Sample Boiler Calculations

1.Convert actual steam rating into From and At 100 C

Steam capacity from and at 100 C (212 F) is equivalent steam capacity if operating conditions are reduced to atmospheric pressure.



Steam capacity = 8000 kg/hr at 10.5 kg/cm2 saturated
Feed water inlet = 30 C
Heat load = 8000 (664-30) Kcal/hr
= 5.072e06 Kcal/hr = 20.127e06 btu/hr = 5.8976 MW

Publish Post

where Sat steam enthalpy = 664 Kcal/kg
Inlet water enthalpy = 30 Kcal/kg
Steam enthalpy at 100C and 1 atm pressure = 540 Kcal/kg

Therefore, steam capacity F&A 100 C = 5.072e06/540
= 9392 Kg/hr


2. Heat Duty Calculations :

Let us calculate heat duty of a boiler generating 50,000 kg/hr at 65 bar and 485 C
Water inlet temperature = 105 C

Steam & water properties:

Superheated steam enthalpy at 65 bar & 485 C = 808 Kcal/kg
Saturated water enthalpy = 295 Kcal/kg

Heat Duty = 50000 x (808 - 105)
= 35.15e06 Kcal/hr (139.48e06 Btu/hr or 40.87 MW)

Usually 1 – 3% of the water flow is used for blowdown.

Considering 2% blow down , heat in blowdown water = 50000 x 0.02 x (295 – 105)
= 0.19e06 kcal/hr

Total heat duty = (35.15 + 0.19) e06 = 35.34e06 kcal/hr
= 140.24e06 Btu/hr = 41.09 MW

In case of Hot water generator or hot water boiler,

Heat duty = Water flow x Cp of water x Temp gain

For example, 200,000 kg/hr of water is heated from 70 to 90 degC,

Heat Load = 200,000 x 1 x (90-70)
= 4.0e06 Kcal/hr
= 15.873e06 Btu/hr or 4.651 MW

3. Heat Transfer calculations:

Over all heat transfer coefficient,

Uo = 1/(1/Ho+Rm+1/Hi*(TubeOD/TubeID)+Ro+Ri*(TubeOD/TubeID))

Where Ho = Outside heat transfer coefficient
Hi = Inside heat transfer coefficient
Rm = tube metal resistance
Ro = Fouling resistance on outside tubes
Ri = Fouling resistance on inside tubes

Inside Heat Transfer coefficient can be calculated using the following correlation :

NuInside=0.023* (ReInside^0.8)*(PrInside^0.4)

Where NuInside = Hi x TubeID / Gas Cond

Outside heat transfer coefficient during boiling is very high and so resistance offered is negligibly small. There are many correlations available to predict Ho, but Ho can be safely assumed to be about 10000 Kcal/hr/m2/C.

Boiler Efficiency calculations

Efficiency is a very important criterion in Boiler selection and Design. Efficiency figure depends upon the type of boiler as well as on the type of fuel and it’s constituents. For example, efficiency of a Bagasse fired boiler is about 70% where as that of oil fired boilers is about 85 %. Higher moisture content in Bagasse reduces it’s efficiency. So better criterion is efficiency based on LCV or NCV. This is widely used in Europe and efficiency based on HHV or GCV is used in other parts of the world.

There are basically two methods to calculate efficiency of the boilers : Input-Output method and Heat Loss method. In Input-output method, boiler must be in steady running condition and the data of heat input in the form of fuel and air and heat output in the form of steam and other losses is taken.

Here we are going to discuss the second and more popular method. In this method, first we calculate the heat input. Then all heat losses are calculated. Effective heat output is heat input less the heat losses. Output to Input ratio gives the efficiency.

