Boiler Defnition

8/2/2019 Boiler Defnition

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BOILER DEFNITION


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Boiler is a Closed Vessel exceeding 22.75 litres in Capacity which is used expressly for

generating steam under pressure and includes any mounting or other fitting

attached to such vessel, which is wholly or partly under pressure when steam is shut off.


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STANDARD OPERATINGPARAMETERS

TYPE OF BOILER FBC

CAPACITY 12 TPH

WORKING PRESSURE 10.5 KG/CM2

DRYNESS FRACTION 99.9% TYPE OF FUELS HSD/RFO

FUEL CONSUMPTION 1330 KG/HR

TURN DOWN RATIO

THERMAL EFFICIENCY 88.2%

STEAM OUTLET TEMPERATURE 100*c

SAFETY VALVE SET PRESSURES 11.0 & 11.25


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SYSTEMS

AIR SYSTEM

This System starts with F.D fan to supply required combustion air & import energy for

fluidisation .F.D fan is a centrifugal type, direct coupled coupled fan. Ambient air from

fan outlet is heated in recuperative type heater & airheater(i.e air preheater).This air

is distributed evenly to combustion chamber through compartmentalised air box. A

part of combustion air is tapped from air heater outlet and further pressurised by a

P.a fan for pneumatic fuel feeding. P.A fan is also a centrifugal type direct coupled

fan.air flow is measured by using aerofoil meter

BASIC FLOW OF AIR

->F.DHEADER -> COMBUSTION AIR

FORCED DRAFT FAN->AIR PRE-HEATER->|


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(primaryair) ->PRIMARY AIR FAN ->PRIMARY AIR

HEADER->COAL FEEDERS

COMBUSTION CHAMBER

The distributor plate is heart of FBC system.It is made of carbon steel base plate with

air nozzles to distribute fluidised air from airbox uniformly all over the bed. Bed coils

are immersed in the bed to maintain the temperature of 850-900*c by absorbing the

heat. Sufficient free board volume is available above the bed to ensure complete

Combustion OF fuel.

FUEL AND ASH HANDLING SYSTEM

The heat from bunker is fed pneumatically into the bed by pocket feeder and mixing

nozzles located below the bunker. The fuel flow is automatically adjusted by the

control panel according to the steam demand. The ash generated is collected at

bed,bank,air-heater and ESP zones.

FLOW OF ASH


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PANELS->MUD DRUM->ECONOMISER, AIRPREHEATER->ASH BLOWER->E.S.P

FLUE GAS SYSTEM

The hot flue gas generated from combustion chamber is cooled by passing through

waterwall bank tubes and airheater. Balanced draft is maintained by FD and ID fans.

Flue gas is finally let, into the atmosphere through chimney.

FLOW OF FLUE GASES

ECONOMISER->AIRPREHEATER->I.D FAN->CHIMNEY

WATER AND STEAM CIRCUIT

The deareator water is stored in the deareator storage tank. The deareator water at105*c is fed to the boiler feed pump where it is pressurised to the required boiler

pressure. The feed water from feed pump is fed into the steam drum. From steam


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drum the water passes through the bank tubes to mud drum. While passing through

the bank tube the water is heated by the flue gases passing over the tubes. The hot

water enters into the bed coils trough downcomers and further heated up. The watermixture than raises through the water panel tubes and enters into the steam drum

through riser tubes. From the outlet of maintanence steam stop valve in the steam is

taken to process

FLOW OF WATER

FEED WATER TANK->DEAREATOR->ECONOMISER->STEAM DRUM->BANK TUBES-

>MUD DRUM

FLOW OF STEAM

STEAM DRUM->BANK TUBES->MUD DRUM->PANEL TUBES->MAIN STREAM VALVE


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BALANCED DRAFT

A boiler system having I.D fan and F.D fan is called balanced draft system. In this case

the system is designed i.e I.D & F.D fan are operated or amount of air is controlled

such that the furnace is at a slightly negative pressure because w.r.t atmospheric

pressure. The furnace is not operated at higher negative pressure because of

leakages in the fuel burning equipments.

Advantage:

1.Operates at slightly negative pressure assures any leakage will be relatively coolcombustion air leaking into the furnace instead of very hot combustion air

leaking out into atmosphere.

2.The atmospheric air is not drawn inside the furnace which does not create a lossof efficiency otherwise which could have been a waste of energy in heating up

unwanted area.


