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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Absorptivity:
Ability of a surface to absorb radiant energy, expressed as a decimal compared to the absorptive ability of a black body. Also, see emissivity and emittance.
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Adiabatic flame temperature:
The "hot mix temperature" or the theoretical or calculated temperature of a flame resulting from complete combustion with a stoichiometric air-fuel mixture in a perfectly insulated (adiabatic) chamber so that all the combustion energy is absorbed by the combustion gases.
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Air:
The gases contained in the earth's atmosphere. It contains the following approximate constitution: Nitrogen - 78%, Oxygen - 21%, Water vapor - variable to 2%, CO2 - 0.03% (300 ppm), other inert gasses (argon, neon, etc.) variable to 0.09%.
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Air-fuel firing:
Conventional combustion using atmospheric air, as opposed to oxy-fuel firing.
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Air-fuel ratio:
The ratio of the air to the gas volume and the reciprocal of fuel/air ratio. Air/fuel ratio should be controlled with air flow as the primary variable, i.e. with fuel following air flow to avoid producing a rich furnace atmosphere. Usually expressed as a quotient of volumes, i.e.; 10 ft3 air/1 ft3 gas = 10, or 10:1, or 10 to 1.
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Air, primary:
Primary air is the air volume mixed with a fuel prior to ignition. The fuel/air ratio control system adjusts the primary air flow to the combustion system when a demand for heat occurs and the automatic ratio control then makes a corresponding adjustment in the fuel flow.
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Air register:
A type of burner mounting that can admit secondary air to the combustion space through openings around the burner. Also used for primary air in windbox burners.
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Atmosphere (atm):
1) Pressure exerted by a "standard" atmosphere on the surface of the earth at sea level at 45 degrees north latitude which is 29.92"Hg or 760 mmHg or 14.696 psi.
(2) The mixture or chemical makeup of the gases within a furnace, e.g. reducing (rich) atmosphere, oxidizing (lean) atmosphere, or neutral atmosphere.
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Atmospheric ratio regulator:
Also called a zero governor. A diaphragm type regulator that maintains gas pressure at atmospheric or "zero" pressure.
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Atomizing air:
Used for oil-fired systems. That part of the air supplied through a burner (usually about 10%) that is used to break an oil stream into tiny droplets. The atomizing air is also used for combustion after it has broken up the oil stream.
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Auto-ignition temperature:
The lowest temperature required for self-sustained combustion in the absence of a spark or flame. It varies considerably with the nature, size, and shape of the hot surface, and other factors. Some vapors can be ignited by surfaces at temperatures as low as 500°F.
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Automatic fuel shutoff valve:
A valve for stopping the fuel flow automatically when a dangerous situation develops. The valve is closed by a spring force tripped by de-energizing a hold-open mechanism when any connected interlock senses a dangerous condition.
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Automatic reset fuel shutoff valve:
An automatic fuel shutoff valve that automatically reopens as soon as a normal operating condition is restored.
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Available heat:
The heat that is left available for heating the load and balancing wall, conveyor, and opening losses, after the stack loss is subtracted from the gross heat input. It represents the quantity of heat remaining for useful purposes and the best possible efficiency for a furnace or process. Available heat can be calculated from estimates of the flue gas exit temperature and the percent excess air.
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Black body:
An emitter or receiver of radiation (usually solid) with a maximum capability to emit or receive heat or light radiation. The emissivity and the absorptivity of a black body are each 1.0. This is a theoretical concept used as a basis to measure or compare radiation emitting and absorbing capabilities of various materials and surface conditions. Usually applied to solids, but also used for liquids, gases, and flames.
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Black body radiation:
The theoretical rate of radiation from a black body at a given temperature.
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Block valve or blocking valve:
A redundant fuel shutoff valve for protection in case of failure of the primary fuel shutoff valve. Usually automatic or manual reset type.
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Blow-off:
Lifting of a flame because feed stream velocity exceeds flame velocity. The phenomenon occurs when a flame moves away from a burner and often results in the flame being extinguished. Also called Liftoff.
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Blower:
A fan used to push air through combustion systems and to burners. May be integral with the burner or piped with distribution manifolds to banks of burners, lances or nozzles. In industrial furnaces, the blowers are usually centrifugal fans that develop air pressures from 0.5 to 3 psi (3.5 to 20.7 kPa)
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British thermal unit (Btu):
The quantity of energy required to heat one pound of water from 59°F to 60°F at standard barometric pressure of 30 inches of mercury.
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Burner:
A device with air, or oxygen, and fuel orifices that delivers those items to the burner quarl or combustion chamber that positions a flame in such a manner that continuous self-sustained ignition is accomplished. Burner designs vary and determine the flame character (size, shape, velocity, luminosity, completeness of combustion, and noise and emissions minimization). Most burner assemblies include mixing, proportioning, piloting, and flame monitoring devices. Many are designed specifically to enhance either radiation or convection heat transfer within a furnace.
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Burner block:
Refractory block with a conical or cylindrical hole through its center. The block is mounted in such a manner that the flame fires through this hole. The brick helps to maintain ignition, and reduces the probability of flashback or blow-off. Also called a burner tile, combustion tile, combustion block, refractory tile, refractory block, or quarl.
