Heat of evaporation is given by steam to the process and in turn steam gets condensed Heat transfer coefficients during condensation are highest and thus saturated steam at exact pressure should be used for process heating.
The most common use for steam that is put to work in process or heating plants is a coil heated vessel.
The vessel is filled with the product to be heated, and steam is admitted to the coil. The steam then gives up its
enthalpy of evaporation to the metal wall of the coil which transfers it to the product. Hot water is formed as the steam condenses, and runs down to the bottom of the coil. This condensate, as it is properly known, must be then recovered for further use.
The correct steam pressure for heating process
Steam should be always used at the lowest possible pressure. At low pressure, heat content ( enthalpy of evaporation ) is high and heat transfer would be efficient. Less steam would be consumed in the process making it economical. For example, if final product temperature of 110 oC is to be attained in a coil reactor, considering the suitable temperature difference across the reactor, say 15 oC, the required steam temperature is 125 oC. Then the corresponding steam pressure is 2.5 to 3.0 bar g.