Cochran Boiler: Working, Parts, Applications, & Construction

Fire tube boilers are used all over the world for various industrial and household applications. The Cochran boiler is one of the most famous types of fire tube boilers. It is a vertical boiler. This article describes the Cochran boiler working, components, applications, advantages, and disadvantages.

What is a Cochran Boiler?

The Cochran boiler is a vertical drum shaft, natural draft, low pressure, solid fuel-fired, fire tube natural circulation boiler. It is a multi-tubular boiler. It comprises a furnace that is internally fired.

It is an improved model of the simple vertical boiler. It contains multiple horizontal fire pipes. It is more efficient than a conventional vertical boiler.

The Cochran boiler is available in a variety of sizes with an evaporative capacity between 150 kg and 3000 kg per hour. They have operating pressures up to 15 bar.

It contains a firebox and a cylindrical shell. These parts have a hemispherical shape. The boiler shell’s hemispherical crown provides the most space and ability to bear the pressure of the steam within the boiler.

One of the major benefits of the firebox’s hemispherical crown is that it is more capable of withstanding extreme heat.

A short tube connects the combustion chamber to the firebox. A series of tubes are used to transfer the combustion chamber’s flue gases to the smokebox.

These gases then travel from the smokebox to the chimney, from where it is released into the environment. To bear the extreme heat, the combustor is coated with firebricks.

The shell has a manhole near the top of the hemispherical crown. The manhole is used to enter the boiler for maintenance. A grate is located at the bottom of the firebox. The fire hole is used to feed coal to the grate and is kept closed by a fire door.

 Cochran Boiler Working

The Cochran boiler working is no different than other kinds of boilers. This boiler usually works in the following way:

• The coal fuel is first added to the grate through a firebox.
• The furnace is then filled with air.
• The fire hole of the boiler delivers a fire spark to the coal in the grate.
• The fuel combustion process begins as soon as the coal catches a fire spark in the presence of air.
• Heat is produced inside the furnace as a result of fuel combustion, and this heat is transmitted to the flue gases.
• The temperature of the flue gases rises as they are heated and passed through a flue pipe from the furnace into the combustion chamber (or burner).
• The hot flue gases enter the fire tubes when they get to the combustion chamber. These fire tubes are encircled by boiler water.
• Convection is the mechanism by which heat is transferred to the water, and when water is heated, it becomes hotter and turns into steam as a result.
• The released steam begins to rise higher before being stored in the hemispherical dome.
• The Cochran boiler contains an anti-priming pipe which is used to separate the dry steam from the wet steam.
• The dry steam is utilized to power the steam turbine, and the steam flow is regulated by a steam stop valve.
• Electricity is generated as the steam strikes turbine blades. This further rotates the coil of the generator. The Rankine cycle governs the steam turbine’s operation.
• When coal is burned in the furnace, hot gases and smoke are produced. The smoke and exhaust hot gases are transferred to the smokebox after exchanging heat of the hot gases, from where they are expelled to the environment by a chimney.
• The boiler has an ash pit which is utilized to gather the ash of burned coal.

Components of Cochran Boiler

The Cochran boiler has the following components:

• Combustion chamber
• Boiler shell
• Furnace
• Flue pipe
• Fire tubes
• Grate
• Manhole
• Stop valve
• Fire hole
• Pressure gauge
• Chimney
• Feed pump
• Safety valve
• Feed check valve

1) Combustion Chamber

The fuel is burned in the combustion chamber. The combustion chamber is filled with solid fuel that is manually fed from the outside. Due to the combustion of the fuel, heat is produced. The flue gases receive the generated heat and use it to further heat the water and make steam.

2) Boiler Shell

The boiler shell is made of steel plates that are bent into a cylinder shape and then welded or riveted together. Endplates are used to seal the ends of the shell. The shell has sufficient capacity to hold both steam and water.

3) Furnace

The furnace is the area in which the fuel is burned. It is usually located below the shell and above the grate. The firebox is another name for the furnace.

Read More: Gas Furnace Working and Types

4) Flue Pipe

The short pipes that link the combustion chamber and firebox are known as flue pipes. A flue pipe carries the warm flue gases from the grate into the combustion chamber.

5) Fire Tubes

These are horizontal tubes. They are placed inside the combustion chamber. The main function of these tubes is to transfer heat from hot flue gases to water.

