Heat Transfer Operation Questions and Answers Part-3

1. If in a double pipe heat exchanger, we require to have a constant wall temperature, we use it in a counter-flow direction.
a) True
b) False

Answer: b
Explanation: When we want to have a constant wall temperature in double pipe HE, parallel flow is a better choice of operation over counter-flow as in the latter the equipment experiences varying temperature regimes.

2. Which is the major mean of heat transfer in a Double Pipe heat exchanger?
a) Convection
b) Conduction
c) Radiation
d) Combined Convection and Conduction

Answer: d
Explanation: In a double pipe HE, the fluid is in a constant uniform motion inside the pipes, it is the phenomena of forced convection that removes the fluid from the wall by dynamic mixing which increases the overall bulk temperature of the fluid. The separating wall is the heat transfer medium that carries heat from the hot fluid to the cold fluid through conduction.

3. Consider we have a Double Pipe Heat Exchanger, with the inner tube of diameter 20mm (neglect thickness) and outer tube of diameter 30mm. We have two fluids A & B, we desire to have their flow rates as 15m3/s and 21m3/s respectively. Which fluid pass arrangement would be best suited for the HE?
a) Parallel flow with A in inner pipe and B in the annulus
b) Counterflow with B in inner pipe and A in the annulus
c) Counterflow with A in inner pipe and B in the annulus
d) Parallel flow with B in inner pipe and A in the annulus

Answer: c
Explanation: When we are required to provide the best arrangement for a double pipe HE, we consider to keep the fluid with higher flow rate on the side which has larger flow area, this helps to reduce the pressure drop for the longevity of the equipment.
Flow Area in Annulus = π(Do2 – Di2)/4 = 392.7mm2, larger area hence fluid B
Flow Area in inner pipe = π(Di2)/4 = 314.2mm2, Fluid A.

4. How many times do we have to calculate for Nusselt number in a Double Pipe Heat Exchanger?
a) 1
b) 2
c) 3
d) 4

Answer: b
Explanation: For a double pipe HE, we have to calculate Nusselt number twice, once for the inner pipe to calculate Convective Heat Transfer Coefficient for the inner pipe. Similarly, the second time for the annulus side.

5. In an operation when we want to heat a stream of liquid by Steam, we usually keep steam on the annular side and fluid on the inner side.
a) True
b) False

Answer: a
Explanation: It is best suited when we keep the steam on the annular side because it is easier and mechanically more feasible to remove condensate from the Annular side as film condensation decreases heat transfer coefficient.

6. Which of the following is not a subset of the category of Tubular Heat Exchangers?
a) Double pipe
b) Finned pipe
c) Shell and Tube
d) Spiral tube

Answer: b
Explanation: Finned type heat exchangers belong to the category of extended surface heat exchangers whereas tubular type HE covers double pipe, shell and tube and spiral tube HE.

7. In an operation where we want to heat a stream of liquid by Steam, we have the option to use extended fins. Then which of the following is best suited?
a) Steam on the annular side with the fins on the cold liquid side
b) Steam on the annular side with the fins on the steam side
c) Steam on the inner side with the fins on the cold liquid side
d) Steam on the inner side with the fins on the steam side

Answer: b
Explanation: We keep steam on the annular side to withstand high flow rate, and fins on the gas side to increase its heat transfer coefficient as it is usually low on the gas side.

8. When gas is used as a fluid in a double pipe heat exchanger, which one of the following is not true?
a) The gas side has a low heat transfer coefficient
b) Extended fins are used on the gas side to increase the Heat Transfer coefficient
c) The gas side heat transfer coefficient is the highest
d) Fins increase necessary heat transfer area

Answer: c
Explanation: Fins are generally used to increase the Heat transfer Area when the heat transfer coefficient on that fluid side is comparatively low. Hence as gases usually have lower heat transfer coefficients, fins are kept on this side.

9. Consider we have a Double pipe Heat Exchanger, with inner tube of diameter 20mm (neglect thickness) and outer tube of diameter 30mm. We have two fluids A & B (KA = 15W/mK and KB = 20W/mK) we desire to have their flow rates as 15m3/s and 21m3/s respectively. If their Nusselt number after calculation is Nui = 429 and Nuo = 530. What is the overall heat transfer coefficient of the equipment is no fouling exists?
a) 168.4 W/m2K
b) 168.4×103 KW/m2K
c) 188.4 W/m2K
d) 168.4 KW/m2K

Answer: d
Explanation: Here we have Nui = 429 = hi×ID/KA as A is in the inner tube for lower flow rate owing to smaller flow area, and Nuo = 530 = ho×OD/KB. We have ID = 20mm, OD = 30mm, KA = 15W/mK and KB= 20W/mK. Hence hi = 321.75 KW/m2K and ho=353.33 KW/m2K. As H = (hi-1+ho-1)-1 = 168.4 KW/m2K.

10. For the calculation of Overall Heat transfer coefficient for fouling factor R, which one of the following expression do we use?
a) (hi-1+ho-1 + R-1)
b) (hi-1+ho-1 + R)-1
c) (hi + ho + R)-1
d) (hi-1+ho-1 + R-1)-1

Answer: d
Explanation: When a heat exchanger is used with a fouling liquid, it leaves traces/deposits on the surface of the separating wall which reduces the overall heat transfer coefficient of the HE. This extra factor which reduces it is specified as the dirt factor (R), which is represented as H = (hi-1+ho-1 + R-1)-1.