Avoid over sizing the boiler. Over sizing leads to recurring on-off cycles in a boiler which lowers boiler efficiency.
An oversized boiler helps meet process heating demands quickly and shuts off
till heat is required in the process again. This is referred to as boiler "short
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
decreases. Thus, during switch on and switch off more heat is carried away
with the air to the stack, reducing the over all efficiency.
This also causes the heat in the combustion chamber to be carried away by
the air purge, which results in greater stack loss and thereby reduced boiler
Hence, oversizing boilers must be avoided. In case of existing establishments
with oversized boilers, by analysing the plant load pattern, it is possible to
make adjustments to the boiler so as to minimise the losses.
Illustration: 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.