boiler new.docx

8/9/2019 boiler new.docx

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amount of heat released is normally e'pressed in !T8s 6!ritish thermal units7 or Calories. %or industrial applications in the8.9., we generally use the term !T8. - !T8 is defined as :the amount of heat necessary to raise one pound of water one

degree %ahrenheit from ; to 1;%.< - Calorie is defined as :the energy required to raise one =g of water one degree

Celsius..#

)ydrogen? 11.I to1(.> #( to#+ >.+ to 1#

9ulfur? .+ to 1. .1 to .* .5 to (.+

&'ygen? 1. to 1.+ .>#
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Hitrogen? to .1 .+ to I

-sh? to .1 .1 to .+

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O"#$en

The o'ygen needed to support combustion comes from theairthat surrounds us. -ir is a mi'ture of gases consisting mainly of

about #1 percent o'ygen and about 5I percent nitrogen of volume. The remaining 1 percent consists of small amounts ofargon, carbon dio'ide, and other gases.

$ven in a simple wood fire o'ygen plays a part in combustion. - chemical reaction occurs between the o'ygen of the air and

the woodfuel. The nitrogen and other gases in the air do not enter into the reaction, but do carry away the gases ofcombustion.

/f a cover is put over the fire, the o'ygen will be used up and the fire will go out. /f the cover is removed before the fire is

completely e'tinguished, o'ygen becomes available and the fire continues.

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%eat

The EEignition temperatureEE is the temperature that will start a fuelto rapidly ignite with o'ygencausing combustion to takeplace. - chemist would call this process EEo'idationEE. Combustion is a form of o'idation that produces heat and light.

/f you have ever tried to light a large log in a fireplace with a match, you notice it is very difficult. The large log does not

release heat rapidly enough to maintain the ignition temperature of the log. /f you shave off pieces of the log and light them

with a match, the results will be much different. The burning shavings produce enough heat to maintain ignition. This sameconcept applies to liquid fuel. Trying to light a container of oil would be difficult, but atomiFing the fuel will allow it to burn

easier.

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%o& Fuels Burn

9ome things burn more readily than others. -sfuelis heated, several different gases are released. These gases are hydrogen,carbon mono'ide, and hydrocarbons similar to methane. %inally, all that is left is solid carbon and impurities. /f airis added to

the solid carbon,o'ygenfrom the air will penetrate the surface and break away atoms of carbon. The carbon atoms combine

with the o'ygen. The products of combustion are carried away by the moving air. This process continues until the burnablecarbon has disappeared and only impurities remain.

&nly vapors burn, not liquids or solids. $ach type of fuel has a different volatility. EEEJolatilityEEE is a measure of how rapidly

the liquid turns into vapors. The vapors still must be raised to at least its flash point before ignition can occur.

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Fuel'Air Ratio
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%ossilfuelsburned in a boiler contain two basic elementsB hydrogen and carbon. /f these elements are combined, thecompound is called EEhydrocarbonEE. Thefuelgas used for ignition is a hydrocarbon. - chemical analysis of the fuel determines

how muchairmust be mi'ed with it for complete combustion. The relationship between fuel and air is called the EEfuelair

ratioEE.

/f you have ever worked on an older car with a carburetor, you probably ad@usted the fuelair Kmi'ture.K The mi'ture istypically ad@usted by controlling the amount of fuel entering a carburetor. 9upplying too much fuel is called a EErichEE mi'ture

and causes e'cess emissions or smoke from the e'haust. 9upplying too little fuel is called a EEleanEE mi'ture and causespoorheatgeneration and a rough running engine.

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Fuels

%uels can generally be classified as gaseous, liquid, or solid. /n cases where a solid fuel is finely ground, such as pulveriFed

coal, and can be transported in anairstream, its control characteristics approach those of a gaseous fuel.iquid fuels,as they

are atomiFed and sprayed into a furnace, also have control characteristics similar to those of a gaseous fuel. The controltreatment of a solid fuel that is not finely ground is quite different from that of a gaseous or liquid fuel.

Lhether a fuel is a gas, a liquid, or a solid is determined by the ratio of its two primary chemical ingredients, carbon and

hydrogen. Hatural gas has an )C ratio of in e'cess of .(. %uel oil has an )C ratio of above .1. 9ince hydrogen is thelightest element and the molecular weight of carbon is si' times that of hydrogen, a decrease in the )C ratio increases the

specific gravity and the density of the fuel.

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Basic an( I(eal Combustion

-n idealfuelburning system would have the following characteristicsB

Ho e'cesso'ygenor unburned combustibles in the end products of combustion

- low rate of au'iliary ignition4energy input to initiate the combustion process

-n economic reaction rate between fuel and o'ygen compatible with acceptable nitrogen and sulfur o'ide

formation

-n effective method of handling and disposing of the solid impurities introduced with the fuel

8niform distribution of the product weight and temperature in relation to the parallel circuits of heatabsorbing

surface

- wide and stable firing range, fast response to changes in firing rate, and high equipment availability with low

maintenance

/n actual practice, some of these characteristics must be compromised to achieve a reasonable balance between combustion

efficiency and cost. %or e'ample, firing a fuel with no e'cessairabove the theoretical amount would require an infinite

residence time at temperatures above the ignition point at which complete burnout of the combustibles takes place. Thus,every firing system requires a quantity of air in e'cess of the ideal amount to attain an acceptable level of unburned carbon in

the byproducts of combustion leaving the furnace. This amount of e'cess air is an indicator of the burning efficiency of the

firing system.
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Talk Page

Basic Combustion

-s we have learned, combustion can occur when the following conditions are metB

%uel

)eat

&'ygen

Chemical reaction

-ll flammable material has a K%-9) P&/HTK and an K/0H/T/&H P&/HT.K

The flash point of fuel is the lowest temperature at which sufficient vapors are given off for in a momentary flash

when an ignition source is applied near the surface.

