Draught or Draft refers to the pressure difference between the burner and the atmosphere.  This pressure difference or draught causes the air to flow from the burner to the atmosphere.  The residue of combustion such as waste gases, soot, etc are carried away by the flow of air.

Draught also has a great role to play in combustion.  The flow of fresh air into the burners is necessary for proper combustion.  Hence, the draught system should be designed such that the combustion can take place properly. 

The draught of a combustion system can be measured using a manometer when the furnace is in operation.  One end of the manometer is connected to the furnace while the other end is left open to the atmosphere.  The pressure difference indicates the draught of the system. 

Types of Draught in Boilers


  • Natural Draught where the draught occurs naturally due to the pressure difference between the furnace and the atmosphere. 
  • Induced Draught where the draught occurs by means of fans which create a negative pressure in the furnace causing fresh air to enter
  • Forced Draught where the draught occurs due to fans which provide combustion air and create a positive draught in the furnace.  This  drives the air through the chimney

Steam Jet Draught


Steam Jet Draught refers to the Draught created by using a jet of steam.  The steam generated by the boiler can be used for this purpose.  The jet of steam is used to create an airflow which will cause the flue gases to exit through the chimney.

If the steam jet is applied near the stack of the chimney, the negative pressure it creates draws the flue gases from the furnace into the Chimney.  This is known as induced draught.

If the jet is applied below the grate, the steam pushes the flue gases in the direction of the chimney.  This is a forced draught.

The Steam Jet draught is a simple mechanism.  No external equipment such as compressors or blowers are required.  The steam when used below the grate cools the firebars and prevents the clinkers from sticking to the bars.


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Pressure is a very important parameter in boilers.  The boiler and all the connected equipment are designed to withstand the pressure developed by the steam.  Pressure, is an important criterion to classify boilers.

These boilers have an operating pressure of less than 10 bar.  Natural circulation is sufficient for these boilers.  Typical application are in industries.

High pressure boilers


High pressure boilers have an operating pressure of 10 to 14 bar.  They have forced circulation.

Super high pressure boilers


Super high pressure boiler are also used for utility applications.  The operating pressure is above high pressure boilers but generally lesser than 17 bar. 

Super critical boilers


Supercritical boilers have an operation pressure higher than 22.5 bar

Miniature Boilers


These are boilers with very small capacity with a pressure less than 6.8 atmospheres and a gross volume less than 0.1415 cubic metres.


The Critical pressure in boiling a liquid is that pressure above which there is no clear change of state between the liquid and the vapour phases.  Simply put, water turns into vapour without boiling.  Above a pressure of 22.1 MPa, water reaches this state.  

In supercritical boilers, water is boiled at a very high pressure.  At that high pressure, there is no clear distinction between the water and vapour phases .  The fluid can  no longer be called liquid or vapour.  It becomes what is known as a super-critical fluid.

Supercritical Boilers are generally used in Turbine systems.  When the supercritical fluid drives the turbine, it loses pressure.  As the pressure drops below the critical point, the supercritical fluid becomes a mixture of water and steam which then passes through the condenser.

Supercritical Boilers are boilers in which the working fluid is above the critical pressure.  At this pressure, water changes into steam without boiling.  This intermediate state is known as a super critical liquid.  
Supercritical boilers are used in Turbine systems.  

Advantages of Supercritical Boilers


The advantages of supercritical boilers over sub critical boilers are

Efficiency


Supercritical boilers are more efficient that sub critical boilers.  They consume less fuel.  The efficiency rating of supercritical boilers is in the range of 32 - 38 % while that of ordinary boilers is in the range of 32% - 38%.

Reduced Operating Costs


As the efficiency increases, there is a natural reduction in fuel costs which translates into reduced operating costs

Lower Emissions


Due to less fuel being burnt, there are lower emissions.

Higher Initial Costs


The downside is that super-critical boilers have higher initial costs as the boiler and the systems have to be designed to withstand higher pressures.  

Advanced Water Chemistry


Supercritical boilers require very pure water.  Even small levels of impurities can cause deposits on the turbine blades. 


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Deaeration refers to the process of removing dissolved gases such as oxygen and carbondioxide from the water in a boiler. Dissolved Oxygen in Water causes corrosion by the formation of rust on the

surfaces of the boiler and the piping (rust). Carbondioxide which is dissolved in the water forms carbonic acid which also causes corrosion.

Classification of Deaerators in Boilers


Hence, it is essential that these two gases are removed from water. 

Deaerators can be classified into 

Mechanical Deaerators

 
These Deaerators separate the gases by a mix of high temperature and mechanical action Chemical Deaerators 

Chemical Deaerators


Chemical deaerators work by passing the water through chemicals which absorb the oxygen and the carbondioxide. 

Vacuum Deaerators


Vacuum Deaerators or Membrane Contractors work by passing the water through hollow fibres. The water is made to pass on the outside of the hollow fibre. A vacuum is created on the inside. The gases pass through the membrane on to the inside and drawn into the vacuum pump. 

Vacuum Deaeration is a method of removing dissolved gases from water. Removing dissolved gases from water is necessary as they can cause corrosion.
   

Working of Vacuum Deaerators


The principle on which Vacuum Deaerators are based is called Henry's Law. 

Henry's Law states that the gas solubility in a solution reduces as the partial pressure of the gas above the solution decreases. The Deaerator consists of a tower with baffles. The tower is made of galvanized or reinforced steel. Water is drawn to the top of the tower and made to fall through the baffles. The falling water is in the form of thin films. 

This creates a large contact area between the water and the air. A vacuum pump creates a vacuum in the inside of the tower. This lowers the air pressure on the inside. The dissolved gases in the water are drawn to the vacuum and are removed. In some vacuum deaerators the water is increased in lower the solubility. The amount of vacuum to be created depends on the temperature of the water. Vacuum deaerators are highly efficient and can deliver water with dissolved oxygen less than 5 ppb (parts of billion). 

Chemical deaeration is the use of Chemicals to remove the dissolved gases, usually oxygen. Chemical deaeration is usually used after Mechanical deaeration. Even after mechanical deaeration, all the oxygen will not be removed. 

