Heat Transfer Test

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Heat Transfer Test - 6

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Question 1
4 / -1

The condensate film runs down a vertical flat surface where saturated steam is allowed to condense. Condensation’s local coefficient of heat transfer

A
Increases with increasing thickness of film

B
Increases with increasing temperature differential between the surface and vapour

C
Decreases with increasing distance from the top of the surface

D
Remains constant at all heights of the surface

Solution

It decreases with increasing thickness of condensate film.
Question 2
4 / -1

A steam condenser is transferring 250 k W of thermal energy at a condensing temperature of 65 degree Celsius. The cooling water enters the condenser at 20 degree Celsius with a flow rate of 7500 kg/hr. If overall heat transfer coefficient for the condenser surface is 1250 W/m² K, what surface area is required to handle this load?

A
7.08 m²

B
5.08 m²

C
4.08 m²

D
1.08 m²

Solution

Q = U A α _m. So, A = 7.08 m².
Question 3
4 / -1

What is the value of overall heat transfer coefficient ammonia condensers?

A
200-750 W/m²K

B
250-2500 W/m²K

C
1500-1750 W/m²K

D
800-1400 W/m²K

Solution

It is an electrical device that stores a vast amount of energy.
Question 4
4 / -1

Which of the following is having highest value of overall heat transfer coefficient?

A
Steam

B
Steam condensers

C
Feed water heaters

D
Alcohol condensers

Solution

Feed water heaters have an overall heat transfer coefficient of 8500 W/m²K, whereas steam, alcohol condensers, and ammonia condensers have heat transfer coefficients of 5000 W/m²K, 630 W/m²K and 1400 W/m²K.
Question 5
4 / -1

For laminar film condensation on a vertical plate, the local heat transfer coefficient at the lower edge of the plate is given by

A
[k ³ p² g h _{f g}/4 δ l (t _sat – t _s)] ^0.125

B
[k ³ p² g h _{f g}/4 δ l (t _sat – t _s)] ^0.1

C
[k ³ p² g h _{f g}/4 δ l (t _sat – t _s)] ^0.25

D
[k ³ p² g h _{f g}/4 δ l (t _sat – t _s)] ^0.5

Solution

The rate of condensation heat transfer is higher at the upper end of the plate than at the lower end.
Question 6
4 / -1

The outer surface of a vertical tube is 1.25 m long and outer diameter is 50 mm is exposed to saturated steam at atmospheric pressure. If the tube surface is maintained at 80 degree Celsius by the flow of cooling water through it, determine the rate of heat transfer to the coolant

A
32397 W

B
19879 W

C
48476 W

D
25234 W

Solution

h _v = 0.943 [k ³ p ² g h _{f g}/δ l (t _sat– t _s)] ^0.25 = 5046.8 W/m ² k and heat flow rate = 19879 W.
Question 7
4 / -1

What is the value of the overall heat transfer coefficient for air to heavy tars and liquid?

A
As low as 25 W/m²K

B
As low as 15 W/m²K

C
As low as 45 W/m²K

D
As low as 55 W/m²K

Solution

It is an incompressible fluid which has constant volume independent of pressure.
Question 8
4 / -1

Exhaust gases (c _P= 1.12 k J/kg K) flowing through a tubular heat exchanger at the rate of 1200 kg/hr are cooled from 400 degree Celsius to 120 degree Celsius. This cooling is affected by water (c _P= 4.18 k J/kg K) that enters the system at 10 degree Celsius at the rate of 1500 kg/hr. If the overall heat transfer coefficient is 500 k J/m² hr degree, what heat exchanger area is required to handle the load for counter flow arrangement?

A
1.758 m²

B
6.758 m²

C
8.758 m²

D
3.758 m²

Solution

m _hc _h(t _{h 1} – t _{h 2}) = m _cc _c(t _{c 2} – t _{c 1}).
Question 9
4 / -1

What is the value of the overall heat transfer coefficient for air condensers?

A
250-900 W/m²K

B
200-1950 W/m²K

C
350-780 W/m²K

D
200-350 W/m²K

Solution

It consists of an air coil which is used to remove heat from fluids.
Question 10
4 / -1

Heat transfer coefficients for natural convection across the surface of a vertical pipe and a vertical flat plate at the same height. What could be the cause(s) of this?

(i) Same height
(ii) Both vertical
(iii) Same fluid
(iv) Same fluid flow pattern

Select the correct answer

A
iii and iv

B
iv

C
i and ii

D
i

Solution

The fluids must be same so their flow pattern.

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Heat Transfer Test - 6

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Heat Transfer Test - 6

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Question 1

Increases with increasing thickness of film

Increases with increasing temperature differential between the surface and vapour

Decreases with increasing distance from the top of the surface

Remains constant at all heights of the surface

Solution

Question 2

7.08 m2

5.08 m2

4.08 m2

1.08 m2

Solution

Question 3

200-750 W/m2 K

250-2500 W/m2 K

1500-1750 W/m2 K

800-1400 W/m2 K

Solution

Question 4

Steam

Steam condensers

Feed water heaters

Alcohol condensers

Solution

Question 5

[k 3 p 2 g h f g /4 δ l (t sat – t s)] 0.125

[k 3 p 2 g h f g /4 δ l (t sat – t s)] 0.1

[k 3 p 2 g h f g /4 δ l (t sat – t s)] 0.25

[k 3 p 2 g h f g /4 δ l (t sat – t s)] 0.5

Solution

Question 6

32397 W

19879 W

48476 W

25234 W

Solution

Question 7

As low as 25 W/m2 K

As low as 15 W/m2 K

As low as 45 W/m2 K

As low as 55 W/m2 K

Solution

Question 8

1.758 m2

6.758 m2

8.758 m2

3.758 m2

Solution

Question 9

250-900 W/m2 K

200-1950 W/m2 K

350-780 W/m2 K

200-350 W/m2 K

Solution

Question 10

iii and iv

iv

i and ii

i

Solution

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7.08 m²

5.08 m²

4.08 m²

1.08 m²

200-750 W/m²K

250-2500 W/m²K

1500-1750 W/m²K

800-1400 W/m²K

[k ³ p² g h _{f g}/4 δ l (t _sat – t _s)] ^0.125

[k ³ p² g h _{f g}/4 δ l (t _sat – t _s)] ^0.1

[k ³ p² g h _{f g}/4 δ l (t _sat – t _s)] ^0.25

[k ³ p² g h _{f g}/4 δ l (t _sat – t _s)] ^0.5

As low as 25 W/m²K

As low as 15 W/m²K

As low as 45 W/m²K

As low as 55 W/m²K

1.758 m²

6.758 m²

8.758 m²

3.758 m²

250-900 W/m²K

200-1950 W/m²K

350-780 W/m²K

200-350 W/m²K