Avoid over sizing the boiler. Over sizing leads to frequent on-off cycles in a boiler which lowers boiler efficiency.
Boiler short cycling occurs when an oversized boiler quickly satisfies process heating demands, and then shuts down until heat is again required.
Most boilers operate on the cross limiting control scheme, where the air flow increases before the fuel flow increases and it decreases after the fuel flow decrease. Thus during switch on and switch off greater heat is carried away with the air to the stack, reducing the over all efficiency.
Thus frequent switch on and switch off causes the heat in the combustion chamber to be carried away by the air purge. This result’s in greater stack loss thereby reducing the boiler efficiency.
Thus oversizing boilers must be avoided. In case of existing establishments with oversized boilers, by analyzing the plant load pattern, it is possible to make adjustments to the boiler so as to minimize the losses.
The illustration below shows increased stack loss through variation of excess air as against the steam pressure in the boiler. As seen the oxygen content in the stack peaks when boiler cuts in and cuts out.