Fan - A machine consisting of a rotor and housing for moving air or gases at relatively low-pressure differentials.
Fan performance - A measure of fan operation in terms of volume, total pressures, static pressures, speed, power input, and mechanical and static efficiency, at a stated air density.
Fan performance curves - The graphical presentation of total pressure, static pressure, power input, and mechanical and static efficiency as ordinates and the range of volumes as absciassas, all at constant speed and air density.
Feedwater regulator - A device for admitting feedwater to a boiler automatically on demand. Practically a constant water level should result.
Firebox - The equivalent of a furnace. A term usually used for furnaces of locomotive boilers and similar types of boilers.
Firetube - A tube in a boiler having water on the outside and carrying the products of combustion on the inside.
Firing rate control - A pressure temperature or flow controller which controls the firing rate of a burner according to the deviation from pressure or temperature set point. The system may be arranged to operate the burner on-off, high-low or in proportion to load demand.
Flame - A luminous body of burning gas or vapor.
Flame detector - A device which indicates if fuel, such as liquid, gaseous, or pulverized, is burning or if ignition has been lost. The indication may be transmitted to a signal or to a control system.
Flame propagation rate - Speed of travel of ignition through a combustible mixture. (See flame speed)
Flame speed - The rate at which a flame can propagate in a combustion mixture. If the flame is lower than the speed of the reacting flow, the flame may lift off the burner. If the flame speed is higher than the speed of the reacting flow the flame may flash back into the burner.
Flammability limits - 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 from a furnace in a direction opposed to the normal flow, usually caused by the ignition of an accumulation of combustible gases.
Flashback - A phenomenon occurring only in pre-mix gas burners when the flame speed overcomes the gas-air mixture flow velocity exiting the gas tip. The flame rushes back to the gas orifice and can make an explosive sound when flashback occurs. Flashback is most common when hydrogen is present in fuel gas.
Flashing - The process whereby a drop in pressure or increase in temperature cause vaporization.
Flue - A passage for products of combustion.
Flue gas - The gaseous products of combustion in the flue to the stack.
Foaming - Formation of steam bubbles on the surface of boiler water due to high surface tension of the water. See carryover.
Forced circulation - The circulation of water in a boiler by mechanical means external to the boiler.
Forced-draft fan - A fan supplying air under pressure to the fuel burning equipment.
Fouling - The accumulation of refuse in gas passages or on heat absorbing surfaces which results in undesirable restrictions to the flow of gas or heat.
Fuel NOx - NOx that is formed from nitrogen that is organically bound to the fuel molecule. Fuel NOx is most often a problem with liquid fuel or coal burning. Once the nitrogen has been cracked from the fuel molecule, the mechanism follows basically the same path as the prompt NOx mechanism.
Furnace - An enclosed space provided for the combustion of fuel.
Furnace arch - Uppermost part of a radiant furnace (also called the "bridgewall", a term which came from the original furnace designs and has remained in use). The last area in an upflow furnace before the convection section.
Furnace Draft - The negative air pressure generated by buoyancy of hot gases inside a furnace. The temperature difference between gases within the furnace and in the atmosphere along with furnace and stack height basically determine the amount of draft generated by the furnace. Draft is generally measured in negative inches of water column. ("-w.c."; 27.7 inches w.c. = 1 psig)
Furnace explosion - A violent combustion of dust or gas accumulations in a furnace or combustion chamber of a boiler.
Furnace release rate - The heat available per square foot of heat-absorbing surface in the furnace. That surface is the projected area of tubes and extended metallic surfaces on the furnace side including walls, floors, roof, partition walls, and platens and the area of the plane of the furnace exit which is defined as the entrance to the convection tube bank.
Furnace volume - The cubical contents of the furnace or combustion chamber.