Cornish Boiler: Definition, Parts, Operation, Merits and Demerits

Cornish boiler is a type of steam boiler with a single fire tube, single flue, or single furnace. It was used in the nineteenth century and is named after the county of Cornwall in the UK.

It is a fire tube type, that is, a type of boiler in which hot gases or the products of the combustion pass through the furnace tube surrounded by water, thereby boiling it by heat transfer and producing steam as a consequence, which is to expand in the steam engine or steam turbine for the production of work.

In its construction, it is a horizontal shell-type boiler with flat end plates. The diameter of the shell of a typical cornish boiler is around 1.25 to 2 meters, with a length that may range from 5 to 8 meters. The diameter of the flue tube inside the cornish boiler is 0.8 times the diameter of the shell.

cross sectional view of cornish boiler

It is sufficient to produce 2000 kg of steam per hour and is designed to operate at a pressure of up to 11 bar. With little design advancements, such as with shortened flue tubes and smoke tubes, the pressure may go up to 14 bar with a steam generation capacity of no less than 3000 kg per hour.

When fitted with smoke tubes, the cornish boiler is called the multi-tubular boiler.

Main Parts

The main parts of the cornish boiler are mentioned below:

1. Shell

It is the horizontal cylinder or the main body of the cornish boiler (or any other shell-type steam boiler) that retains water and the furnace.

cornish boiler labeling diagram

2. Furnace

It is designed to be located at one end of the boiler. Its sole purpose is to burn the fuel, usually coal, and produce an extremely hot exhaust directed to pass through the fire tube to transfer heat to the water for producing steam.

3. Flue Tube

It is a pipe that is intended for the hot flue (exhaust) gases to pass through and exit into the atmosphere through the chimney or stack. In some cases, it is aligned not with the center line but is located a few centimeters eccentric to ease internal cleaning. Patents claim that such eccentricity causes improved water circulation due to turbulent water flow.

4. Side Flue and Bottom Flue

side flue and bottom flue tube

Combustion gases rise above the grate and collect at the trailing end of the boiler, having transferred heat to the water. They continue to flow into the two side flues and, by passing through them, now accumulate at the front end of the boiler to pass through the bottom flue and finally exit via. Chimney into the atmosphere. It is shown below in a clearer illustration.

5. Grate

It is typically designed to be at the bottom of the furnace. It supports the combustion process by maximizing the oxidation of the fuel to be burned in the furnace.

6. Ashpit

It collects the burned gas residue in the form of ash at the bottom of the grate.

7. Combustion Chamber

It is the key area of the furnace in which the combustion of fuel takes place, as a result of which hot gases produced are destined to flow through the tubes.

It is designed to be at the front end of the boiler next to the firebox. In the boiler design,  the combustion chamber’s optimization is a crucial aspect.

8. Galloway Tubes

galloway tubes of cornish boiler

These are the special tubes that are inserted into the flue tube in a criss-cross fashion, as shown in Figure 2. Their purpose is to improve the flowability of the circulated water. A few of them are oriented vertically, while others are diagonally.

They permit the diversion or spread of the current of flue gases flowing in them and cause scrubbing action of the gases at the outer surface of the tube.

9. Water Space

It is the area within the shell which contains water. The level of water in the water space is monitored continuously for the smooth operation of the cornish boiler.

10. Safety Valve/s

Safety valves are pressure relief valves for releasing excess pressure built up in the shell to prevent any explosions in case pressure goes off limits.

11. Chimney

Flue gases leave the cornish boiler through a vertical pipe called a chimney.

12. Brick Arc

It serves to prevent the admission of ash into the shell tubes and miscellaneous internal parts of the boiler.

13. Blow-off Valve

As evident from its name, it is used to purify water space from the impurities deposited over time by blowing water at high speed and driving it by steam flow.