The fire tubes are also used to transfer the flue gases from the combustor to the smokebox.

Read More: Boiler Mountings and Accessories 

6) Grate

It is an area in the combustor where fuel is combusted. The grate is often made of cast-iron bars with gaps between them to allow air to travel through. The grate surface refers to the region of the grate where fire occurs.

7) Man Hole

A manhole is used to inspect, repair and clean the shells of the boiler.

8) Stop Valve

The stop valve’s primary function is to regulate the steam or water flow in accordance with the boiler demand.

9) Fire Hole

The fire hole is located at the base of the combustion chamber. It is usually used to fire fuel in the furnace.

10) Pressure Gauge

It displays the boiler’s internal steam pressure. The pressure gauge is attached to the front of your boiler.

11) Chimney

The smokebox is connected to the chimney. The chimney is placed at the top of the boiler. When the exhaust gases leave the fire tubes, they flow from the smokebox to the chimney, from where they go into the atmosphere.  

12) Feed Pump

Water is transferred from the reservoir to the boiler by using a feed pump. Several types of feed pumps are used to pump water into the boiler, including centrifugal pumps, gear pumps, diaphragm pumps, reciprocating pumps, and piston pumps.

13) Safety Valve

The main function of the safety valve is to stop the steam pressure from going more than a specified limit. The valve opens and releases the steam into the atmosphere when the pressure rises over the design pressure. It prevents your boiler from explosion.

When the steam pressure becomes lower than the specified limit, the valve automatically closes. It is a spring-controlled valve.

14) Feed Check Valve

The check valve is used to regulate the water flow from the feed pump to the boiler. It also stops the water from flowing backward into the pump when the pump pressure is lower than the boiler pressure or when the feed pump stops operating.

Read More: Bensor Boiler Working and Applications

Cochran Boiler Applications

• Cochran boilers are used in refining units.
• They are employed in small power plants.
• They are used in chemical processing divisions.
• Pulp and Paper manufacturing units.

Cochran Boiler Advantages

• The Cochran boiler has a low initial installation cost
• It needs low installation space in the horizontal direction.
• It is easy to install.
• This boiler has an easy operation.
• It is easy to shift from one place to other.
• This boiler has the ability to use all types of fuels.
• The Cochran boiler has a high volume-to-area ratio.

Cochran Boiler Disadvantages

• It is very hard to maintain.
• It also has a limited pressure range.
• It requires a large installation space in the vertical direction.
• It has a low generation capacity.

FAQ Section

Which fuel types are compatible with the Cochran boiler?

In the Cochran boiler, you may use all types of fuels. This boiler is ideal for applications that need a small amount of steam. It provides around 70% thermal efficiency while burning coal and about 75% when burning oil. The proportion of the heating surface area to the grate area ranges from 10:1 to 25:1.

What conditions allow for the usage of a Cochran boiler?

When there is a need for a small amount of steam, Cochran boilers are employed since they need less installation area and are, therefore, more cost-effective. 

What type of boiler is the Cochran boiler?

The Cochran boiler is a vertical drum shaft, natural draft, low pressure, internally fired, solid fuel-fired, fire tube natural circulation boiler. It is multi-tubular. It is an improved version of the simple vertical boiler.

What is the operating pressure of the Cochran boilers?

The Cochran boiler has the ability to produce 3500 kg per hour of steam at an operating pressure of 6.5 bar.

Is the Cochran boiler a low-pressure boiler?

The Cochran Boiler produces very little steam at a rapid rate. This boiler is a vertically constructed fire tube boiler. It needs more space in a vertical position. The fire tube boiler only produces low-pressure steam because water transforms into steam slowly.

What is the shell diameter of a Cochran boiler?

The height of the Cochran boiler shell is 6 meters, while the shell diameter is 2.75 meters.

What is the Cochran boiler used for?

• Pulp and Paper manufacturing units.
• Used in small power plants.
• Cochran boilers are used in refining units.
• Chemical processing divisions.

What are the parts of Cochran boiler?

• Feed check valve
• Combustion chamber
• Feed pump
• Boiler shell
• Furnace
• Flue pipe
• Fire tubes
• Safety valve
• Grate
• Manhole
• Chimney
• Stop valve
• Fire hole
• Pressure gauge