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Carbon dioxide, ultimate %:
The percentage of carbon dioxide appearing in the dry flue gases when a fuel is burned with its chemically correct fuel/air ratio. The theoretical maximum % CO2 possible.
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Carbon monoxide, CO:
A product of incomplete combustion (pic); a colorless odorless gas harmful to humans if inhaled at a concentration >400 ppm (0.04%) for more than one hour.
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CH4:
Methane (CH4), first in the paraffin series of hydrocarbons: C2H6 = ethane; C3H8 = propane; C4H10 = butane; C5H12 = pentane; C6H14 = hexane; C7H16 = heptane; C8H18 = octane; C9H20 = nonane; etc.
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Combustion air:
Main air. All of the air supplied through a burner other than that used for atomization. May or may not include air induced through the burner register by a negative pressure in the combustion chamber.
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Combustion control:
A device that (manually or automatically) proportions combustion air to fuel over the whole operating range of the burner system.
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Conduction:
The transfer of heat through a material by energy passing through it (from molecule to molecule) without gross displacement of the particles.
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Conductivity:
The rate of heat flow and the ability of a material to conduct heat, measured in Btu/hr, joules/hr, or kW flowing through a square foot or square meter of cross-sectional area, and through an inch, foot, or meter thickness with one degree (F, C, K) of temperature difference across that thickness. Commonly expressed in Btu per hour per square foot of surface, per foot or inch of thickness, per degree F temperature difference across the thickness (Btu / hour / ft2 in °F).
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Convection:
Transfer of heat by moving masses of matter (gas or liquid). Convection currents are set up by either mechanical agitation (forced convection) or because of differences in density at different temperatures (natural convection). Convection heat transfer is more prevalent at lower temperatures (or where temperature uniformity is more important than energy efficiency) and velocity is the major factor.
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Corebuster:
Device inserted inside a heat transfer (or radiant) tube to turbulate the flow and increase velocity and heat transfer.
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Damper:
A type of valve used to control flow in large ducts, usually for air or flue gas. May be metal or refractory and of a variety of configurations such as butterfly, guillotine, louver, etc. Often automatically power-actuated, counter-balanced, with mechanical advantage mechanisms.
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Deficiency of air:
A supply of air inadequate for complete combustion of a fuel. This is the same as an excess of fuel.
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Delayed mixing:
A process in which the fuel and air leave the burner nozzle unmixed, and thereafter mix relatively slowly, largely through diffusion. This results in a long, luminous flame, called a diffusion flame, luminous flame, or long flame.
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Delta p:
Pressure difference or pressure drop.
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Delta t:
Temperature difference or temperature change (either down or up).
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Diffusion flame:
A long luminous flame created by the slower diffusion mixing (delayed mixing) of parallel fuel and air streams in laminar flow; or, in a broader sense, any flame in which combustion follows from the gradual mixing of air and combustible gas after these have been introduced separately into the combustion region.
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Direct-fired:
A combustion heating process in which direct radiation and convection contact the load without a muffle or radiant tube separating the poc (products of combustion) from the product being heated.
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Direct spark ignition:
Direct electric ignition, use of an electric arc, as from a spark plug or other igniter, to light the main flame of a burner without the use of a gas or oil pilot as an intermediate step.
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Dissociation:
The breaking up of combustion products into combustibles and oxygen (or compounds containing less oxygen), accompanied by absorption of heat. This usually occurs at high temperatures and is one of the factors limiting the maximum temperature of a flame.
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Draft:
A difference of pressure that causes a flow of air or gases through a furnace or chimney.
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Efficiency:
The percentage of gross Btu input that is realized as useful Btu output of a furnace. The useful output is divided by the input and expressed as %. Some use thermal efficiency, fuel efficiency, and furnace efficiency interchangeably. Use care in differentiating between thermal efficiency and combustion efficiency, furnace efficiency, fuel efficiency, heating (or heat transfer) efficiency as they may not be synonymous. Combustion efficiency is a measure of how well a fuel is burned and applied in a process.
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EGR = FGR:
Exhaust gas recirculation = flue gas recirculation. Can be internal or external. FGR increases the flow of poc in a furnace. Internal recirculation increases the mass flow rate causing (a) a lowering temperature gradient along the flow path (thereby improving temperature uniformity of the furnace loads) and (b) increased convection heat transfer to the loads because of increased velocity. External recirculation is more effective in reducing NOx emissions because the external gas is cooler. However its fuel usage is greater. Both internal and external recirculation enhance convection heat transfer and lower NOx.
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Emissivity, e:
A measure of the ability of a material to radiate energy. The ratio, expressed as a decimal fraction, of the radiating ability of a given material to that of a black body (a "black body" emits radiation at the maximum possible rate at any given temperature and has an emissivity of 1.0.). Emissivity denotes a property of the material, whereas 'emittance' refers to an actual geometry or surface condition. The emissivity and absorptivity of most materials are nearly the same and are often used interchangeably. In industrial heating engineering, it is usually the absorptivity that is of most concern.
