Power Plant Engineering

What is a Cochran Boiler? How Does It Work?


If you’re interested in learning about a Cochran boiler, it’s essential to understand the functioning of a Fire tube boiler. In our previous article, we delved into the operational principles of a Fire tube boiler. In this article, we will discuss the working principles of a Cochran boiler. Why it is considered a Fire tube Boiler?

This classification is because, based on the arrangement of water and hot gases, the hot flue gases flow through small tubes that are immersed within the water. As the fire or hot flue gases are present within these tubes, this is also classified as a Fire tube boiler.

When we examine the orientation of the shell axis in a Cochran boiler, it becomes apparent that it is a vertical boiler. Additionally, the combustion chamber is an integral component of the boiler, So it is also referred to as an internally fired boiler. Let us start with the definition.

What is a Cochran Boiler?

A Cochran boiler is also known as a Vertical-type boiler and is also classified as an Internally fired boiler. This is commonly used in small-scale industries due to its limited steam generation capacity. It represents an advanced version of a Portable boiler, designed to reduce the heating surface area respectively. Its vertical or longitudinal structure allows it to occupy minimal floor space. Furthermore, by adding more fire tubes, the heating surface of this boiler can be expanded.

Diagram of a Cochran Boiler


Cochran Boiler Parts

The important parts of a Cochran Boiler are given below.

  • Cylindrical Shell: This is the main body of a boiler and the shape is cylindrical type. The top head of this boiler is designed in such a way that it looks dome-shaped or you can say hemispherical type. To avoid leakage, it is reverted and welded properly. This hemispherical or dome-shaped head gives a higher volume-to-area ratio, which increases the production of steam capacity.
  • Firebox (furnace): A firebox is placed over the grate surface of this boiler. Here the structure of the firebox or furnace is similar to the boiler shell’s head e.g. hemispherical type or dome-shaped. Practically it plays a mediator role between the fire holes and the combustion chamber. This furnace is designed in such a way that it deflects back the un-burnt charge particle and directs the flue gases into the combustion chamber.
  • Fire tubes: The fire tubes are horizontal tubes whose one end is joined with the combustion chamber and another with the smoke box. The flue gas flows from the combustion chamber to these firetubes and is then exhausted by the chimney through the smoke box. These firetubes are used to exchange heat energy from their surface to the water by convection process. As this is a firetube boiler, the regular flow of hot gases could damage its strength and longevity. So to make these tubes more reliable, they are 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 and high-pressure boiler.
  • Chimney: This is an open chamber provided at the upper portion of this boiler. Generally, the lower part of the chamber is attached to the smoke-box and the upper part is opened to the atmosphere. The hot flue gases are coming out from fire tubes to the smokebox and exhausted out through this open chamber which is called a chimney.
  • 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 heating 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 great surface.
  • Firebricks lining: The firebricks lining is attached to the outer part of a combustion chamber but is present inside the boiler. Generally, it prevents the overheating of the combustion chamber. In simple words you can say, it acts as an insulator.
  • Firehole: The small hole is provided at the bottom section of this boiler and is attached to the firebox to place fuel (e.g. coal, oil). Also through this hole, the air is entered into the firebox and combustion chamber, which helps for burning the fuel easily.
  • Manhole: This hole is present in the upper part of this boiler. It is provided for inspection, cleaning, and repair of the interior part of a boiler shell. But the question is why is it named so? Through this hole, a man enters into this boiler, and he can easily identify the integral defects. The size of the manhole is equal to the size of a human.
  • Flue Pipe: The firebox and combustion chamber in a Cochran boiler are connected through a short pipe or the channel which is called a flue pipe. The requirement of this device in the boiler is that it allows the hot flue gases to flow from the grate surface into the combustion chamber.
  • Combustion Chamber: This is a closed chamber, which is placed above the flue tubes and just below the head of this boiler. Another part is attached to the small fire tubes. Through this chamber, the hot flue gases pass into the fire tubes. This chamber provides greater heat to the boiler because of its greater surface.
  • Safety Valve: The safety valve is a mechanical device and also the most important part of a Cochran boiler’s construction. It preserves human life as well as property in a thermal powerplant. Normally it prevents an explosion when there is an excessive internal pressure of steam inside this boiler. This mechanical device is designed in such a way that it automatically opens and releases excess internal steam pressure. Similarly, it closes automatically when the internal pressure decreases to the prescribed value.
  • 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 in this boiler. Where is it located? It is mounted on 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.
  • Pressure Gauge: This indicates the pressure of steam in a boiler. It is mounted on the upper section of this boiler. The main purpose of this mechanical device is that it is used for monitoring the internal pressure of a Cochran boiler. In most cases, a Bourdon-type pressure gauge is widely used in a steam boiler.
  • Water Level Indicator: This is an important part of boiler fittings 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. The lower portion of the indicator is dipped in water and the upper portion is connected with the steam chamber in the boiler. These fittings may be seen in the front section of a Cochran boiler. In case of urgency, if the water level falls 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 a steam stop valve is to shut off or regulate the flow of steam from the boiler to the steam pipe or from the steam pipe or delivery tube to the engine respectively. 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.
  • 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 the 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.
  • 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 any damages due to overheating caused by the low water level inside the boiler. This is the last safe mechanical device in this boiler. When the water level inside the boiler is extremely 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.

