Power Plant Engineering

What is a Cornish Boiler? How Does it Work?


Cornish Boiler has a long horizontal cylinder with a single large flue tube that contains fire. So this is categorized under the Fire tube Boiler type. But if we consider the design and construction then this is quite similar to the Lancashire Boiler which we have studied earlier. Let’s study the parts, applications, working principles, advantages, and disadvantages of the Cornish Boiler.

What is a Cornish Boiler

A Cornish Boiler is a fire tube boiler. As the Boiler shell is placed horizontally, it is also a horizontal boiler. This can be moved from one place to another, so it is a portable boiler. As the combustion chamber is present inside the boiler, it is an internally fired boiler. However, performance-wise, the Lancashire Boiler is more efficient than the Cornish Boiler.

Definitely, I will make a separate article on the difference between the Lancashire Boiler and the Cornish Boiler. The most important thing is that the fire tube is surrounded by the chamber filled with water and this flue tube is not placed in the center of the boiler.

Cornish Boiler Diagram


Parts of Cornish Boiler

The important parts Cornish boiler are described below.

Cornish Boiler Mounting

These are pre-installed safety devices for which they work smoothly.

  • Grate: It is like a base or floor section inside the combustion chamber, where the fuel (solid fuel e.g. coal) is burnt. This base plate is made up of cast iron. Why? Because of its high heat capacity. You can see that there is some space between them so that air can pass through them. The whole surface area of a grate where the fuel is burnt is known as a grate surface.
  • Combustion chamber: This is a closed chamber inside a boiler where combustion e.g. burning of fuel takes place. This chamber provides greater heat to the boiler.
  • Ash pit: This is a chamber provided below the grate surface to collect the burnt products e.g. ash.
  • Fire tube: Here in this boiler, it is a single large flue tube that is immersed in water. By burning the fuel i.e. Coal flue gas is generated and flows to the down part of this boiler. Then the side flue is extracted into the atmosphere through a chimney. This fire-tube is used to exchange heat energy from its surface to the water by the convection process. As this is a fire-tube boiler, the regular flow of hot gases could damage its strength and longevity. So to make this tube more reliable, this is made up of seamless steel e.g. carbon steel or alloy steel. Remember the materials of these tubes are different for low pressure, medium pressure as well as high-pressure boiler.
  • Blow-off cock: This is a mechanical device that is used for periodical cleaning by extracting mud water and sediments from the bottom section of a boiler. It also minimizes the corrosion property of the material or shell of the boiler. Another use of this device is for emptying the boiler whenever the boiler is not in operation.
  • Manhole: This hole is present in the upper part of this boiler. This hole is provided for inspection, cleaning, and repair of the interior part of a boiler shell. But the question is why is it named so? Because through this hole, a man is entered into a boiler, and he can easily identify the integral defects. The size of the manhole is equal to the size of a human.
  • Chimney: This is an open chamber provided at the top of the boiler and it is connected to the smokebox. The flue gases come out from firetubes flow through the smokebox and release out through a chimney.
  • Brick arch: This is placed inclined over the grate and prevents the entry of ash, dust, and burnt fuel particles into the fire tubes. Also, it deflects the hot flue gases to travel a definite path before entering into the fire tubes of the boiler.
  • Fusible Plug: This is a cylindrical metal with a threaded structure made up of bronze, brass, or gunmetal and the core is filled with tin which has a low melting point. It is used to protect the boiler against damage due to overheating caused by the low water level in the boiler. This is the last safety device in a steam boiler. When the water level inside the boiler is drastically low, the top portion of the fusible plug is melted and forms an opening through which the high-pressure steam is released with the remaining water. In this way, it warns the operator of the danger before the top section of the boiler runs completely dry, which could result in a severe explosion.
  • Feed check valve: This mechanical device is a non-return valve that is placed at the end of the delivery pipe of the feedwater pump. Where is it located? It is mounted on the front part of the boiler shell and slightly below the normal water level. The primary function of this valve is that it allows a unidirectional flow of water from the feed pump to the boiler shell.
  • Water level indicator: This is an important part of this boiler fitting that indicates the level of water in the boiler constantly to an observer. It is a safe mechanical device upon which the correct working of the boiler depends. Here we can see that the lower portion of the indicator is dipped in water and the upper portion is connected with the steam chamber in the boiler. This fitting may be seen in the front section of a fire-tube boiler. In case of urgency, if the water level falls down then the water will be resupplied by the operator through the feed pump. It is mostly located in the steam boiler.
  • Steam stop valve: The main function of the steam stop valve is to shut off or regulate the flow of steam from the Cornish boiler to the steam pipe or from the steam pipe or delivery tube to the engine respectively. It is designed in such a way that the main body of the steam stop valve is made up of cast steel and the valve and nuts are made up of Brass with excellent resistance and limited dimensional tolerance.
  • Safety valve: The safety valve is a mechanical device and is also the most important part of the construction of this boiler. It preserves human life as well as property in a Thermal power plant. Normally it prevents the explosion when there is an excessive internal pressure of steam inside a boiler. This mechanical device is designed in such a way that it automatically opens and releases the excess internal pressure. Then it closes automatically when the internal pressure decreases to the prescribed value.
  • Pressure Gauge: This indicates the pressure of steam in a boiler. It is mounted on the upper section of the boiler. The main purpose of this mechanical device is that it used for monitor the internal pressure of a boiler. In most cases, a Bourdon-type pressure gauge is widely used in a steam boiler.

