INTERVIEW SHORT QUESTION ANSWERS-HEAT EXCHANGERS-1         Introduction HT          Fin HT       Q.A. HT     Q.A.HEX 2


Q. Explain multi-pass heat exchangers? When are these used?

These heat exchangers are of three types.

1. Firstly multi-pass shell heat exchanger

In these, fluid passes through more than one shell and the other fluid passes through a single tube pass, it is multi-shell pass heat exchanger.

2. Secondly multi-tube pass heat exchanger

when fluid passes through tubes (U-tubes) more than once and only once through a shell, it is multi tube pass heat exchanger.

3. Thirdly multi shell-multi tube pass heat exchanger

Here one fluid passes through tubes (normally U-tubes) more than once while the other fluid passes through shell more than one, it is called multi shell-multi-tube pass heat exchanger.

Example    2:4 multi-pass heat exchanger

First figure represents the number of passes in the shell.

Second figure represents the number of passes in the tube.

These multi-pass heat exchangers are used to increase the rate of heat transfer economically.

Q  Describe a ‘header’? In which type of  heat exchanger are these used?

There are two headers in a shell and tubes type heat exchanger. One each is placed at each end. These supply the fluid in the tubes on one end and collect the fluid at the other end.

Q. What are baffles? Why and in which type of heat exchangers are these used?

Baffles are used in shell and tubes heat exchangers. Baffles are thin plates inserted in the shell at regular intervals. These baffles are arbitrary partitions and support the tubes. Baffle plate diameters are of shorter size than the inside diameter of the shell and hence leave the passage for flow in the shell across the tubes. Due to the baffles, the flow becomes ‘cross flow’ across the tubes between two baffles in the shell and thus the rate of heat transfer increases.

Q . Justify when one fluid is undergoing phase change, the direction of flow is immaterial for finding LMTD.

When one of the fluids is undergoing phase change, its temperature remains constant. The two end temperature differences in parallel and counter flow will be same. Therefore, the LMTD will remain the same. Hence during phase change of a fluid, direction of flow makes no difference in the rate of heat transfer.