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Emittance:
The ability of a surface to emit (or radiate) energy, as compared with that of a "black body" with an emittance of 1.0. In contrast, emissivity denotes a property of the bulk material independent of geometry or surface condition, but emittance refers to an actual piece of material.
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Endothermic reaction:
A chemical reaction that absorbs heat and that heat must be supplied to continue the reaction.
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Equilibrium:
As applied to a furnace, the condition that exists when its walls have absorbed all the heat they can hold at a specific furnace temperature, so that any further flow of heat to the walls results in an equal amount of heat being transferred to the outside.
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Excess air, abbreviated XSAir:
The air remaining after a fuel has been completely burned, or air supplied to a combustion reaction in addition to that required for chemically complete (stoichiometric) combustion. A lean air/fuel ratio.
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Excess oxygen:
Like excess air, this is an indication of how lean or how oxidizing the combustion reaction is. For most fuels, the % excess oxygen in the flue gas is about 1/5 of the % excess air (up to about 3% oxygen or 15% excess air, above which point the ratio is progressively less than 1/5).
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Exit temperature:
The temperature of combustion gases as they leave a furnace.
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Exothermic reaction:
A chemical reaction that liberates (releases) heat, such as the burning of a fuel.
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Firing rate:
The rate at which air, fuel, or a fuel-air mixture is supplied to a burner or furnace. It may be expressed in volume, weight, or heat units supplied per unit time.
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Flame:
A typically visible shape within which combustion occurs, sometimes referred to as the "flame envelope".
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Flame character:
The nature of a flame -- e.g. length, size, shape, color, luminosity, velocity -- usually determined by the design of the flame holder and refractory quarl and by pressures, velocities, and directions of fuel jets and air jets.
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Flame holder:
Burner nozzle, a part of a burner that positions the flame, determines the character of the flame (length, shape, luminosity, velocity), and provides flame stability.
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Flame monitoring device:
Flame "sensor", flame "scanner", can be referred to as a flame safety device. These devices, utilized for flame surveillance -- ultraviolet detector, flame rod, flicker detector, infrared detector, photocell, thermopile, bimetal warp switch -- sense the presence of flame and cause fuel to be shut off in the event of flame failure.
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Flame retaining nozzle:
Any burner nozzle with built-in features to hold the flame at high mixture pressures.
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Flame temperature:
Theoretical flame temperature is calculated in the same way as the hot mix "adiabatic" temperature, but usually for stoichiometric air/fuel ratio. It may or may not be corrected for dissociation.
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Flame velocity:
The speed at which a flame progresses into a mixture relative to the speed of the mixture. Also called flame speed, ignition velocity, rate of flame propagation. The latter sometimes refers to flame front movement in a tube whereas the other forms usually refer to measurements in quiescent mixtures or in perfectly streamlined (laminar) flames. The turbulence encountered in the tube measurements usually results in velocities about twice as great as by the other methods.
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Flameless combustion:
A furnace condition where the combustion reaction has been diluted by internal flue gas recirculation of poc and inerts to the point where the reaction temperature is so low that the flame is invisible. The combustion reaction is at such a low temperature that it fails to supply energy for luminosity.
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Flammability limits:
The maximum and minimum percentages of a fuel in a fuel-air mixture which will burn. Sometimes called limits of inflammability.
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Flashback:
The phenomenon that occurs when a flame front moves back through a burner nozzle (and possibly back to the mixing point). Flashback occurs because the flame velocity exceeds the fuel-air mixture velocity through the burner nozzle.
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Flow coefficient:
A correction factor used for figuring volume flow rates through an orifice. This factor includes the effects of contraction and turbulence loss (covered by the coefficient of discharge), plus the compressibility effect, and the effect of an upstream velocity other than zero. Since the latter two effects are negligible in many instances, the flow coefficient is often equal to the coefficient of discharge.
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Flue, exhaust:
The opening in a furnace through which the poc exit the furnace and enter the exhaust stack. These openings are often refractory-lined and convey furnace exhaust gases away from personnel, usually through the roof of the building.
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Flue gas:
All gases or products of combustion (poc) that leave a furnace by way of a flue including gaseous products of combustion, water vapor, excess oxygen and nitrogen. Exit gases from recuperators, waste heat boilers, regenerators, and other heat recovery devices are termed "waste gases" or "stack gases".
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Flue gas analysis:
A statement of the quantities of the various compounds of a sample of flue gas, usually expressed in percentages by volume.
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Flue gas loss:
Also called stack loss. The sensible heat carried away by the dry flue gas plus the sensible and latent heat carried away by the water vapor in the flue gas.
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Forced draft:
Usually produced by a fan located in the inlet air passage to the furnace, forced draft results from the difference in pressure that blows air into a furnace.
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Fuel:
Any substance used for combustion as a heat source.
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Fuel-air ratio:
The ratio of the fuel supply flow rate to the air supply flow rate when both rates are measured in the same units under the same conditions; the reciprocal of air/fuel ratio. (These terms are often used interchangeably in qualitative discussions.)