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Disadvantage: Need proper control of furnace pressure for safety reasons and for

maintaining proper working efficiency of boiler

MAINPARTS OF BOILER

FEED WATER TANK DEAREATOR ECONOMISER AIR PREHEATER COMBUSTION CHAMBER STEAM DRUM MUD DRUM ASH BLOWER E.S.P I.D FAN AND F.D FAN


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CHIMNEY COAL FEEDER FANS

FEED WATER TANK.

One of the main parts of feedwater system is deareator feedwater tank. The massbalance between steam produced by boiler and the feedwater should be maintained

in a fluctuating load conditions based on firing rate and steam demand. Furthermore

feed water system should be controlled to supply enough water into steam boiler.

Steam boiler has some systems which running together to produce high quality

steam. One of the systems is feedwater system. The required water must faces watertreatment system before supplied into steam boiler, so the water has required

parameter contents about silica, scaling, conductivity, PH, dissolved oxygen and so
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on. The main consideration is how to remove oxygen to prevent corrosion and

remove scaling which can isolate heat transfer process in the pressure parts of steam

boiler (water wall, header, economizer, etc).

Feed Water Tank Receives Water from Both Feed Pumps and Steam condensate

Pipeline. The Feed Water Supplied Is De-mineralised And Passed Through Degasser

Such That It Has A P.H


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steam is injected to deareator to remove gases content and dissolved oxygen and

then discharge them through deareator venting. In addition to remove oxygen

content, low pressure steam is also used to increase temperature up to 130 140 C.The water which gas content and dissolved oxygen had been removed enters to feed

water pumps.

DEAREATOR PRINCIPLE

The removal of dissolved gases from boiler feed water is an essential process in a

steam system. The presence of dissolved oxygen in feed water causes rapid localizedcorrosion in boiler tubes. Carbon dioxide will dissolve in water, resulting in low pH

levels and the production of corrosive carbonic acid. Low pH levels in feed water

causes severe acid attack throughout the boiler system. While dissolved gases and

low pH levels in the feed water can be controlled or removed by the addition of

chemicals, it is more economical and thermally efficient to remove these gasesmechanically. This mechanical process is known as deaeration and will increase the

life of a steam system dramatically.


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Deaeration is based on two scientific principles. The first principle can be described

by Henry's Law. Henry's Law asserts that gas solubility in a solution decreases as the

gas partial pressure above the solution decreases. The second scientific principle thatgoverns deaeration is the relationship between gas solubility and temperature. Easily

explained, gas solubility in a solution decreases as the temperature of the solution

rises and approaches saturation temperature. A deaerator utilizes both of these

natural processes to remove dissolved oxygen, carbon dioxide, and other non-

condensable gases from boiler feed water. The feed water is sprayed in thin films into

a steam atmosphere allowing it to become quickly heated to saturation. Spraying

feed water in thin films increases the surface area of the liquid in contact with the

steam, which, in turn, provides more rapid oxygen removal and lower gas

concentrations. This process reduces the solubility of all dissolved gases and removes

it from the feed water. The liberated gasesare then vented from the deareator.


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With these principles in mind, the dissolved oxygen is removed. This system reduces

dissolved oxygen concentration to less than 0.005 cc/liter (7 ppb), and completely

eliminates the carbon dioxide concentration.The deareator tank receives from condensate vam steam as well. The water from

deareator tank enters into the feed water pumps which pump the water to the

economiser.

ECONOMISERS

Economisers are mechanical devices intended to reduce energy consumption, or to

perform another useful function like preheating a fluid. The term economizer is usedfor other purposes as well.In simple terms, an economizer is a heat exchanger.

In boilers, economizers are heat exchange devices that heat fluids, usually water, up to

but not normally beyond the boiling point of that fluid. Economizers are so named

because they can make use of the enthalpy in fluid streams that are hot, but not hotenough to be used in a boiler, thereby recovering more useful enthalpy and improvingthe boiler's efficiency. They are a device fitted to a boiler which saves energy by using
http://en.wikipedia.org/wiki/Fluidhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Boilinghttp://en.wikipedia.org/wiki/Enthalpyhttp://en.wikipedia.org/wiki/Enthalpyhttp://en.wikipedia.org/wiki/Boilinghttp://en.wikipedia.org/wiki/Heat_exchangerhttp://en.wikipedia.org/wiki/Fluid


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the exhaust gases from the boiler to preheat the cold water used to fill it (thefeed

water).

