Boiler Efficiency basically depends upon the heat transfer taking place through the furnace shell and tubes. Better the heat transfer greater is the efficiency. Efficiency also depends on the stack temperature, combustion efficiency, and radiation losses.
Radiation losses
The boiler is designed to transfer a specific amount of heat through the designed surface area. For this, the fuel should be fired at the specified rate. But we must also notice that the heat transfer area remains constant always. This implies that some part of the generated heat is always lost as radiation losses. Therefore when the boiler is operated at lower loads, a lesser amount of fuel is fired. This means a lesser amount of heat is generated out of which a part is going to be utilized to heat the boiler heating surface area which is constant. Hence steam to fuel ratio will be less at lower loads when compared to the rated load. Since the steam to fuel ratio is less at lower loads it is said that the efficiency of the boiler dops at lower loads.
To simplify things we can put it this way. Suppose the boiler heat transfer area is designed for 1000Kcal/hr. The radiation losses for this surface are say 10 Kcal/hr i.e. 1%. So when the boiler is operated at lower loads, lesser fuel is fired and in turn, lesser heat is generated say 500 Kcal/hr. But the radiation losses remain 10Kcal/hr, as the surface area doesn’t change. This means 10Kcal (2%) of the total 500Kcal generated is lost. Thus radiation losses increase. Thus it is advised not to run boilers on lower loads.
Low Fire Operation
During the operation at lower loads, the combustion is less efficient. This is because of the increased oxygen % in the flue gases. Generally, the oxygen % is 2-3% greater at low fire operation than what at high fire. Thus the oxygen % rises above the ideal 3-3.5% and efficiency drops.
Startup and shut down losses
Start-up and shut-down losses are mandatory in every boiler. Burners are incorporated with pre-purge and post-purge which actually is a safety measure. During start-up, the burner does not start firing immediately. Instead, it purges air for a period of 30 seconds before the actual atomization. The purpose of Pre-purge is to blow away the residual exhaust flue gases that exist in the furnace and the boiler tubes since the boiler is shut down. Similarly, a post purge cycle is carried out after the shutdown. These purging cycles blow away hot flue gases which actually is a loss. This problem gets aggravated when the boiler is operated on lower loads. Reason: if the loads drop below the turn-down ratio, the burner trips and the boiler shuts down. This would get reduced if the loads are higher and do not fluctuate.
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