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


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 meant by Duty of a heat exchanger?

The rate of heat to be 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 =Cmin (Thi –Tci)

q.max is the maximum rate of heat transfer

Rate of heat capacity Cmin is smaller of mh.cph and mc. cpc

hot fluid inlet temperature = Thi 

cold fluid inlet temperature = Tci

mhis mass flow rate of hot fluid

mc. is mass flow rate of cold fluid

cph is specific heat of hot fluid

cpc      is specific heat of cold fluid

Q. In which case, will the outlet temperature of a cold fluid be higher than outlet temperature of a hot fluid in a heat ex-changer?

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