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.