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
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:
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.