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Fuel-lean:
Less fuel than required for stoichiometric combustion.
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Fuel oil:
A petroleum product used as a fuel. Common fuel oils are classified as:
#1 = distillate oil for vaporizing type burners.
#2 = distillate oil for general purpose use and for burners not requiring #1.
#4 = blended oil intended for use without preheating.
#5 = blended residual oil for use with preheating facilities (usual preheat temperatures are 120° to 220°F)
#6 = residual oil, for use in burners with preheaters permitting a high viscosity fuel (usual preheat temperatures are 180° to 260°F).
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Fuel primary:
A system of air/fuel ratio control in which the demand for heat adjusts the fuel flow to the combustion system, and the automatic ratio control then makes a corresponding adjustment in the air flow.
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Fuel-rich:
More fuel than required for stoichiometric combustion.
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Fuel train:
The fuel handling system between the source of fuel and the burner. It may include regulators, shutoff valves, pressure switches, flow meters, control valves.
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Furnace pressure:
The gauge pressure that exists within a furnace combustion chamber. The furnace pressure is said to be positive if greater than atmospheric pressure, negative if less than atmospheric pressure, and neutral if equal to atmospheric pressure.
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Gas gravity:
The ratio of the weight of a given volume of a gas to the weight of an equal volume of air (0.0765 lb/ft3). Sometimes called specific gravity.
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Gas-jet mixer (inspirator):
A mixer using the kinetic energy of a jet of gas issuing from an orifice to entrain all or part of the air required for combustion.
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Gas mixer:
Any device for mixing gas and air, such as a fan, aspirator or inspirator.
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Gauge pressure:
The difference between atmospheric pressure and the pressure the gauge is measuring.
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Gravity, specific:
A measure of the density of a liquid relative to that of water (62.43 lb/ft3). By contrast, see gas gravity.
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Heat content:
The total of latent and sensible heat stored in a substance minus that contained at an arbitrary set of conditions chosen as the base or zero point. It is usually designated h, in Btu per pound, but may also be expressed in such units as Btu per gallon and Btu per cubic foot if the pressure and temperature are specified.
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Heat flux:
The rate of flow of heat through a unit area usually expressed in in Btu/hr ft2.
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Heat of combustion:
The heat released by combustion of a unit quantity of a fuel, measured in calories, joules or Btu.
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Heat receiver:
Heat sink or load being heated.
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Heat recovery:
Getting back heat energy that might otherwise be lost up the stack of a furnace, boiler, heater, incinerator, kiln or oven. Heat recovery can be accomplished by addition of an unfired load preheat section, waste heat boiler, or air preheater (see recuperator or regenerator). Some engineers consider oxygen enrichment and oxy-fuel firing as forms of heat recovery.
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Heat transfer:
Flow of heat by conduction, convection, or radiation. This term is often used to mean heat transfer rate. The flow rate of thermal energy is expressed in units such as Btu/hr.
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Heating capacity:
Weight of load that can be heated in unit time through a specified temperature range without overheating. The heating capacity can be expressed as heating capacity per unit of hearth area or per unit of furnace volume.
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Heating value:
The heat obtained from combustion of a specified amount of fuel and its stoichiometrically correct amount of air, when both start at 60°F (16°C). The Gross or higher heating value, hhv, = the total heat released. The Net or lower lhv = hhv minus the latent heat of vaporization of the water vapor formed by the combustion of hydrogen in the fuel. In the USA, heating value is assumed to be hhv unless otherwise specified. In European practice, nhv or lhv is normally used.
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High fire:
A relative term meaning that the input rate to a burner or combustion chamber is at or near its maximum.
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Higher heating value:
Gross heating value = the total heat obtained from combustion of a specified amount of fuel and its stoichiometrically correct amount of air, both being at 60°F when combustion starts, and the combustion products being cooled to 60°F before the heat release is measured. By contrast, see net or lower heating value.
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Hydrocarbon:
Any of a number of compounds composed of carbon and hydrogen atoms.
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Ideal combustion:
Perfect combustion = stoichiometric combustion = 'on ratio' = combustion occurring at stoichiometric air/fuel ratio. See stoichiometric ratio.
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Ignition:
The act of starting combustion (usually with a spark).
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Ignition temperature:
The lowest temperature at which a fuel-air mixture can proceed as flame with an oxidation rate that releases heat faster than heat is lost to the surroundings.
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Inches H2O:
Inches wc = inches of water gauge = inches of water column. 1.732"wc = 1 ounces per square inch (osi). A measure of pressure, referring to the height of a column of water in a water manometer. One inch of water column equals a pressure of 0.578 osi.
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Inches of mercury column:
Inches Hg = a unit used in measuring pressures. One inch of mercury column equals a pressure of 0.491 psi.
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Incomplete combustion:
Combustion in which fuel is only partially burned and is capable of being further burned under proper conditions. An example is the case of carbon burning to form CO. With more air, it would burn to CO2.
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Indirect-fired heating:
A heating device in which the poc do not contact the load being heated--separated by a muffle or radiant tube.
