Modern boiler plants are increasingly focused on energy efficiency, reducing their physical footprint, and minimizing capital and installation costs. Companies are striving to keep their total costs of operation as low as possible to remain competitive in the global economy. One of the challenges is having a steam supply that can handle large variations in demand. If the boiler is too small to handle peak demands, the plant cannot operate as designed. If the boiler is sized to handle short bursts of steam, the boiler may be oversized and have higher energy input versus output due to lower efficiencies at lower loads.
Boiler efficiency can be impacted significantly when the boiler is oversized for various reasons. Combustion is much more efficient when operated at higher firing rates. When burners are continually cycling from high fire to low fire, or even turning off, efficiency is lost. During the restart of the burner, the combustion air fan must be run without lighting the burner to purge out any fuel due to leakage. During shutdown, any remaining fuel must be burned off from the combustion chamber as well. Both of these waste fuel and energy. Boilers also lose energy from radiant heat losses from their shells. At lower firing rates, the percentage of energy lost is greater than at higher firing rates, again lowering the efficiency. All of these stops and starts also cause wear on the equipment, increasing maintenance costs.