Heat losses in fired boiler are :

a) Dry gas losses

b) Loss due to moisture in fuel

c) Loss due to moisture formed during combustion

d) Loss due to moisture in combustion air

e) Unburnt fuel loss

f) Loss due to radiation from Boiler to surroundings

g) Manufacturers Margin OR unaccounted losses

Sample Case :

Let us calculate Boiler efficiency of coal fired boiler. Ambient temp is 80 F and Back End Temperature (Exh gas temp) is 302 F. The percent composition of Coal is as under:

Carbon , C - 76.0 ; Hydrogen, H₂ - 4.1 ; Nitrogen , N₂ - 1.0 ; Oxygen, O₂ - 7.6 ; Suphur, S - 1.3 ; Moisture, H₂O - 3.0 ; Ash - 7.0 ;

The Combustion calculations of the above fuel is already explained in detail in the other article.

From the above calculations, Unit Wet Gas, Kg / Kg of fuel = Unit Wet Air + (1-Ash)

= 13.12 + (1-0.007)

= 14.05

Unit Dry Gas, Kg / Kg of fuel = Unit Wet Gas – (Moisture in Air + Water produced during combustion)

= 13.484

Higher Heating Value, HHV or Gross Calorific Value, GCV in BTU/Lb

= 14600.C + 62000 (H2-O2/8) + 4050.S

Lower Heating Value, LHV or Lower Calorific Value, LCV or Net Calorific Value, NCV, BTU/lb

= HHV – 1030(9.H2 + Moisture)

Let us use HHV and LHV notation.

HHV = (14600 x 76 +62000 (4.1-7.6/8) + 4050 x 1.3 )/100

= 13101.65 BTU/lb (7278.7 Kcal/kg )

LHV = 13101.65 – 1030(9*4.1+3)/100

= 12690.6 BTU/lb (7050 Kcal/kg)

Calculations of the Losses based on Higher Heating Value:

a) Dry gas losses:

Exhaust gases always leave the boiler at a higher temp than ambient. Heat thus carried away by hot exhaust gases is called Dry gas losses

Heat Losses, La = UnitDryGas x Cp x (Tg-Ta) x 100/HHV

= 13.478 x 0.24 x (302 -80) x 100 / 13101.65

= 5.48 %

b) Loss due to Moisture in fuel :

The moisture present in the fuel absorbs heat to evaporate and get superheated to exit gas temperature.

Lb = MoistureInFuel x (1089-Ta+0.46xTg)x100/HHV

= 0.03 x (1089 – 80 +0.46 x 302) x100 / 13101.6

= 0.263 %

c) Loss due to Moisture Produced during combustion :

Lc = MoistureProduced x (1089-Ta+0.46xTg)x100/HHV

= 0.369 x (1089 – 80 +0.46 x 302) x100 / 13101.6

= 3.23 %

d) Loss due to Moisture in air :

Ld = MoistureInAir x Cp of Steam x (Tg-Ta) x 100/HHV

= 0.0132 x 12.95 x 0.46 x (302 - 80) x100 / 13101.6

= 0.133 %

Here, Moisture in Air = 0.0132 lb/ lb of dry air at 60% Relative Humidity

Cp of steam = 0.46

e) Unburnt fuel loss :

This is purely based on experience. Unburnt fuel loss depends up on type of Boiler , grate, grate loading and type of fuel. For Bio-Mass fuels, it ranges from 1.5 to 3 %, for oils from 0-0.5 and almost nil for gaseous fuels.

Let us consider Unburnt fuel loss, Le = 2.5 % for Coal.

f) Radiation Loss:

Radiation Loss is because of hot boiler casing loosing heat to atmosphere. ABMA chart gives approximate radiation losses for fired boilers.

Let us take a radiation Loss , Lf = 0.4 % in this case.

g) Manufacturer’s margin :

This is for all unaccounted losses and for margin. Unaccounted losses are because of incomplete combustion carbon to CO, heat loss in ash ..etc. This can be 0.5 to 1.5 % depending up on fuel and type of boiler.

In this case, let us take, Manufacturer’s margin Lg = 1.5%.

