In some type of heat exchanger, the temperature of both the fluids is varying and there fore logarithmic mean temperature will have to be calculated. The temperature changes of water and hot fluid may be represented graphically by the following figure. During the process the hot fluid will reject heat in the following manner:

(a) To cool down to saturation temperature, (process DC)
(b) To liquefy (process CB)
(c) To subcool the liquid (process BA)

At the same time the water will receive heat and will get heated from tc1 to tc2.

This type of condenser may be assumed to consist of three sections i.e. (i) desuperheater (ii) liquefier (iii) subcooler.

Here if t_{h1, 2, 3 & 4} is known then LMTD of each section can be calculated and average mean temperature difference is approximately given by:

 ΔT _m =                                  total heat rejection (HR) (kJ/min)                        (eq. 1.35)

                       (HR_D-C/ ΔT _{m (D-C)}) + (HR_C-B/ ΔT _{m (C-B)}) + (HR_B-A/ ΔT _{m (B-A)}) 

And then with this LMTD values for Q and/or U can be calculated.

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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