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Higher feedwater temperature leads to increased 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 100°C, at atmospheric pressure. Thus, the closer the feedwater temperature to 100°C, the closer the output generated to rated capacity.

In most plants, feedwater temperature is rarely maintained at 100°C. 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

Where, Energy to be added = (Total enthalpy 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.

Feedwater Temp. 40°C

Feedwater Temp. 100°C

Maximum steam generation capacity

8.71 tph

9.64 tph

Steam to fuel ratio