Application of Cochran Boiler

Due to the low steam production rate and low pressure, these boilers are used in small scale industries which are listed below.

  • Refining Industry
  • Chemical Industry
  • Paper Mill

Specification of Cochran Boiler

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

  • The Operating and Working pressure: 6.5 bar to 15 bar (Maximum)
  • Steam generating capacity: 2000 kg/hr to 3500 kg/hr
  • Boiler Shell diameter: 2.75 m
  • Heating surface area: 120 m2
  • The overall efficiency of the Boiler: 70 to 75%

How does Cochran Boiler work?

The Cochran boiler consists of an external cylindrical shell with a dome or hemispherical shaped head. The hemispherical head of the boiler shell gives maximum space and strength to combat the pressure of steam inside the boiler.

Water is supplied to the boiler through the feed check valve and coal is introduced to the grate through the fire door respectively. By burning fuel hot gases are produced in the furnace (dome-shaped or hemispherical) and enter the combustion chamber through a flue pipe. Then it passes through the horizontal fire tubes and heat transfer takes place by the process of convection from the surface of the tube to the water that surrounds the tube.

The flue gases entered the smokebox through the fire tubes and finally discharged into the atmosphere via the chimney. Now the ash is formed & collected through the ash pit. The steam is collected through an anti-priming pipe on the top of the boiler. 

Note:- Cochran boiler and Economizer system will operate up to 96% efficiency, based upon an NCV (Net Calorific value) of 34828KJ/m³ for Natural gas and 35863 KJ/L for Distillate fuel oil.

Advantages of Cochran Boiler

The advantages of the Cochran boiler are given below.

  • The initial setup cost of this boiler is very low.
  • According to its design and structure, this boiler requires less floor area.
  • The most important part of this boiler is that it can work on all types of fuel (e.g. solid, liquid, and gas).
  • Due to its large reservoir, the fluctuation in load in a Cochran boiler can be met easily.
  • Let us come to its handling and operation, and it is quite easy.
  • This is a portable boiler means it can transport from one place to another.

Disadvantages of Cochran Boiler

The disadvantages of the Cochran boiler are given below.

  • The steam generation capacity in this boiler is low compared to others.
  • It is very difficult to inspect the Cochran boiler and its maintenance.
  • It requires a high room head for installation, as it has a dome-shaped head.
  • It has a limited pressure range (which means you can’t say this is a high-pressure boiler).
  • It has a low heating surface (due to great height and small shell dia).

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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