What Is Cochran Boiler?
A Cochran Boiler is one of the multi-tubular vertical fire tube boilers that have several horizontal fire tubes. Cochran is the modification of a simple vertical boiler where the heating surface has been increased employing some fire tubes. So, in comparison to the simple vertical boilers, the efficiency of this boiler is much better.
The Cochran boiler has an external cylindrical shell and a firebox. The shape of the shell and firebox is hemispherical and the hemispherical head of the boiler provides maximum space and strength for withstanding the pressure built up in the Boiler. Also, the hemispherical crown of the firebox is helpful to resist intense heat.
The Cochran boiler was produced by Cochran & Co. of Annan, Scotland. It is widely used in marine practice, either fired directly by coal or oil fuels or else used for heat recovery from the exhaust of large diesel engines. Where such a boiler may be heated either by the exhaust gases of the main propulsion plant or else separately fired when in port (usually by oil rather than coal) it is referred to as a composite boiler.
The boiler is a cylindrical vertical water drum with a hemispherical domed top. This domed shape is strong enough not to require staying. The firebox is another hemispherical dome, riveted to the base foundation ring to give a narrow water space.
The fire tubes are arranged in a single horizontal group above this, mounted between two flat vertical plates that are inset into the boiler barrel. The first of these plates form a shallow combustion chamber and is connected to the firebox by a short diagonal neck.
The combustion chamber is of the “dry back” form and is closed by a steel and firebrick plate, rather than a water jacket. The exhaust from the fire tubes is into an external smokebox and a vertical flue. For maintenance access to the tubes, a manhole is provided in the hemispherical dome.
A typical Cochran boiler, as illustrated, might be 15 feet (4.57 m) high and 7 feet (2.13 m) in diameter, giving a heating surface of 500 square feet (46.45 m2), and a grate area of 24 square feet (2.23 m2). Working pressure is between 100 and 125 psi (6.9 and 8.6 bar; 690 and 860 kPa).
Where composite firing is used, there are several possible arrangements for the heating gases. Most use a double-pass tube arrangement where another dry back combustion chamber routes the gases from one tube bank to return through the other.
Some arrangements use a separate tube bank for the heat recovery exhaust gases or the direct firing gases, others pass the exhaust gases into the top of the (unlit) firebox. A pure heat-recovery boiler may have no firebox at all, other than a shallow domed plate for strength.
Cochran also offers a modified “Sinuflo” fire-tube, which is claimed to offer better heat transfer from the gases. This is bent into a number of horizontal sinusoidal waves, rather than being straight.
The Cochran boiler was not applied to locomotives but was used for a pair of experimental steam railmotors built for the GNSR by Andrew Barclay in 1905. These were not successful and were scrapped after a few years, although one coach body survives in a poor state.
Parts of Cochran Boiler
There are more than twenty parts and components involved in a single Cochran boiler. We decided to focus only on the most important of these parts and give a brief description of what they do and what role they carry in the boiler. Below we listed nine of them.
- Boiler Shell: The boiler shell is made of steel plates into a cylindrical form and it is riveted or welded together. The ends of the shell are closed by endplates. The boiler shell should have enough capacity to store water and steam.
- Combustion Chamber: The combustion chamber is below the boiler shell for burning the fuel to generate steam from the water in the shell.
- Grate: The grate is a platform in the combustion chamber where fuel is burnt. The grate is generally a cast-iron bar and there is space between them so the air can pass through it. The surface area of the grate where fire takes place is called a grate surface.
- Furnace: The furnace is above the grate and below the boiler shell and in the furnace, the fuel is burned.
- Fire Tubes: The fire tubes are the horizontal tube between the combustion chamber. The flue gases from the combustion chamber flow to the smokebox via some fire tubes. These fire pipes are used to exchange heat from hot flue gases to water.
- Fire Hole: It is at the bottom of the combustion chamber for firing fuel inside the furnace.
- Chimney: The chimney is provided at the top of the boiler and it is connected to the smokebox. These exhaust gases are coming out from fire tubes and pass-through smokebox and exhaust through a chimney.
- Manhole: The manhole is for cleaning, repairing, and inspecting the boiler shell.
- Flue Pipe: The firebox and combustion chamber are connected through a short pipe and these pipes are called flue pipes. The hot flue gases from the grate flow into the combustion chamber via flue pipe.
How Does a Cochran Boiler Work?
The first step that a Cochran boiler takes to work is the fuel inserted into the firebox and placed on the grate. The fuel is ignited through the fire hole installed at the right bottom of the boiler. So, the fuel is burnt in the firebox, and due to the burning of the fuel, smoke and hot flue gases come out. The flue pipes direct the hot flue gases into the combustion chamber.
From the combustion chamber, hot gases enter into the fire tubes. The fire tubes are surrounded by water. The hot gases inside the tubes transfer the heat from the hot gases to the water. Due to the exchange of heat, the temperature of the water starts increasing and it gets converted into steam.
The steam produced rises upward and is collected at top of the boiler in the hemispherical dome. An anti-priming pipe is installed at top of the boiler which separates the water from the steam and makes it dry steam. This dry steam is then transferred to the turbines through the steam stop valve.
The hot flue gases and smoke after rejecting heat, are directed to the smokebox. From the smokebox, the burnt gases and smoke is discharged into the atmosphere through the chimney.
A fusible plug is also provided at the top of the combustion chamber. When the temperature of the combustion chamber crosses the permissible level, the fusible plug melts, and the water through the combustion chamber enters into the furnace of the boiler and stops the fire. In this way, a big fire accident can be prevented to take place and also protects the boiler from damage.
Main Features of a Cochran Boiler
Below we have prepared a list of features and characteristics of this boiler for you to read.
- It is portable
- It has a natural circulation
- It is a low-pressure boiler
- you can use any type of fuel for running a Cochran Boiler
- This is suitable for small capacity requirements
- It has about 70% thermal efficiency with coal firing and about 75% thermal efficiency with oil firing
- The ratio of grate area to heating surface area varies from 10:1 to 25:1
Applications of Cochran Boilers
Cochran boilers are used in power generation plants where large quantities of steam from 500 kg/s with high pressures approx. 160 bar and high temperatures reach up to 550o C.
In the following, we listed some other applications of Cochran Boilers:
- It is used in Refining units.
- It is used in Paper manufacturing plants.
- You can find Cochran boilers in Chemical processing divisions.
- And it is used in a variety of process applications in various industries
Advantages of Cochran Boiler
- This requires less floor area.
- Easy to operate and handle.
- Lower pre-installation costs.
- It has a higher ratio of area to volume.
- It is easy to transport from one place to another.
- It can use all types of fuel.
Disadvantages of Cochran Boiler
- Low steam generation pass.
- Due to the vertical design, its installation requires a high room head.
- It has a limited speed limit.
- Difficulty changing and maintaining.