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Induced air:
Air flowing in a furnace through openings as the furnace pressure is a lower pressure than atmospheric. Also, air brought in a furnace by entrainment in a high velocity stream.
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Induced draft:
A method for conveying flue gas, where combustion air is pulled through the burners and poc through the furnace by an induced draft fan, which develops more negative pressure (more suction) in the combustion system than can be created by natural draft alone. Induced drafts can be produced by natural or artificial means.
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Inerts:
Gases and materials that are not capable of combustion reactions, including those already oxidized, i.e.; N2 and CO2.
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Input rate:
The quantity of heat, fuel or air supplied per unit time, measured in volume, weight, or heat units.
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Inspirator:
Inspirator mixer = gas-jet mixer -- a mixer using the kinetic energy of a jet of gas issuing from an orifice to entrain all or part of the air required for combustion.
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Laminar flow:
Streamline flow, viscous flow, or the flow of a viscous fluid in which the particles of fluid move in straight lines parallel to the direction of flow.
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Latent heat:
Thermal energy absorbed or given off by a substance without changing its temperature, as when melting, solidifying, evaporating, condensing, or changing crystalline structure. "Latent flue loss" refers to the heat lost up the flue in the form of evaporated water formed by the combustion of hydrogen (from fuel).
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Lean ratio:
A proportion of fuel to air in which an excess of air is supplied in relation to the amount needed for complete combustion of the fuel.
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LEL:
Lower explosive limit.
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Light-up:
The entire procedure of igniting a burner or system of burners.
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Limiting orifice valve:
A fuel flow control device, usually manually adjustable, for setting fuel/air ratio.
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Low fire:
A relative term meaning that the input rate to a burner or combustion chamber is at or near the minimum.
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Low-fire start:
The firing of a burner with fuel controls in a low-fire position to provide safer operating conditions during light-off.
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Low pressure switch:
A device to monitor liquid, steam, or gas pressure and arranged to shut down the burner if the pressure falls below a preset low pressure limit (normally open).
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Lower heating value (lhv):
Also, net heating value (nhv). The gross heating value minus the latent heat of vaporization of the water vapor formed by the combustion of the hydrogen in the fuel. For a fuel with no hydrogen, net and gross heating values are the same.
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Main air:
Combustion air, or the air supplied through a burner.
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Manifold:
Header, or a supply pipe from which a number of branch pipes are fed.
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Manometer:
Generally a U-shaped tube, with liquid in the bottom of the U, used for measuring gauge pressure or pressure differences of fluids and gases. The U-tube is partially filled with a liquid and pressure lines are connected to the two ends of the U-tube. The liquid level rises in the low-pressure side, and falls correspondingly in the high-pressure side. The difference in height of the two liquid columns is proportional to the difference in pressure and is measured in inches or millimeters of liquid column. Manometers also are designed in inclined and well-type (single tube) designs.
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Manual reset automatic fuel shutoff valve:
Manual reset valve = M-R valve -- a fuel shutoff valve that automatically closes by spring action when its hold-open mechanism is electrically or pneumatically tripped by any connected interlock sensing a dangerous condition. It must be reopened by hand after the dangerous condition is rectified and the hold-open mechanism re-energized.
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Mass flow air/fuel ratio control:
Fully metered air/fuel ratio control with temperature compensation to correct volumetric metering to weight flow metering.
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Mechanical mixer:
A device that uses mechanical means to mix gas and air and compress the mixture to a pressure suitable for delivery to its point of use. These utilize either a centrifugal fan or mechanical compressor with a proportioning device on its intake.
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Minimum firing rate:
The lowest input rate for a burner or a process.
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Minimum ignition temperature:
The lowest temperature at which combustion of a given fuel can start. (Sometimes shortened to "ignition temperature".)
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Mixer, gas:
A device used to mix gas and air before delivery to a burner; an aspirator, an inspirator, or a fan mixer.
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Mixture, lean:
An air-fuel mixture containing too little fuel or too much air for perfect combustion.
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Mixture, rich:
An air-fuel mixture containing too much fuel or too little air for perfect combustion.
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Modulating control:
Proportional control, but sometimes used to refer to any system of automatic control that provides an infinite number of control positions, as opposed to systems with a finite number of positions such as two-position control.
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Net heating value:
Lower heating value, the gross heating value minus the latent heat of vaporization of the water vapor formed by the combustion of the hydrogen in the fuel. For a fuel with no hydrogen, net and gross heating values are the same.
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Neutral atmosphere:
An atmospheric condition in firing a furnace or kiln that is neither oxidizing nor non-oxidizing (i.e., reducing).
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NO:
Nitric oxide, or nitrogen monoxide, formed in flames, electric arcs, and many other places; colorless gas, readily reacts with oxygen to form nitrogen dioxide.
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NO2:
Nitrogen dioxide, formed from NO in the presence of sunlight and VOCs; red-brown gas, causes smog, acid rain.
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Nonoxidizing:
Not capable of oxidizing. Usually refers to the atmosphere in a furnace or kiln. Also used to describe a burner flame when insufficient O2 is present to complete combustion (reducing).