In an economizer the water passes through tubes surrounded by hot gases. But this isbut a detail. The important thing is this: In a boilerthe burning gases heat the water to

produce steam for various uses. In an economizer, some of the heat energy that would

otherwise all be lost to the atmosphere is instead used to heat the water and/or air that

will go into the boiler, thus saving fuel.

AIR PREHEATER

An air preheater (APH) is a general term to describe any device designed to heat air

before another process (for example, combustion in a boiler) with the primary

objective of increasing the thermal efficiency of the process. They may be used alone

or to replace a recuperative heat system or to replace a steam coil.

The purpose of the air preheater is to recover the heat from the boiler flue gas whichincreases the thermal efficiency of the boiler by reducing the useful heat lost in the

flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or
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chimney) at a lower temperature, allowing simplified design of the ducting and the

flue gas stack. It also allows control over the temperature of gases leaving the stack

(to meet emissions regulations, for example).

COMBUSTION CHAMBER

The distributor plate is heart of FBC system.It is made of carbon steel base plate with

air nozzles to distribute fluidised air from airbox uniformly all over the bed. Bed coils

are immersed in the bed to maintain the temperature of 850-900*c by absorbing the

heat. Sufficient free board volume is available above the bed to ensure complete

Combustion OF fuel.

STEAM DRUM

A steam drum is a unit which is rated at a certain temperature and pressure to force

the boiler feedwater into the vapour phase (steam). As the vapour phase is formed

the liquid level in the drum decreases. It is important to maintain control of the liquid

level in the boiler drum to prevent damage to the unit. Complex control schemes


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have been developed control the level of the drum by adjusting the rate of flow, the

inlet temperature etc. The steam formed in the boiler drum still contains a certain

percentage of liquid content. This liquid contained in the steam is referred to as the

quality. The two phase system that is formed (liquid & vapour) in the boiler drum is

then cycled through the boiler several times before it is sent to the super heater unit.

Steam drum receives feed water from feed water control station & is distributed by

perforated feed pipe connected inside steam drum. Water flows to the water drum

through part of the convection bank tubes,which are not baffled at steam drum,

acting as down comers. Water is distributed from water drum to furnace side

wall,baffle wall & rear walls. Furnace side,baffle ,boiler side,rear wall tubes & part of

the steam drum,on absorption of heat from the furnace & convert the water into

water steam mixture & this conversion keeps the tubes cool within their safe

permissible operating temperature. Steam drum receives the steam water mixture

from the above tubes. From the baffles of steam drum this mixture rises upwardthrough the scrubbers provided at the top of the drum.Scrubbers provide a tortuous

path to the steam and during this passage,strips any traces of moisture from


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steam.Saturated dry steam then flows to the main steam line to common steam

distribution header

CONVECTION TUBES

The convection tubes are where the cooler water being fed to the boiler drum and

the liquid/steam mixture exiting the boiler drum exchange heat in order to furtherraise the temperature of the boiler feedwater. The heat exhcange between the liquid

feedwater stream and the liquid/steam exit stream occurs through the use of a fluid

as the medium of heat transfer. This type of heat transfer is referred to as

convection.

MUD DRUM


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The Mud drum is a unit which is located beneath the steam drum to collect the solid

materials which precipitate out of the boiler feedwater due to the high presssure and

temperature conditions of the boiler. The process by which suspended solids are

collected in the boiler is referred to as cycling. Cycling occurs because the boiler

feedwater is sent throught the boiler drum a number of times. This is done to

produce the maximum amount of steam per unit volume of feedwater. On each run

through the boiler drum, a portion of the boiler feedwater is vaporized. This results in

an increase in the solids concentration in the boiler drum. Eventually, the solids

concentration hinders the ability to maintain steam generation efficiency. At this

time, a stream of compressed air is used to blow the solids into the mud drum

beneath the boiler. The mud drum then stores these materials for later disposal. This

process of using compressed air to remove the collected suspended solids is referred

to as blowdown. moved by the deaerator

ELECTROSTATIC PRECIPIRATOR


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An electrostatic precipitator (ESP), or electrostatic air cleaner is a particulate

collection device that removes particles from a flowing gas (such as air) using the

force of an induced electrostatic charge. Electrostatic precipitators are highly efficient

filtration devices that minimally impede the flow of gases through the device, and can

easily remove fine particulate matter such as dust and smoke from the air stream. In

contrast to wet scrubbers which apply energy directly to the flowing fluid medium, an

ESP applies energy only to the particulate matter being collected and therefore is

very efficient in its consumption of energy (in the form of electricity) of the

electrostatic precipitator

The most basic precipitator contains a row of thin vertical wires, and followed by a

stack of large flat metal plates oriented vertically, with the plates typically spaced

about 1 cm to 18 cm apart, depending on the application. The air or gas stream flows

horizontally through the spaces between the wires, and then passes through the

stack of plates.