Total Losses = La + Lb + Lc + Ld + Le + Lf + Lg

= 5.48 + 0.263 + 3.23 + 0.4 +0.133 +2.5 + 1.5

= 13.506 %

Therefore, Efficiency of the boiler on HHV basis = 100 – Total Losses

= 100 – 13.506

= 86.494 %

Efficiency based on LHV:

Efficiency based on LHV = EfficiencyOnHHV x HHV/LHV

= 86.494 x 13101.6/12690.6

= 89.29 %

article from : http://www.firecad.net/

Boiler Economizers

A boiler economizer is a device that reduces the overall fuel requirements a boiler requires which results in reduced fuel costs as well as fewer emissions - since the boiler now operates at a much higher efficiency. Boiler economizers recover the "waste heat" from the boiler's hot stack gas from transfers this waste heat to the boiler's feed-water. Because the boiler feed-water is now at a higher temperature that it would have been without a boiler economizer, the boiler does not need to provide as much additional heating to produce the steam requirements of a facility or process, thereby using less fuel and reducing the fuel expenses. Boiler economizers also help improve a boiler's efficiency by extracting heat from the flue gases discharged from the final super-heater section of a radiant/reheat unit or the evaporative bank of a non-reheat boiler. Heat is transferred, again, back to the boiler feed-water, which enters at a much lower temperature than saturated steam.

Boiler Economizers are a series of horizontal tubular elements and can be characterized as bare tube and extended surface types. The bare tube includes varying sizes which can be arranged to form hairpin or multi-loop elements. Tubing forming the heating surface is generally made from low-carbon steel. Because steel is subject to corrosion in the presence of even low concentrations of oxygen, water must be practically 100 percent oxygen free. In central stations and other large plants it is common to use deaerators for oxygen removal.

Boiler Inspection

Introduction

Normally boiler inspection will be carried out onboard the ship by a port state control and during the dry dock. They are used to carry out the inspection and see the working condition of the boiler. During the inspection they will conduct an in-depth analysis of the boiler condition considering various factors to find the working condition of the boiler. If necessary they will replace damaged parts of the boiler needed for continued safe operation.

Need For Boiler Survey or Inspection

1. Boilers are inspected to maintain the Class requirement.
2. Regular internal inspection and external examination during such survey constitute the preventive maintenance schedule the boiler goes through to have a safe working condition.

Frequency of Boiler Survey

1. Water tube high pressure boilers are surveyed at two year intervals.
2. All other boilers, including exhaust gas boilers, are surveyed at two yearly intervals until they are eight years old and then surveyed annually.

Planning for Boiler Survey

1. Confirm time available, manpower, and time required.
2. Check before shutting down boiler.
3. Check for spares e.g. manhole door joints, gauge glass, packing and steam joints.
4. Check the tools required e.g. gagging tool, torque spanner, rope, chain block etc.
5. Check manual for special instruction and past records.
6. Steam requirement for the next port should be considered e.g. Tankers require steam in discharged Port.
7. Briefing to other engineers of work involved.

Shutting Down the Boiler for Inspection

Before inspection is to be carried out, the boiler which is firing should be shut down. These are the steps to be followed before shutting down the boiler for inspection.

1. Inform the chief engineer and inform the duty officer in the bridge.
2. Change over M/E, A/E, and Boiler to diesel oil.
3. Top up diesel oil service tank, stop heavy oil and lube oil purifiers.
4. Stop all tank and tracing steam heating and carry out soot blowing.
5. Change over from automation to manual firing of boiler.
6. Stop the firing of the boiler and purge boiler for three to five minutes.
7. Switch off power and off the circuit breaker for forced draught fan, FO pump, feed pump, and combustion control panel. Hang necessary notices.
8. Shut main steam-stop valve and shut all fuel valves to boiler.
9. Let the boiler cool down, do not blow down now.
10. When the boiler pressure is about 4 bars, carry out blow down.
11. When boiler pressure is slightly higher than atmospheric pressure, open the vent cock to prevent formation of vacuum.
12. Let the boiler cool down.
13. Once sufficient cooled, open top manhole door first with all safety precaution.
14. Mark the nut on the top manhole, slacken the dog-nut, and secure it with a rope.
15. Knock the manhole door gently, but do not open it as it may contain steam or hot water.
16. Conform nothing coming out; open the door fully with the help of securing rope.
17. Do not open immediately open the bottom door, since the boiler is still hot and if opened relatively cool current of air will pass through the boiler causing a thermal shock.
18. Allow further cool down before opening bottom manhole door.
19. Open the bottom manhole door with the same precautions and open the furnace side door also.
20. Ventilate foe period of 12 to 24 hours.
21. Then check for oxygen, flammable vapor, and toxic gasses.
22. If it is safe, prepare for entry.