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NOx:
Nitrogen oxides, specifically defined by the USEPA as NO plus NO2. NOx is formed in some combustion reactions, particularly with flame temperatures above 2800°F. To minimize NOx formation, the mixing and thermodynamics of flames are designed (a) to have the chemical burning take place in stages so that some burning occurs below combustion chamber temperature and the balance only slightly above furnace gas temperature and (b) to minimize the concentrations of free N and O ions. NOx formation is largely dependent on combustion reaction temperature.
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Nozzle:
An opening, port, orifice, or jet tube through which a gas or fluid flows. For a burner, the part that delivers air, fuel, or an air-fuel mixture to a combustion chamber.
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On-off control:
A control scheme that turns the input on or off, but does no proportioning or throttling of the flow rates as is the case with a modulating control.
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Osi:
Ounces per square inch; a measure of pressure i.e.; 1 Osi = 1/16 psi.
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Oxidizing atmosphere:
A furnace atmosphere with an oversupply of oxygen, thus tending to oxidize materials placed in it.
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Oxidizing flame:
Oxidizing or lean flame or fire resulting from combustion of a mixture containing too much air and too little fuel. This kind of flame produces an oxidizing atmosphere.
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Oxy-fuel combustion:
A system for operating a burner with 100% oxygen instead of air.
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Oxygen concentration:
The % oxygen concentration (by volume) in an air-oxygen mixture or in the total of separate air and oxygen streams being fed to a burner or combustion chamber. (Normal air has a % O2 of 20.9; oxy-fuel has a % O2 of 100.)
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Oxygen-enriched combustion:
Burning fuel with a mixture of air and pure oxygen (anywhere from 20.9% and 100% oxygen) to improve efficiency or produce higher flame temperatures. The degree of oxygen enrichment is designated by the % oxygen concentration in the air-oxygen mixture.
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Oxygen trim (O2 trim):
An air/fuel ratio control system that uses an oxygen sensor in the flue gas as a feedback signal to control the amount of excess oxygen (excess air).
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Percent air:
The actual amount of air supplied to a combustion process, expressed as a percentage of the amount theoretically required for complete combustion.
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Percent excess air:
The percentage of air supplied in excess of that required for complete combustion. For example, 120% air equals 20% excess air.
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Perfect combustion:
The combining of the chemically correct proportions of fuel and air in combustion so that both the fuel and oxygen are totally consumed.
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pic (products of incomplete combustion):
Such as CO, H2, aldehydes. The pic, when they exist, are mixed with poc (products of combustion). See Poc in glossary
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Pilot:
A small flame used to light a burner. An interrupted pilot (sometimes called ignition pilot) is automatically spark-ignited each time that the main burner is to be lighted. It burns during the flame-establishing period and/or trial-for-ignition period, and is automatically cut off (interrupted) at the end of the main burner flame-establishing period, while the main burner remains on. Interrupted pilots are preferred / required for industrial heating operations. Two types of pilot control not suitable for industrial furnaces, ovens, kilns, incinerators, or boilers are:

1. A continuous pilot (sometimes called a constant pilot, standby pilot, or standing pilot) burns without turndown throughout the entire time that the burner assembly is in service, whether or not the main burner is firing.
2. An intermittent pilot is automatically ignited each time there is a call for heat, and maintained throughout the entire run period. It is shut off with the main burner at the end of heat demand.

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Poc (Products of Combustion):
Poc are combustion gases (usually assumed stoichiometric or lean combustion unless specified as containing pic = products of incomplete combustion) in a combustion chamber, heat recovery device, pollution reduction equipment, or stack that consist of CO2, H2O, N2, and O2 but may also include pic CO, H2, aldehydes, other complex hydrocarbons; and sometimes particulates, sulfur compounds, and nitrogen compounds. May be termed flue gas, stack gas, exit gas--depending on position. Should not be called waste gas because of confusion with by-product fuels.
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Power stack:
A furnace exhaust system using a fan, in addition to natural draft, to remove products of combustion from the furnace and flue system. The gases may be pulled through a hot fan (induced draft fan), or inspirated by the Venturi effect of an air jet.
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Ppm = parts per million:
Referring to concentration, usually of a pollutant or contaminant in air, water, or food. The "parts" may be volumes in a million volumes or weights in a million weights. For gases it is usually by volume. 1 ppm = 0.0001%, or 1 pound in 500 tons, or 1 ounce in 7530 gallons of water.
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Preheated air:
Air heated prior to its use for combustion. Frequently the heating is done by hot flue gases.
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Pre-ignition purge:
Pre-purge, an acceptable method for scavenging the combustion chamber, boiler passes, and breeching to remove all combustible gases before the ignition system can be energized.
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Pressure drop:
The difference in pressure between any two points along the path of flow of a fluid.
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Pressure-reducing regulator:
Pressure regulator = line pressure regulator -- a device used to maintain a constant pressure in a fuel supply line regardless of the demand. It cannot maintain a pressure greater than its inlet pressure.
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Pressure-relieving regulator:
Pressure relief valve -- a device used to maintain a constant pressure in a fuel supply line regardless of flow by bleeding off some fuel to atmosphere or to return to a tank.