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A negative voltage of several thousand volts is applied between wire and plate. If the

applied voltage is high enough an electric (corona) discharge ionizes the gas around

the electrodes. Negative ions flow to the plates and charge the gas-flow particles.

The ionized particles, following the negative electric field created by the power

supply, move to the grounded plates.

Particles build up on the collection plates and form a layer. The layer does not

collapse, due to electrostatic pressure (given from layer resistivity, electric field, andcurrent flowing in the collected layer).

Plate precipitators are commonly marketed to the public as air purifier devices or as a

permanent replacement for furnace filters, but all have the undesirable attribute of

being somewhat messy to clean. A negative side-effect of electrostatic precipitation

devices is the production of toxic ozone and NOx. However, electrostatic


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precipitators offer benefits over other air purifications technologies, such as HEPA

filtration, which require expensive filters and can become "production sinks" for

many harmful forms of bacteria.

The two-stage design (charging section ahead of collecting section) has the benefit of

minimizing ozone production which would adversely affect health of personnel

working in enclosed spaces. For shipboard engine rooms where gearboxes generate

an oil fog, two-stage ESP's are used to clean the air improving the operating

environment and preventing buildup of flammable oil fog accumulations. Collectedoil is returned to the gear lubricating system.

With electrostatic precipitators, if the collection plates are allowed to accumulate

large amounts of particulate matter, the particles can sometimes bond so tightly to

the metal plates that vigorous washing and scrubbing may be required to completely

clean the collection plates. The close spacing of the plates can make thoroughcleaning difficult, and the stack of plates often cannot be easily disassembled for


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cleaning. One solution, suggested by several manufacturers, is to wash the collector

plates in a dishwasher.

Some consumer precipitation filters are sold with special soak-off cleaners, where theentire plate array is removed from the precipitator and soaked in a large container

overnight, to help loosen the tightly bonded particulates.

The main components of an ESP are:

- Collecting Plates

- Discharge Electrodes

- Rappers

- Hoppers


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1. Collecting plates are made from rolled steel, and are welded together in the

factory to reduce the installation time at the jobsite. Each plate contains electrodes

which are positively charged. When the particulate gas enters the electrostatic

precipitator and is struck with a negative charge electrode, the positively charged

plates act as a magnet and pull the particulate gas to them.

2. The plates have both top and bottom stiffeners and plates which allow better

mounting and the ability to deal with more abuse from the rappers. The plates are

arranged to form a series of gas passages.3. There can be anywhere from 30-35 passages inside the electrostatic precipitator

shell. The plates are placed parallel to the incoming particulate gas.

B. Discharge Electrodes


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1. Discharging electrodes are a high voltage unit that negatively charges the

particulate gas as it enters. These electrodes were once wires that were suspended

from the ceiling and weighted at the bottom, but are now a rigid mast.

2. There are many types of configurations of discharge electrodes which can be

tailored to your needs. The most common types are the two spiked and multiple

spiked electrodes.

Figure 4 shows the two spiked discharge electrode.

3. The discharge electrode is mounted to a frame in between the collecting plates.

The negatively charged particles that pass by the electrodes and then the counter

charge of the collecting plates; make the particulate like the magnet on a refrigerator.

C. Rappers


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1. There are many types of rappers. Rappers are used to dislodge the particulate

from the collecting plates. Some types of rappers are mechanical, pneumatic, and

the MIGI rapper.

2. Mechanical rappers work like a hammer and chisel. The hammers are attached to

the rods which are attached to a rolling cam above. The cam is turned by an external

motor and gear.

3. The pneumatic rapper uses compressed air to operate. Pneumatic powered

rappers work really well as long as the conditions permit and the factory hascompressed air on tap.

4. MIGI stands for magnetic impulse gravity impact rapper. The MIGI uses magnetic

coils to drive the hammer up and down. The magnetic coil is wrapped around the

hammer. When the magnet is electrically energized, the hammer will be pulled up,

similar to the way a positive and negative magnet are attracted. Once the electricity

is turned off from the magnet, the hammer falls, similar to the way two negative or

positive magnets repel.


