Question 2.10: Use of LMTD to rate and size a heat exchanger. Calculate the...

An Introduction to Mechanical Engineering

Use of LMTD to rate and size a heat exchanger.

Calculate the LMTD for parallel- and counter-flow cases that have the same inlet and outlet stream temperatures. The cold enters at 20°C, and exits at 40°C.

2.56

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Referring to Figure 2.56, the temperature profile is distinctly different in the counterflow and parallel-flow operation. In the parallel case,

LMTD = \frac{(80  –  10)}{[\ln (80/10)]}

LMTD = 33.7°C

and in the counter flow case,

LMTD = \frac{(60  –  30)}{[\ln 60/30]}

LMTD = 43.3°C

Note: The parallel arrangement has the lower LMTD – therefore needs a larger surface area for the same heat transfer, i.e. a larger heat exchanger. This shows the advantage of the counter-flow arrangement.

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