The ignition point is the temperature at which the ignited material provides enough heat to maintain combustion.

-s a flammable mi'ture is heated, the rate of chemical combination of o'ygen with the carbon and hydrogen

increases. $ventually, the combination rate becomes high enough to be continuously self4supporting. Le call this

EEcombustionEE.

"egardless of the fuel, it must be vaporiFed in order to burn. &il, a liquid, and coal, a solid, must be heated to the point wheregaseous vapors are rapidly given off. /ts these vapors which burn, H&T the solid or liquid. This is what makes it possible, for

e'ample, to put out a match in a bucket of light oil that is below its flash point.

Hatural gas consists primarily of methane 6C)*7. The heat is released as the carbon 6C7 and hydrogen 6)#7 combine 6react7with o'ygen and produce water 6)#7 and carbon dio'ide 6C.

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I(eal Combustion

$fficient combustion of anyfueldepends on its chemical and physical characteristics, and how well it is mi'ed with

combustion air.Three important factors 4 time, temperature, and turbulence 4 control the completeness of combustion and

influence the design of boiler equipment and operating practices.

T/M$4 Hormally, combustion reactions are so rapid that the time to complete them seems instantaneous. - good

e'ample is the combustion of gasoline in an internal combustion engine. )owever, natural gas or a droplet of oilwill travel several feet in the furnace and require a finite period of time between the start of ignition and the

completion of burning.

T$MP$"-T8"$4 /f a mi'ture of air and fuel is heated gradually, a temperature will be reached at which

outside heatis no longer required and rapid combustion occurs. This temperature is referred to as the ignitiontemperature and is defined as the temperature at which more heat is generated by the combustion process than is

lost to the surrounding atmosphere. -t this point, combustion becomes self4sustaining. !elow this point, the fuelair

mi'ture will not burn freely and continuously unless e'ternal heat is supplied.

T8"!8$HC$4 /f the fuel andairare mi'ed in swirling paths, instead of each flowing in streamlined paths,
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combustion will be greatly improved because the mi'ing of fuel and air is more complete. The proper amount of airfor a given amount of fuel means nothing if the two are not mi'ed.

-t oil burning plants, the oils burned must be heated on their way to the burner. This accomplishes two tasks. %irst, the oil

flows more readily when heatedN secondly, it atomiFes better.

C-8T/&HO

9trict adherence to fuel oil temperature limits is advised. Carbon formation

tends to increase at high temperatures, leading to clogged strainers and burnertips, and at e'tremely high or low temperatures, the booster pumps tend to

KcavitateK 6lose prime7, a condition that could lead to equipment damage. -t

low fuel oil temperatures, inadequate combustion will occur at the burners.

This leads to a loss of boiler efficiency and if not corrected, possibly tohaFardous furnace conditions.

The speed at which the chemical reaction between the carbon, hydrogen, and o'ygenoccurs is crucial to flame performance.

!y atomiFing the oil into very small droplets, more surface area of the oil is available for the o'ygen to come in contact with.The more surface area, the quicker the reaction will occur.

0ood combustion is very rapid, has a high flame temperature, and is very turbulent. Turbulence is a key factor in boiler

furnace combustion. /f the turbulence is high, the mi'ing of the o'ygen and fuel will be good, therefore, combustion willoccur very rapidly and the result will be a high flame temperature. /f the turbulence is low, mi'ing will not be as goodN

therefore, more time is required for complete combustion and the result is lower flame combustion and a lower flame

temperature.

The chain of events is as followsB

1. %irst, the fuel is gasified.

#. 9econd, it is mi'ed with air so the mi'ture is in the flammable range.

(. Third, the mi'tures temperature is raised to the ignition point.

*. %ourth, combustion takes place with time, temperature, and turbulence.

The precise amount of air required to complete combustion with no e'cess is called Ktheoretical air.K /n real combustion

systems, an e'cess amount of air is required above the theoretical amount to complete combustion. This is because mi'ing of

the fuel and air 6turbulence7 is not perfect and some of the o'ygen does not come in contact with the fuel while in the flameFone where temperatures are sufficient for combustion. This additional amount of air is commonly referred to as Ke'cess airK

and is e'pressed as Kpercent e'cess air.K 9ince e'cess air is supplied to the combustion process, all of the available o'ygen inthe air will not be used.

The o'ygen that is not used is referred to as Ke'cess o'ygenK and is e'pressed as Kpercent e'cess o'ygen.K The quantity of

e'cess air required is dependent on several parameters including boiler type, fuel properties, and burner characteristics. is

preferred for monitoring furnace performance for the following reasonsB

The relation of to e'cess air is relatively invariant with fuel composition, whereas C relations vary
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considerably.

C measurements require more precision than e'cess measurements to obtain the same accuracy.

$'cess is more associated with e'cess air, i.e., as e'cess air goes to Fero, e'cess follows.

$'cess instrumentation is generally less e'pensive and more reliable.

Lhen measuring or C, beware of stratification of gases within the duct and the possible intrusion of outside air through

leaks in the breaching, air heater seals, etc. Multiple point measurements as close to the boiler outlet as possible are preferred.

The chemical reactions occurring between the fuels and o'ygen within the flame areB

EEEC 4 C )eat $nergy #)# 4 )#& )eat $nergyEEE

/f there is a lack of o'ygen, some of the carbon will not be completely o'idiFed and carbon mono'ide will be formed. -notherway to look at a lack of o'ygen is to recogniFe that since fuel flow can be ad@usted, we can also say that the furnace is Kfuel

rich.K This condition is to be avoided. $'cess fuel will carry over to other portions of the furnace, through the air preheaters,and out of the stack as black smoke 6T)/9 /9 - K%8$ "/C)K C&H2/T/&H T& !$ -J&/2$27. Lhen this happens, only a

portion of the carbons heat energy is released.