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Primary air:
The first stream of air to mix with fuel at a burner, also see secondary air.
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Proportional control:
A mode of control in which there is a continuous linear relation between value of controlled variable and the position of the final control element.
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Proportioning:
Maintaining the desired ratio of fuel to air.
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Proportioning valves:
A system for air/fuel ratio control using air and fuel valves that are opened and closed in proportion to one another by a common shaft, a mechanical linkage or an electronic "linkage".
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Pulse-controlled firing:
A control scheme for furnaces with multiple industrial burners wherein all burners are operated on/off or high to very low, and modulated by varying their ratio of time on to time off. This enhances convection heat transfer because burners operate only at full fire. Individual burner cycle times are usually "stepped" to start at slightly different times so as to increase furnace temperature uniformity.
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Purge, post:
An acceptable method for scavenging the combustion chamber, boiler passes and breeching to remove all combustible gases after flame failure controls have sensed pilot and main burner shutdown and fuel shutoff valves are closed.
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Purging:
Eliminating an undesirable substance from a pipe, piping system, or furnace by flushing it out with another substance, as in purging a furnace of unburned gas by blowing air through it.
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Quarl:
Another term for a burner lock or tile (a refractory-lined hole), mounted through a combustion chamber wall, in which air and fuel are injected, and/or a burner flame is fired. The quarl is usually designed to enhance flame stability by adding the minimum ignition energy required to begin and sustain chemical reaction. The burner tile may influence the flame stability and character. The inside passage of a quarl may be cylindrical or conical, diverging or converging.
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Quenching of flames:
The process in which the reactants (poc) in a flame are rapidly cooled. This usually prevents the reaction from reaching completion in the localized area where quenching occurs. Large scale quenching may result in incomplete combustion.
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Radiant tube:
A tubular muffle through which a burner is fired for indirect heating of furnace loads. The metal alloy or ceramic tube wall transfers heat to the load without poc contact by a combination of radiation and convection from its outer surface. This provides process heating with reduced risk of scale formation or damaging reactions on the product surface. A prepared atmosphere (friendly to the material being heated) may be piped into the furnace space outside the radiant tubes.
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Radiation:
A heat transfer mode in which heat travels very rapidly (at light speed) in straight lines without heating the intervening space (except it will heat triatomic gas molecules such as CO2 and H2O). Heat can be radiated through a vacuum, through many gases, and through a few liquids and solids. Radiation heat transfer is most prevalent at high temperatures.
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Ratio regulator:
A proportional control device that regulates the downstream pressure in the pipeline in which it is located to maintain proportional pressures in fuel and air lines in a pressure control system, thus producing proportional flow of fuel and air.
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Recuperator:
A piece of equipment that makes use of the energy contained in hot flue gases to preheat air for combustion. The flue (poc) gases and air flow are in separate adjacent passageways so that heat is transferred from the hot gases, through the separating wall, to the cold air.
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Recuperator effect (also regenerator effect):
The bonus gain from preheating air, by virtue of the more intense heat transfer from a hotter flame, in addition to the savings from having the combustion air preheated so that less fuel is used in getting the air and fuel up to flame temperature.
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Reducing atmosphere:
A rich non-oxidizing furnace atmosphere used for protection of some metals and ceramic materials. It may be produced by supplying inadequate air to the burners, i.e.; incomplete combustion. Opposite of an oxidizing atmosphere.
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Reducing flame:
Reducing fire, a rich flame or fire; that is, one resulting from combustion of a mixture containing too much fuel and too little air, producing a reducing atmosphere.
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Refractory block:
Refractory tile = burner block = burner tile = combustion tile = combustion block = burner refractory – a piece of refractory material molded with a conical or cylindrical hole through its center. The block is mounted in such a manner that a burner flame fires through this hole. The block helps to maintain continuous combustion and reduces the probability of flashback and blow-off with premix burners.
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Regenerator:
A cyclic heat interchanger which alternately receives heat from gaseous combustion products and transfers heat to air before combustion.
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Regulator:
A device that detects a change in a process variable, and automatically energizes a mechanism that will correct the deviation in that variable so as to return it to a preset value. A controller is externally powered, but a regulator uses energy from the system that it is regulating. In current practice, a controller is sometimes called a regulator.
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Regulator, gas pressure:
A spring loaded, dead weighted or pressure-balanced device that can maintain the gas pressure to the burner supply line within ±10% of the operating pressure at any one rate from maximum to minimum firing rates, with variations in inlet pressure of ±40% of the rated inlet pressure.
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Rich ratio:
A proportion of fuel to air containing too much fuel or too little air for complete combustion. More precisely, a fuel-rich ratio, or an air-lean ratio, or oxidant-lean ratio.
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Sankey diagram:
A pictorial, or visual aid, method for analyzing how heat is spent in a furnace, boiler, oven, kiln, incinerator, or heater. It shows the portion of heat available and where losses occur by use of arrows of widths proportional to the magnitude of the heat flow.