 A chemical known as an oxygen scavenger is used. This ensures that all the oxygen has been removed. A common oxygen scavenger is Sodium Sulphite. Sodium sulphite reacts with the trace amounts of oxygen. Sodium sulphite, however, cannot be used at high pressure as it can decompose to acidic gases which can increase corrosion. 

Another oxygen scavenger is hydrazine. Hydrazine reacts with oxygen and produces volatile compounds which do not dissolve. Hydrazine also does not cause corrosion. However, the downside is that it is a carcinogen (cancer causing substance) and thus has to be used very carefully. It may be banned in the future. 
Oxygen Attack refers to the corrosive action of dissolved action on the boiler.  Dissolved oxygen causes pitting on the boiler surface.  Oxygen enters the boiler through the feed water.  Though, the deaerators remove a large amount of oxygen, the oxygen that remains can cause corrosion. 
When the feed water is heated, the oxygen becomes even more aggressive resulting in severe corrosion.

If the water contains ammonia, this results in corrosion of components containing copper and copper alloys such as bearings. 

Corrosion also results in deposits on the heat transfer surfaces which affect efficiency. 

Corrosion caused by oxygen is usually localized.  Oxygen Corrosion can also be extensive. 
Oxygen Attack is not monitored and prevented can result in failure of the boiler components. 
Heating the feed water reduces its solubility and reduces the dissolved oxygen.  Mechanical deaerators can further reduce the dissolved oxygen level.  Finally, chemical deaerators such as sodium sulphite can scavenge the remaining oxygen ions. 


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Significance of Blowdowns


 Blow down in Boiler is a very important procedure.  The Blow down helps flush the boiler of impurities which may accumulate as the water evaporates.  If the blow down is not carried out, the impurities can reach dangerous levels which can result in the formation of scales in the pipelines and the formation of sediments due to precipitation.  Scaling and deposit formation reduces the heat transfer between the boiler and the water and affects the efficiency. 

Impurities can also cause foaming which results in the loss of water which gets carried away in the steam. 

There are two ways in which blowdowns can be carried out in Boilers.  The Bottom blowdown and the Surface blowdown. 

The Bottom blowdown is done by opening a drain at the bottom of the boiler.  The boiler pressure pushes the impurities and deposits out. 

Surface blowdown is done to remove the impurities which have formed a foam on the surface of water.  The foam needs to be removed for optimum heat transfer.  A pipe placed at the water level in the steam drum is used for this purpose.  Opening the pipe causes the water on the surface to be vented. 

The duration and frequency of blowdown depends on the boiler design and the conditions of operation.  It also depends on the levels of the contaminants in the feedwater and thus on overall water quality.  The boiler blowdown rate should be determined uniquely for each boiler installation. 

Surface Blowdown


Surface Blowdown in boiler is carried out to remove dissolved substances at the surface of the water.  That is, it is used to remove impurities which are in the liquid or dissolved phase.  Impurities which are in solid phase precipitate to the bottom where they are removed by the bottom blowdown.

The impurities and dissolved substances tend to form a layer of foam on the surface of the water.  This layer of foam needs to be removed to reduce the level of the dissolved substances. 

Since the blowdown involves removing water from the surface, it is called surface blowdown.
Surface blowdown is done by a pipe which is made to float a few inches below the water surface.  The pipe is connected to the outlet by means of a swivel joint.  The pipe can thus freely float in the water.  The pipe is held afloat by means of a float.

The pipe has a needle valve at its end.  The size of the valve opening can be adjusted based on the frequency and amount of blowdown required during each session.  Today, Automatic blowdown controllers which can control the rate and volume of the blowdown are also available. 

Bottom Blowdown


Bottom blowdown in boilers is used to remove impurities which have fallen to the bottom as precipitates.  These impurities are in the solid phase.  Bottom blowdown is done by means of a valve connected to the bottom of the valve.  When the valve is opened, the impurities are flushed out by the boiler pressure.

The steam collected during the blowdown can be removed into a steam flasher and a heat exchanger.  The heat can be recovered and the steam can be recirculated after passing through the flash tank and the deaerator.

The duration and frequency of the blowdown is determined on factors such as size of the boiler, water quality and the location and the operating load.

A proper blowdown programme improves efficiency and reduces maintenance costs.
 

Boiler Blowdown Rate


The Boiler Blowdown rate refers to the rate at which the blowdown should occur in an operating boiler.  It describes the blowdown in kilograms per hour.

The Boiler blowdown rate depends on the quantity of the impurities and the limits of tolerance for the employees.  The Blowdown rate is a product of the steam consumption and the ratio of the level of the TDS to the difference between the maximum allowable TDS and the actual TDS. 

qBD = qS fc / (bc - fc) 

where
qBD    the blowdown rate in kg/hour
qS is the rate of steam consumption in kg/hour
fc is the total dissolved substances in ppm
bc is the limit of the total dissolved substances in ppm



 The Boiler Blowdown percentage refers to the amount of boiler water drained during a blowdown to the total quantity of the boiler feed water.  This is a very useful value

The formula is 





This value is a very important parameter.  The boiler blowdown percentage can range from 1% for high quality feed water to 20 % for low quality feed water. 

Types of Blowdowns


Two types of blowdown can be carried out in boilers. They are,

Intermittent blowdown


Intermittent blowdown, as the name suggests, is the blowdown performed at frequent intervals.  The general rule is to do the blowdown for 2 minutes in 8 hours.

This method requires increases in the feedwater input to the boiler.  Feedpumps of large size may be required for this method.

With each blowdown, a significant amount of energy is lost. 

Continuous blowdown


Continuous blowdown involves a steady discharge of concentrated boiler water and its replacement by a constant input of feed water.  TDS and steam purity are maintained at a given load.

Once the discharge rate of the blowdown and the feed rate are set, it requires no operator intervention.

The heat lost during continuous blowdown can be recovered by blowing it into a flash tank and generating flash steam.

The blowdown which leaves the flash tank will still have heat which can be recovered.  This is done by using a heat exchanger to heat the make-up water.

Package blowdown heat recovery systems which can be customized are available.

Benefits of blowdown control


The benefits of blowdown control are


  • Reduced cost of pretreatment.
  • The quantity of makeup water required is less.
  • The maintenance downtime is less.
  • The boiler life is increased.
  • The amount of chemicals to treat the water is less.


