Article Index

Flame speed- Is the measured rate of expansion of the flame front in a combustion mixture. Flame propagation is the result of strong coupling between chemical reaction, transport processes of mass diffusion & heat conduction and fluid flow.

Flammability- Is the ability of a substance to burn or ignite, causing fire or combustion.

Flammability limits- Refer to the range of compositions, for fixed temperature and pressure, within which an explosive reaction is possible when an external ignition source is introduced. The upper and lower bounds of the fuel/air mixture which will support combustion. The upper flammability limit indicates the maximum fuel concentration in air that will support combustion. The lower flammability limit indicates the minimum fuel concentration in the air that will support combustion. Outside of these bounds the mixture does not burn.

Flareback- A burst of flame back or out from a furnace or similar space in opposition to the normal direction of the draft.

Flashback- Is the unwanted intrusion of flame behind the burner nozzle resulting in uncontrolled fire within the premix supply conduit, occurring only in pre-mix gas burners when the flame speed overcomes the gas-air mixture flow velocity exiting the gas tip.

Flashing- The process of producing steam by discharging water into a region of pressure lower than the saturation pressure that corresponds to the water temperature

Flash point- The lowest temperature at which a liquid can form an ignitable mixture in air near the surface of the liquid.

Flue- Is a chamber through which smoke, hot gases, and combustion byproducts are vented.

Flue gas- A pipe or channel for conveying exhaust gases to the stack.

Foaming- The continuous formation of bubbles which have sufficiently high surface tension to remain as bubbles beyond the disengaging surface.

Forced circulation- An extra pump is used to increase the circulation rate as compared to that of natural circulation boiler.

Forced draft fan- Fan providing positive air pressure to fuel burning equipment.

Fouling- The buildup of unwanted solid deposits to gas passages or on heat transfer surfaces which results in restriction to the flow of gas or heat.

Fuel- A substance that can be burned to produce energy in the form of heat or power.

Fuel air mixture-The ideal mix of gas to air.

Fuel air ration- Is the mass ratio of air to fuel present in a combustion process.

Fuel Nox- Generic term for the mono-nitrogen oxides NO and NO2, are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures.

Fuel oil- Is a general term for a number of burnable liquids made from crude oil.

Fuel to steam efficiency- This is the ratio of BTU output divided by BTU input on a particular boiler.

Furnace- Is a device used for high-temperature heating.

Furnace pressure- Pressure inside the combustion chamber.

Furnace volume- The heat release rate of the furnace or combustion chamber.

Furnace arch- Is one which spans the fire-chamber and supports; or it may form the ceiling and roof of a metal furnace, uppermost part (bridgewall) of a radiant furnace.

Furnace Draft- The difference between atmospheric pressure and the pressure existing in the furnace or flue gas passage of a boiler.

Furnace explosion- The violent combustion of refuse or fume build in a furnace or combustion chamber.

Fusible plug- A threaded metal cylinder usually of bronze, brass or gunmetal, with a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point that flows away if a pre-determined, high temperature is reached.