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
Illustration:
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
|
12.9
|
14.28
|