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Saturated steam has a higher heat transfer coefficient than super heated steam. Thus use saturated steam for indirect heat transfer applications.

Most of the energy content of steam is the latent heat of evaporation. For indirect heating applications in a process, it is this latent heat of evaporation in steam that is actually utilized. The condensate which leaves the plant equipment contains a portion of heat termed “sensible heat”. In case of super heated steam, steam has to first cool to saturation temperatures before it can condense to release its latent heat. The amount of heat given up by superheated steam as it cools to saturation temperature is relatively small in comparison to the latent heat of evaporation. Thus 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.

Illustration:

Although superheated steam contains a large amount of heat energy, this energy is in three forms; enthalpy of water, enthalpy of evaporation (latent heat) and enthalpy of superheat. The bulk of the energy is in the enthalpy of evaporation, and the energy in the superheat represents a smaller proportion.

For example, take superheated steam at 10 bar a and 300°C, then:

Enthalpy of water = 181 kcal/kg

Enthalpy of evaporation = 481.9 kcal/kg

Enthalpy of superheat = 65.7 kcal/kg

Superheated steam requires a larger heat exchange area which if not provided leads to reduced heat exchange.