Combustion of fuel in furnace and burner design


The heat producing constituents of the fuel are hydrogen, carbon and sulphur.

The calorific value of the combustion processes measured in mega joules for each Kg of fuel burnt

The main cause of heat loss with the process is that taken away by nitrogen. Therefore, to achieve maximum efficiency the excess air should be kept to a minimum. However there is a limit to the reduction in the excess air in that the combustion process must be fully completed within the furnace and within a finite time.

The main type of combustion process is called the suspended flame. The flame front remains in the same position relative to the burner and quarl.. The fuel particles pass through the flame completing their combustion process and exiting at the same rate as the fuel entering.

Primary Flame-To burn oil the temperature must be raised to vaporisation temperature, this can not be done in heaters due to gassing but is done by radiant heat in the flame. The lighter hydrocarbons in the atomised spray are rapidly heated and burnt in the primary flame. The heavier fractions pass through this achieving their vaporisation temperature. The primary flame is essential to good combustion. By design the primary flame exists where it receives maximum reflected heat from the shape of the quarl. The size of the primary flame ( shown smaller than actual in drawing) just fills the quarl space. Too large and impingement leads to carbon deposits building up. Too small unheated secondary air reduces combustion efficiency. The tip plate creates vortices reducing the mixing time for the air/fuel and reduces the forward speed of the flame

Secondary Flame-Here the heavier fractions are burnt. The velocity of the air and fuel must be matched to the required flame propogation rate.

Combustion in furnace space

For proper combustion of fuel in the furnace and adequate supply of air must be supplied and intimately mixed with a supply of combustible material which has been presented in the correct condition.

Air- it is the purpose of the register, swirler vanes and (vortice) plates, and quarl to supply the correct quantity of air for efficient combustion suitably agitated to allow proper mixing.

The air is generally heated on larger plant to;

Fuel It is the purpose of the burner to present the fuel in suitable condition for proper combustion. Generally this means atomising the fuel and giving it some axial (for penetration) and angular (for mixing) velocity. For effective atomisation the viscosity of the fuel is critical, for fuels heavier than gas or diesel oils some degree of heating is required. It should be noted that the temperature of the fuel should not be allowed to raise too high as this can not only cause problem with fuel booster pumps but also can cause flame instability due to premature excessive gassification (is that a real word-answers to the normal address)
The smaller the droplet size the greater the surface areas/volume ratio is, this increases evaporation, heating and combustion rate.

Combustion zones

Register- supplies the correct quantity of excess air. Too little allows incomplete combustion, smoking, soot deposits and flame instability. Too much excess air reduces combustion efficiency by removing heat from the furnace space, may cause 'white' smoking and promote sulphurous deposits. In addition too much excess air increases the proportion of sulphur trioxide to dioxide promoting increase acid corrosion attack in the upper regions.
The register and to some extent the quarl determine the shape of the flame, short and fat for side fired boilers, long and thin for roof fired.

Flame burning off the tip- may occur after initial ignition or after a period of high excess air. The effect of this is to move the primary flame away from the quarl thereby effecting the combustion process leading to black smoke and flame instability. Two methods of bringing the flame back are to reduce excess air and introduce a hand ignitor to ignite the fuel correctly, or to rapidly close then open the register damper


There are six main types of burner in common use;

Turndown ratio ratio of minimum to maximum flow ( roughly the square root of the ratio of maximum to minimum pressure)

Pressure jet

This is the simplest and oldest design of burner. Atomisation of the fuel is achieved by forcing the fuel under pressure through an orifice at the end of the burner, the pressure energy in the fuel is converted to velocity. Spin is given to the fuel prior to the orifice imparting centrifugal force on the spray of fuel causing it to atomise.

The disadvantage of this burner is its low 'Turn-Down' ratio (in the region of 3.5). The advantage is that it does not require any assistance other than supplying the fuel at the correct pressure. Due to this it is still seen even on larger plant were it is used as a first start or emergency burner.