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Fouling in Boilers


 Fouling is a phenomenon where the hot flue gases and the ash precipitate and settle down in the places where the flue gases exit the boiler.

This layer which is formed reduces the gas flow into the Selective Catalytic Reduction tubes.  This can result in poor effluent treatment of the gases.
Fouling is generally removed by soot blowers. 

Slag Formation in Boilers


Slag formation occurs when the temperature of the gases exiting the furnace is above the fusion temperature of the fly ash.  At these temperatures, the fly ash melts and gets deposits on the sides of the furnace.

Slag Formation can lead to problems such as
  • reduced heat transfer from the combustion gases inside the furnace to the water
  • It can lead to further overheating of the boiler gases which, in turn, leads to further deposition.
  • Leads to unpredictable behaviour in the boiler
It is necessary to maintain furnace temperature below the fusion temperature of the ash.  The fusion temperature of the ash can be obtained by testing ash samples at laboratories. 
Slagging can also be prevented or minimized by a Slag Screen Arrangement in Boilers.


Thermal Spray in Boilers


Thermal Spray is a protective coating made on the tubes of the boiler.  The Thermal spray prevents corrosion, damage to the tubes and unscheduled breakdowns.   The material used for coating is usually an alloy.

Alloys based on Iron with added Chromium are used.  Low carbon steel can also be used as a thermal spray as it resembles the weld overlay.  Aluminium based thermal sprays are also used. 

There are different methods of applying the Thermal spray.  The metal is melted by using an electric arc or a gas flame and sprayed on to the tubes.

Scaling in Boilers


Evaporation of water causes the impurities and minerals in the water to concentrate.  Scale Formation in boilers when impurities precipitate from water.  Scaling also occurs when matter which is suspended settles down to the bottom.

Scaling forms usually on heat transfer surfaces.  Scaling affects the heat transfer and thus the overall efficiency of the boiler.  Severe scaling can cause blockages which can be very expensive to remove. 
Some of the contaminants which can form scales are calcium, magnesium, silica.  Calcium and Magnesium form their carbonate and sulphate salts.  These salts get deposited on the tubes and other internal surfaces of the boiler and other equipment. 

Scaling can be prevented by using good demineralized water as feed water.  Effective water treatment and maintaining good water chemistry can prevent scaling to a large extent. 

Silica in Boiler Water


Ordinary Silica is insoluble in water.  But when silica combines with other materials such as lime and soda, it can form scales which are very difficult to remove.  Soda and lime are used in softening units.

Use of silica based lubricants in the thermal plant as well can also result in silica entering the boiler water.  Another source is the presence of unreacted silicon in the feed water. 

If silica is not removed in time, it forms deposits in the turbine nozzles and change the direction of the steam.  The velocities and pressure drops are changed inside the turbine resulting in reduced efficiency.  Uneven nozzle flow can result in torsional vibration due to uneven loading of the blades.  This can result in vibrations. 

Silica deposits in the boiler are difficult to remove.  They equipment has to be dismantled and physically cleaned.  Blasting aluminium oxide on the surface is also a method used in the removal of silica deposits. 


Superheated Steam


 When water is heated to the boiling point at a given pressure, its temperature rises.  However, once the boiling point is reached, the temperature stops to rise.  The energy which is fed to the water is used to convert the water to vapour. 

The heat which is given to the water converts it into the vapour state.  This is known as the latent heat of vaporization.  The steam produced in this stage contains water droplets.  The temperature does not rise till all the water has been converted into steam. 

The steam which contains suspended droplets of water is called saturated Steam.
Saturated Steam is used widely in the industry for drying, heating.  It is also used in cooking as it has a high energy transfer coefficient. 

If the steam is heated further, all the water is converted into steam.  The steam, at this stage, is called superheated steam. 
     

Saturated Steam


Superheated Steam is steam which does not have any water droplets.  When saturated steam is heated,  the water droplets which are suspended get converted into steam.  Superheated steam is also called dry steam.  Superheated Steam is used to drive turbines.  

Superheated steam has a lower density and higher temperature. 
The main advantage of superheated steam is that there are no droplets.  In turbines, where the steam moves at high velocity, any water droplet which hits the turbine blades can seriously damage the blades or cause corrosion. 

Superheated Steam can store large quantities of internal energy and can release them during expansion.  This is utilized in turbines.  Superheated Steam has higher change in specific volume when it cools.  This enables better efficiency in turbine operation. 

Superheated Steam has low oxygen.  Hence, there is a reduced risk of corrosion in components using superheated steam. 
    

Steam Washing


 Steam Washing in boilers refers to the "washing" of the steam with fresh water or steam with condensate.  The objective of steam water is to remove the impurities in steam such as silica.  Silica in steam is mostly in the vapour state.  Silica can deposit in the blades of turbines and affect the efficiency and the operation.


When water which is at a colder temperature than steam is sprayed on the steam, the silica condenses and gets carried away by the water. 

The washing is usually carried in many stages for better efficiency. 

 

Steam Separation in Boilers


The steam that is generated from the boiler is wet steam.  This steam has water droplets suspended in it.  The water in steam can damage the components of a boiler system by way of corrosion.  It has lower energy carrying capacity.

Steam Separators are devices used to separate the suspended water from the steam.  There are many different methods to achieve this.  One method is the use of baffles which are placed in the path of the steam.  The baffles collect the water from the steam. 

Another method is by using the centrifugal principle.  The steam is passed through a chamber where there is a rotary device which spins the steam.  Due to the centrifugal principle, water which has a higher mass is separated from the steam and collected. 

Another method is by passing the steam through a wire mesh known as a demister.  The water particles tend to collect in the mesh while the steam alone passes. 

Steam separators usually use more than one method to separate the water droplets from the steam. 

Carryover in Boiler Steam


Carryover is the escape of moisture and impurities such as silica, copper, sodium, etc. along with the steam.  These impurities which are carried away by the steam can affect the piping and other equipment in the system.

Carryover occurs in two ways.  The first is due to mechanical action wherein the high velocity steam carries with itself small droplets of water which carry the impurities .  This is called mechanical carryover.  The second way in which carryover occurs is called vaporous carryover. 
In vaporous carryover, the impurities are carried over by steam. 