Carbon mono'ide is, itself, a fuel, with a heating value of *,(++ !T8lb. /t is, therefore, classified as a EEcombustibleEE. !y

comparison, pure carbon has a heating value of 1*,+ !T8lb. &il fuel contains small amounts of sulfur and ash which aretransformed into sulfur o'ides and particulates. These are important from the standpoint of boiler operation and structural

integrity. The oil ash 6particulates7 sticks to tubes and boiler surfaces and leads to boiler fouling if not cleaned regularly6usually with soot blowing equipment7. EEE- portion of the sulfur o'ides combines with the water vapor 6formed in the

combustion process7 to form sulfuric acid, which is the primary chemical responsible for boiler corrosion.EEE

? ! L$/0)T

%uel Carbon )ydrogen &'ygen Hitrogen 9ulfur )eating Jalue 6!tulb7

G# &il I5. 11.> . 44 .+ 1>,*1

G &il I. 1.> .5 .* 1.+ 1I,+

Hatural 0as 5+ #(.+ 44 1. 44 #(,1

Talk Page

)aseous Fuels

The most used gaseous fuelis natural gas. Hatural gases vary in their chemical analysis and, thus, in their heating values. Theaverage heating value is appro'imately 1, !tu per standard cubic foot, but may range from >+ to over 1,1 !tuscf.
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Hote that, in all cases, the amount of methane is over I? by volume.

Table #B 9elected 9amples of Hatural 0as from 89 %ields

9ample Ho. 1 # ( * +

9ource of 0as P- 9o. C- &) - &=

-nalyses

Constituents,? by vol

)#)ydrogen 444 444 1.I# 444 444

C)* Methane I(.* I*. >(.(( >. I*.1

C#)* $thylene 444 444 .#+ 444 444

C#) $thane 1+.I 1*.I 444 +. .5

C& Carbon mono'ide 444 444 .*+ 444 444

C Carbon dio'ide 444 .5 .## 444 .I

H# Hitrogen .I .+ (.* +. I.*

&'ygen 444 444 .(+ 444 444

)#9 )ydrogen sulfide 444 444 .1I 444 444


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8ltimate,? by wt

9 9ulfur 444 444 .(* 444 444

)# )ydrogen #(.+( #(.( #(.# ##.I #.I+

C Carbon 5+.#+ 5*.5# >.1# >.# *.I*

H# Hitrogen 1.## .5 +.5 (. 1#.>

&'ygen 444 1.## 1.+I 444 1.*1

9pecific 0ravity 6rel. to air7 .( .( .+5 . .(

)igher heat value

!tucu. ft. Q;% and ( in. )g 1,1#> 1,11 >* 1,# >5*

!tulb of fuel #(,15 ##,>* ##,55 #1,I#* #,1

Hatural gas is the only ma@or fuel that is delivered by the supplier as it is used. EEE%igure #EEE shows a typical supply system for

natural gas.


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EEE%igure #B 0as Pressure "educing and Metering -rrangementEEE

Talk Page

Li*ui( Fuels

The most common liquidfuelis fuel oil, a product of the refining process. Lhile crude oil as produced from the well is

sometimes used, the most common fuel oils used for boiler fuel are the lightweight Ho. # fuel oil and the Ho. grade of

heavy residual fuel oil. EEE%igure (EEE shows a typical supply system for oil fuel.
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EEE%igure (B Typical %uel &il Pumping and )eating -rrangementEEE

/f the fuel is Ho. # fuel oil, heating of the fuel is normally unnecessary. /f the fuel is a heavy oil such as Ho. , it is usually

necessary toheatthe oil in the tanks so that it can be easily pumped through the system. /f heavy fuel oil in a tank is unused

for a period of time, the tank heating may cause the evaporation of some of the lighter constituents, ultimately making the oiltoo thick to remove from the tank by any normal means.
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EEE%igure *B %uel &il Temperature vs. JiscosityEEE

Most burners are designed for a viscosityof 1(+ to 1+ 9aybolt universal seconds 69987. - very important aspect of oil firing

is viscosity. The viscosity of oil varies with temperatureB the hotter the oil, the more easily it flows. /ndeed, most people are

aware that heavy fuel oils need to be heated in order to flow freely. Lhat is not so obvious is that a variation in temperature,

and hence viscosity, will have an effect on the siFe of the oil particle produced at the burner noFFle. %or this reason, thetemperature needs to be accurately controlled to give consistent conditions at the noFFle.

Talk Page

P!#sical Combustion Re*uirements

/n the previous section, we discussed the requirement for combustion using the fire triangle. The same process holds true in a
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furnace. EECombustionEE is the rapid o'idation offuelin a mi'ture of fuel andairwithheatproduced and carried by the mass offlue gas generated. Combustion takes place only under the conditions shown in EEE%igure +EEE.

EEE%igure +B Combustion "equirementsEEE

Time, Temperature, and Turbulence are the three EEETsEEE of combustion. - short period of time, high temperature, and very

turbulent flame indicates rapid combustion. Turbulence is the key because fuel and air must be thoroughly mi'ed if the fuel isto be completely burned. Lhen fuel and air are well mi'ed and all the fuel is burned, the flame temperature will be very high

and the combustion time will be shorter. Lhen the fuel and air are not well mi'ed, complete combustion may not occur, the

flame temperature will be lower, and the fuel will take longer to burn.

ess turbulence and longer burning has been known to produce fewer nitrous o'ides 6Ho'7. /n some cases, combustion has

been delayed or staged intentionally to obtain fewer nitrous o'ides or to obtain desired flame characteristics. The fuel must be

gasified. The oil must be atomiFed so that the temperature present can turn it into gas. The ignition temperature and flametemperature are different for different fuels if all other conditions are the same. Typical ignition temperatures when mi'ed

with air are shown in EEETable (EEE.