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5. All three are effective in what they do. Rappers knock the solid particulate off of

the collecting plates where it is collected and trucked away.

D. Hoppers

1. Hoppers look like upside down triangular prisms. They are generally made of steel,

and their only purpose is to store particulate.

2. Once the rappers have done their job, it is then time to collect the fallingparticulate. Once the particulate has entered the hopper, it is stored there until it is

emptied and the particulate is carried away by a conveyor.

3. Most hoppers are heated so that the presence of moisture will be minimized. The

worst thing that can happen is that the solid particulate gets wet and hardens in the

hopper. This will cause the hoppers to be unable to be emptied causing seriousissues.


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PRINCIPLE

1. Once all the previous items have been created, they are placed in a shell. This

shell is basically the home of the precipitator. Almost all of the previous components

are located in the shell. The shell is typically comprised of carbon steel. It has holeson either side for the inlet and outlet ducts. The shell is also insulated to reduce the

risk of condensation build up.

2. Condensation will form when the flue gas which can leave the refinery at 200oF

hits the inside of a cold precipitator. Also condensation will interfere with the way in

which the electrodes work and render them useless because the condensation willcollect on the walls and will begin to collect the particulate before it can be properly

taken care of.


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3. The hoppers are one of the major external elements. Yes, there are wires, piping

and ductwork on the outside, but in the end the hoppers collect the particulate.

They, just like the shell, are typically made of carbon steel. Their main job is to collect

what is rapped off the collecting plates. Once the particulate has been collected, and

the hoppers are full, valves and access doors are used to evacuate the hoppers. The

hoppers can have piping running to them for a vacuum operated system which will

pull the particulate from the hoppers and bring it to a remote storage facility; or they

can have trucks driven underneath them that will physically truck the ash away.

4. Inside of the shell, the collecting electrodes are assembled parallel to the inlet

duct. They are assembled this way because it is the most economical and efficient

way to collect the particulate.

5. The discharge electrode is a different story. The discharge electrode runs

perpendicular to the inlet and collecting electrodes. As the flue gas comes out of theinlet and enters the precipitator the rigid mast discharge electrodes greet the flue gas


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and negatively charge them; allowing the positively charge plates that are running

parallel to the inlet to collect the flue gas.

6. The other major external element is the rapper. It sits on top of the roof of theprecipitator, and is programmed to deliver its powerful strike within a certain timed

interval. The hammer end strikes the top of the collecting plates, making all the

collected particulate fall into the hoppers below.

7. Electricity is the major power source to operate just about everything on the

precipitator. It is used to power the electrodes, both positive and negative, andpowers the rappers. It can also be used to power the vacuum system on the

evacuation of the hoppers, if the hoppers have this option. Most precipitators run

auxiliary transformers to subsidize the amount of energy needed to keep the

precipitators running.

8. The final element of an electrostatic precipitator is the outlet duct. Although notreally covered in this procedure very much, it does hold one vital role. It is where the

new clean gas will leave the precipitator. So in the end that final piece of ductwork


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gives the gas a final exit strategy from inside the shell where all the work had been

done previously.

ASH BLOWER

A boiler that has been running for a long time is sure to have soot deposits on its

tubes and furnace.

This is especially prominent in boilers using heavy fuel oil for burning. Although thefuel will be heated and filtered before combustion, it still contains a lot of impurities.

The by-products of combustion as well as imperfect combustion cause the soot to

form. The soot is deposited on the heating surfaces.

During the combustion of the fuel oil in a steam boiler, hot gases are formed. These

hot gases are used to heat up the water in the boiler to form steam.


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With the deposits of soot, a lot of the heat energy is not able to be transferred to the

water, but instead is lost through the chimneystack. The soot layer acts as a heat

insulator for the tubes and shells of the furnace. The heat is unable to reach the

water.

This not only causes the boiler efficiency to be lowered, but a more serious problem

can also occur. The soot can catch fire!

A soot fire can be detrimental to the strength of the boiler because it can cause

serious localized hotspots to occur at the tubes. These localized hotspots can evenreach temperatures that weaken the materials of the tubes.

Soot blowers are installed to blow away these soot deposits. Steam is normally used

as a medium for blowing away the soot.

The operation of the soot blowers goes like this:

1. Steam is channeled to the soot blower pipeline.


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2. The operator of the soot blower will open a drain valve to drain off any water in

the steam. This is to make sure that the steam going through the soot blower is dry.

