Boiler Efficiency - Parameters affecting it

Energy savings does not always imply efficiency improvement in boilers therefore, we need to understand the factors which are affecting it. So basically one should know all those parameters that influence efficiency.

For this one should know how to calculate the energy savings from parameters that do not influence thermal efficiency.

For very clear understanding between an increase in steam generation and improvement in efficiency, I have always emphasized that process engineer should be very clear about his options, definitions, & actual process happening due to modification proposed in the name of energy efficiency.

Recall the general efficiency equation of a boiler

(1) Eff = Absorbed Heat / Energy Input

The equation says nothing about the fact that not all adsorbed heat is useful heat. For instance the blow down is certainly “adsorbed” heat but rarely “useful”. In other words the energy in the blow down will be lost to ambient but any change in the blow down rate will not affect the system efficiency.

Equation (1) was converted into another equation


(2) Eff = 1 - Losses / Energy Input


It was shown that equation (1) and equation (2) are equal and should give the same result.

The energy input to the system is in the most simple case the energy of the fuel and the enthalpy of the combustion air. However one may add to the ener0gy input the steam necessary to atomize the fuel, or the electricity needed to power all electric motors of the boiler. In large power plant boilers it is especially important to draw a system boundary and prepare a list of all energy flows that enter and leave the system boundary.

Similarly the sum of losses depends on what we call a loss. Some obvious losses are the energy in the stack gas, the radiation and convection loss, and the refuse loss. However blow down is not considered a loss and therefore excluded from the sum. In fact the norms state no blow down is allowed during efficiency testing.

Using the efficiency simulator one will notice that system efficiency does not change at all if the following parameters are changed

• The steam pressure
• The steam temperature
• The blow down fraction
• The percentage of condensate return
• The condensate return temperature

The above parameters do not enter equation (2) and consequently the system efficiency will not change.

Nevertheless lowering the steam pressure or temperature or increasing the condensate return and temperature will certainly save fuel. There is absolutely nothing wrong with the definition of efficiency except the fact that we may save fuel by not changing the system efficiency at all.

Whenever Ps , Ts, xBD, xcon,Tcon, change the steam output changes as well, but the efficiency stays the same. Consequently we cannot calculate the fuel savings by the equation

(3) Fuel Saving % = ( Eff new - Eff old ) / Eff new


Another peculiarity are the savings one may achieve by preheating the air or the feedwater. The efficiency definition does not provide for entering the temperature of preheated air or feedwater, because preheating devices such as an economizer and air preheater are inside the system boundary. In other words preheating of the combustion air and the feedwater is taken into account through lowering the stack gas temperature.

So basically, the major purpose of writing it to again clarify that fuel saving does not necessarily mean the increase in efficiency. Currently I have worked an oil water emulsion technique which makes nano particle mix of fuel saving 3-5% fuel but there is no change in the efficiency of the system as none of the boiler parameter is changing. This makes saving just by using water as fuel.

So be Careful.


This Article is reproduced from the paper of GTZ from BEE India website.