An important parameter in the design and monitoring of heat exchangers is the overall heat transfer coefficient, U, between the two fluids. A value for U can be easily obtained by knowing the followings:

1.      Mass flow of the fluid,

2.      Specific heat of the fluid,

3.      Difference in temperature of the fluid across the heat exchanger,

4.      Inlet and outlet temperature of both the fluids involved in heat exchanger and

5.      Area of the heat transfer surface.

From the equation         Q = m.cp. Δt

Calculating Q in watts:

Where m = mass flow in kg/hr of kg/sec

            Cp= specific heat or heat content in KJ/Kg K

            Δt= Temperature difference across the heat exchanger.

Calculating LMTD from the following equation with the inlet and outlet temperatures of the hot and cold fluid across the heat exchanger and with the knowledge of type of heat exchanger using:

(For multipass heat exchangers)

ΔT _m =    (T2-t1) – (T1 – t2)                                                                           

               ln [(T2-t1)/(T1-t2)]

Where ΔT _m = log mean temperature difference,

            T1  = inlet shell side fluid temperature,

            T2  = outlet shell side fluid temperature,

            t1   = inlet tube side temperature,

            t2   = outlet tube-side temperature,

 OR

(for single pass heat exchanger)

Δt_m = (T1-t2) +(T2-t1)                                    

                        2

We can calculate U from the equation                Q = UA Δt_m

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.

U =      .   Q   .                                                                                                 (eq. 1.14)

            A Δt_m

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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