The sum of the mechanical and the vaporous carryover gives the total carryover. 
Causes of Carryover

The causes of carryover in boilers  are
  • Boiler Design
  • Fluctuating load on the boiler.  This causes the water level to rise and fall owing to the change in steam pressure.  This results in small droplets of water being ejected from the water surface into the steam above.
  • High quantity of impurities and dissolved substances
  • Overloading of the boilers
  • High water level
Use of efficient steam separation devices can minimize carryover.  A well designed blowdown schedule will help drain impurities and greatly reduce carryover.  Antifoam agents can also reduce carryover. 

 

Condensate Slug in Steam Lines


A Condensate slug occurs when condensate which collects in a steam line and moves through it at high speed encounters a point of resistance such as a bend or a closed valve. 

The condensate travels at the speed of the steam in the line.  When this condensate meets a point of resistance such as a bend in the pipeline or a pressure reducing valve, a tremendous impact occurs.  The shock wave travels through the system getting reflected many times.  This results in hammering.  This can cause damage to the pipelines and even injury or death to personnel.

Hence, adequate precautions to prevent such a scenario. 

Steam Traps should be fitted at points where the line makes sharp turns.  Steam Traps should also be placed before pressure reducing valves and valves which are mostly closed and at the end of steam lines. 

Function of Flash Tank


The Flash tank is used to flash the condensate at high temperature into steam. The primary function of the Flash Tank is to reduce the pressure of the condensate.  Some of the condensate "flashes" into steam in the Flash Tank.This low pressure condensate can be reintroduced into the boiler or condensate through the low pressure lines.  The steam can be used in place of live steam in the boiler system.

The Flash Tank serves to preserve the heat content of the condensate while reducing the pressure to a safe value.  The Flash Tank is a very important component of closed systems.  In fact, the flash tank is what makes closed systems possible


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Pulverised Coal


 Pulverised Coal is coal which has been powdered to a fine size.  Coal is powdered or pulverised by passing into through a pulveriser.  A Pulveriser is a machine which consists of rollers which crush coal into a fine size. Today, almost all coal fired plants used Pulverised Coal. 

The Pulverised coal is mixed with hot air.  This air and coal mixture is fed to a burner in the furnace which ignites it.  Modern pulverisers can grind coal to a very fine size of the order of microns.  When the pulverised coal mixes with air, it flows almost like a fluid stream.  

Pulverised coal burns more efficiently as it has a higher surface area.  Sometimes, pulverised coal is mixed with other fuels such as biomass. 

 Cyclone furnaces are used in boilers to burn poor quality coal which are not suitable for normal pulverized coal combustion.  These coal particles require higher temperatures and more oxygen to burn. 

Cyclone furnaces, as the name suggests, are able to provide a cyclone of air which results in greater turbulence which results in better mixing of the coal and air.  The cyclone of air also causes greater exposure of the surface area to the flame. 

The particles are whirled about in the air flow which results in greater exposure of the surface of the coal particle to the atmosphere. 
    

Stoichiometric Combustion in Boilers


Stoichiometric Combustion is the ideal combustion where at the end of the combustion process no fuel or air is left behind.  Thus all the carbon in the fuel is converted to carbon dioxide and the sulphur is converted to sulphur dioxide.  If residues such as carbon, carbon monoxide or sulphur remain, then the combustion is not stoichiometric.

Stoichiometric combustion cannot be achieved practically.  By adjusting the air/fuel mixture, the actual combustion can be made as close to the stoichiometric combustion as possible. 
    

Excess Air in Combustion


 When fuel is burned in the combustion chamber of the boiler, the calculated amount of air which is required for the fuel to burn is never sufficient.  Excess air needs to be fed to the combustion chamber.  This air which is in excess of the calculated amount is known as excess air.


Excess Air refers to the additional air which is fed to the combustion chamber of the boiler to ensure that the fuel gets burnt properly.  Excess Air is provided by means of blowers.  in the case of supercharged boilers, it is provided by compressors. 

Different types of fuel have different requirements for excess air. Gas may require up to 10% of excess air.  Fuel will require up to 20 % and coal will require large amounts of excess air up to 60 %.

Draft in Combustion


 Draft or draught is an important factor in the combustion of fuel in the boiler.  Draft refers to the difference in pressure in the boiler furnace to the pressure on the top of chimney.

This pressure difference is necessary for the flow of fresh air into the boiler and for the removal of flue gases out of the boiler.  The draft should be optimal.  The draft has a direct influence on the fuel/air mixture in the furnace.  A higher draft will result in more air being sucked into the furnace.  This will result in higher combustion. 

A lower draft will not be able to remove the flue gases properly from the furnace.  This will result in less air entering the furnace.  This, in turn, will result in incomplete combustion which affects the efficiency.  Incomplete combustion will also result in more pollution.
   

Stack Temperature for Boilers


 The Stack temperature of the boiler is a very important parameter in Boiler design.  The stack temperature is the temperature of the flue gases when they reach the stack or the chimney.

A low temperature of the stack temperature indicates that little heat is carried away by the flue gases and that the boiler is operating efficiently.  The stack temperature is a very important specification at the time of boiler purchase.

If the temperature is too low, it can result in cold corrosion. 

The Stack temperature will vary with the time of the year as it is also dependent on ambient temperature. 


 When the fuel is burnt in the furnace of a boiler, all the energy in the fuel is not available to heat the boiler.  Some of the energy is lost in the form of losses.
The Losses in combustion of a boiler are categorized in to the following types.

Loss in the Flue gases


The heat generated by burning the fuel is present in the flue gases.  When these gases escape into the atmosphere, some amount of heat also escapes with the gases.

Hydrogen Losses


This refers to the heat used in evaporating moisture or water present in the fuel.  This is particularly significant in coal-fired boilers.

Losses due to improper combustion


The improper combustion of fuel due to poor quality or inadequate air also results in loss of potential energy

Losses due to Convection and Radiation


The furnace of the boiler is insulated.  Despite this, some heat escapes from the furnace to the atmosphere.