Shutting Down Boiler

Preparation for Entry

These are the steps to be carried out before entering the boiler for inspection.

1. Prepare a long rope, wooden plank oxygen analyzer, safety hand lamp, and safety torch attached with rope.
2. Get a pouch to carry tools and keep track of the number of tools to be brought into boiler.
3. Personnel safety protection wear, e.g. helmet, safety shoes, hand gloves, etc.
4. No extra instruments to be brought in and clear pocket contents as it may fall into boiler.
5. Keep an emergency breathing apparatus ready.
6. Remain in communication and ensure proper lighting.
7. Check boiler internals before making an entry, e.g. foothold and handhold.

Pneumatic Spreader Fired Boiler With Fire Bars

RFP (Radiant Furnace Smoke tube with Pneumatic Feeding)

RFP SERIES
Automatic feeding of fuels. Both auto and manual feeding of fuels simultaneously is possible.
Maximum flexibility in the choice of fuels.
More free board area. Hence complete combustion.

RANGE :
Capacity : 2 to 15 TPH
Working pressure : Upto 42 Kg/cm²
Fuels : Solid fuels like Coal, Wood (Waste / chips also husk, Baggasse, groundnut / Coconut Shell etc.,
Optional : Oil and gas.

Manual Fired Boiler With Fire Bars

RFS SERIES
Fixed grate furnace. Manual feeding
No refractory work. Hence minimum maintenance work.
RANGE :
Capacity : 2 to 12 Tons per hour (TPH)
Working pressure : Upto 32 Kg/cm²
Fuels : Solid fuels like Coal, Wood, Wood waste, Groundnut / coconut Shells, Baggasse (Bale), saw dust etc.,

How to Build a Steam Boiler

A steam boiler is a vessel that contains water and a heat source that turns the water into steam. The boiler transfers heat from the source to the water vessel, thereby creating steam. This steam exits the vessel through a pipe and is transported to another location, where it can be used for cleaning, to power equipment, to provide heat or for a number of other functions. These instructions are for a basic steam boiler that can be used to provide heat to residential and small commercial buildings.

Instructions

1. 
   

   

   
   
   Steam Boiler Vessel

   Order the appropriate boiler vessel and chimney. The vessel and chimney should be made of heavy duty cast iron and can be ordered from a metal foundry. The vessel and chimney should be made as separate parts and should arrive from the foundry as two different pieces.

2. Insert the T-pipe at the base of the chimney into the appropriate inlet valve on the side of the vessel. The diameter of the inlet valve will be slightly larger than the diameter of the T-pipe. Put some of the chemical pipe sealant on the outside of the chimney's T-pipe and push the chimney until the T-pipe has fully entered the vessel's inlet valve. You may have to rotate the chimney slightly as you push in order to get the T-pipe fully inserted.

3. Fully seal the T-pipe to the vessel's inlet valve by wrapping some of the Teflon tape around the attachment area.

4. 
   
   
   Copper Base Plates

   Build the boiler's furnace. The copper base plates will be responsible for generating heat within the furnace. Insert the bolts into the threaded holes on the inside wall of the furnace compartment using the mechanical wrench, and hang the base plates on the bolts. Run the electrical cord from the base plates to the outside of the vessel where it can be plugged into an electrical outlet.