Table (B Typical /gnition Temperatures

%8$ TP$ /0H/T/&H T$MP$"-T8"$

ight %uel &il ;%

)eavy %uel &il 5+;%

Hatural 0as 1,;%

Hote that the gases have the highest temperature required for ignition. iquids have the lowest ignition temperatures when

properly atomiFed and mi'ed with air. Hatural gas cannot be ignited if less than *? of the theoretical air required for
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combustion is present.

%or any fuel, a precise amount of combustion air is needed to furnish theo'ygenfor complete combustion of that fuelscarbon and hydrogen 6see EEE%igure EEE7. The precise amount of air is called the EEtheoretical airEE for that particular fuel.

EEE%igure B !asic Combustion Chemistry and Products of CombustionEEE

The amount of carbon and o'ygen for complete combustion of carbon is represented by the formulaB

EEEC DC 1*,1 !tulb ;CEEE

Twelve pounds of carbon combine with (# pounds of o'ygen to form ** pounds of carbon dio'ide heat.

The formula for combustion of hydrogen isB

EEE#)# D #)#& 1, !utlb )#EEE

%our pounds of hydrogen combine with (# pounds of o'ygen to form ( pounds of water.

- simple e'ample of the many incomplete combustion reactions resulting in intermediate hydrocarbon compounds is thepartial combustion of carbon, resulting in carbon mono'ide rather than carbon dio'ide. /n this case, some of the potential heat
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from the carbon remains in the carbon mono'ide.

EEE#C D#C& *,(*+ !tulb CEEE

Twenty4four pounds of carbon combine with (# pounds of o'ygen to form + pounds of carbon mono'ide. Lith the right

conditions of time, temperature, and turbulence, and by adding more o'ygen to the carbon mono'ide, it will further o'idiFe to

carbon dio'ide, releasing additional heat energy.

EEE#C& D #C *,(*+ !tulb C&EEE

-s indicated, the combustion process produces heat, but a low percentage of this heat is not useful in transferring heat to the

boiler water. -s hydrogen combines with o'ygen to form water, the combustion temperature vaporiFes the water into

superheated steam. This vaporiFation absorbs latent heat. -s the gases pass through the boiler and e'it from the system, thegases retain the vaporiFed water in the form of superheated steam and the heat is lost from the process. The hydrogen content

of the fuel determines this amount of heat loss. /t is important to keep in mind that combustion air must be furnished for thetotal combustion or on the basis of the ))J, while only the )J has any effect on the heat transfer of the system.

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Air

The air supply for the combustion process must be adequate for theoretical combustion and also provide Ke'cess airK to ensure

complete combustion. -s shown in the graph of EEE%igure 5EEE, as the air is increased the combustion is improved. &nce the

e'cessive air becomes too great, the loss ofheatreduces boiler efficiency.
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EEE%igure 5B !oiler $fficiency 0raphEEE

$'cess air can be determined by the amount ofo'ygenin the flue gas and calculated byB

$'cess air 6?7 D

whereB = D .> for gas and .>* for oil.

Typical measurements of o'ygen in the flue gas are shown in EEETable *EEE.

Table *B Typical /gnition Temperatures

$RC$99 -/" "$S8/"$2 -T %8 C-P-C/T
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%uel? &'ygen in flue

gas

? $'cess air,

minimum

Hatural 0as 1.+ to ( 5 to 1+

%uel &il . to ( ( to 1+

-n adequate flow of air and combustion gases is required for the complete and effective combustion of fuel. %low is createdand sustained by the stack and fans. EE2raftEE is the difference between atmospheric pressure and the static pressure of the

combustion gases in a furnace. The flow of gases can be created by four methodsB

%orced draft

/nduced draft

!alanced draft

Hatural draft

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Force( Drat

%orced draft boilers operate with the airand combustion products maintained above atmospheric pressure.%ansat the inlet to

the boiler system, called EEforced draftEE 6%27 fans, provide sufficientpressureto force the air and flue gas through the system.%2 fans supply the necessary air for fuelcombustion and must be siFed to handle the stoichiometric air plus e'cess air needed

for burning the fuel. They also provide air to make up for air heater leakage and for some sealing air requirements.

"adial airfoil 6centrifugal7 or variable pitch 6a'ial7 fans are preferred for %2 service. %2 fans operate in the cleanestenvironment in the plant associated with a boiler. Most %2 fans have inlet silencers and screens to protect the fans from

entrained particles in the incoming air.

!oth the air temperature at the power plant and the elevation above sea level affect air density and, therefore, are a directinfluence on fan capacity.

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In(uce( Drat

/nduced draft boilers operate with airand combustion pressure below atmospheric. 9tatic pressure is progressively lower as

gas travels from the inlet to the induced draft fan. /nduced draft 6/27 fans e'haust combustion products from the boiler. /n

doing so, they create sufficient negative pressure to establish a slight suction in the furnace 6.# to .+ inches of water7. -nairfoil centrifugal fan is typically used.

Hatural draft boilers operate with draft formed by the stack alone.
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Talk Page

Balance Drat

!alanced draft boilers have a forced draftfan at the boiler inlet and aninduced draftfan at the system outlet. This reduces

both flue gas pressure and the tendency of combustion gases to escape the furnace. Most modern boilers are balanced draft.

The %2 fans supply combustionair. The forced draft fans, in con@unction with trim, maintain the properfuelair ratiofor

maintaining proper combustion and furnace safety. %low is controlled by modulating the inlet vanes controls airflow.

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Air %eaters

-ir preheaters reclaim someheatfrom the flue gas and add it to the air required for combustion. 8se of preheated air willspeed up combustion at all loads, improve combustion at low loads and increase efficiency.

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Burners an( Controls

!urners are the devices responsible forB

Proper mi'ing offueland airin the correct proportions, for efficient and complete combustion

2etermining the shape and direction of the flame

Coal, as a boiler fuel, tends to be restricted to specialiFed applications such as water4tube boilers in power stations. Thissection reviews the most common fuels for heating boilers.

