This set of Heat Transfer Operations Multiple Choice Questions & Answers (MCQs) focuses on “Temperature Enthalpy Diagrams”.

1. If the coolant is not in multiple-phase, and if the overall heat transfer coefficient is constant, then the assumptions underlying the logarithmic mean temperature difference (LMTD) __________

a) Are Valid

b) Are Invalid

c) Not all but some are valid

d) Single assumption invalid

View Answer

Explanation: For LMTD (logarithmic mean temperature difference) to be valid, the first condition is that the coolant should be in single-phase, and the overall heat transfer coefficient should be reasonably constant.

2. If the coolant is single-phase, and if the overall heat transfer coefficient is reasonably constant, then the formula for the area of the condenser can be ________

a) A=\(\dot{Q}/U \Delta T_{LMTD} \)

b) A=\(\dot{Q}/U \Delta T_{LMTD} \)

c) A=\(\dot{Q}/U \Delta T_{LMTD} \)

d) A=\(\dot{Q}/U \Delta T_{LMTD} \)

View Answer

Explanation: The correct expression is A=A=\(\dot{Q}/U \Delta T_{LMTD} \)

where the heat transfer coefficient is the averaged over value of all the zonal heat transfer coefficients and the temperature difference should be the Saturation deviation temperature of the fluid and not the LMTD.

3. With condensation occurring in fluid mixtures which might be having a non-condensing gas, the variation of the equilibrium temperature with enthalpy can be a _________ function.

a) Linear

b) Non-Linear

c) Quadratic

d) Parabolic

View Answer

Explanation: There is no specific function like the moisture isotherm-sigmoid expression for an example rather the enthalpy relation of the non-condensable gas follows a non-linear regime that cannot be represented as a definite equation.

4. The thermal design of condenser is therefore considerably ___________ than that of a single-phase heat exchanger.

a) Same

b) More complicated

c) Less complicated

d) Not relatable

View Answer

Explanation: In the thermal design of condenser, it is impossible to assign a single demanding temperature difference and an overall heat transfer coefficient to the exchanger based on it, and hence we need a zonal or stepwise calculation of the surface area which is far more complicated than a HE.

5. In condensation from mixtures, with or without a non-condensing gas, then the assumptions underlying the logarithmic mean temperature difference (LMTD) are valid.

a) True

b) False

View Answer

Explanation: In condensation from mixtures, with or without a non-condensing gas, the variation of the equilibrium temperature with enthalpy is not linear and in the thermal design of condenser, it is impossible to assign a single demanding temperature difference and an overall heat transfer coefficient to the exchanger based on it, and hence we need a zonal or stepwise calculation of the surface area which is far more complicated than a HE.

6. In condensation from mixtures, with or without a non-condensing gas, the assumptions underlying the logarithmic mean temperature difference (LMTD) are invalid, so what is the method that we can adopt to cope with this problem?

a) Zonal Distribution

b) Taking linear groups of enthalpy curves for LMTD calculations

c) Apply LMTD anyway

d) Apply average LMTD

View Answer

Explanation: In the thermal design of condenser, it is impossible to assign a single demanding temperature difference and an overall heat transfer coefficient to the exchanger based on it, and hence we need a zonal or stepwise calculation of the surface area which is far more complicated than a HE because here we are taking linear groups of enthalpy curves for LMTD calculations.

7. Which one of the following plot correctly represents the temperature-enthalpy relationship of a mixture?

a)

b)

c)

d)

View Answer

Explanation: The correct diagram is the one where the curve is non-linear for low temperature and linear for higher temperatures, hence the diagram is

8. Which one of the following statement is correct about the temperature-enthalpy relationship of a mixture?

a) Sub-cooled at the entrance

b) Sub-heated at the entrance

c) Superheated at the entrance

d) Superheated at the exit

View Answer

Explanation: The Enthalpy curve is represented as –

Here the graph does not start from the origin and is not a linear function throughout.

9. Which one of the following is the correct labelling of the temperature-enthalpy relationship curve of a mixture?

a)

b)

c)

d)

View Answer

Explanation: The coolant should always have the linear curve as it is not a mixture of non-condensable gases and hence the curve given below best represents it.

10. Which one of the following bold line represents the coolant line in the temperature-enthalpy relationship of a mixture?

a) 1

b) 2

c) 3

d) 4

View Answer

Explanation: The bold line number 1 is correct because the coolant should always have the linear curve as it is not a mixture of non-condensable gases and hence the curve given below best represents it.

11. The temperature-enthalpy relationship of a mixture represents the coolant flow in __________

a) Single phase

b) Double phase

c) Triple phase

d) Any phase possible

View Answer

Explanation: The coolant should always have the linear curve as it is not a mixture of non-condensable gases and is a single phase operating fluid, hence the curve given below best represents it.

12. Why do we divide the temperature-enthalpy relationship of a mixture into zones?

a) To calculate LMTD for each zone

b) To calculate Tsat – TLIQUID for each

c) To calculate resistance for each

d) To make the solving process easier

View Answer

Explanation: In the thermal design of condenser, it is impossible to assign a single demanding temperature difference and an overall heat transfer coefficient to the exchanger based on it, and hence we need a zonal or stepwise calculation of the surface area which is far more complicated than a HE because here we are taking linear groups of enthalpy curves for LMTD calculations.

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