Basic design procedure and theory

The general equation for heat transfer across a surface is:

Q = UA Δtm                                                                                           (eq – 1.7)

Where Q = heat transferred per unit time (W)

U = the overall heat transfer coefficient (W/m2  oC)

A –heat-transfer area, m2,

Δtm = the mean temperature difference, the temperature driving force, oC.

The overall coefficient is the reciprocal of the overall resistance to heat transfer, which is the sum of several individual resistances, is given by:

1 = 1 + 1   + (doIn(do/di)) + do >< 1 + do >< 1                                 (eq – 1.8)

Uo  ho   hod               2kw           di       hid   di       hi

Where   Uo = the overall coefficient based on the outside area of the tube, W/m2  oC,

ho = outside fluid film coefficient, W/m2  oC,

hi = inside fluid film coefficent, W/m2  oC,

hod = outside dirt coefficient (fouling factor), W/m2  oC,

hid = inside dirt coefficient, W/m2  oC,

kw = thermal conductivity of the tube wall material, W/m2  oC,

di = tube inside diameter, m,

do= tube outside diameter, m.

The magnitude of the individual coefficients will depend on the nature of the transfer process (conduction, convection, condensation, boiling or radiation), on the physical properties of fluids, on the fluid flow-rates, and on the physical arrangement of the heat transfer surface