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Water Level in Boiler


Maintaining the proper level in a Boiler is a very important aspect of boiler control.  A boiler should have a reliable water level controller.  There are many automatic controllers which automatically keep the level of the water within the specified limits.  The boiler can be seriously damaged if the level of water is not maintained.

Boilers will also have alarms which alert the operator if the water level falls or exceeds the safe limits. If the water level rises above the safe limit, it can result in priming and carryover.  If the water level increases, it can exceed the horizontal limit of the boiler, the surface area for steam generation will then reduce.

If the water level falls below the safe limits, the tubes of the boiler will be exposed.  This will result in the tubes getting overheated and they can rupture.

The level of water in the boiler is also dependent on the pressure.  If the steam pressure is reduced, the water level rises.  If the pressure is increased, the level can fall.  
 

Boiler Feed Water Pumps


Boiler Feed Water Pumps are used to supply water into the boiler.  The water may be fresh water or the water from the condenser.    The size of the Boiler Feed Water Pumps depends on the capacity of the boiler.

The operation of the Boiler Feed Water Pumps is dependent on the water level.  A level switch is used to switch the pumps on or off.

There are generally two pumps with one running and one as a standby.

Boiler Feed Water Pumps are of the centrifugal types.  They are usually multistage pumps.    The pumps are driven electrically or by a turbine.  Turbine driven pumps are preferred as the cycle efficiency of the boiler increases.  

Foaming in Boilers


Foaming refers to the formation of froth or bubbles in the boiler.  Foaming is caused by high concentration of dissolved solids in boiler water.  When a bubble forms in water that has high levels of dissolved solids, the dissolved solids surround the bubble and make it tougher.  The bubble does not break.  When more bubbles collect, foam is formed. 

Foaming can be prevented by maintaining good water quality.  The level of dissolved solids should be maintained as low as possible. 

The presence of oil in the water can also cause foaming.

Anti-foaming agents are available which prevent the formation of bubbles and foam. 
 

Boiler Water Sampling


Boiler Water sampling involves taking a sample of boiler water to analyse for dissolved substances. This is necessary to determine the level of TDS (Total Dissolved Substances) in the water and, consequently, the quantity of the blowdown.

Samples taken from points such as the level gauge glass, inlet for the feed water or the level control mechanisms are usually inaccurate.

It is dangerous to take a sample from the boiler shell as the water will be at pressure and may flash into steam causing injuries to the operator.

The safe method of taking boiler water samples is by use of a small heat exchanger.  In this method, cold water is used to cool the sample being taken.  This eliminates any risk of flashing.  The sample is also more acccurate.

Another method is to use a TDS sensor.  The sensor reaches into the shell of the boiler and can continually monitor the TDS in the water boiler. 
 

Priming in Boilers


Priming refers to the carryover of small droplets of water along with the steam.  Prime is undesirable as it causes corrosion to the steam line and other equipment.
 
Priming is caused due to many reasons such as high water levels, poor boiler design and fluctuating loads.

When the load in the boiler suddenly increases, the steam pressure in the boiler drops.  This causes the level of water to surge.   This surge of water level causes priming.  Small droplets of water are thrown up into the steam above the water.  This water can carry with it dissolved substances such as chloride, silica, copper, etc.  This is known as carryover.

Priming can be prevented by operating the boiler at steady loads and by maintaining good water chemistry which prevents foaming.

Kettling in Boilers


Kettling refers to the sound of boiling water coming from the boiler.  It is similar to the sound coming from a tea kettle.  Hence, the name. 
Kettling noises can have a number of causes.  Some of them are

Deposits


If the water quality is not properly maintained, deposits can form on the bottom of the boiler.  This is particularly in areas where the water is hard, resulting in limescale deposits.  The deposits prevent proper heat transfer between the water and boiler.  This uneven heating can cause kettling.

Flow rate


If the flow rate of the water is not proper, the water can get overheated as the "dwell time" will be more. this can result in kettling.  The flow rate of the boiler should be reviewed. 

Improper Thermostat Settings


If the thermostat settings are not proper, they may result in overheating. 

Improper Burner functioning


If the burner of the boiler is not properly function, kettling can result to uneven heating. 

Improper Installation


Improper installation can also cause kettling.  If the noise occurs soon after installation, do contact the manufacturer.

Kettling Noises should always be investigated as they affect efficiency and may point to other problems


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Methods of Firing a Boiler


Firing refers to the application of heat to a boiler.  Firing is done by different methods.  

Fuel fired Boilers
The most common method is by burning fuel such as wood, oil, coal or gas.

Waste Heat Recovery Boilers
These boilers function by recovering the heat from the gases or effluents of other industrial processes.  For instance, the exhaust gas from a power plant can be used to heat a boiler. 

Electrically heated Boilers
These boilers are used for heat generation.  They are not used for power generation.  They are generally of small capacity.  They can be used in hospitals for sterilizing equipment.  They can be used for laundry and for domestic heating purposes. 

Nuclear Powered Boilers
These boilers are used in Nuclear Power Plants.  The nuclear fission reaction which occurs in the reactor produces enormous amounts of heat.  This heat can be used to heat water and produce steam.  This steam is used in power plants to generate electricity or to drive submarines

Tangential Firing


 Tangential Firing in Boilers is a widely used method in the combustion of fuel, usually, coal.  This method ensures efficient firing of the fuel.  The Tangential Firing method also results in reduced emissions.


In this method, finely powdered coal is blown into the combustion chamber along with air through a series of burner nozzles.  As the coal and air mixture enters the combustion chamber four burners placed in the wall tangential to the fireball are used to fire the mixture.

The method gets its name as the flame from the burners strikes the burning fireball at a tangent.  The burning mass rotates ensuring complete combustion of the fuel particles. 

 

Co-Firing


Co-firing or Co-combustion refers to the combustion of two fuels simultaneously.  For example, biomass fuel can be burnt with coal.  In paper plants, the pith, which is a byproduct of the manufacturing process, can be burnt along with coal.

The fuel which is added to the main fuel is called additional or auxiliary fuel.

Co-firing has many advantages. Co-firing is cheap as it uses fuel which would otherwise have gone waste.  This reduces the cost of steam generation.

Co-firing also has environmental benefits.  Co-firing results in more efficient combustion.  The green house gases emitted are lesser than those emitted when burning a single fuel.