Some accessories are fitted with the boiler (e.g. superheater, economizer, and air preheater), which are responsible for increasing the efficiency of the boiler.

Cornish Boiler Application

  • Generally, these are used in the Paper industry, Chemical industry, Sugar industry, and some small-scale industries.
  • Also, this is used in the Marine/ Shipping industry.

Specifications of Cornish Boiler

These data may slightly vary according to the Boiler Manufacturing Companies.

  • Shell diameter ranges from 1.25 to 2.75 m
  • The length of the boiler shell ranges from 4 to 7 m
  • Maximum Pressure 12-13 bar
  • Steam Production Rate:- 1350 kg/hr
  • Heating surface area:- 120 m2
  • No. of flue tubes:- 1
  • Height of the Boiler:- 5 – 6 m
  • The efficiency of the Boiler:- 70 – 75%

Working of Cornish Boiler

Generally in a Cornish boiler, solid fuel e.g. coal is added in the grate area, which is present inside the combustion chamber. A Firebrick is present in the end part of the grate which prevents the entry of the ash, dust, and burnt fuel particles into the fire tube. It produces hot gases by burning the fuel and moves into a single large fire tube. This fire tube is placed inside the boiler shell and is surrounded by water. Then the heat is transferred from the surface of the fire tube to the water by the process of convection heat transfer. Now the water started boiling and converted into saturated steam.

When the hot gases reach the end part of the fire tube, it is divided into two sections as shown in the figure (Cornish Boiler front view). Then the burnt gases reached the front part of the boiler through the side channels. Again the side flue moves downward to the bottom part of the boiler. In this section, the bottom flue is extracted out into the atmosphere through a Chimney.

The ash is now deposited over the ash pit which can be extracted out easily. In order to maintain the water level inside the boiler, a Water level indicator is mounted on it. A brick chamber is placed below the boiler to regulate the flow of flue gases which increases the efficiency. The saturated steam is again heated by using some accessories like an economizer, and superheater to get superheated steam. By using these devices, the efficiency of the boiler also increases.

Advantages of Cornish Boiler

The advantages of this boiler are given below.

  • The design and construction of the cornish boiler are simple.
  • This is also compact and portable.
  • It has low construction and maintenance costs.
  • The fluctuations of load could be overcome during the operation.

Disadvantages of Cornish Boiler

The disadvantages of this boiler are given below.

  • The steam production rate in this boiler is very low.
  • It requires a large floor area as this is a horizontal Boiler.
  • This is a low-pressure boiler (Operating pressure below 15 bar).

This is all about “Cochran Boiler”. Thanks for giving your valuable time. If you have any doubts related to this boiler please don’t hesitate, to ask in the comments section below. If you have any prior experience related to this boiler and know more about it, please share it with us. We promise that we will solve your doubts as soon as possible. And if you really like this article, don’t forget to share it with your colleagues and friends. Good Luck.

MechStudy Team

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