Another disadvantage over assisted atomisation burners is the lack of cooling from stam or air means the burner must be removed when not in use from lit boilers to prevent carbonising in the tube

Spill type pressure jet

The method of atomisation is the same as for simple pressure jet type. The burner differs in that a proportion of the supplied fuel may be spilled off. This allows for increased turn down ratio

Variable orifice pressure jet

Fuel Pressure entering the burner acts against a spring loaded piston arrangement. Increasing pressure causes the piston to pull a spindle away from the tip, this has the effect of enlarging a closed swirl chamber and uncovering ports. In this way atomisation efficiency is maintained over a greater fuel supply pressure range

Steam assisted atomisers.

This can refer to both external and Internal steam/fuel mixing although conventionally they refer to external mix. In these no mixing of the steam and fuel occurs within the burner itself.

Fuel is supplied to a standard pressure tip atomiser. Steam passes around the fuel passage and exists through an open annulus having being given an angle of swirl to match the fuel spray. At low fuel pressure the steam, supplied at constant pressure throughout turndown, provides for good atomisation. At higher fuel pressure the pressure tip provides for the atomisation.

For first start arrangements compressed air may be used.

Steam atomisation

The two main types of internal mixing (the most common) ar the 'Y' jet and the Skew jet .

Y- Jet

Skew jet

The main advantage of this design over the 'Y' jet is the reduced 'bluff' zone due the reduced pitch diameter of the exit holes.

Venturi register

Matched to a venturi register, a very stable efficient flame is formed. The Fuel/Steam mix exits the nozzle in a series of conic tangents, fuel reversals inside the fuel cone allow efficient mixing with air over a wide 'Turn-Down ratio (20:1). In addition this type of nozzle is associated with reduced atomising steam consumption (0.02Kg per Kg fuel burnt) Venturi and conventional register throat design

Matched to a venturi register, a very stable efficient flame is formed. The Fuel/Steam mix exits the nozzle in a series of conic tangents, fuel reversals inside the fuel cone allow efficient mixing with air over a wide 'Turn-Down ratio (20:1). In addition this type of nozzle is associated with reduced atomising steam consumption (0.02Kg per Kg fuel burnt) Venturi and conventional register throat design


Manufactured by Kawasaki is said to offer the following advantages;

Atomisation is achieved primarily by the energy of ultrasonic waves imparted onto the fuel by the resonator tip which vibrates at a frequency of 5 MHz to 20 MHz under the influence of high speed steam or air impinging on it. Extremely small droplet sizes result which allow for a very stable flame.

Spinning Cup

Fuel is introduced onto the inner running surface of a highly polished fast spinning cup (3 to 7000 rpm). Under centrifugal force this fuel forms a thin film.
Due to the conical shape of the cup the fuel flows to the outer edge spilling into the primary atomising air stream. The fuel is broken into small droplets and mixed with the primary air supplied by the shaft mounted fan. Secondary air is supplied by an external fan for larger units.
Packaged units of this design have the air flow valve controlled by the fuel supply pressure to the distribution manifold.

The spinning cup offers the following advantages;

Blue flame

This highly efficient and clean burning method is very close to stoichiometric combustion. Under normal conditions a portion of the hot gasses from the combustion process is recirculated. Fuel is introduced into the gas were it is vaporised. The resultant flame is blue with little or no smoke


This is the name given to an assembly of vane air swirler plates etc fitted within the boiler casing in association with each burner ,its functions is to divide air into primary and secondary streams and to direct them such as to give the correct air flow pattern.

The air must pass through the air check to enter the register . In some cases the check can be formed by the swirl vanes themselves by rotating them about their axis, in other cases a sliding sleeve is used.

The inner primary air flows until it reaches the tip plate ( stabiliser ) then spills over to form a series of vortices which reduces the forward velocity of the air. This retains the primary flame within the quarl . The outer , secondary air passes over the swirler vanes which causes the air to rotate thus assisting the mixing of air and fuel.

The secondary air shapes the flame, short and fat for side fired, longer and thinner for roof fired.
A small amount of cooling air is allowed to flow to the tip plate and atomiser tip.

It is important that the air check forms a tight seal otherwise thermal shock can damage the quarls when the burner is not in use The front plate is usually insulated , the complexity of the air control is related to the TDR .The steam jet types have the steam providing additional energy for the mixture of air and fuel.