5. Assemble the drain. Apply some chemical pipe sealant to the threaded part of the steel reducer and screw the steel reducers to the bottom of the boiler vessel using the pipe wrench. Seal the reducer in place using the Teflon tape. Use the pipe wrench to screw the steel elbow to the reducer, and then screw one piece of the 4-inch stainless steel pipe to the elbow using your hands. Seal all attachments using the chemical sealant on the threads and the Teflon tape on the outside of the attachment areas. Finally, screw the 5-inch copper tubing into the stainless steel pipe using your hands, and seal the attachment area. The tubing will carry excess water from the boiler, so the other end of the tubing must attach to an outlet pipe or end in a place where the excess water can be released.

6. 
   Bronze Safety Valve

   Attach the bronze safety valve to the top of the boiler's vessel using the metal screws and the Phillips-head screw driver. You can purchase a safety valve from an industrial supply company such as WW Grainger.

7. Apply chemical sealant to the threads of the second steel reducer, and use the pipe wrench to screw the reducer into the valve at the top of the vessel's dome. Seal the attachment with the Teflon tape, and screw the second elbow to the reducer using the pipe wrench. Next, screw one end of the second 4-inch piece of stainless steel pipe into the reducer and attach the other end to the building's main heating line. The pipe will transport the steam from the furnace to the heating line, which will distribute it to the building's radiators. Be sure to seal all attachments with the chemical sealant and the Teflon tape.

8. Screw the rubber hosing into the bottom outlet on the boiler's vessel using your hands, and seal the attachment with the chemical sealant and the Teflon tape. Run the hose from the furnace to the furnace's water supply.

How to Maintain a Steam Boiler

Steam boilers are as effective in generating heat as they are potentially dangerous. With loads of heat burning and pumping through the boiler, safety must be the number one concern when operating a steam boiler. Maintaining your boiler correctly is one of the best ways to avoid accidents and keep your boiler safe and efficient. Follow these steps to properly maintain your steam boiler.

Instructions

1. Clean your boiler. Draining and cleaning your boiler will keep the parts working correctly and will help prevent damage to the mechanism. Additionally, cleaning your boiler will allow you to inspect it for flaws, corrosion and damage.
2. Maintain the proper water level. Studies have found that most boiler accidents and injuries are caused by a low water level. Aside from the danger to people, a low water level is also bad for the boiler itself. So, check your boiler's water level on a regular basis to make sure it's correct.
3. Keep all moving parts clean and oiled. Valves are especially crucial in maintaining a steam boiler and should be easily turned open and shut. Many of a boiler's valves play a vital role in keeping the boiler running safely, so test all moving parts and valves for optimal functionality.
4. Keep a boiler log. Keeping a log of incidents, malfunctions, inspections and cleanings will help you just when you need it. Like a medical patient's charts, a boiler log communicates valuable information to you and boiler professionals without costing you much time or money.

Supercritical steam generator

Steam generation power plant.

Supercritical steam generators are frequently used for the production of electric power. They operate at supercritical pressure. In contrast to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over 3,200 psi/22.06 Mpa or 3,200 psi/220.6 bar) that actual boiling ceases to occur, the boiler has no liquid water - steam separation.
There is no generation of steam bubbles within the water, because the pressure is above the critical pressure at which steam bubbles can form. It passes below the critical point as it does work in a high pressure turbine and enters the generator's condensor. This results in slightly less fuel use and therefore less greenhouse gas production. The term "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" actually occurs in this device.

Water tube boiler

Diagram of a water-tube boiler

Another way to rapidly produce steam is to feed the water under pressure into a tube or tubes surrounded by the combustion gases. The earliest example of this was developed by Goldsworthy Gurney in the late 1820s for use in steam road carriages. This boiler was ultra-compact and light in weight and this arrangement has since become the norm for marine and stationary applications. The tubes frequently have a large number of bends and sometimes fins to maximize the surface area. This type of boiler is generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which can contain the pressure with a thinner wall. It can however be susceptible to damage by vibration in surface transport appliances. In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections. These sections are mechanically assembled on site to create the finished boiler.

