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The surface area A of heat exchangers required for a given service is determined from

where

Q	= rate of heat transfer
U	= mean overall heat transfer coefficient
	= mean temperature difference

For a given heat transfer service with known mass flow rates and inlet and outlet temperatures the determination of Q is straightforward and can be easily calculated if a flow arrangement is selected (e.g. logarithmic mean temperature difference for pure countercurrent or cocurrent flow). This is different for the overall heat transfer coefficient U. The determination of U is often tedious and needs data not yet available in preliminary stages of the design. Therefore, typical values of U are useful for quickly estimating the required surface area. The literature has many tabulations of such typical coefficients for commercial heat transfer services. Following is a table with values for different applications and heat exchanger types. More values can be found in the sources given below.

The ranges given in the table are an indication for the order of magnitude. Lower values are for unfavorable conditions such as lower flow velocities, higher viscosities, and additional fouling resistances. Higher values are for more favorable conditions. Coefficients of actual equipment may be smaller or larger than the values listed. Note that the values should not be used as a replacement of rigorous methods for the final design of heat exchangers, although they may serve as a useful check on the results obtained by these methods.

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