HEAT EXCHANGERS QUESTION ANSWERS CLASS NOTES

HEAT EXCHANGERS

QUESTION ANSWERS

 CLASS NOTES

Most common method of

heat transfer is through the

use of heat exchangers. There

are many types of heat exchangers.

Finned tube heat exchangers are

most common.

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, one fluid passes through more than one shell.  The other fluid passes through a single tube pass. It is multi-shell pass heat exchanger.

2. Secondly multi-tube pass heat exchanger

when a fluid passes through tubes (U-tubes) more than once. The second fluid passes 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. The other fluid passes through more than one shell. It is 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 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  header 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 thin plates. These have number of holes in it. These are placed in the shell at regular intervals. These baffles are arbitrary partitions and support the tubes. Diameters of the baffle plates are smaller than the inside diameter of the shell.  Hence there is a passage for flow in the shell across the tubes. Due to the baffles, the flow becomes ‘cross flow’ across the tubes. Thus the rate of heat transfer increases. Shell and tubes heat exchangers use baffles.

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.

Q.  Which of the arrangement of heat exchangers (HEX) is better, (i) parallel flow,(ii) Counter flow. Explain the reasons.

Counter flow HEX is better because of the following reasons:

(i) Average temperature difference is more.

(ii) There is less variation in temperature difference all along the length of the HEX.

(iii) Higher rate of heat transfer

(iv)  Economical

Q  What is Duty of a heat exchanger?

The rate of heat transferred is the duty of a heat exchanger i.e. 50 kW duty heat exchanger.

Q .Describe plate heat exchangers.

It consists of parallel plates. Fluid flowing in alternate parallel plates. Fluid enter from same side of plates say lower side and leaves at the upper side. It is like having distributors on both sides. After flowing through say four plates the fluid enters another set of parallel plates. In these it enters at the top and leaves at the bottom. These are suitable for any single or multi-phase heat transfer.

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

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

Q . Define effectiveness of a heat exchanger.

It is a ratio of actual rate of heat transfer to the maximum rate of heat transfer

Є = q^._actual / q^._max

Where  q^._max =C_min (T_hi –T_ci)

q^._max is the maximum rate of heat transfer

Rate of heat capacity C_min is smaller of m_h^.c_ph and m_c^. c_pc

hot fluid inlet temperature = T_hi 

cold fluid inlet temperature = T_ci

m_h^. is mass flow rate of hot fluid

m_c^. is mass flow rate of cold fluid

c_ph is specific heat of hot fluid

c_pc      is specific heat of cold fluid

Q. The outlet temperature of a cold fluid is higher in a counter or parallel flow heat exchanger. Comment.

 It is possible in a counter flow heat ex-changer.

https://mesubjects.net/wp-admin/post.php?post=14136&action=edit               MCQ HEAT EXCHANGERS

https://mesubjects.net/wp-admin/post.php?post=1344&action=edit                  LMTD PARALLEL & CONUTER

https://mesubjects.net/wp-admin/post.php?post=1335&action=edit                    HEX CLASS NOTES