Cochran Boiler Construction and Working

Published: 1 Oct 2025

What is Cochran Boiler?

A Cochran boiler is a vertical, multi-tubular, fire-tube boiler widely used in small industries for steam generation. It is an improved version of the simple vertical boiler where multiple fire tubes replace the single tube, increasing the heating surface and efficiency. The boiler generally uses coal or oil as fuel, and the produced steam is supplied for industrial processes or small power generation. Its compact size, easy transport, and simple operation make it one of the most popular fire-tube boilers.

Cochran Boiler Construction and Working

Cochran Boiler Constructions

The Cochran boiler has a cylindrical vertical shell with a hemispherical top and bottom. The hemispherical crown strengthens the boiler to withstand high steam pressure. At the bottom of the shell is the furnace and grate, where the fuel is burned, and below it lies the ash pit for collecting burned residue.

The combustion chamber is lined with fire bricks to prevent heat loss. Hot gases from the furnace pass into horizontal fire tubes surrounded by water, transferring heat to generate steam. A smoke box and chimney are provided for the exit of gases. At the top of the boiler, steam collects in a dome-shaped space before being supplied through the steam stop valve.

Cochran Boiler Parts

1. Boiler Shell

The cylindrical body of the boiler holds water and steam. Its hemispherical top provides strength and space for steam collection.

2. Combustion Chamber

The space where fuel is burned to produce hot gases. It is lined with fire bricks to prevent overheating.

3. Grate

A platform where solid fuel like coal is burned. It allows air to pass through for proper combustion.

4. Fire Tubes

Horizontal tubes through which hot gases pass. They transfer heat to the surrounding water to generate steam.

5. Fire Hole

An opening at the bottom of the combustion chamber. It is used to insert and ignite the fuel.

6. Furnace (Firebox)

The area above the grate where fuel combustion takes place. Also called the firebox, it generates heat for steam production.

7. Ash Pit

Located below the grate to collect residual ash from burned fuel. It allows easy removal and cleaning.

8. Chimney

A vertical pipe at the top of the boiler for releasing flue gases. It maintains proper draft for combustion.

9. Fire Brick Lining

Heat-resistant bricks lining the combustion chamber. They prevent damage and improve heat retention.

10. Man Hole

An access point for inspection and maintenance inside the boiler. Operators can enter to repair or clean internal parts.

11. Flue Pipe

A short passage connects the furnace to the combustion chamber. It directs hot gases to the fire tubes for heat transfer.

12. Water Level Indicator

Shows the current water level inside the boiler. It ensures the boiler is neither underfilled nor overfilled.

13. Pressure Gauge

Measures the steam pressure inside the boiler. It helps monitor safe operation.

14. Safety Valve

Releases excess steam to prevent boiler pressure from exceeding safe limits. Usually spring-controlled.

15. Steam Stop Valve

Controls the flow of dry steam from the boiler to external equipment. It regulates the steam supply efficiently.

16. Anti-Priming Pipe

Separates water droplets from steam before it leaves the boiler. Ensure only dry steam is supplied.

17. Blow-Off Valve

Removes accumulated impurities like mud and salt from the bottom of the boiler. Helps maintain efficiency and prevents damage.

18. Feed Check Valve

Controls the supply of water from the feed pump into the boiler. Prevents the backflow of steam into the feed line.

Working of Cochran Boiler

1. Fuel Combustion in Furnace

Coal or oil is fed onto the grate and ignited through the fire door. As the fuel burns, it releases hot gases inside the furnace, raising the temperature significantly. This combustion process is the primary source of energy for steam generation.

2. Transfer of Heat to Water

The hot gases produced in the furnace enter the combustion chamber, which is surrounded by water. The hemispherical shape of the chamber allows maximum heat transfer, ensuring that water absorbs energy efficiently. This rapid heat exchange helps speed up the process of steam generation.

3. Flow of Hot Gases through Fire Tubes

After leaving the combustion chamber, the hot gases pass through multiple horizontal fire tubes inside the boiler shell. These tubes are completely surrounded by water, and as gases travel through them, they transfer more heat. This increases the overall heating surface area and improves the boiler’s efficiency.

4. Formation and Collection of Steam

As water absorbs heat, it gradually converts into steam. The light steam rises and collects at the top of the shell, forming a steam space. The dome-shaped top ensures strength and provides enough storage for accumulated steam.

5. Separation of Moisture from Steam

Before the steam is supplied to machines, it passes through the anti-priming pipe. This pipe helps remove any water droplets mixed with the steam. As a result, only dry steam is sent forward, preventing damage to turbines or equipment.

6. Exit of Steam through Stop Valve

The dry steam is finally released from the steam space via the steam stop valve. This valve regulates the flow of steam, ensuring that it is supplied at the required pressure for industrial use. From here, the steam moves to turbines, engines, or processing units.

7. Exhaust of Waste Gases through Chimney

The cooled gases that have given up most of their heat energy are collected in the smokebox. From here, they are expelled through the chimney into the atmosphere. This continuous removal of gases maintains proper airflow and sustains combustion inside the boiler.

Uses of Cochran Boiler

The Cochran boiler is widely used where moderate steam is required. Common applications include:

Textile industries for dyeing and fabric processing.
Food industries for sterilization and cooking processes.
Chemical plants for heating and processing.
Paper mills for pulp preparation.
Refineries and small power plants for auxiliary steam generation.
Ships and marine units serve as auxiliary boilers.

Cochran Boiler Specifications

Typical specifications of a Cochran boiler are:

Efficiency: 70% to 75%

Steam Capacity: 150 to 3000 kg/hr (maximum up to 4000 kg/hr)

Working Pressure: 7 bar (maximum up to 15 bar)

Heating Surface Area: About 120 sq. meters

Shell Height: Approximately 5.75 meters

Shell Diameter: Approximately 2.75 meters

Advantages of a Cochran Boiler

Simple in design and easy to install.
Low initial cost compared to other boilers.
Compact size, requiring less floor space.
Can operate with coal or oil as fuel.
Hemispherical design withstands high steam pressure.
Easy to transport from one place to another.

Disadvantages of the Cochran Boiler

Limited steam generation capacity.
Lower efficiency compared to modern high-pressure boilers.
Maintenance and cleaning are difficult due to the limited space.
Requires high headroom because of vertical design.
Not suitable for large-scale power plants.

Conclusion

The Cochran boiler is a reliable and efficient fire-tube boiler ideal for small-scale industries and auxiliary steam generation. Its simple design, compact structure, and low installation costs make it highly popular. Although it has limitations like low steam capacity and a restricted pressure range, it remains one of the most practical boilers for small and medium applications.