High pressure water tube boilers generate steam rapidly at high temperatures that can be increased by lengthening the tubes. The superheater section of the tubes.

Superheater

A superheated boiler on a steam locomotive.

L.D. Porta gives the following equation determining the efficiency of a steam locomotive, applicable to steam engines of all kinds: power (kW) = steam Production (kg h^-1)/Specific steam consumption (kg/kW h).

A greater quantity of steam can be generated from a given quantity of water by superheating it. As the fire is burning at a much higher temperature than the saturated steam it produces, far more heat can be transferred to the once-formed steam by superheating it and turning the water droplets suspended therein into more steam and greatly reducing water consumption.

The superheater works like coils on an air conditioning unit, however to a different end. The steam piping (with steam flowing through it) is directed through the flue gas path in the boiler furnace. This area typically is between 1,300–1,600 degree Celcius (2,372–2,912 F). Some superheaters are radiant type (absorb heat by thermal radiation), others are convection type (absorb heat via a fluid i.e. gas) and some are a combination of the two. So whether by convection or radiation the extreme heat in the boiler furnace/flue gas path will also heat the superheater steam piping and the steam within as well. It is important to note that while the temperature of the steam in the superheater is raised, the pressure of the steam is not: the turbin or moving pistons offer a "continuously expanding space" and the pressure remains the same as that of the boiler. The process of superheating steam is most importantly designed to remove all droplets entrained in the steam to prevent damage to the turbine blading and/or associated piping. Superheating the steam expands the volume of steam, which allows a given quantity (by weight) of steam to generate more power.

A steam turbine with the case opened

When the totality of the droplets are eliminated, the steam is said to be in a superheated state.In a Stephensonian firetube locomotive boiler, this entails routing the saturated steam through small diameter pipes suspended inside large diameter fire tubes putting them in contact with the hot gases exiting the firebox; the saturated steam flows backwards from the wet header towards the firebox, then forwards again to the dry header. Superheating only began to be generally adopted for locomotives around the year 1900 due to problems of overheating of and lubrication of the moving parts in the cylinders andsteam chests. Many firetube boilers heat water until it boils, and then the steam is used at saturation temperature in other words the temperature of the boiling point of water at a given pressure (saturated steam); this still contains a large proportion of water in suspension. Saturated steam can and has been directly used by an engine, but as the suspended water cannot expand and do work and work implies temperature drop, much of the working fluid is wasted along with the fuel expended to produced it.

Firetube Boiler

The next stage in the process is to boil water and make steam. The goal is to make the heat flow as completely as possible from the heat source to the water. The water is confined in a restricted space heated by the fire. The steam produced has lower density than the water and therefore will accumulate at the highest level in the vessel; its temperature will remain at boiling point and will only increase as pressure increases. Steam in this state (in equilibrium with the liquid water which is being evaporated within the boiler) is named "saturated steam". For example, saturated steam at atmospheric pressure boils at 100 °C (212 °F).

Saturated steam taken from the boiler may contain entrained water droplets, however a well designed boiler will supply virtually "dry" saturated steam, with very little entrained water. Continued heating of the saturated steam will bring the steam to a "superheated" state, where the steam is heated to a temperature above the saturation temperature, and no liquid water can exist under this condition.

Physical layout of the four main devices used in the Rankine cycle

Most reciprocating steam engines of the 19th century used saturated steam, however modern steam power plants universally use superheated steam which allows higher steam cycle efficiency.

Solid fuel firing

In order to improve the burning characteristics of the fire, air needs to be supplied through the grate, or more importantly above the fire. Most boilers now depend on mechanical draft equipment rather than natural draught.

The stack effect in chimneys: the gauges
represent absolute air pressure
and the airflow is indicated with light grey arrows.
The gauge dials move clockwise with increasing pressure.