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Oil Burners

-s previously mentioned, oil must be atomiFed for optimal combustion. &il burners are classified according to the method

used for atomiFationB

-ir4atomiFing burners

9team4atomiFing burners

Mechanical4atomiFing burners

The ability to burnfueloil efficiently requires a high fuel surface area4to4volume ratio. $'perience has shown that oilparticles in the range of # to * m are the most successful. Particles which areB

!igger than * EEEEm tend to be carried through the flame without completing the combustion process

9maller than # EEEEm may travel so fast that they are carried through the flame without burning at all
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EEE%igure IEEE shows a typical burner assembly.

EEE%igure IB Typical !urner -ssemblyEEE

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Pressure +et Burners

- pressure @et burner 6EEE%igure >EEE7 is simply an orifice at the end of a pressuriFed tube. Typically, thefueloilpressureis in the

range of 1+ to #1I P9/.

/n the operating range, the substantial pressure drop created over the orifice when the fuel is discharged into the furnaceresults in atomiFation of the fuel. Putting a thumb over the end of a garden hosepipe creates the same effect.
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EEE%igure >B Pressure 3et !urnerEEE

Jarying the pressure of the fuel oil immediately before the orifice 6noFFle7 controls the flow rate of fuel from the burner.

-dvantages of pressure @et burnersB

"elatively low cost

9imple to maintain

2isadvantages of pressure @et burnersB

/f the plant operating characteristics vary considerably over the course of a day, then the boiler will have to be taken

off4line to change the noFFle.

$asily blocked by debris, this means that well maintained, fine mesh strainersare essential.

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Rotar# Cup Burners

/n a rotary cup burner 6EEE%igure 1EEE7,fueloil is supplied down a central tube, and discharges onto the inside surface of a

rapidly rotating cone. -s the fuel oil moves along the cup 6due to the absence of a centripetal force7, the oil film becomes

progressively thinner as the circumference of the cap increases. $ventually, the fuel oil is discharged from the lip of the coneas a fine spray.
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EEE%igure 1B "otary Cup !urnerEEE

!ecause the atomiFation is produced by the rotating cup, rather than by some function of the fuel oil 6e.g.,pressure7, the

turndown ratio is much greater than the pressure @et burner.

9ome advantages of rotary cup burners are that they are robust, have a good turndown ratio, and fuelviscosityis less critical.The ma@or disadvantage of rotary cup burners is they are more e'pensive to buy and maintain.

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)as Burners

-t present, gas is probably the most common fuelused in the facilities. -tomiFation is not an issue with a gas, and proper

mi'ing of gas with the appropriate amount ofairis all that is required for combustion. Two types of gas burners in use are

low4pressure and high4pressure.

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Lo&,Pressure Burner

These operate at low4pressure, usually between #.+ and 1 mbar. The burner is a simple venturi device with gas introduced in

the throat area and combustion airbeing drawn in from around the outside 6EEE%igure 11EEE7.

EEE%igure 11B ow4Pressure 0as !urnerEEE

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%i$!,Pressure Burner

These operate at higher pressures, usually between 1# and 15+ mbar, and may include a number of noFFles to produce a

particular flame shape.

Talk Page

Dual Fuel Burners

The usual arrangement is to have afueloil supply available on site, and to use this to fire the boiler when gas is not available.

This led to the development of Kdual4fuelK burners 6EEE%igure 1#EEE7. These burners are designed with gas as the main fuel, buthave an additional facility for burning fuel oil.
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EEE%igure 1#B 2ual %uel !urnerEEE

The following procedure is an e'ample of how the changeover from gas to oil is accomplishedB

1. /solate the gas supply line.

#. &pen the oil supply line and switch on the fuel pump.

(. &n the burner control panel, select oil firingN6this will change the airsettings for the different fuel.7

*. Purge and refire the boiler.

This operation can be carried out in quite a short period. /n some facilities, the changeover may be carried out as part of aperiodic drill to ensure that operators are familiar with the procedure, and any necessary equipment is available.

)owever, because fuel oil is only Kstandby,K and probably only used for short periods, the oil firing facility may be basic. &n

more sophisticated plants, with a highly rated boiler plant, the gas burner6s7 may be withdrawn and oil burners substituted.

There is more to a burner than @ust blowing fire into a boiler or another heating device. 3ust what is a burner supposed to doU

Provide heat to a boiler.

Control the outlet temperature orpressureof a boiler.

Provide a high turndown so that it does not shut off over the full range of boiler load demands.
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!urn the fuel in the most efficient way possible to keep fuel consumption low.

The following are some basics about how a burner functions. Hatural gas will be used as the basic fuel, but fuel oils follow

the same rules. !efore we start, here are a couple of terms and their meaning that youll need to understand.

EEE$'cess -irEEE4 The e'tra amount of air added to the burner above that is required to completely burn the fuel

EEETurndownEEE4 The ratio of the burners ma'imum !T8) firing capability to the burners minimum !T8) firing capability.

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C!emistr# o Combustion

Hatural gas is primarily composed of methane, or C)*. Lhen mi'ed with the proper amount ofairand heated to the

combustion temperature, it burns. EEE%igure 1( EEEshows the process with the amount of air andfuelrequired for perfect

combustion.

EEE%igure 1(B Combustion ProcessEEE

Perfection is absolutely impractical, however. $'tra or e'cess air must be added to assure safe burner operation. %orced

draftburners use fans to supply air for combustion. The fan on a burner moves a constant volume of air, not molecules. -ny

change in temperature or barometric pressure causes a change in the number of air molecules that the fan moves.
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efficiency.

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E"cess Air- Eicienc# an( Turn(o&n

-n important function of burners is turndown. This is usually e'pressed as a ratio and is based on the ma'imum firing ratedivided by the minimum controllable firing rate.

