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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