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Higher feedwater temperature increases the boiler output with reduced fuel consumption.

The boiler’s capacity is stated in ‘from and at’ rating which shows the amount of steam in kg/hr which the boiler can create ‘from and at 100oC’, at atmospheric pressure. Thus the closer the feedwater temperature to 100oC the closer the o/p generated to rated capacity.

Across most plants the feedwater temperature is rarely maintained at 100oC. It is typically ambient or slightly higher than ambient. This leads to a poor steam to fuel ratio and lowers boiler capacity.

The following formula helps calculate the steam output from the boiler:

Steam output = Boiler rating (TPH) x 540 Kcal/kg

(Energy to be added) Kcal/kg


Energy to be added = (Latent heat of evaporation at boiler operating pressure – Feedwater temperature)

540 Kcal/kg = Latent heat of evaporation for water at atmospheric pressure


Consider a 10 ton/hr capacity oil fired boiler with an 80% operating efficiency.