EEE%igure 1*EEE shows a simplified burner head. Theairis brought into the head by means of a forced draftblower orfan. The

gas is metered into the head through a series of valves. /n order to get proper combustion, the air molecules must bethoroughly mi'ed with the gas molecules before they actually burn.

EEE%igure 1*B 9implified !urner )eadEEE

The mi'ing is achieved by burner parts designed to create high turbulence. /f insufficient turbulence is produced by the

burner, the combustion will be incomplete and samples taken at the stack will reveal carbon mono'ide as evidence.

9ince the velocity of air affects the turbulence, it becomes harder and harder to get good fueland air mi'ing at higherturndown ratios since the air amount is reduced. Towards the highest turndown ratios of any burner, it becomes necessary to

increase the e'cess air amounts to obtain enough turbulence to get proper mi'ing. The better burner design will be one that isable to properly mi' the air and fuel at the lowest possible airflow or e'cess air.

EEE%igure 1+EEE graphically displays how e'cess air affects the efficiency and operating cost of a boiler. The data was compiledon an actual boiler.
https://www.myodesie.com/index.php/forum/index/talkpage/id/3054https://www.myodesie.com/index.php/wiki/index/returnEntry/id/Combined%20Fundamentals#Airhttps://www.myodesie.com/index.php/wiki/index/returnEntry/id/Combined%20Fundamentals#Airhttps://www.myodesie.com/index.php/wiki/index/returnEntry/id/3054#Forced%20Drafthttps://www.myodesie.com/index.php/wiki/index/returnEntry/id/HVAC%20Systems#Fanshttps://www.myodesie.com/index.php/wiki/index/returnEntry/id/HVAC%20Systems#Fanshttps://www.myodesie.com/index.php/wiki/index/returnEntry/id/3054#Fuelhttps://www.myodesie.com/index.php/forum/index/talkpage/id/3054https://www.myodesie.com/index.php/wiki/index/returnEntry/id/Combined%20Fundamentals#Airhttps://www.myodesie.com/index.php/wiki/index/returnEntry/id/3054#Forced%20Drafthttps://www.myodesie.com/index.php/wiki/index/returnEntry/id/HVAC%20Systems#Fanshttps://www.myodesie.com/index.php/wiki/index/returnEntry/id/3054#Fuel


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EEE%igure 1+B $ffect of $'cess -ir on !oiler $fficiencyEEE

EEE%igure 1EEE shows the savings realiFed with a 1 horsepower load at various efficiencies caused by different e'cess airlevels.


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EEE%igure 1B $ffect of $'cess -ir on %uel CostsEEE

There are several strong reasons why high turndown and low e'cess air are important. The first is the operating cost of the

burner. ou have seen how e'cess air affects the operating cost, but the turndown ratio of a burner has a big affect as well.

$very time the burner starts and stops there is a cost associated. -ir is always blown through the boiler to ensure that there isno unburned fuel remaining. These purges make the boiler work like a chiller because it takes energy out of the system. Two

other reasons for having a high turndown relate to lowered maintenance costs and better process or heating control.

2o not confuse turndown with Kfully modulatingK burners. )aving a fully modulating burner with only the typical turndownof 1.5 to 1 is like having a car that can only go between speeds of +> MP) and 1 MP). /t is a Kfully modulatingK car,but try

driving it to the grocery store. ou would not only look silly, but think of the how the gas mileage would drop.

EEE%igure 15EEE shows how the turndown ratio of a burner impacts the fuel cost needed to run a 1 horsepower boiler forheating. Lhen you combine the effects of low e'cess air and high turndown, the operating cost savings can range from 1?

to 1+? below a brand new burner that does not have those characteristics.


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EEE%igure 15B $ffect of Turndown on %uel CostsEEE

Process control is enhanced with a high turndown. /f the load is smaller than the burner can turn down to, it cycles on and off.

Lhen off, thepressureor temperature falls off. &n some boilers, we have seen steam pressures drop from 1 psig at burner

shutdown to about * psig before the burner comes on again. That can cause problems in a manufacturing plant that depends

on constant steam pressure. $ven on hot water heating systems, control problems occur because of low turndown boilers.Jalves hunt and temperature control becomes erratic. Lith a high turndown, those fluctuations are eliminated because the

burner tracks the load down to the point where it shuts off only when the load is very slight. There is enough stored energy inthe system to take up the small fluctuations at that point.

Maintenance costs are reduced with a high turndown burner because there is much less thermal cycling taking place in the

boiler. Lhen a burner cycles, the refractory and metal parts e'pand and contract. -lthough those materials are built to take it,their life is prolonged if everything stays the same temperature. 0as valves, ignition transformers, etc. are all less prone to fail

if they never have to cycle. /f the burner stays on, they dont have to turn on and off and, therefore, last longer.

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Steam &ngineering Tutorials'The (oiler )ouse

(oiler &iciency and Combustion

A #roa$ o%er%ie& o' the co(#u)tion *roce))+ inclu$ing #urner ty*e) an$ control)+ an$ heat out*ut an$ lo))e),

This Tutorial is intended to give a very broad overview o the combustion process! which is an essential component o

overall boiler eiciency. Readers re*uiring a more in+depth knowledge are directed towards specialist textbooks and burner

manuacturers.

(oiler eiciency simply relates energy output to energy input! usually in percentage terms,

&*uation -../

0)eat exported in steam0 and 0)eat provided by the uel0 is covered more ully in the ollowing two Sections.

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)eat exported in steam

This is calculated 1using the steam tables2 rom knowledge o,

The eedwater temperature.

The pressure at which steam is exported.

The steam lowrate.

)eat provided by the uelCaloriic value

This value may be expressed in two ways 03ross0 or 04et0 caloriic value.

3ross caloriic value

This is the theoretical total o the energy in the uel. )owever! all common uels contain hydrogen! which burns with oxygen

to orm water! which passes up the stack as steam.