14. Water Level Indicator

It is a glass gauge indicator that is used to measure the water level in a drum or shell. Usually, two or three glass tubes are mounted at the outer casing of the Cornish boiler. Internally, they are linked to the various shell positions to determine water level.

It is programmed to turn the feed water pump on once the level of water drops dangerously low until it reaches the minimum design level. It is considered a safety feature of any boiler to prevent dry-firing as well as water over-filling conditions.

15. Feed Water Pump

It pumps water into the boiler via. External sources such as water tanks, lakes, and reservoirs for the steady generation of steam. Its design varies from manufacturer to manufacturer according to need and application.


In principle operation, the fuel is burned in the combustion chamber of the furnace. As a result of the oxidation of the fuel or firing coal, the hot gases thus produced in the chamber are directed to flow through the tube submerged in the water.

In the water space, due to heat transfer from the hot gases, the water converts into hot steam, which collects above the water space and is used to perform key operations such as rotating the shaft of the steam turbine for electricity generation.

The safety valves operate when the pressure inside the cylinder exceeds the safe pressure limit. In that case, a certain quantity of steam is purged off so that a safe pressure can be maintained inside the cornish boiler as estimated by the boiler’s design engineer.


The merits of using a cornish boiler are mentioned below:

  • On account of its compact design due to the single firebox inside a cylindrical shell, which makes it relatively smaller, it is used where space is a key concern.
  • It is simple in design and construction. Cornish boilers are a perfect choice, from the fabrication to their operation and maintenance.
  • Concrete advancements in the controls of boiler technology have made the Cornish boiler today safer and more reliable than when it was designed in the nineteenth century due to the scarcity of resources in instrumentation and control.
  • The design of the long flue of the cornish boiler maximizes the heat transfer with effective combustion in applications where consistent steam generation needs are to be met.
  • Because it generates steam at moderate pressures, it is suitable in situations where steam with high pressure and mass flow rate is not a necessity.


On the industrial level, there are certain limitations of using Cornish Boiler, which are articulated below:

  • It has a limited capacity to produce large amounts of steam due to the smaller water space it retains. Because of this, they cannot be preferred for running heavy-duty machinery such as large steam turbines or commercial steam engines.
  • The rate of steam generation is quite low with Cornish boilers on account of its engineering design. This characteristic limits their use where steam is required at a rapid rate.
  • It operates at a relatively low steam pressure, thereby limiting its application to small-scale commercial steam engines.
  • It is prone to overheating and, in extreme cases, dangerous boiler explosions due to its single-flue design as well as the low water level in the water storage area.
  • The fuel is inefficiently burned in the combustion chamber of the Cornish boiler.
  • Since its structural design is such that it is a single flue type, removing soot and cleaning other deposits poses a hefty challenge during boiler maintenance, whether reactive or preventive.
  • It is bigger and thereby requires large space, which is sometimes difficult to manage in industrial practices such as in small to medium-sized commercial or domestic power plants.


There were widespread applications of the cornish boiler, which are mentioned below point by point:

  • It was used on the industrial level where low to moderate-pressure steam generation was required. In the past, its applications in the textile industry were rife. Steam generated by it was used in textile operations such as spinning, weaving, etc.
  • It was affirmatively used on mining sites with steam engines for pumping water out of the mines.
  • It was also used in the food processing industry in the operations requiring steam, such as cooking food, canning, and others.
  • It was kept in small buildings for heating purposes during cold weather conditions. 
  • In power generation at a small scale, a cornish boiler was encouraged to be used.

Advancements in the Cornish Boiler Design

Over time, multiple engineering endeavors have helped improve the design of cornish boiler in terms of its optimum heat transfer rate with minimal heat loss to achieve higher thermal efficiency and performance, increased steam pressure, feedwater pre-heating mechanisms, more reliable safety protocols and automation, of effective combustion and modeled fuel consumption, of boiler insulation, of advanced construction materials of shell, tube and baffles and avant-garde operation in general.