Question  set 162

Design Considerations For Fins

1.      The following factors need consideration for the optimum design of fins
(i) Cost
(ii) Space considerations
(iii) Weight considerations
Choose the correct option
a) i only
b) i and ii only
c) i, ii and iii
d) ii only
Answer: c
Explanation: The design will be considered optimum when the fins require minimum cost of manufacture are light in weight.

 

2.      A heating unit is made in the form of a vertical tube of 50 mm outside diameter and 1.2 m height. The tube is fitted with 20 steel fins of rectangular section with height 40 mm and thickness 2.5 mm. The temperature at the base of fin is 75 degree Celsius, the surrounding air temperature is 20 degree Celsius and the heat transfer coefficient between the fin as well as the tube surface and the surrounding air is 9.5 W/m2 K. If thermal conductivity of the fin material is 55 W/m K, find the amount of heat transferred from the tube with fin
a) 1234 .98 W
b) 1004.84 W
c) 6539.83 W
d) 3829.46 W
Answer: b
Explanation: Heat flow rate convicted from the base, Q b = h A b (t 0 – t INFINITY) and heat flow rate convicted from the fins, Q f = n k A C m (t 0– t a).

 

3.      The fins would be effective for heat conduction if the ratio P k/h A C is
a) Greater than 5
b) Less than 5
c) Equal to 5
d) Varies between 2 to 9
Answer: a
Explanation: The ratio perimeter divided by area multiply by length must be greater than 5.

 

4.      Consider the following statements pertaining to heat transfer through fins
(i) They must be arranged at right angles to the direction of flow of working fluid
(ii) The temperature along the fin is variable and accordingly heat transfer rate varies along the fin elements
(iii) Fins are equally effective irrespective whether they are on the hot side or cold side of the fluid
(iv) Fins are made of materials that have thermal conductivity higher than that of wall
Identify the correct statements
a) iii and iv
b) i and iv
c) ii and iii
d) i and ii
Answer: d
Explanation: The statements made at serial number 3 and 4 are wrong. Fins are located on the side where the convective coefficient has a low value.

 

5.      An increase in fin effectiveness is caused by a high value of
(i) Convective coefficient
(ii) Thermal conductivity
(iii) Circumference
(iv) Area
Identify the correct statement
a) i and iii
b) iii and iv
c) ii and iv
d) ii and iii
Answer: c
Explanation: Refer to the expression for fin effectiveness, an increase in fin effectiveness is caused by high value of circumference and thermal conductivity.

 

6.      A steel strap is serving as a support for the steam pipe. The strap is welded to the pipe and bolted to the ceiling. The junction between the support strut and the ceiling is adiabatic, and the outside temperature of steam pipe is 105 degree Celsius. The strut AB is 60 cm high and AD = BC = 12.5 cm. It is 0.3 cm thick. Workout the rate at which heat is lost to the surrounding air by the support strut. It may be assumed that thermal conductivity for steel is 45 W/m degree, the total outside surface coefficient is 17 W/m2 degree and the surrounding air is at 32 degree Celsius

a) 178 W
b) 168 W
c) 158 W
d) 148 W
Answer: a
Explanation: α x/α 0 = t x – t a/t 0 – t a = cos m (l – x)/cos ml.

 

7.      Choose the correct option regarding fin efficiency and fin effectiveness
a) 2 Fin effectiveness = A FIN/A B (Efficiency of fin)
b) 3 Fin effectiveness = A FIN/A B (Efficiency of fin)
c) Fin effectiveness = A FIN/A B (Efficiency of fin)
d) ½ Fin effectiveness = A FIN/A B (Efficiency of fin)
Answer: c
Explanation: On simplify the equations of fin efficiency and fin effectiveness we get the result.

 

8.      The handle of a saucepan, 30 cm long and 2 cm in diameter, is subjected to 100 degree Celsius temperature during a certain cooking operation. The average unit surface conductance over the handle surface is 7.35 W/m2 degree in the kitchen air at 24 degree Celsius. The cook is likely to grasp the last 10 cm of the handle and hence the temperature in this region should not exceed 38 degree Celsius. What should be the thermal conductivity of the handle material to accomplish it? The handle may be treated as a fin insulated at the tip
a) 18.36 W/m degree
b) 17.36 W/m degree
c) 16.36 W/m degree
d) 15.36 W/m degree
Answer: b
Explanation: α x/α 0 = t x –ta/t 0 –t a= cos m (l – x)/cos ml. Now, for a circular handle of diameter d, P/A = 4/d.

 

9.      Let us assume a square section fin split longitudinally and used as two fins. This will result in
a) Increase or decrease in heat transfer depending on the material of fin
b) Heat flow remains constant
c) Decrease in heat transfer
d) Increase in heat transfer
Answer: d
Explanation: Heat transfer will definitely increases because it split into two fins i.e. more surface area.

 

10.  Mark the false statement regarding effectiveness of fin
a) A high value of film coefficient adversely affects the fin effectiveness
b) Fin effectiveness is improved if the fin is made from a material of low conductivity
c) Fin effectiveness is improved by having thin fins
d) It can also be improved by having closely spaced fins
Answer: b
Explanation: It should be of high thermal conductivity.