Question:-
A counter flow heat exchanger is used to heat water from 20°C to 80°C by using hot exhaust gas entering at 140°C and leaving at 80°C . the log mean temperature difference for the heat exchanger is
Option (A)
80°C
Option (B)
110°C
Option(C)
60°C
Option(D)
Not determinable as zero is involved
Correct Option:
(C)
Question Solution:
We use L-hospital rule for differentiation
q1 = q2 = q
Let (q1/q2) = x
qm = ((q1-q2)/in(q1/q2))={q2(x-21)/in x}
qm = ltx®1{(q2) /(1/x)}= q2
\ qm = q2 = q1
question-answer-faq-3052