Modern design


Furnace explosions caused by oil vapour and air present in furnace may lead to catastrophic failures. Low impact explosions are oftern referred to as Blow back.

Usually adequate purging is provided within the combustion control however makers timings should be strictly followed .

N.B. This is particularly important with membrane wall boilers where the pressure wave is contained within a strong cell which if ruptured, has disastrous consequences.

A typical cause  are leaking fuel valves or insufficient purgeing following a flame failure.

Air Heaters

Reasons for their use

These are fitted for three main reasons

Additionally the air heater forms a convenient way to warm through a boiler in standby using steam from a secondary source.

For Air heaters which remove heat from the flue gas and transfer it to the air a limit must be placed on the amount of heat remove to ensure that the dew point is not reached before the gas is expelled from the funnel. Failure to do this can lead to the formation of acids which attack sensitive parts of the system. For this reason all gas/air  tube or regenerative type heaters have a low load by pass arrangement.

For water tube boilers gas air heaters are only considered where the temperature at inlet to economiser is greater than 200oC. Due to greater heat transfer efficiency between gas/water economisers are preferred to gas/air exchangers.

Types of air heaters

Lungstrom gas/air heater (regenerative)

The drum contained within the cylindrical casing is formed into segments into which are placed removable ( for cleaning) baskets, consisting of vertical plates (to give minimum resistance to flow) The drum slowly rotates, about 4rev/min, driven via a flexible coupling,gear train, clutch and thrust bearing by one of two electric motors; one mounted on top the other below.

As the drum rotates a segment will enter the gas side, here it removes heat from the gas, it continues to rotate until entering the air side where it will give up its heat to the air. The heat transfer is very efficient, however, soot and corrosive deposits quickly build up in the mesh and hence an effective soot blowing method is essential. This normally takes the form of an arm , pivoted at the circumference of the drum with a single nozzle at the other end. This sweeps across the drum rather like a record arm. One of these arms are fitted top and bottom.

Gas leakage to the air side is prevented by the air being at a higher pressure and by fine radial clearance vanes fitted in the drum segments.

By passes for both air and gas sides are fitted to prevent fouling with the reduced gas flow and temperature, also during manoeuvring when the possibility of different gas/air flow rates occurring leading to high metal temperatures and possible fires.

Failure by uptake fires is not uncommon with this as in most gas/air heater designs.

The main advantages are considered to be very high efficiencies, small foot print relative to a tubular heat exchanger and ease of maintenance with replaceable relatively cheap baskets

Tubular gas/air heater

Shown above is the horizontal tube type air heater which was less susceptible to choking with soot than the vertical types sometimes found with older scotch boilers.

To aid cleaning water washing was sometimes carried out to aid the sootblowers

Bled steam air heater

The use of individual banks and 'U' tubes allow for ease of isolation when these become perforated without large loss of heating surface. The tubes were expanded into the headers and made of cupronickel with copper fins.


The maximum efficiency possible for a plant is given by the Carnot cycle and can be calculated using the formula

Efficiency = T1- T2/ T1

Where T1 is the maximum temperature in a cycle ( kelvin ), and T2 is the minimum temperature in a cycle.

For the steam plant these equate to boiler  outlet temperature and the exhaust temperature of the turbine.

Hence, to increase final temperatures at boiler outlet conditions either; the boiler pressure can be increased, or the degree of superheat can be increased. Boiler pressure increase is ultimately limited by the scantling requirements,more importantly however, the energy stored within the steam is little increased due to the reduction in the latent heat.

Increasing the degree of Superheat not only increases the temperature but also greatly increases the heat energy stored within contained another advantage would be that the onset of condensation through the turbine would be delayed. However this increases the specific volume which would require excessively large plant. Also there would be insufficient pressure drop for efficient expansion through the turbine. There would also be little allowance for feed heating.

There is therefore a combination of increased Pressure and Superheat to give the increased efficiency potential allied with practical design parameters.

Limit of Superheat

Superheated steam, having a lower specific heat capacity then water does not conduct heat away as efficiently as in water cooled tubes, and hence the tube metal surface temperature is higher.

