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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 Deg.C, at atmospheric pressure. Thus, the closer the feedwater temperature to 100 Deg.C, the closer the output generated to rated capacity.

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

Illustration:

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

Feedwater Temp. 40 Deg C

Feedwater Temp. 100 Deg C

Maximum steam generation capacity

8.71 tph

9.64 tph

Steam to fuel ratio

12.9

14.28