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Heat Transfer - Engineering

Q1:

Log mean temperature difference (LMTD) cannot be used, if

A heat transfer co-efficient over the entire heat exchanger is not constant.

B there exists an unsteady state.

C the heat capacity is not constant and there is a phase change.

D none of these.

ANS:D - none of these.

Log Mean Temperature Difference (LMTD) method cannot be used if the heat transfer coefficient over the entire heat exchanger is not constant. LMTD method assumes that the heat transfer coefficient remains constant throughout the heat exchanger. If the heat transfer coefficient varies significantly along the length or across the surface of the heat exchanger, the LMTD method becomes inaccurate. Therefore, the correct option is: heat transfer coefficient over the entire heat exchanger is not constant.