1. 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.
2. 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.
3. Gases cannot be used in Double pipe heat exchangers because of their high flow rates.
a) True
b) False
Explanation: In a double pipe heat exchanger we can use any type of fluid for the purpose, be it a gas or a liquid, their flow rate does not matter until it crosses the pressure limit.
4. If we know the output and input temperatures of the heat exchanging fluids, then which one of the following calculation is not required to determine the number of bends in tube for the equipment?
a) Pressure drop in the equipment
b) Overall heat transfer coefficients
c) Total heat transfer area required
d) Pipe length
Explanation: The pressure drop is calculated only to determine whether the equipment would work for a given flow rate and not the temperature requirements
5. Which of the following has the maximum Log mean temperature difference for a Double Pipe Heat Exchanger?
a) Counter-flow
b) Parallel Flow
c) Cross Flow
d) Split Flow
Explanation: The mean temperature difference for a counter-flow operation is uniform throughout the length of the tube and hence gives a higher value to the LMTD.
6. We can use mean temperature difference too in place of LMTD if we can calculate it.
a) True
b) False
Explanation: We can use mean temperature difference in place of LMTD but it is impossible to continuously measure the temperature difference along the whole length of the tube of the HE. Hence we use LMTD for a better approximation.
7. How many types of flow operations exist for a double pipe heat exchanger?
a) 1
b) 2
c) 3
d) 4
Explanation: There exist only two possible flow operations for a double pipe heat exchanger which are Counter-flow and Parallel flow operations.
8. Which one of the following is the determining reason for heat transfer in double pipe HE?
a) Conduction
b) Natural Convection
c) Forced Convection
d) Radiation
Explanation: Forced convection is the major means of heat transfer taking place in the HE. In a double pipe HE, the fluid is in a constant uniform motion inside the pipes, it is the phenomena of forced convection that removes the fluid from the wall by dynamic mixing which increases the overall bulk temperature of the fluid.
9. Which one of these is not true when the steady state is reached by the heat exchanging fluids in a double pipe HE?
a) When the two liquids have same temperature
b) When their temperatures become stable
c) Wall temperature becomes constant
d) Rate of heat transfer becomes constant
Explanation: When the steady state is reached by the fluids in the HE, their temperatures become stable but not necessarily equal, as the heat transfer rate becomes constant, heat is still entering the tubes.
10. We can apply LMTD only when?
(i) There is no change in Specific heats
(ii) Overall heat transfer coefficient is constant
(iii) No heat loss
(iv) No pressure drop
a) (ii)(iii)(iv)
b) (i)(ii)(iii)
c) (ii)(iii)
d) (i)(ii)(iv)
Explanation: The LMTD has limitations on its applicability which usually are no change in specific heats overall heat transfer coefficient is constant and no heat loss, but it is totally independent on the pressure drop that takes place in the equipment.