Evaluation of heat Required:
Heat which is required to generate steam is different from 'total heat' or enthalpy of steam. Heat required for generating steam means heat needed to produce steam from water the initial temperature of which is tºC (say) which is always more than 0ºC. Total heat or enthalpy of steam means heat needed to generate steam from water the initial temperature of which is 0°C. If initial temperature of water is 0°C, then of course heat needed to generate steam becomes equal to total heat or the enthalpy of steam.
heat required to generate steam can be given by QDry=hf + hfg - h' kJ/kg,
where h' = heat required to raise 1 kilogram water from 0°C to given initial temperature (say t°C) of water
= mst =1 × 4.2 × (t - 0) = 4.2t kJ [sp. heat of water = 4.2 kJ/kg K].
(b) When steam is wet saturated, heat needed to generate steam can be given by
Qwet = hf + x.hfg - h' kJ/kg.
(c) When steam is superheated, heat required can be given by
Qsup =hf + hfg + CP(tsup - ts) - h' kJ/kg.