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Heat Exchanger Calculations

Two basic formulas are used in all heat exchanger calculations. These are:

Heat transfer Btu/hr = lbs/hr x sp ht x temp. change °F

In the case of water, the specific heat can usually be considered as 1.0 and the formula expressed as:

Btu/hr = flow rate in GPM x 500 x temp. change °F (1)

 where:
500 = 8.33 lbs/gal x 60 min/hr conversion to lbs/hr
Heat transfer in Btu/hr = (U)(A)(LMTD) (2)

where:
U = overall heat transfer coefficient
A = heat transfer area in sq. ft.
LMTD = log mean temperature difference
= (large temp. diff.) - (small temp. diff.)

 all divided by:
the natural log of (large temp. diff - small temp. diff)

The range of the heat transfer coefficient "U" varies widely, depending upon the physical conditions involved, such as the solution viscosities, surface fouling, fluid velocities and turbulence, constant or change of state, and metal resistance.

For example, water-to-water heat exchangers typically have "U" value ranges like:

These can be contrasted to "U" values of 4 to 8 for air-to-air or steam-to-air heat transfer, which is why these are usually plate-fin coils with very large surface areas.

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