This is because natural draught is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as chimney height. All these factors make effective draught hard to attain and therefore make mechanical draught equipment much more economical. There are three types of mechanical draught:

1. Induced draught: This is obtained one of three ways, the first being the "stack effect" of a heated chimney, in which the flue gas is less dense than the ambient air surrounding the boiler. The denser column of ambient air forces combustion air into and through the boiler. The second method is through use of a steam jet. The steam jet or ejector oriented in the direction of flue gas flow induces flue gases into the stack and allows for a greater flue gas velocity increasing the overall draught in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The third method is by simply using an induced draught fan (ID fan) which sucks flue gases out of the furnace and up the stack. Almost all induced draught furnaces have a negative pressure.
2. Forced draught: draught is obtained by forcing air into the furnace by means of a fan (FD fan) and ductwork. Air is often passed through an air heater; which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draught furnaces usually have a positive pressure.
3. Balanced draught: Balanced draught is obtained through use of both induced and forced draft. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draft fan works in conjunction with the forced draft fan allowing the furnace pressure to be maintained slightly below atmospheric.

Combustion

The source of heat for a boiler is combustion of any of several fuels, such as wood, coal,oil, or natural gas. Nucleas fission is also used as a heat source for generating steam.

Nuclear fission

Heat recovery steam generators (HRSGs) use the heat rejected from other processes such as gas trubines.

Multi-tube boilers

A significant step forward came in France in 1828 when Marc Ssguin devised a two-pass boiler of which the second pass was formed by a bundle of multiple tubes. A similar design with natural induction used for marine purposes was the popular “Scotch” marine boiler. Prior to the Rainhill trials of 1829 Henry Booth, treasurer of the Liverpool and Meanchester Railway suggested to George Stephenson, a scheme for a multi-tube one-pass horizontal boiler made up of two units: a Firebox surrounded by water spaces and a boiler barrel consisting of two telescopic rings inside which were mounted 25 copper tubes; the tube bundle occupied much of the water space in the barrel and vastly improved heat transfer.

The firebox of a coal-fired train steam engine.


Section of typical boiler and firebox

Old George immediately communicated the scheme to his son Robert and this was the boiler used on Stephenson's Rocket, outright winner of the trial. The design was and formed the basis for all subsequent Stephensonian-built locomotives, being immediately taken up by other constructors; this pattern of fire-tube boiler has been built ever since

Cylindrical fire-tube boiler

An early proponent of the cylindrical form, was the American engineer, Oliver Evans who rightly recognised that the cylindrical form was the best from the point of view of mechanical resistance and towards the end of the 18th Century began to incorporate it into his projects. Probably inspired by the writings on Leupold’s “high-pressure” engine scheme that appeared in encyclopaedic works from 1725, Evans favoured “strong steam” i.e. non condensing engines in which the steam pressure alone drove the piston and was then exhausted to atmosphere.
The advantage of strong steam as he saw it was that more work could be done by smaller volumes of steam; this enabled all the components to be reduced in size and engines could be adapted to transport and small installations. To this end he developed a long cylindrical wrought iron horizontal boiler into which was incorporated a single fire tube, at one end of which was placed the fire grate. The gas flow was then reversed into a passage or flue beneath the boiler barrel, then divided to return through side flues to join again at the chimney (Columbian engine boiler). Evans incorporated his cylindrical boiler into several engines, both stationary and mobile. Due to space and weight considerations the latter were one-pass exhausting directly from fire tube to chimney. Another proponent of “strong steam” at that time was the Cornishman, Richard Trevithick. His boilers worked at 40–50 psi (276–345 kPa) and were at first of hemispherical then cylindrical form. From 1804 onwards Trevithick produced a small two-pass or return flue boiler for semi-portable and locomotive engines.