3. Once the steam is considered dry, the drain valve is shut off, and the soot bloweris turned. Most soot blowers are constructed in such a way that when it is rotated, a

steam port is uncovered and steam can enter into it.

4. The steam shoots out from the soot blower tube that is inside the boiler fireside.

Many small holes for the steam to emerge are drilled along the length of the tube. As

the tube rotates, the position of the steam jet will also move with it. After a fullrotation, all the areas around the soot blower tube should be clear of soot. (However,

it depends on how thick the soot is, the speed at which the soot blower is rotated,

and how efficient the soot blowing is)

5. After completing the soot blowing, the steam supply is shut off again.


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The operation of the soot blower can be done manually or by remotely controlled

motor drive. A motor driven soot blower will usually include some means of turning

manually in case the motor fails.

The soot that is blown away from the heat transfer surfaces of the boiler will be

carried together with the hot gases out through the chimneystack.

If there is a dust collector, the soot will be caught by it, otherwise it goes out to the

environment as particles of carbon and ash.

CHIMNEY

A chimney is a structure for venting hot flue gases or smoke from a boiler, stove,

furnace or fireplace to the outside atmosphere. Chimneys are typically vertical, or as

near as possible to vertical, to ensure that the gases flow smoothly, drawing air into

the combustion in what is known as the stack, or chimney, effect. The space inside achimney is called a flue. Chimneys may be found in buildings, steam locomotives and

ships.


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The height of chimneys plays role in their ability to transfer flue gases using stack

effect, the dispersion of pollutants at higher altitude helps to ease down its influence

on surroundings. In the case of chemically aggressive output, the tall chimney allows

partial or complete self-neutralization of chemicals in the air before they reach the

ground. The dispersion of pollutants over greater area reduces their concentrations in

compliance with regulatory limits.

Industrial chimneys are commonly referred to as flue gas stacks and are typically

external structures, as opposed to being built into the wall of a building. They are

generally located adjacent to a steam-generating boiler or industrial furnace and the

gases are carried to it with ductwork. Today the use of reinforced concrete has

almost entirely replaced brick as a structural component in the construction of

industrial chimneys. Refractory bricks are often used as a lining, particularly if the

type of fuel being burned generates flue gases containing acids. Modern industrial

chimneys sometimes consist of a concrete windshield with a number of flues on the


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inside. The height is to ensure the pollutants are dispersed over a wider area to meet

legislative or safety requirements.

Chimney draft

When coal, oil, natural gas, wood or any other fuel is combusted in a stove, oven,

fireplace, hot water boiler or industrial furnace, the hot combustion product gases

that are formed are called flue gases. Those gases are generally exhausted to the

ambient outside air through chimneys or industrial flue gas stacks (sometimes

referred to as smokestacks).

The combustion flue gases inside the chimneys or stacks are much hotter than the

ambient outside air and therefore less dense than the ambient air. That causes the

bottom of the vertical column of hot flue gas to have a lower pressure than the

pressure at the bottom of a corresponding column of outside air. That higher

pressure outside the chimney is the driving force that moves the requiredcombustion air into the combustion zone and also moves the flue gas up and out of

the chimney. That movement or flow of combustion air and flue gas is called "natural


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draught/draft", "natural ventilation", "chimney effect", or "stack effect". The taller

the stack, the more draught or draft is created. There can be cases of diminishing

returns: if a stack is overly tall in relation to the heat being sent out of the stack, the

flue gases may cool before reaching the top of the chimney. This condition can result

in poor drafting, and in the case of wood burning appliances, the cooling of the gases

prior to exiting the chimney can cause creosote to condense near the top of the

chimney. The creosote can restrict the exit of flue gases and may pose a fire hazard.

Designing chimneys and stacks to provide the correct amount of natural draught or

draft involves a number design factors, many of which require trial-and-error

reiterative methods.

As a "first guess" approximation, the following equation can be used to estimate the

natural draught/draft flow rate by assuming that the molecular mass (i.e., molecular

weight) of the flue gas and the external air are equal and that the frictional pressureand heat losses are negligible:


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where:

Q = chimney draught/draft flow rate, m/s

A = cross-sectional area of chimney, m (assuming it has a constant cross-section)

C = discharge coefficient (usually taken to be from 0.65 to 0.70)

g = gravitational acceleration, 9.807 m/s

H = height of chimney, m

Ti = average temperature inside the chimney, K

Te = external air temperature, K.


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Combining two flows into chimney: At+Af

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Boiler Defnition