# Heat Transfer Operations Questions and Answers – Double Pipe Heat Exchangers – Dirt Factor and Overall Heat Transfer Coefficients

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This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Double Pipe Heat Exchangers – Dirt Factor and Overall Heat Transfer Coefficients”.

1. Fouling factors increase the overall heat transfer conductance and hence decrease the overall heat transfer coefficient.
a) True
b) False

Explanation: Fouling factors or dirt factors increase the overall heat transfer resistance and not the conductance which then decreases the overall heat transfer coefficient.

2. Which one of the following cannot be determined by knowing fouling factors?
a) When to clean the equipment
b) When the equipment will stop to work
c) Its value is zero for a new heat exchanger
d) Its value is infinity for an old heat exchanger

Explanation: Fouling factor for a new heat exchanger is zero but is never infinity for any equipment it must have some value even if the heat exchanger is old.

3. Which of the following methods can determine the fouling factor of a Heat Exchangers?
a) A dirty Heat Exchanger
b) A clean Heat Exchanger
c) A dirty and a clean Heat Exchanger
d) A dirty and a fully choked Heat Exchanger

Explanation: To determine the fouling factor of a heat exchanger, we need to know the heat transfer coefficient of a clean HE and of a dirty HE. As according to the following formula:
$$\frac{1}{U_D} = \frac{1}{U_C} + R_D$$
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4. How many types of readily identifiable Fouling exist?
a) 1
b) 2
c) 3
d) 4

Explanation: Fouling can be classified to four readily identifiable types which are: Chemical, Biological, Deposition and Corrosion fouling.

5. Which one of the following is not a collective class of fouling?
a) Biological fouling
b) Deposition Fouling
c) Chemical Fouling
d) Iron fouling

Explanation: Fouling can be classified to four readily identifiable types which are: Chemical, Biological, Deposition and Corrosion fouling. In the given options, Iron fouling is not a collective class as it belongs to Corrosion fouling.
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6. For the calculation of Dirty Overall Heat transfer coefficient UD, for given fouling factor Rf and Dirt Factor Rd, which one of the following expressions do we use?
a) $$\frac{1}{U_D} = \frac{1}{U_C} + R_D$$
b) $$\frac{1}{U_D} = \frac{1}{U_C} + \frac{1}{R_D}$$
c) UD = UC + RD
d) UD = $$U_C+\frac{1}{R_D}$$

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 (RD) or the fouling factor (Rf), which is represented as $$\frac{1}{U_D} = \frac{1}{U_C} + R_D$$.

7. Deposition fouling can be avoided or controlled by monitoring the flow velocity of the fluid so that sediments don’t settle out.
a) True
b) False

Explanation: Deposition fouling is the phenomenon when particles contained within the fluid settle on the surface of the heat exchanger when the velocity of the fluid falls below a critical value. This is within the control of the heat exchanger designer as we can manage the flow velocity to monitor deposits.

8. The phenomenon of scaling on the surface of a Heat Exchanger due to the hardness of water where salts get deposited on the surface is an example of ____ fouling.
a) Chemical
b) Deposition
c) Biology
d) Corrosion

Explanation: The phenomenon explained above belongs to the class of chemical fouling as scaling due to hardness of water occurs as a result of decreased solubility of the salts due to increased temperature in the heat exchanger.

9. Algal deposits on the tube surfaces of the heat exchanger belong to which class of fouling?
a) Chemical
b) Deposition
c) Biology
d) Corrosion

Explanation: The phenomenon explained above belongs to the class of biological fouling as algae are living organisms that thrive in waters with high BOD and get deposited due to increased temperature in the heat exchanger resulting in decreased BOD.

10. Which one of the following fouling phenomenon is under the control of the operator?
a) Chemical
b) Deposition
c) Biology
d) Corrosion

Explanation: Deposition fouling is the phenomenon when particles contained within the fluid settle on the surface of the heat exchanger when the velocity of the fluid falls below a critical value. This is within the control of the heat exchanger operator as we can manage the flow velocity to monitor deposits.

11. Nickel alloy based Heat exchangers are used to avoid _____ fouling.
a) Chemical
b) Deposition
c) Biology
d) Corrosion

Explanation: Corrosion fouling is the phenomenon which occurs when a particular fluid acts corrosively on the surface of the heat exchanger causing it to form a layer of non-conducting material on the equipment. Hence using alloys helps us to reduce this fouling.

12. Which one of the following fouling deposits on a heat exchanger cannot be removed by using wire brushes or high speed water jets?
a) Chemical
b) Deposition
c) Biology
d) Corrosion

Explanation: Corrosion fouling is the phenomenon which occurs when a particular fluid acts corrosively on the surface of the heat exchanger causing it to form a layer of non-conducting material on the equipment. This fouling being a permanent damage to the tube surface, using wire brushes does not help us remove it.

13. Which of the following statements are not true about fouling in a Heat Exchanger?
a) It decreases the heat transfer coefficient in both sides
b) Temperature of the hot fluid remains hot and the cold fluid remains cold
c) Pressure drop decreases
d) Efficiency decreases

Explanation: When a heat exchanger faces fouling, its heat transfer coefficient decreases, pressure drop increases due to scaling, efficiency decreases and net heat transferred too decreases.
Explanation: When a heat exchanger faces fouling, its heat transfer coefficient decreases, but the decrease is not linear. It is somewhat similar to the plot of critical thickness of insulation. Along with the coefficients, the net pressure drop increases due to scaling, efficiency decreases and net heat transferred too decreases.

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