This has led to the external superheat design and parallel steam flows in an effort to keep metal temperatures within limits
For mild steel, upto 455
oC superheat is possible; for higher temperatures, up to 560oC the use of chrome molybdenum steels is required. The use of special alloy steels introduces manufacturing and welding difficulties.
It can be seen that there is a requirement for some form of superheat temperature control

Positioning of the superheater

Integral (FW D-type)

Earlier designs mounted the  superheater tube bank within the generating tube bundle.

The superheater was exposed to elevated temperatures and the  design suffered from heavy slagging of the tubes, particularly the superheater bank, caused by the vanadium bearing ash of the increasingly poorer quality fuel blends.

This ash caused a heavy bonding slag deposit which often bridged the gap between the tubes. This slagging attached to the hot surfaces of the superheater support tube led to wastage and failure.

Increasing slagging would eventually lead to blockage and hence reduced gas path with increased gas velocities over the smaller number of tubes, this led to overheating and failure.

Access for cleaning was limited, this and the problems outlined above led to the external superheater design

Modern designs placed the superheater outside of the Combustion area in the External Superheat Design (ESD)

Roof firing ensured the Superheater was protected from direct radiant heat and flame impingement however the size of the superheater had to be increased  to allow the same degree of superheat at outlet.

The positioning of the superheater banks allowed for easier inspection and cleaning. More effective sootblowing could also be employed as well as simplified mounting arrangements.

The secondary superheater, mounted below that of the primary superheater was of the parallel flow type,  this means the steam flow is in the same direction as the gas flow.

This ensured that the lower temperature attemperated steam ( steam which has had some cooling effect applied as part of the maximum superheat control) was in the tubes in the highest temperature zone. In modern Radiant Heat boilers it is common to mount the primary superheater below that of the secondary and use parallel flow throughout; this ensure adequate cooling throughout.


Designs of Superheater banks and mounting arrangements


This design was typically found in the earlier Integrated Superheater arrangement

The tubes were supported by a support plate which hung off a special increased diameter water cooler tube called the support tube. As the supports were situated in a high temperature zone they were susceptible to failure.

Division plates were welded into the headers, these allowed the steam to make many passes increasing the efficiency of the bank. Small hole were formed in these plates to allow for proper drainage, failure of these plates caused short circuiting, overheating and subsequent failures. Failure of a single tube, although possible leading to a restriction in the flow meant that the heating surface was reduced by only a small amount.

External (melesco type)

In this design there are no baffles fitted inside the header, instead the steam makes a multipass over the gas flowby way of the many limbs or bends of each tube.

The disadvantage of this system is that if a tube should fail then a significant reduction in heating surface would occur. Simpler, more reliable support methods are possible allied to the easier access and sootblowing arrangement.

This type of superheater has the advantage that the number of expanded or welded joints are reduced.

With this design the initial passes are made of Chrome Molybdenum steel. a transition piece attaches this to the mild steel passes.
The inlet header is made out of mild steel and the outlet an alloy steel.

Methods of Tube Connections


Only used in superheaters for temperatures up to 450oC.

Tube ends must be cleaned and degreased and then drifted and roller expanded into the hole, the end of the tube must be projecting by at least 6mm. The bell mouth must have an increase of diameter of 1mm per 25mm plus an additional 1.5mm.

It is important that the tube enter perpendicular into the head, a seal will be assured if the contact length is greater than 10mm, if it is not possible to enter perpendicularly then the contact length should be increased to 13mm.
For larger diameter pipes then grooved seats are used.


Welding gives advantages over expanding in that access to the internal side of the header is not so important and so the number of handhole doors can be much reduced eliminating a source of possible leakage. welding also generally provides a more reliable seal.

The disadvantage is that heat treatment before and following welding is required. The criticality of this is driven by the differing thicknesses of material of the drum tube plate and tube wall.

 To simplify the requirements stubs are welded on to the newly constructed  drum and heat treatment applied to the whole. Blanks are then welded onto the  stubs to provide for the pressure test.