In certain countries, the governments incentivize cofiring as it is beneficial to the environment.




Soot Blowers


Soot Blowers are devices used for cleaning in boilers.  The Soot Blower is a mechanical device which cleans the deposits due to fouling in the furnace.  The Soot Blower consists of a cleaning medium such as steam, compressed air or water.

It directs jets of the medium on the surfaces and tubes to be cleaned.  Special injectors which can precisely focus the jets on to the tubes are used.  Removing the soot deposits greatly improves the efficiency of the boilers. 

There are different types of Soot Blowers such as the insertable soot blowers which can be inserted into the tubes as well as the rotating kinetic type of soot blowers. 

The soot blower can be operated manually by an operated or it can be made automatic to operate at specific intervals.  Boilers will have a specific cleaning cycle which depends on the size of the boiler, type of coal used, operating load, etc.

Soot Blowers which are powered by compressed air will have a dedicated air compressor for soot blowing.  The soot blowing equipment is generally galvanized to withstand the corrosive effect of the gases

 

Screen Tubes


In radiant super heaters, where the tubes of the superheater are placed directly in the furnace, special tubes are used to protect the superheater tubes from the high temperature of the furnace.  These tubes are called Screen Tubes as the screen the superheater tubes from the intense heat.

Slag Screen


The Slag Screen is a bank of specialized Boiler tubes which is fitted at the entrance of the convection passes in a boiler furnace.  The slag screen cools the hot gases and the ash before the entry to the convection shaft.  This minimizes the risk of slag. 

The slag screen can be arranged in an inline arrangement where the slag screen tubes are parallel to the boiler tubes or a staggered arrangement where the tubes are at an angle to the boiler tubes.

 Water Walls Tubes are tubes which run along the inner surface of the boiler.  The function of the water wall tubes is to transfer the heat of the furnace to the water which flows in them.

Water Wall Tubes are more efficient than water tubes which were used earlier.  Water wall tubes help to better capture the heat of the boiler.  Since they are located in the walls, they prevent the heat from leaving the boiler and improve its efficiency.  They also keep the boiler surroundings cooler.

The water Wall Tubes are connected to the steam drum by means of risers. 

Super Heater


The Super heater Tubes serve to super heat the steam from the steam drum.  Steam from the steam drum is saturated steam.  It may still have droplets of water in it.  This steam is passed through tubes passing through the furnace to heat it.  Superheated steam has very high energy and is used to drive the turbines.

The capacity of a boiler is dependent on the amount of superheated steam it can generate.  Super heater tubes must be able to withstand very high temperature and pressure.  The steam-water ratio is designed such that it does not overheat.    Hence, super heater tubes are one of the most important parts of the boiler. 

Super heaters are classified into

  1. Radiant Super heaters which are placed directly in the furnace
  2. Convectional Super heaters which are placed in the path of the hot gases
  3. Separately fired super heaters are heated by a separate heat source.

Steam Drum


Steam drum is the reservoir at the top of the water tube boiler.  The water tubes which carry water are connected to the Steam drum.  The steam drum serves to extract the steam from the water and send it for superheating.  The difference in density causes the steam to rise.

The pressure in the steam drum regulates the steam generation within the boiler.  As the steam is extracted, further steam is generated.

The water is sent back to the water drum for further heating. 


Lagging in Boilers


Lagging in Boilers refers to the covering over the boiler.  The principal function of the lagging is to cover the insulation and prevent heat loss from the boiler.  It also provides a cool surface over the boiler for fitment of accessories.  It provides a formal shape to the boiler after the insulation has been fitted.

Lagging is a essential part of boiler design.  Lagging is usually made of aluminium or steel.  The Lagging should be adequately supported so that it maintains its shape. 

The surface of the Lagging should also be provided with drainage so as to drain the excess water in case of rain or other water leakage.  This is important particularly in outdoor design. 

Adequate allowance should be made in the design for expansion and contraction of the lagging during operation as the temperature varies. 

The expected lifetime of the insulation and lagging is 15 years.

Level Indicators are very important instruments in any boiler.  Level indicators tell the operator how much water is there in the boiler.  The Level indicator is a vertical tube made of glass which can withstand high temperature.  This glass is mounted on the boiler drum.  One end of the level indicator is placed below the water level, the other end is placed above the water level.  This is called the steam end.

As the level of water rises and falls in the boiler, the level in the indicator also changes.  The glass tube of the indicator is covered by another protective layer of toughened glass.
The indicator has a drain cock which can be opened to drain the water in the level indicator.  This is used to check the functioning of the level indicator. 

The level indicator has two safety balls at each of the inlets, the water inlet and the steam inlet.  If the glass of the level indicator gets broken, the balls seal the two outlets and prevent leakage.
The Level indicator is a simple device which provides a direct reading of the level inside the boiler. 

Conductivity Probes for Level Monitoring in the Boiler
Maintaining level is an important aspect of Boiler Control.  Automatic Boiler Controls work by sensing the level of water in the boiler based on conductivity and and taking the necessary action.  Level monitoring is done by means of conductivity probes which are inserted through the boiler wall.

These probes are placed at different levels of the boiler.  When the water reaches the particular level, the contacts are bridged and a current flows through the probes and a signal reaches the controller.  If the water level falls no current flows signalling that the level has fallen.

These probes are specially designed to withstand the temperature and pressure of the boiler.

Steam Throttling Valve


The function of a throttling valve is to reduce the steam flow rate of the boiler.  This is necessary to prevent steam flashing.  

If a throttling valve is not used, a false temperature reading may reach the boiler blowdown controller.  This will result in wrong operation of the boiler blowdown.  The boiler throttling valve must be installed before the sensor. 
    

Baffles in Boilers


 Baffles are used in Boilers to reduce turbulence in the flow of the hot combustion gases over the boiler tubes.  The baffles maintain proper velocity of the gases which enables efficient energy transfer.

Baffles also guide the fly ash and slag to the proper place for deposition from where they an be easily removed.  

If the Baffles are damaged, it will result in overheating at certain places and poor heating in others.


Boiler Burners


The Burner of the Boiler is the source of heat in boilers which are powered by natural fuel such as gas or oil.  It is the place where the fuel is burnt to produce energy.

