The performance of boilers need to be evaluated periodically to ensure that the boiler operates satisfactorily.
- Different boilers are designed to generate steam at different pressures and also during operation the steam generation pressure could be different from that envisaged at the time of designing it.
- Therefore, we need to relate the ability of Steam Generators boiler to raise steam to standard conditions.
- For all boilers the stand conditions considered are :
Temperature of feed water = 100oC,
steam generated is dry saturated at 100oC.
Accordingly for this condition the enthalpy of vaporisation (latent heat) at 100oC is found to be 2257 kJ/kg.
The term ‘Equivalent Evaporation’ is defined as the amount of water evaporated from saturated liquid water at 100oC to dry saturated steam at 100oC.
Let us consider a boiler generating mw kg of steam per hour at a pressure p and temperature T.
For this given p and T the enthalpy of steam generated is h. (The value of h needs to be taken from steam tables by considering T and p and depending up on T and p steam could be wet, saturated or super heated).
Let the specific enthalpy of water entering boiler be hf.
The heat gained by water in boiler per hour = mw (h – hf). The equivalent evaporation (me) from an definition is given as