On completion of a successful test  and subsequent inspections, tubes are welded to the tubes. As the material thicknesses  of the stub wall and tubes are the same the requirements for heat treatment is much reduced

A Backing ring may be fitted  to the inside of the tube.The purpose of the backing ring fitted to the conventional attachment method is to prevent the weld metal breaking through into the tube

Melric Joint

The Melric joint offers the following advantages over the conventional method;

The stub bosses can be readily blanked off externally in the event of tube failure and so do not require the access to the header internal side

Reason for superheating steam

Basically the control of temperature is to protect the superheater by preventing the metal temperatures reaching a dangerously high level reducing mechanical strength and leading to failure.

Water flowing through a tube conducts heat away much more effectively than steam due to its higher specific heat capacity. This means that tubes carrying water have a metal temperature much closer to the fluid passing through it.

Where superheat temperatures upto 455oC are in use then the use of mild steel is not a problem, for superheat temperatures above this then alloys of chrome molybdenum steels are used (upto 560oC), difficulties in welding means that there use is restricted to only within the highest temperature zone and a transition piece fitted to connect to remaining mild steel tubing.

Superheat temperature control is therefor fitted to ensure superheat temperature does not exceed design limits.

Methods of regulating superheat temperature

a, By regulating the gas flow over the superheater by means of dampers

The balance line prevents any tendency for the control unit to steam under conditions of low feed flow say due to sudden load change or especially when flashing ( several of these have been burnt out due to incorrect flashing procedure)

The control unit operates the linkages via a control arm, if the superheat is too high then gasses are diverted to flow over the control unit and less gas now flows over the superheat bank.

The control arms and the dampers were very susceptible to damage caused by operating in the hot gas path. Also this control was very sensitive to excess air which can raise the superheat temperature by increasing the heat energy removed from the furnace.

Babcock and wilcock selectable superheat

This design gave a wide range of temperature control, it operated in a similar manner to the Foster Wheeler ESD II. The gas path is separated by a baffle which has flaps located above the tubes operation of which can determine the superheat temperature, as the superheater only extends across one path it is made out of 'W' rather than 'U' tubes.

This design suffered from similar problems to the ESD II with regard to flaps and flap linkages susceptibility to corrosion.

b, By use of multi furnace boilers

Babcock and wilcock Controlled superheat)

The superheat temperature was regulated by changing the position of lit burners within the boiler, shutting off burners in the main furnace and replacing them with flames in the wing furnace had the effect of reducing the superheat temperature as the gasses are cooler when the reach the superheater bank. In this way the superheat temperature could be varied by 60oC.

The advantage of this system was the superheat temperature could be maintained over a wide variation of load. To prevent reversal of flow in the intermediate generating bank a baffle plate is fitted in the water drum which allows the first two rows of the bank to be isolate from the rest and to be supplied by their own two downcomers.

Difficulty was encountered in maintaining the correct air/fuel ratio during differential firing of the two furnaces.
During flashing only the wing furnace is used to give better protection for the superheater

c,Use of air cooled attemperators

Air cooling effect of the double casing is lost in this arrangement so additional insulation must be fitted to ensure that the casing temperature does not exceed safe handling limits.

As air is a relatively poor cooling medium large attemperators are required allied to increased FD fan output required to overcome frictional resistance losses. There is an overall increase in weight, size and initial cost which led to the system being superseded by the regulated gas flow method and then by water or spray cooled attemperation

d, Use of separately fired superheater

In very rare use, normally limited to tank boilers

e, Use of boiler water attemperator (external mounting)

Superheat control is achieved by diverting a proportion of the steam through the simple tubular heat exchanger attemperator

f, Water cooled attemperator (internal)

Shut off valves have to be fitted to the attemperator as in the event of tube leakage the boiler will empty in to the attemperator as it is at a slightly higher pressure due to frictional losses in the superheater.

g, Water spray attemperation

This the most common form of attemperation in use, it consists of two spray nozzles which spray feed water into the steam as it passes from the primary to secondary superheaters. The water receives heat from the steam and thereby reduces the superheat of the steam. To prevent thermal shocking of the transfer pipe, a thin flexible inner tube is fitted.

The spray valves work in series with one reaching its maximum capacity before the second comes into use, the control system takes as its measured value both the outlet temperature and either steam or air flow (load). The spray valves are often designed to be of the air to open variety so in the event of air failure they will fail safe open.

