Efficiency may be defi ned as the ratio of output to input. In a boiler plant, the input is the total amount of heat in the fuel consumed while the output is represented by the total amount of heat in the steam generated.
The difference between the input and the output in any case represents the various losses, some of which are controllable and some uncontrollable. The heat equivalent of these losses, especially the avoidable ones, is of great importance to the power plant engineer, as a careful study of the variables affecting these losses often leads to clues for reducing them.
To this end, a heat balance is of inestimable value, for it provides the engineer with a complete accounting of the heat supplied to the plant and its distribution to the various units.
A complete heat balance is composed of the following items, each of which will be discussed individually:
1. Loss resulting from the evaporation of moisture in the fuel (mois- ture loss, fuel).
2. Loss from the evaporation of moisture in the air supplied for com- bustion (moisture loss, air).
3. Loss of the heat carried away by the steam formed by the burning of the hydrogen in the fuel (hydrogen loss).
4. Loss of the heat carried away by the dry fl ue gases (dry gas loss).
5. Loss from burning to carbon monoxide instead of carbon dioxide (incomplete combustion loss).
6. Loss from unburned combustible in the ash and refuse (combus- tible loss).
7. Loss due to unconsumed hydrogen, hydrocarbons, radiation, and unaccounted (unaccounted loss).
8. Heat absorbed by the boiler.
9. Total heat in 1 lb. of fuel consumed.
These items are computed on the basis of the number of Btu per lb. of fuel; either, “As Received,” “As Fired,” “Dry,” or “Combustible” being selected as the pound base. Whichever base is selected, all items in the heat balance have the same base.
For example, suppose that the “As Fired” base is chosen as a basis for computations, then whenever the words or symbols indicating “per lb. of fuel” appear, the inference “per lb. of fuel AS FIRED” is intended.