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To deliver the same heat transfer rates as saturated steam, superheated steam requires larger heat transfer area. Use saturated steam for indirect heat transfer applications.

A major portion of the heat content of steam is the latent heat of evaporation. The latent heat of evaporation is released at the instant saturated steam converts to condensate. The sensible heat is carried away along with condensate.

In case of super saturated steam, the steam is heated beyond its saturation temperature. Thus, super heated steam has to first cool to saturation temperatures before it can condense to release the latent heat. As long as steam is super heated it has a lower heat transfer co-efficient as maximum heat transfer occurs when saturated steam transfers its latent heat. Thus, with superheated steam the heat exchanger area required to achieve the required heat transfer will be larger. If heat exchangers designed for saturated steam are used for super heated steam, the required heat transfer rates will not be achieved.

Illustration:

The overall 'U' (heat transfer co-efficient) value for superheated steam will vary throughout the process, but will always be much lower than that for saturated steam. It is difficult to predict 'U' values for superheated steam, as these will depend upon many factors, but generally, the higher the degree of superheat, the lower the 'U' value.

Typically, for a horizontal steam coil surrounded with water, 'U' values might be as low as 43 to 86 kcal/m2)°C for superheated steam but 1036 kcal/m2)°C for saturated steam.