The burners in Boilers should combust the fuel with very low emissions.  They are sometimes provided with an air source such as a fan to ensure proper combustion with little residue.

Duel Fuel burners can burn both oil and gas.  Common fuels are Furnace Oil, Light Oil, Natural Gas and Liquefied Petroleum Gas. 

Boiler Burners can range in capacity from 200 kW to 15000 kW

Modern Burners in Boilers have sophisticated electronic controls which algorithms for precise air-fuel mixtures for optimum efficiency. 

 

Economizer in Boilers


The Economizer in a boiler is used to preheat the water which is fed into the boiler by using the exhaust gases of the boiler.  In this way, it is able to "economize" or save energy.  The heat of the exhaust gases will be in the range of 380 to 550 degrees Celsius.  By utilizing this heat energy, the economizer increases the overall efficiency of the boiler. 

The Economizer is in the form of vertical tubes in which the water flows.  The gases on the way to the exhaust stack transfer their heat to the economizer.  The temperature of the inlet water to the economizer should not be too low as that can result in fouling and corrosion. 

The outlet temperature of the economizer is also below the boiling point of the water.
  

Shell of the Boiler


 The Shell of the boiler refers to the body of the boiler.  The Shell is made up of steel plates which have been rivetted  or welded together. 

The end plates are at the top and the bottom of the shell. 
The Shell is designed to withstand high pressure and temperature.  It is also designed to resist corrosion. 

 

Membrane Contactors in Boilers


Membrane Contactors are used to deaerate water in a boiler.   Membrane Contactors are getting increasingly popular.  Membrane Contactors use a membrane which is made of hydrophobic material with pore size of the order of 0.03 micrometer.

On one side of the membrane, a gas, usually nitrogen,  is passed at low pressure.  On the other side, the water to be deaerated is passed.  Since the membrane is hydrophobic, the water does not pass through it.  But the gases which are dissolved pass through the membrane to the other side with low pressure. 

The deaeration process can be controlled by varying the pressure of the gas and its concentration. The capacity of the contactor can be increased by adding more membranes. The membrane contactor method can produce water with a dissolved oxygen concentration of less than 1 ppb (parts per billion).

The advantages of using Membrane Contactors are
  • Absence of Emulsions
  • No Flooding even at high flow rates.
  • No density difference between the fluids is required.
  • High surface areas

Vacuum Pumps


Vacuum Pumps are used in Boiler Systems to evacuate the air from the piping.  Air reduces the heat transfer and acts as a thermal insulator.  It allows impedes the flow of steam.  The Vacuum pump is used to evacuate air from the pipelines.

The gases present in the air such as oxygen and carbon dioxide can dissolve with air and cause corrosion.  Using a Vacuum pump can remove these gases and lower the dissolved gas level in the system preventing corrosion.

Once air is removed from a system and a vacuum is created, steam requires a very low pressure to flow across the system. 

Since the pressure of the system decreases, the boiling point of water also decreases.  This results in a reduction of the fuel consumption.  Vacuum pumps can also be used for lifting the condensate  to the receiver.  This is necessary to prevent water hammering.

Vacuum pumps are specified in the amount of air they can move at a given vacuum. They can produce vacuums of the range of 5, 10 and 15 inches of Hg.


 Insulation in Boiler system is a critical piece of equipment.  It is factor which has a direct relationship with efficiency and cost.  Heat Energy is produced in the boiler.  This energy is transferred using steam as a medium throughout the system.

Poor or damaged insulation can also result in condensate formation which can cause problems such as hammering.

The Insulation in Boiler systems has three main functions

  • To prevent heat leakage from the boiler and the system
  • To protect personnel from accidental contact with hot surfaces.
  • To ensure a tolerable working temperature in the boiler room and surrounding for the operating personnel

There are many different materials used for Boiler Insulation. Glass Wool is a common material. Other insulation Materials are Styrofoam, fibreglass and polystyrene.

Spray On Ceramic Insulation in Boilers

Spray On Ceramic Insulation is a popular method of insulation in recent times. The insulation consists of a ceramic material which is sprayed on to the surface of the boiler and piping using a special equipment.

 Ceramic insulation is more expensive than fibreglass insulation. Once sprayed, the material solidifies into a foam type material. One advantage of this method is that it is easy to apply. The downtime required is very less.

It is lightweight and can also be easily applied to irregular surfaces and edges . Ceramic insulation is also a corrosion inhibitor. It can also withstand moisture better.

Special types of ceramic insulation can be sprayed on to the surfaces of hot equipment. This can reduce downtime.


Condensing and Non Condensing Boilers


A condensing boiler has a special heat exchanger to remove heat from the flue gases.  The flue gases are thus at a lower temperature.  The overall efficiency of the boiler is increased.  Condensing boilers can have an efficiency as high as 95%.

In many countries, the use of condensing boilers is mandatory.  Many governments give financial incentives to install condensers in boilers. 

Fired and Unfired Boilers


Fired Boilers are boilers in which steam is generated by the direct application of heat.  Heat is generated by burning fuels such as coal, fuel or gas
. 
In Unfired Boilers, steam is generated by an indirect source such as hot water or steam from another process.  No fuel is burnt in these kind of boilers.  

Utility Boilers


Boilers which are used in power generation applications are called utility boilers.  Utility boilers generally use coal as fuel. 

Gas and Oil are also used by utility boilers.  These boilers are of large capacity of the order of hundreds of Megawatts.  These boilers are used to drive a turbine which is coupled to a generator.

Water Tube Boilers


Water tubes boilers are boilers where the water flows in pipes above a furnace or a burner.  The hot gases in the burner warm the water.  Water tube boilers are used for high capacity applications.  They can built for very high pressure designs.

The water tubes are connected to a steam drum on top.  The steam rises to the steam drum from where it is collected.  High efficiency is possible with water tube boilers.  Since the water flows through the tubes, very good water chemistry needs to be maintained to prevent deposits and corrosion.  

Large heating surface can be obtained by adding more number of tubes.  The heat transfer is by convection.  The flow rate of the water in the tubes is higher and hence greater efficiency is possible. 
The water tube boiler can control its own pressure and is known as a self controlled boiler.  This is because if steam is extracted from the drum, the pressure inside falls.  This causes the water in the tubes to flow faster.  This results in more heat transfer and the original pressure is quickly attained.