The gross caloriic value o the uel includes the energy used in evaporating this water. %lue gases on steam boiler plant are

not condensed! thereore the actual amount o heat available to the boiler plant is reduced.

5ccurate control o the amount o air is essential to boiler eiciency,

Too much air will cool the urnace! and carry away useul heat.

Too little air and combustion will be incomplete! unburned uel will be carried over and smoke may be produced.

Table -../

%uel oil data

Table -..6

3as data


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4et caloriic value

This is the caloriic value o the uel! excluding the energy in the steam discharged to the stack! and is the igure generally

used to calculate boiler eiciencies. In broad terms,

4et caloriic value 7 3ross caloriic value + /89

Where,

C : Carbon

) : )ydrogen

; : ;xygen

4 : 4itrogen

5ccurate control o the amount o air is essential to boiler eiciency,

Too much air will cool the urnace! and carry away useul heat.

Too little air and combustion will be incomplete! unburned uel will be carried over and smoke may be produced.

In practice! however! there are a number o diiculties in achieving perect 1stoichiometric2 combustion,

The conditions around the burner will not be perect! and it is impossible to ensure the complete

matching o carbon! hydrogen! and oxygen molecules.

Some o the oxygen molecules will combine with nitrogen molecules to orm nitrogen oxides 14;x2.

To ensure complete combustion! an amount o 0excess air0 needs to be provided. This has an eect on boiler eiciency.

The control o the airyoto

5greement o /??@. These countries agreed to take positive and individual actions to,


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Re$uce the e(i))ion o' har('ul ga)e) to the at(o)*here -5lthough carbon dioxide 1C;62 is the least potent

o the gases covered by the agreement! it is by ar the most common! and accounts or approximately A89 o the

total gas emissions to be reduced.

Ma.e /uanti'ia#le annual re$uction) in 'uel u)e$ - This may take the orm o using either alternative! non+

polluting energy sources! or using the same uels more eiciently.

In the B>! the commitment is reerred to as 0The B> 4ational 5ir uality Strategy0! and this is having an eect via a number

o laws and regulations. ;ther countries will have similar strategies.

Technology

$ressure rom legislation regarding pollution! and rom boiler users regarding economy! plus the power o the microchip have

considerably advanced the design o both boiler combustion chambers and burners.

#odern boilers with the latest burners may have,

Re+circulated lue gases to ensure optimum combustion! with minimum excess air.

Sophisticated electronic control systems that monitor all the components o the lue gas! and make adDustments to

uel and air lows to maintain conditions within speciied parameters.

3reatly improved turndown ratios 1the ratio between maximum and minimum iring rates2 which enable eiciency

and emission parameters to be satisied over a greater range o operation.

)eat losses

)aving discussed combustion in the boiler urnace! and particularly the importance o correct air ratios as they relate to

complete and eicient combustion! it remains to review other potential sources o heat loss and ineiciency.

)eat losses in the lue gases

This is probably the biggest single source o heat loss! and the &ngineering #anager can reduce much o the loss.

The losses are attributable to the temperature o the gases leaving the urnace. Clearly! the hotter the gases in the stack! the

less eicient the boiler.

The gases may be too hot or one o two reasons,

The #urner i) *ro$ucing (ore heat than i) re/uire$ 'or a )*eci'ic loa$ on the #oiler0

o This means that the burner1s2 and damper mechanisms re*uire maintenance and re+calibration.

The heat tran)'er )ur'ace) &ithin the #oiler are not 'unctioning correctly+ an$ the heat i) not #eing

tran)'erre$ to the &ater0


37/45


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(urner turndown

5n important unction o burners is turndown. This is usually expressed as a ratio and is based on the maximum iring rate

divided by the minimum controllable iring rate.

The turndown rate is not simply a matter o orcing diering amounts o uel into a boiler! it is increasingly important rom an

economic and legislative perspective that the burner provides eicient and proper combustion! and satisies increasingly

stringent emission regulations over its entire operating range.

5s has already been mentioned! coal as a boiler uel tends to be restricted to specialised applications such as water+tube

boilers in power stations. The ollowing Sections within this Tutorial will review the most common uels or shell boilers.

;il burners

The ability to burn uel oil eiciently re*uires a high uel surace area+to+volume ratio. &xperience has shown that oil particles

in the range 68 and G8 Hm are the most successul.

$articles which are,

(igger than G8 m tend to be carried through the lame without completing the combustion process.

Smaller than 68 m may travel so ast that they are carried through the lame without burning at all.

5 very important aspect o oil iring is viscosity. The viscosity o oil varies with temperature, the hotter the oil! the more easily

it lows. Indeed! most people are aware that heavy uel oils need to be heated in order to low reely. What is not so obvious

is that a variation in temperature! and hence viscosity! will have an eect on the sie o the oil particle produced at the burner

nole. %or this reason the temperature needs to be accurately controlled to give consistent conditions at the nole.

Pressure jet burners

5 pressure Det burner is simply an oriice at the end o a pressurised tube. Typically the uel oil pressure is in the range @ to

/F bar.

In the operating range! the substantial pressure drop created over the oriice when the uel is discharged into the urnace

results in atomisation o the uel. $utting a thumb over the end o a garden hosepipe creates the same eect.


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%ig. -../$ressure Det burner

Jarying the pressure o the uel oil immediately beore the oriice 1nole2 controls the lowrate o uel rom the burner.

)owever! the relationship between pressure 1$2 and low 1%2 has a s*uare root characteristic! K$ %! or knowing the

lowrate $ %6.

%or example i,

%6: 8.F %/$6: 18.F26$/$6: 8.6F $/

I the uel lowrate is reduced to F89! the energy or atomisation is reduced to 6F9.

This means that the turndown available is limited to approximately 6,/ or a particular nole. To overcome this limitation!

pressure Det burners are supplied with a range o interchangeable noles to accommodate dierent boiler loads.

