Heat transfer coefficients can also be considered as reciprocals to heat transfer resistances. The overall resistance is the sum of the individual resistances illustrated in Figure 200-2.

Individual inside and outside film and fouling resistances are customarily referred to the tube surface at which they occur, whereas the overall resistance and overall heat transfer coefficient are customarily referred to the outside tube surface. Individual resistances are therefore added as follows:

1/U = (1/hi + Rfi)(Ao/Ai) + Rw + 1/ho + Rfo

(Eq. 200-5)

where:

hi = Inside film coefficient, Btu/hr × °F × ft2

Rfi = Inside fouling resistance, hr × °F × ft2/Btu

Ao = Outside area, ft2

Ai = Inside area, ft2

Rw = Tube wall resistance, hr × °F × ft2/Btu

ho = Outside film coefficient, Btu/hr × °F × ft2

Rfo = Outside fouling resistance, hr × °F × ft2/Btu

The tube wall resistance is given by:

where:

do = Tube O.D., inches

di = Tube I.D., inches

kw = Tube wall thermal conductivity, Btu/hr × °F × ft

Estimation of film coefficients and the factors that control them are discussed below.

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