The general equation for heat transfer across a surface is:

Q = UA Δt_m                                                                                           (eq – 1.7)

Where Q = heat transferred per unit time (W)

            U = the overall heat transfer coefficient (W/m^{2  o}C)

            A –heat-transfer area, m²,

            Δt_m = 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   + (d_oIn(d_o/d_i)) + d_o >< 1 + d_o >< 1                                 (eq – 1.8)

U_o  h_o   h_od               2k_w           d_i       h_id   d_i       h_i

Where   U_o = the overall coefficient based on the outside area of the tube, W/m^{2  o}C,

            h_o = outside fluid film coefficient, W/m^{2  o}C,

            h_i = inside fluid film coefficent, W/m^{2  o}C,

            h_od = outside dirt coefficient (fouling factor), W/m^{2  o}C,

            h_id = inside dirt coefficient, W/m^{2  o}C,

            k_w = thermal conductivity of the tube wall material, W/m^{2  o}C,

            d_i = tube inside diameter, m,

            d_o= 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.

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Introduction | Combined heat transfer process | Heat transfer in cooling tower | Variables affecting performance of CT heat transfer | Heat transfer within cooling system (heat exchanger) | Types of heat exchanger | Basic design procedure and theory | Designing a test heat exchanger | Log Mean Temperature difference | L.M.T.D. Correction factors | Overall heat transfer coefficient | Elaborated method for calculating U values | Effect of scale formation | Condensation of steam | Condenser, where the hot fluid temperature varies | Significance of pressure | Significance of flow rate | Methods of checking steam condenser performance | Common conversion factors