Modern Superheat temperature control system

The main system components are a P+I+D Mater controller (reverse acting, hence output increases for measured values above setpoint ) in which the desired final superheat temperature is set, working in cascade is a P+I slave controller whose output controls the spray attemperator control valve.

There is a temperature transmitter on the inlet to the secondary superheater (Tx1, fitted after the spray) and a secondary superheater outlet temperature transmitter (Tx2).

Tx1 output Mv1 is fed to both the master and slave controllers, in the slave controller this forms the measured value Tx2 output Mv2 is fed to the master controller and forms the measured value, here it is compared to the required set point entered. The output Op1 is sent to the computing relay.

Master controller Op1 = -(Desired set point - Mv2)
(reverse acting)

In the computing relay the signal is added to the rate of change of air flow signal, as the air flow is taken from the combustion control circuit it forms a load signal. In this way the circuit has the ability to react quickly to load changes before they actually begin to effect the temperatures.

The output of the computing relay is fed to the slave controller as its set point Sp2 the set point for the slave controller now has the error of the final superheat and an amount by which the volume rate of air flow ( and hence boiler load) would tend to change the superheat contained within.

The set point Sp2 is compared in the slave controller to the output from the secondary inlet transmitter Tx1 signal Mv1.

Slave controller Output Op2 = Setpoint Sp2 - Mv1

The use of the controllers in cascade speeds up response to system changes.

Computing relay Output SP1 = OP1 + d/dt (air flow)

It is necessary to add the air flow signal as this has a direct effect on the superheat temperature. If there was a load demand increase the combustion control would increase fuel and air to the boiler, this would cause an increase in the superheat steam temperature as there would be some lag until the steam flow increased due to the increased fuel . Once the steam flow has stabilised then the increased steam flow will match the increased gas temperature and so the temperature will reduce. It can be seen then that only during the transition period when the fuel/air has increased but the steam has not that the increased spray is required, this is why the rate of change of air flow rather than volume is used in the control system.

If the measured superheater outlet temperature drops then Mv2 drops, OP1 decreases (the master controller is reverse acting), this is fed through the computing relay and so the set point Sp2 for the slave controller decreases. The setpoint of the slave controller has now fallen below the measured value and hence its output will decrease. This signal OP2 is fed to the spray valve which will shut in increasing the superheat temperature .

If the load on the boiler increases the output of the computing relay increases and hence the set point Sp2 increases, the output of the slave controller Op2 increases and hence the spray valve starts to open even though the increased air flow and hence gas temperature passing over the superheater is yet to be detected in the superheated steam either Tx1 or Tx2, in this way problems of process and control lags can be negated.

The output of Tx1, Mv1 is fed not only to the slave controller but also to the master controller; Its function here is to prevent the master controller from saturating and hence speeding its response under certain conditions. It does this by feeding the integral bellows via the integral restrictor in the controller rather than the more normal feedback arrangement of the output feeding the Integral bellows via the restrictor. In this way the master controller always takes account of the inter temperature.

With the normal layout in low load conditions, should Mv2 fall below the setpoint the Integral action will force the controller into saturation if the temperature fails to recover. This can happen as even with the spray valves shut there may not be enough energy in the flue gasses to heat the steam upto the required temperature in low loads.

By using the output from Tx1, Mv1 then the controller will fail to go into saturation as the integral bellows will receive a signal Mv1 rather than its falling output Op1.

Other additional fittings

Shown on the diagram is a fitting sometime used to protect the system in the event of failure of the spray control valve, this takes the form of a thermostat set so that should the temperature fall below a certain value it will operate a solenoid valve fitted before the spray control valve to shut off the feed. It can be seen that in the event of loss of superheat control , and hence with the spray valve failed open, some form, albeit very coarse , of superheat control can be maintained by use of the thermostat and solenoid valve.

There is alarms fitted to both the inlet and outlet from the secondary superheater as well as a main engine trip due to high superheat temperature. A boiler trip may be fitted for low superheater outlet temperature.