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Secondary air:
The second stream of air to mix with fuel at or near a burner. Usually, secondary air is the volume of air supplied to the flame after ignition (also see tertiary air). In an atomizing burner, the atomizing air might be considered to be primary air and the main or combustion air to be secondary air. In an open burner, all air through the burner (atomizing and main) may be considered to be primary and all air through the air register to be secondary.
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Sensible heat:
Heat, the addition or removal of which results in a change in temperature, as opposed to latent heat.
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Setpoint:
The value chosen to be maintained by an automatic controller, i.e.; set point temperature or selected air/fuel ratio, or selected pressure to be controlled.
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Soak (soaking):
To hold the load in a kiln or furnace at one temperature for a time to allow equalization of temperature throughout the load.
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Soot:
A black substance, consisting of very small particles of carbon or heavy hydrocarbons, which appears in smoke resulting from incomplete combustion.
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Spark-ignited pilot:
An electrically-ignited small flame used to light a main burner. See pilot.
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Specific heat:
The amount of heat required to raise a unit weight of a substance through one degree temperature rise. 1 Btu/lb °F = 1 cal/gm °C.
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Spuds:
A gas orifice -- a small drilled hole for the purpose of limiting gas flow to a desired rate; a flame holder; a small-port premix nozzle.
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Stability, combustion:
That quality of a burner enabling it to remain lighted over a wide range of air/fuel mixture ratios and input rates without benefit of a pilot or spark.
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Stack loss:
Flue gas loss, the sensible heat carried away by the dry flue gas plus the sensible and latent heat carried away by the water vapor in the flue gas.
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Staged combustion air:
An arrangement of burner components and surrounding injector nozzles that add air to the fuel or fuel to the air in stages. For example a dual-fuel (or combination) burner may have atomizing air as the primary air stage, 1st stage combustion air as the 2nd stage combustion air, and 2nd stage combustion air as tertiary air. Pilot air is not usually considered part of any of the above. Staging is sometimes accomplished with peripheral air or fuel jets around a burner proper to reduce NOx formation by lengthening a flame (delayed mixing), which results in a lower average reaction temperature.
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Static pressure:
The pressure, or force, pushing outward on the inside of a wall per unit area. For flow in a pipe, it is measured by a gauge connected to the side of the pipe, perpendicular to the direction of flow.
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Stoichiometric ratio:
Pronounced "sto-key-o-metric". Refers to the combustion, flame, or air/fuel ratio that is chemically correct, perfect, or ideal; i.e. no excess fuel or oxidant can be detected. This correct ratio will leave no unused fuel nor oxygen after combustion.
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Swirl:
Spinning or spiral motion of a fluid; usually an aid to combustion stability.
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Temperature sensor:
Such as a thermocouple (T/c, or tc) for observation, input control, or high limit protection.
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Tertiary air (pronounced tur'-she-ary):
A third supply of air to a burner introduced downstream from the secondary air. Example: a dual-fuel low NOx burner with staged air might have atomizing air as the primary air, combustion (or main) air as the secondary air, and the staged air as the tertiary air.
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Thermal conductivity, k:
The ability of a material to conduct heat, measured in flow of Btu per hour through a square foot of cross sectional area and one foot (or inch) of thickness with 1°F of temperature difference across this thickness.
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Thermal head:
The difference in temperature between the source of heat (furnace refractory or poc) and the receiver of heat (the furnace load). Increasing this difference in potential increases the rate of heat transfer.
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Thermal NO:
Nitric oxide formed by temperature effects as opposed to that formed by fixed nitrogen from the fuel (fuel NO).
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Thermal turndown:
Achieving a lower effective input to a furnace by adding excess air through burners -- in effect turning down the thermal efficiency when a lower minimum input is required than achievable by valve-throttling turndown. One way to accomplish temperature control by thermal turndown is to hold the air flow constant while reducing fuel input.
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Timed trial-for-ignition:
Period of time during which the programming flame supervising controls permit the burner fuel valves to be open before the flame sensing device is required to detect the flame.
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Tramp air:
Infiltrated air, usually undesirable added excess air that "leaks" into a furnace, not helping the combustion reaction, or the heating process, and generally increasing temperature non-uniformity.
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Turndown:
The ratio of maximum to minimum input rates. Turndown ratio is abbreviated t/d and measures the difference between the high-fire rate and low-fire rate.
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UEL:
Upper explosive limit.
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Upper limit of flammability:
The maximum percentage of fuel in an air fuel mixture which can be ignited. Above this percentage, the mixture will be too rich to burn.
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Venturi:
A converging and then diverging flow nozzle, used for metering and for creating a suction such as in eductors and ejectors.
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Volume, combustion:
The space occupied by the fuel while it is actually burning, including both the flame and invisible combustion zone.
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Wall loss:
The heat lost from a furnace or tank to or through its walls.
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Waste gases:
By-product fuel. See discussion under poc and flue gases.
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Wobbe Index:
Wobbe Number, an index used to show fuel interchangeability. Wobbe Index = gross heating value in Btu/ft3 divided by gas gravity.
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Zone control:
That section of a furnace within which temperature is controlled by one temperature measurement (and usually with one control valve).
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