Trevithick's engine of 1806 is built around

an early example of a flued boiler (specifically, a return-flue type)

The Cornish boiler developed around 1812 by Richard Trevithick was both stronger and more efficient than the simple boilers which preceded it. It consisted of a cylindrical water tank around 27 feet (8.2 m) long and 7 feet (2.1 m) in diameter, and had a coal fire grate placed at one end of a single cylindrical tube about three feet wide which passed longitudinally inside the tank. The fire was tended from one end and the hot gases from it travelled along the tube and out of the other end, to be circulated back along flues running along the outside then a third time beneath the boiler barrel before being expelled into a chimney. This was later improved upon by another 3-pass boiler, the Lancashire boiler which had a pair of furnaces in separate tubes side-by-side. This was an important improvement since each furnace could be stoked at different times, allowing one to be cleaned while the other was operating.

Lancashire boiler, from Fairbairn's lecture

Railway locomotive boilers were usually of the 1-pass type, although in early days, 2-pass "return flue" boilers were common, especially with locomotives built by timothy Hackworth.

Haycock and wagon top boilers

Haycock and wagon top boilers

Newcomen steam engine.
– Steam is shown pink and water is blue.
– Valves move from open (green) to closed (red)

For the first Newcomen engine of 1712, the boiler was little more than large brewer’s kettle installed beneath the power cylinder.

Kettle from a Korean tea house.

Because the engine’s power was derived from the vacuum produced by condensation of the steam, the requirement was for large volumes of steam at very low pressure hardly more than 1 psi (6.9 kPa) The whole boiler was set into brickwork which retained some heat.

Pump to demonstrate vacuum

A large vacuum chaber

A wall built in Flemish bond

A voluminous coal fire was lit on a grate beneath the slightly dished pan which gave a very small heating surface; there was therefore a great deal of heat wasted up the chimney. In later models, notably by John Smeaton, heating surface was considerably increased by making the gases heat the boiler sides, passing through a flue. Smeaton further lengthened the path of the gases by means of a spiral labyrinth flue beneath the boiler. These under-fired boilers were used in various forms throughout the 18th Century. Some were of round section (haycock). A longer version on a rectangular plan was developed around 1775 by Boulton and Watt (wagon top boiler). This is what is today known as a three-pass boiler, the fire heating the underside, the gases then passing through a central square-section tubular flue and finally around the boiler sides.

Component of prime mover in boiler machine

The steam generator or boiler is an integral component of a steam engine when considered as a prime mover; however it needs be treated separately, as to some extent a variety of generator types can be combined with a variety of engine units. A boiler incorporates a Firebox or Furnace in order to burn the fuel and generate heat; the heat is initially transferred to water to make steam: this produces saturated steam at ebullition temperature saturated steam which can vary according to the pressure above the boiling water.

The higher the furnace temperature, the faster the steam production. The saturated steam thus produced can then either be used immediately to produce power via a turbine and alternator , or else may be further superheated to a higher temperature; this notably reduces suspended water content making a given volume of steam produce more work and creates a greater temperature gradient in order to counter tendency to condensation due to pressure and heat drop resulting from work plus contact with the cooler walls of the steam passages and cylinders and wire-drawing effect from strangulation at the regulator. Any remaining heat in the combustion gases can then either be evacuated or made to pass through an economiser, the role of which is to warm the feed water before it reaches the boiler.

Boiler Machine (steam generator)

A boiler or steam generator is a device used to create steam by applying heat energy to water . Although the definitions are somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure

(1–300 psi/0.069–20.684 bar; 6.895–2,068.427 kpa), but at pressures above this it is more usual to speak of a steam generator.

An industrial boiler, originally used for supplying steam to a stasionary steam engine

A boiler or steam generator is used wherever a source of steam is required. The form and size depends on the application: mobile steam engines such as steam liocomotives, portable engines and steam-powered road vehicles typically use a smaller boiler that forms an integral part of the vehicle; stasionary steam engines, industrial installations and power stations will usually have a larger separate steam generating facility connected to the point-of-use by piping. A notable exception is the steam-powered fireless locomotive, where separately-generated steam is transferred to a receiver (tank) on the locomotive.