Any increase in the temperature of the combustion air or fuel oil increases the efficiency of the boiler. One method is appropriate for large boiler rooms, where incidental heat rising from hot pipes and furnace surfaces can be collected at the ceiling and drawn into the boiler as combustion air. Other, more elaborate, methods can heat the air above this temperature, for an even better boost in efficiency.
A good guideline to keep in mind for large installations is that boiler efficiency increases by about 2 percent for each 100°F rise in the temperature of combustion air. Depending on the boiler capacity, pre-heaters of various expense and complexity can also be considered. Hot flue gases are one convenient source of waste heat. They’re available whenever the boiler is on-line and drawing combustion air. Various types of heat exchangers are available for this air-to-air system, including plate-and-tube arrangements. Rotary wheels are another option, or heat pipes. Another indirect method is runaround coils in the stack and combustion air duct.
The advantage to preheating the fuel oil is to obtain complete atomization. The ideal fuel temperature ranges from 130 to 210°F, depending on the grade of oil. Preheating to this atomization temperature can increase the boiler efficiency by up to 3 percent. Increasing the fuel-oil temperature beyond this value is not advisable, though, as this can result in vapor locking and flame-outs.
Preheating the liquid fuel is a little easier to do than preheating the combustion air. The quantity of thermal energy is less, and the transfer rate of heat to a liquid is more efficient than to air. Possible waste-heat sources are hot flue gases via an air-to-fluid exchanger, boiler blowdown water, condensate return water, or hot-water discharge from a nearby laundry or kitchen.
Another possibility is to take advantage of the high temperature of the boiler jacket by routing the fuel line around the flue or along the surface of the boiler for a short distance. Of course, proper precautions for fire safety are in order for any such installation.
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Excerpt from Energy Conservation Projects.