Question set 157
Fins
1. A very long copper rod 20 mm in diameter extends horizontally from a plane heated wall maintained at 100 degree Celsius. The surface of the rod is exposed to an air environment at 20 degree Celsius with convective heat transfer coefficient of 8.5 W/m2 degree. Workout the heat loss if the thermal conductivity of copper is 400 W/m degree
a) 10.71 W
b) 20.71 W
c) 30.71 W
d) 40.71 W
Answer: b
Explanation: P/A = 4/d and m = (h P/k A) ½ = (4 h/k d) ½ = 2.061.per meter.
2. Common applications of finned surfaces are with
(i) Electrical motors
(ii) Economizers for steam power plant
(iii) Convectors for steam and cold water heating systems
(iv) Cooling coils
Identify the correct option
a) i, ii and iv
b) i, ii and iii
c) i, ii, iii and iv
d) i and ii
Answer: a
Explanation: It should be for hot water heating systems.
3. The extended surface used for the enhancement of heat dissipation is
a) Convective coefficient
b) Fourier number
c) Fin
d) No finned surface
Answer: c
Explanation: The surface area exposed to the surroundings is frequently increased by the attachment of protrusions to the surfaces, and the arrangement provides a means by which heat transfer rate can be improved.
4. It is said that fins can take a variety of forms
(i) Longitudinal fins of rectangular cross section attached to a wall
(ii) Cylindrical tubes with radial fins
(iii) Conical rod protruding from a wall
Identify the correct statement
a) i only
b) i and ii
c) ii and iii
d) i, ii and iii
Answer: d
Explanation: Option b is also known as annular fins.
5. A steel rod (k = 30 W/m degree) 1 cm in diameter and 5 cm long protrudes from a wall which is maintained at 10 degree Celsius. The rod is insulated at its tip and is exposed to an environment with h = 50 W/m2 degree and t a = 30 degree Celsius. Calculate the fin efficiency
a) 56.57%
b) 66.57%
c) 76.57%
d) 86.57%
Answer: b
Explanation: Fin efficiency = tan h ml/ml, where m = (h P/k A) ½ = 25.82 per meter.
6. If the fin is sufficiently thin, so heat flows pertain to
a) One dimensional heat conduction
b) Two dimensional heat conduction
c) Three dimensional heat conduction
d) No heat flow is there
Answer: a
Explanation: As, δ is less than b, so one dimensional heat conduction is there.
7. If heat dissipation for one fin is given by 377.45 k J/hour, then what is the heat dissipation for 12 fins?
a) 7529.4 k J/hour
b) 6529.4 k J/hour
c) 5529.4 k J/hour
d) 4529.4 k J/hour
Answer: d
Explanation: For 12 fins, the heat dissipation will be equal to 12 (377.45) = 4529.4 k J/hour.
8. In order to achieve maximum heat dissipation, the fin should be designed in such a way that has a
a) Maximum lateral surface towards the tip side of fin
b) Minimum lateral surface near the center line
c) Maximum lateral surface at the root side of fin
d) Maximum lateral surface near the center of fin
Answer: c
Explanation: Fins are so designed that lateral surface at the root side of the fin is maximum. This aspect results into higher heat dissipation.
9. A steel rod (k = 30 W/m degree) 1 cm in diameter and 5 cm long protrudes from a wall which is maintained at 10 degree Celsius. The rod is insulated at its tip and is exposed to an environment with h = 50 W/m2 degree and t a = 30 degree Celsius. Calculate the rate of heat dissipation
a) 2.658 W
b) 3.658 W
c) 4.658 W
d) 5.658 W
Answer: b
Explanation: Q = k A m tan h ml (t 0 – t a) = 3.658 W.
10. On a heat transfer surface, fins are provided to
a) Increase turbulence in flow for enhancing heat transfer
b) Increase temperature gradient so as to enhance heat transfer
c) Pressure drop of the fluid should be minimized
d) Surface area is maximum to promote the rate of heat transfer
Answer: d
Explanation: Fins are provided to a heat exchanger surface to augment the heat transfer by increasing the surface area exposed to the surroundings.