| Term | Definition |
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| Fan | A machine consisting of a rotor and housing for moving air or gases at relatively low pressure differentials.
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| Fan Performance | A measure of fan operation in terms of volume, total pressures, static pressures, speed, power input, mechanical and static efficiency, at a stated air density.
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| Fan Performance Curves | The graphical presentation of total pressure, static pressure, power input, mechanical and static efficiency as ordinates and the range of volumes as abscissa, all at constant speed and air density.
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| Feed Pump | A pump that supplies water to a boiler.
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| Feedwater | Water introduced into a boiler during operation. It includes make-up and return condensate.
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| Feedwater Treatment | The treatment of boiler feed water by the addition of chemicals to prevent the formation of scale or to eliminate other objectionable characteristics.
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| FGR | Flue Gas Recirculation or the recirculation of flue gas with combustion air to reduce NOx emissions.
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| Filter | Porous material through which fluids or fluid - and solid mixtures are passed to separate matter held in suspension.
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| Fin | A fin is an extended surface, a solid, experiencing energy transfer by conduction within its boundaries, as well as energy transfer with its surroundings by convection and/or radiation, used to enhance heat transfer by increasing surface area.
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| Fin Tube | A tube with one or more fins.
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| Fired Pressure Vessel | A vessel containing a fluid under pressure exposed to heat from the combustion of fuel.
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| Firetube | A type of boiler design in which combustion gases flow inside the tubes and water flows outside the tubes.
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| 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.
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| Fixed Carbon | The carbonaceous residue less the ash remaining in the test container after the volatile matter has been driven off in making the proximate analysis of a solid fuel.
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| Flame | A luminous body of burning gas or vapor.
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| Flame Detector | A device which indicates if a fuel (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.
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| Flame Propagation Rate | Speed of travel of ignition through a combustible mixture.
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| Flame Safeguard | A control that sequences the burner through several stages of operation to provide proper air purge, ignition, normal operation, and shutdown for safe operation.
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| Flammability | Susceptibility to combustion.
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| Flash Point | The lowest temperature at which, under specified conditions, fuel oil gives off enough vapor to flash into a momentary flame when ignited.
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| 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
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| Flue | A passage for products of combustion.
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| Flue Gas | The gaseous product of combustion in the flue to the stack.
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| FM | Factory Mutual.
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| Foaming | The continuous formation of bubbles which have sufficiently high surface tension to remain as bubbles beyond the disengaging surface.
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| Forced Circulation | The circulation of water in a boiler by mechanical means external to the boiler.
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| Forced-Draft Fan | A fan supplying air under pressure to the fuel burning equipment.
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| Fouling | The accumulation of refuse in gas passages or on heat absorbing surfaces which results in undesirable restriction to the flow of gas or heat.
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| Free Ash | Ash which is not included in the fixed ash.
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| Fuel | A substance containing combustible used for generating heat.
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| Fuel Oil | A liquid fuel derived from petroleum or coal.
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| Fuel-Air Mixture | Mixture of fuel and air.
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| Fuel-Air Ratio | The ratio of the weight, or volume, of fuel to air.
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| Fuel-To-Steam Efficiency | The ratio of heat added to boiler feedwater to produce the output steam to the amount of energy inputted with fuel.
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| Furnace | An enclosed space provided for the combustion of fuel.
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| Furnace Pressure | Pressure occurring inside the combustion chamber; positive if greater than atmospheric, negative if less than atmospheric, and neutral if equal to atmospheric.
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| Furnace Volume | The cubic contents of the furnace or combustion chamber.
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| Fusible Plug | A hollowed threaded plug having the hollowed portion filled with a low melting point material.
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