Combustion products in fuel-fired furnaces are vented into the atmosphere through a stack or a chimney. As these products are mainly hot gases that are less dense and more buoyant than the ambient air, they rise creating a pressure differential between the bottom and top of the furnace. This differential is known as the thermal head and it produces a natural draft or negative pressure in furnaces and boilers.
Furnaces are not immune to wear and tear. Over a period of time, fuel-fired furnaces develop cracks in different areas like doors, joints and hearth seals. Initially, the cracks are small compared to the overall dimensions of the furnace and are ignored. However, due to the pressure differential produced, there is a net flow of air from the outside, through the cracks and openings into the furnace. The cold air entering is heated to the same temperature as that of the exhaust gases in the furnace and leaves through the flue system. Thus, fuel is wasted to heat this air that infiltrates. In addition, air infiltration can cause excessive oxidation of metals and other materials in the furnaces, thus affecting their quality.
The effect of air infiltration on a furnace is governed by a variety of factors. They are listed as follows:
The temperature of the furnace or of the flue gas.
The distance of the crack from the exit point (for exhaust gases) of the furnace.
The distance is measured vertically.
The area of the crack in the furnace.
The time period when the furnace operates at negative pressure.
The velocity with which the flue gas travels through the chimney.
The other conditions at which the furnace operates.
If a furnace has been designed properly, air infiltration will be kept to a minimum. In addition, air leakage in a furnace can be curbed using a variety of measures. The most important part is to find the air leak and repair it. This is usually done by replacing the insulating material or seals in the crack. During the working of the furnace, ensure that the doors are closed properly so that the furnace is sealed shut.
For major air infiltration, install a pressure control system at the point of leakage. It will maintain a slightly positive pressure gradient at this site. If a pressure control system is not viable, a damper may be installed within the chimney. This is a valve that can control the flow of air inside the chimney.