Velox Boilers


A Velox boiler is a supercharged boiler.  The Velox boiler is based on the principle that when the velocity of the exhaust gases exceed the velocity of sound, the heat transfer increases greatly. 

The flue gases are used to drive a gas turbine coupled to a compressor which compresses the gas from the atmospheric pressure to the furnace pressure. 

The Velox Boiler has a very high combustion rate.  It can be quickly started. 
    

Supercharged Boilers


In a Super charged boiler, compressed air is fed to the furnace.  The Super charged boiler has better heat transfer capacity.  The heat transfer required is lesser compared to a conventional boiler.  The exhaust gases which come out with high velocity are used to rotate a gas turbine.  This gas turbine in turn can rotate other auxiliaries.

A compressor is used to produce the compressed air for the furnace.  This compressor is powered by the gas turbine. 

The boiler is easier to control. It is also easier to start and requires lesser personnel to operate it. 

Firetube Boilers


 In Fire Tube boilers, the hot gases from the furnace are passed through a series of tubes.  The tubes are placed in the shell of the boiler which contains the water.  The water is thus heated by the heat from the fire tubes.

These boilers are used in low pressure applications as it is difficult to design the tubes which can withstand the higher pressure of the water in the boiler.  Firetube boilers have a larger water volume and can thus handle sudden load surges better.  They are easier to repair and maintain. 
Firetube boilers are suitable for applications where the load is fixed with little variations.  They may take a long time to warm up as they have a large quantity of water. 
Firetube boilers are not suitable for high pressure applications and for applications which require high steam output. 

Package Boilers


 A Package Boiler is a boiler that has its own burner.  It is small in capacity and is fired by gas or oil.  It has a short start-up time.  It is able to generate a large quantity of steam at high pressure and temperature.


The Package Boiler can be used for different applications.  In some cases, the package boiler is used to generate steam for industrial applications.  The package boiler can also be used for power generation when used with a steam turbine. 

In some cases, the package boiler is used to supplement the output of another large boiler as a peak load source.  In some cases, the boiler can also be used as an emergency generator.  
 

Electrically Powered Boilers


 Electrically Powered Boilers are boilers that are powered by electricity.  These boilers are used for applications such as heating, laundry and in medical applications such as sterilization.  Electrical boilers are not economical at higher capacity.

The advantages of Electrical Boilers are that they are compact and require very few parts.  They are easy to control and use.  They do not produce any residue or exhaust.

Electrical Boilers are usually powered by a three phase supply. 

The Electrical Boiler has a resistor as its heat source.  When electric current flows through the resistor, heat is produced.  This heat can be used to raise the temperature of the water. 
Very precise control of the temperature is possible with modern electronic controls.


Efficiency in Boilers


Efficiency in boilers, is calculated similar to other machines.
Efficiency = Useful output/ Input

In this case, useful output is the heat content in the steam.


Therefore




The heat content in the fuel can be calculated by knowing the calorific value of the fuel. 

The heat content of the steam is measured by calculating factors such as the steam flow rate, steam pressure and the temperature of the feed water.  Using these values, the heat content of the steam can be obtained from the steam tables.

Net Calorific Value and Gross Calorific Value


Calorific value is the total energy contained in fuel.  It is indicated in kilojoules per litre or cubic metre.
All fuels, such as coal, oil or wood contain a small amount of water in the form of moisture.  When fuel is consumed in the furnace, some of the energy is used to evaporate the water contained in the fuel.  This water escapes as steam in the flue gases.

Gross Calorific Value


This includes the total energy in the fuel which  includes the energy used in heating the water.  The Gross calorific value is also known as the higher heating value.

Net Calorific Value


Net Calorific Energy is calculated after subtracting the energy used to evaporate the moisture in the fuel.  Net Calorific Energy is used for boiler efficiency calculation.  The Net calorific value is also known as the lower heating value.
The Net Calorific Value is generally 10 % less than the Gross calorific value.


Boiler Construction

 
The Boiler is a device which generates steam at very high pressure and temperature.  Thus, it should be designed to withstand the mechanical and thermal stress caused by the temperature and pressure.

While selecting materials or boilers, there are also issues such as cost and availability which determine the choice.

In general, Low Pressure Boilers are constructed out of cast iron or steel. 

Miniature Boilers can be made of stainless steel or copper while Power Boilers are designed out of special steels which can withstand very high pressure and temperature.  
 

Classification of Boilers on the basis of pressure


According to the standards of the ASME (American Society of Mechanical Engineers), Boilers can be classified on the basis of pressure into the following types.

Low Pressure Boilers


Boilers with operating steam pressure not exceeding 1.021 atmosphere and a temperature of 394 K. 

Power Boilers


Power Boilers are those boilers whose pressure rating and temperature are above those of Low pressure boilers.

Classification of Boilers on the basis of fuels


Based on the fuel used to produce heat, boilers can be classified into

Solid Fuel Fired boilers


These boilers are fired by either wood or coal.  The fuel may be crushed into fine powder or cut into chips before being fed into the furnace.  The combustion of these fuels will produce ash which can be used for other purposes.

Liquid Fired boilers


These boilers are fired by a liquid such as Furnace oil or diesel.  The Furnace oil may required to be processed to attain the desired viscosity and the temperature before being fed into the furnace.  The furnaces for these boilers have a higher flow rate and smaller volume.  

Gas Fired boilers


These boilers have gas as the fuel.  The gases have very little residue.  The flow rate is higher and the furnace volumes are very small.  

Waste Heat Recovery Boiler


These boilers do not have a separate heat source.  They recover the heat from another industrial process such as exhaust from an engine or the waste heat in a cement plant.  The recovered heat is used to power the boiler.  

Factors which influence the selection of boiler type


The following are some of the factors which determine the type and Size  of boiler
  1. Power to be Generated
  2. Operating Pressure
  3. Type of Fuel and its quality.
  4. Load Factor
  5. Location
  6. Availability of Water
  7. Availability of Area for the Boiler
  8. Cost of Operation and Maintenance


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