<P> For calculations involving the outer stream, the equivalent diameter (or mean hydraulic radius) is used in place of the geometric diameter, as the cross-sectional area of the annulus is not circular . Equivalent diameters are also used for irregular shapes such as rectangular and triangular ducts . For concentric tubes, this relationship simplifies to the difference between the diameters of the shell and the outer surface of the inner tube . </P> <P> D e o = 4 ⋅ A r e a W e t t e d P e r i m e t e r (\ displaystyle D_ (\ mathrm (eo)) = (\ frac (4 \ cdot Area) (WettedPerimeter))) </P> <P> After the heat transfer coefficients (h_ (i) and h_ (o)) are determined, knowing the resistance due to fouling and thermal conductivity of the boundary material (k_ (w)), the Overall Heat Transfer coefficient (U_ (o)) can be calculated . </P> <P> 1 U o . l l R f o + R f i ⋅ D o D i + D o 2 k w ⋅ ln ⁡ D o D i + 1 h o + 1 h i ⋅ D o D i (\ displaystyle (1 \ over U_ (o)). ll (R_ (fo)) + (R_ (fi)) \ cdot (\ frac (D_ (o)) (D_ (i))) + (\ frac (D_ (o)) (2k_ (w))) \ cdot \ ln (\ frac (D_ (o)) (D_ (i))) + (1 \ over h_ (o)) + (1 \ over h_ (i)) \ cdot (\ frac (D_ (o)) (D_ (i)))) </P>

Difference between concentric and shell and tube heat exchanger