It is vital to keep heat losses to a minimum so that efficiency is maximized and more energy is conserved. Heat losses are inevitable, especially through the stack, but great amounts of heat losses may be prevented with the proper measurement and control procedures. Total heat losses are normally tallied by adding the stack losses, the skin/shell losses, and the losses due to the un-burned fuel in ash collection hoppers.
Stack losses will combine the sensible heat losses or dry gas losses and the latent heat losses. Sensible heat losses relate to the heat used to heat the combustion gases exiting the stack; the higher the volume and temperature of the flue gases the larger the dry gas heat losses. Latent heat losses are due to the water vapor in the flue gases (a large amount of energy is used as water evaporates).
Skin/shell losses, which are the losses due to radiation from the boiler walls, can be minimized with proper insulation and in general are relatively small.
In coal and wood fired boilers, ash is normally a by-product of combustion that is collected in hoppers or ash collection areas; it is imperative that the amount of carbon left in those ashes is reduced to extremely small amounts given the resulting heat losses and the negative effects that carbon has in disposing of those ashes.
Measuring Combustion Efficiency
Although combustion efficiency can not be measured directly, it can be calculated by identifying all of the losses that occur during combustion. It is important to consider all factors including sensible heat losses, unburned gases, radiation, and unburned particles. In most instances, the values of the skin losses and latent heat losses are not taken into account. The following equation can be used to calculate combustion efficiency:
Typical combustion process efficiencies:
|Home fireplace:||10 – 40 %|
|Space heater:||50 – 80 %|
|Residential gas furnace with low efficiency atmospheric burner:||70 – 80 %|
|Oil burner heating system:||70 – 85 %|
|Gas powered boiler:||75 – 85 %|
|High efficiency gas or oil condensing furnace:||85 – 95 %|