTU = 0.5, then which will be true?
- (a)
\(\varepsilon \)parallel > \(\varepsilon \)counter
- (b)
\(\varepsilon \)parallel = \(\varepsilon \)counter
- (c)
\(\varepsilon \)parallel < \(\varepsilon \)counter
- (d)
None of these
For evaporators, what will be effectiveness, if NTU = 0.5?
- (a)
\(\varepsilon_p>\varepsilon _c,\varepsilon _p=0.39\)
- (b)
\(\varepsilon_p<\varepsilon _c,\varepsilon _c=0.19\)
- (c)
\(\varepsilon_p=\varepsilon _c=0.39\)
- (d)
\(\varepsilon_p=\varepsilon _c=0.19\)