A$%antage) o' *re))ure 1et #urner)0

Relatively low cost.

Simple to maintain.

i)a$%antage) o' *re))ure 1et #urner)0

I the plant operating characteristics vary considerably over the course o a day! then the boiler will have to be

taken o+line to change the nole.

&asily blocked by debris. This means that well maintained! ine mesh strainers are essential.


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Rotary cup burner

%uel oil is supplied down a central tube! and discharges onto the inside surace o a rapidly rotating cone. 5s the uel oil

moves along the cup 1due to the absence o a centripetal orce2 the oil ilm becomes progressively thinner as the

circumerence o the cap increases. &ventually! the uel oil is discharged rom the lip o the cone as a ine spray.

%ig. -..6

Rotary cup burner

(ecause the atomisation is produced by the rotating cup! rather than by some unction o the uel oil 1e.g. pressure2! the

turndown ratio is much greater than the pressure Det burner.

5dvantages o rotary cup burners,

Robust.

3ood turndown ratio.

%uel viscosity is less critical.

Eisadvantages o rotary cup burners,


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#ore expensive to buy and maintain.

3as burners

5t present! gas is probably the most common uel used in the B>.

(eing a gas! atomisation is not an issue! and proper mixing o gas with the appropriate amount o air is all that is re*uired or

combustion.

Two types o gas burner are in use 0=ow pressure0 and 0)igh pressure0.

=ow pressure burner

These operate at low pressure! usually between 6.F and /8 mbar. The burner is a simple venturi device with gas introduced

in the throat area! and combustion air being drawn in rom around the outside.

;utput is limited to approximately / #W.

%ig. -..-

=ow pressure gas burner

)igh pressure burner

These operate at higher pressures! usually between /6 and /@F mbar! and may include a number o noles to produce a

particular lame shape.

Eual uel burnersThe attractive 0interruptible0 gas tari means that it is the choice o the vast maDority o organisations in the B>. )owever!

many o these organisations need to continue operation i the gas supply is interrupted.


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%ig. -..G

Eual uel burner

The usual arrangement is to have a uel oil supply available on site! and to use this to ire the boiler when gas is not

available. This led to the development o 0dual uel0 burners.

These burners are designed with gas as the main uel! but have an additional acility or burning uel oil.

The notice given by the 3as Company that supply is to be interrupted may be short! so the change over to uel oil iring is

made as rapidly as possible! the usual procedure being,

Isolate the gas supply line.

;pen the oil supply line and switch on the uel pump.

$urge and re+ire the boiler.

;n the burner control panel! select 0oil iring0

1This will change the air settings or the dierent uel2.

This operation can be carried out in *uite a short period. In some organisations the change over may be carried out as part

o a periodic drill to ensure that operators are amiliar with the procedure! and any necessary e*uipment is available.

)owever! because uel oil is only 0stand+by0! and probably only used or short periods! the oil iring acility may be basic.

;n more sophisticated plants! with highly rated boiler plant! the gas burner1s2 may be withdrawn and oil burners substituted.


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Table -..G

Typical turndown ratio available with dierent types o burner

(urner control systems

The reader should be aware that the burner control system cannot be viewed in isolation. The burner! the burner controlsystem! and the level control system should be compatible and work in a complementary manner to satisy the steam

demands o the plant in an eicient manner.

%ig. -..F

Relating boiler output to controls and burner type

The next ew paragraphs broadly outline the basic burner control systems.

;n < o control system

This is the simplest control system! and it means that either the burner is iring at ull rate! or it is o. The maDor disadvantageto this method o control is that the boiler is subDected to large and oten re*uent thermal shocks every time the boiler ires.

Its use should thereore be limited to small boilers up to F88 kg < h.

Advantages of an on / off control system:

Simple.

=east expensive.

Disadvantages of an on / off control system:

I a large load comes on to the boiler Dust ater the burner has switched o! the amount o steam available is

reduced. In the worst cases this may lead to the boiler priming and locking out.

Thermal cycling.

)igh < low < o control system


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This is a slightly more complex system where the burner has two iring rates. The burner operates irst at the lower iring rate

and then switches to ull iring as needed! thereby overcoming the worst o the thermal shock. The burner can also revert to

the low ire position at reduced loads! again limiting thermal stresses within the boiler. This type o system is usually itted to

boilers with an output o up to F 888 kg < h.

Advantages of a high / low / off control:

The boiler is better able to respond to large loads as the 0low ire0 position will ensure that there is more stored

energy in the boiler.

I the large load is applied when the burner is on 0low ire0! it can immediately respond by increasing the iring rate

to 0high ire0! or example the purge cycle can be omitted.

Disadvantages of a high / low / off control system:

#ore complex than on+o control.

#ore expensive than on+o control.

#odulating control system

5 modulating burner control will alter the iring rate to match the boiler load over the whole turndown ratio. &very time the

burner shuts down and re+starts! the system must be purged by blowing cold air through the boiler passages. This wastes

energy and reduces eiciency. %ull modulation! however! means that the boiler keeps iring over the whole range to

maximise thermal eiciency and minimise thermal stresses. This type o control can be itted to any sie boiler! but should

always be itted to boilers rated at over /8 888 kg < h.

Advantages of a modulating control system:

The boiler is even more able to tolerate large and luctuating loads. This is because,

The boiler pressure is maintained at the top o its control band! and the level o stored energy is at its greatest.

Should more energy be re*uired at short notice! the control system can immediately respond by increasing the

iring rate! without pausing or a purge cycle.

Disadvantages of a modulating control system:

#ost expensive.

#ost complex.

(urners with a high turndown capability are re*uired.

Saety

5 considerable amount o energy is stored in uel! and it burns *uickly and easily. It is thereore essential that,


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Saety procedures are in place! and rigorously observed.

Saety interlocks! or example purge timers! are in good working order and never compromised.

Co*yright 2 3456

S*ira7 Sarco Li(ite$

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