Remembering How and When to Use Heat
Exchanger Effectiveness
Calculating heat
exchanger effectiveness allows engineers to predict how a given heat exchanger will
perform a new job. Essentially, it helps engineers predict the stream outlet
temperatures without a trial-and-error solution that would otherwise be necessary.
Heat exchanger effectiveness is defined as the ratio of the actual amount of heat
transferred to the maximum possible amount of heat that could be transferred with an
infinite area.
Two common methods are used to calculate the effectiveness, equations and
graphical. The equations are shown below:
Often times, another variable is defined called the NTU
(number of transfer units):
NTU = UA/Cmin
When NTU is placed into the effectiveness equations and
they are plotted, you can construct the plots shown below which are more often used than
the equations:
Then, by calculating the Cmin/Cmax and
the NTU, the effectiveness can be read from these charts. Once the effectiveness has
been found, the heat load is calculated by:
Q = Effectiveness x Cmin x (Hot Temperature in - Cold
Temperature in)
and the outlet temperatures can be calculated by:
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