Factors Affecting Boiler Efficiency

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Factors Affecting Boiler Efficiency[edit]

Some of the important boiler-efficiency deciding factors are explained in brief below:

Stack Temperature[edit]

It is also referred to as flue gas temperature. It is defined as the temperature level at which the hot exhaust gases make their way out of the boiler. “The flue gas temperature must be a proven value for the efficiency calculation to be reflective of the true fuel usage of the boiler.”[1] Lower than real stack temperature values must always be used for boiler efficiency calculations.

Fuel Specification[edit]

The specification of a fuel source can immensely affect the efficiency of a boiler system. For example, if the hydrogen content within a gaseous fuel source is comparatively high, extra water vapors get generated in the burning process. These water vapors tend to consume heat energy from the boiler for shifting their physical state during combustion. The efficiency of the boiler generally drops if huge loss of water vapors takes place. Due to this, the fuel oil offers greater boiler efficiency as compared to natural gas. “To get an accurate efficiency calculation, a fuel specification that represents the jobsite fuel to be fired must be used.”[2]

Excess Air[edit]

Excess air is defined as the amount of surplus air provided to the burner which is more than the necessary air needed to carry out combustion process. This given excess air mainly acts as a safety air reservoir for combustion in difficult situations such as inadequate air conditions.
However, at the same time, this more than required air tends to consume heat energy produced by combustion which in turn affects the heating efficiency of the boiler. “Seasonal changes in temperature and barometric pressure can cause the excess air in a boiler to fluctuate 5% - 10%. A realistic excess air level for a boiler in operation is 15% if an appropriate safety factor is to be maintained.”[3]

Ambient Temperature[edit]

The efficiency of a boiler also depends upon the ambient air temperature surrounding the boiler. For every 40 degree shift in ambient temperature, the efficiency of a boiler can get affected by at least 1%. Since all the boiler rooms are maintained at moderately warm temperature, majority of the efficiency computations takes upon 80 deg. F as the ambient temperature value.

Radiation & Convection Losses[edit]

These are the losses which emerge due to radiation of heat energy from the boiler. To eliminate the effect of these losses, boiler systems are usually shielded with some sort of insulation material. The presence of these losses extremely influences the efficiency of a boiler. In cases where these losses are not taken into consideration while performing efficiency calculations, accurate fuel consumption value can never be attained.
A boiler must always be designed in such a way that the radiation and convection losses get minimized. These losses tend to increase in proportion to the wind or air velocity prevailing around the boiler. Hence, the boiler systems located in open atmosphere experience more radiation and convection losses as compared to room boilers.

See Also[edit]


Boiler Efficiency


  1. Stack Temperature
  2. Fuel Specification
  3. Excess Air


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