21-04-2012, 05:01 PM
U in Heat Exchangers
U for typ Heat Exchangers.pdf (Size: 286.09 KB / Downloads: 96)
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.