A material in solid form which is capable of maintaining its shape at high tempo (furnace tempo as high as 1650oC) have been recorded.


To protect blr casing from overheating and distortion and the possible resulting leakage of gasses into the machinery space.

To reduce heat loss and ensure acceptable cold faced temperature for operating personnel

To protect exposed parts of drum and headers which would otherwise become overheated. Some tubes are similarly protected.

Act as a heat reservoir.

To be used to form baffles for protective purposes or for directing gas flow.


Must have good insulating properties.

Must be able to withstand high tempo's

Must have the mechanical strength to resist the forces set up by the adjacent refractory.

Must be able to withstand vibration.

Must be able to withstand the cutting and abrasive action of the flame and dust

Must be able to expand and contract without cracking Note: no one refractory can be used economically throughout the boiler


Acid materials- clay, silica, quartz , sandstone etc

Neutral materials-chromite, graphite, plumbago, alumina

Alkaline or base materials- lime, magnesia, zirconia

Note that acid and alkaline refractories must be sepperated


Firebricks- these are made from natural clay containing alumina , silica and quartz. They are shaped into bricks and fired in a kiln

Monolithic refractories- These are supplied in the unfired state, installed in the boiler and fired in situ when the boiler is commissioned.

Mouldable refractory- This is used where direct exposure to radiant heat takes place. It must be pounded into place during installation . It is made from natural clay with added calcided fire clay which has been chrushed and graded.

Plastic chrome ore- This is bonded with clay and used for studded walls. It has little strength and hence stud provides the support and it is pounded inot place.It is able to resist high temperatures

Castable refractory-This is placed over water walls and other parts of the boiler were it is protected from radiant heat . It is installed in a manner similar to concreting in building

Insulating materials- Blocks, bricks , sheets and powder are usually second line refractories. I.E. Behind the furnace refractory which is exposed to the flame. Material; asbestos millboard, magnesia , calcined magnesia block, diatomite blocks, vermiculite etc. all having very low heat conductivity.

Furnace linings

Studded Wall

- these are lined with plastic chrome ore

The amount of studding and the extent of tube surface covered with chrome ore is varied to suit the heat absorption rate required in the various zones of the boiler furnace.

Floor tubes may be situated beneath a 3" layer of brickwork, the tubes are embedded in chrushed insulating material below which is a layer of solid insulation and then layers of asbestos millboard and magnessia.



Membrane Wall

Furnace floors

- Two layers of 50 mm firebrick above the tubes and 100 mm slab insulation below. Tubes in castable insulation are covered with crushed firebrick. Note; Before castable insulation applied ,tubes coated with bitumen to allow expansion clearance when tubes are at working tempo

Front walls

- In front fired boilers these need additional insulation (200 mm) made up of 125 mm mouldable refractory backed by 50 mm castable or slab and 25 mm of asbestos millboard.

Burner openings

- These have specially shaped bricks called quarls or have plastic refractory

Brick bolts

There are two basic types;

using a hole right through the brick

Using a recess in the back of the brick.

A source of weakness is where bricks crack, bolts will be exposed to the direct heat which leads to failure.
Adequate expansion arrangements must be provided. For floor tubes a coating of bitumastic is first applied before the castable refractory is applied. When the boiler is fired the bitumastic is burnt away then a space is left for expansion

Refractory failure

This is one of the major items of maintenance costs in older types of boiler


This is the breaking away of layers of the brick surface. It can be caused by fluctuating temperature under flame impingement or firing a boiler too soon after waterwashing or brick work repair.

May also be caused by failure to close off air from register outlet causing cool air to impinge on hot refractory.


This is the softening of the bricks to a liquid state due to the prescience of vanadium or sodium ( ex sea water ) in the fuel. This acts as fluxes and lowers the melting point of the bricks which run to form a liquid pool in the furnace Eyebrows may form above quarls and attachment arrangements may become exposed Material falling to floor may critically reduce burner clearance and reduce efficiency

Flame impingement may lead to carbon penetrating refractory.


Refractories are weaker in tension than in compression or shear thus, if compression takes place due to the expansion of the brick at high temperature , if suddenly cooled cracking may occur.

Failure of brick securing devices