Cochran Boiler: Construction, Operation, Merits, Demerits, Applications

The Cochran Boiler is one of the modern industrial boilers used today. In its class, it is a vertical-type multi-tubular boiler. It has been designed in different sizes with small to large modifications compared to its conventional design. It is the fire-tube boiler.

It has steam generation or evaporative capacity from 150 to 3000 kg of steam per hour. It can operate with a working pressure of up to 20 bars. It is compatible with multiple fuels. Its thermal efficiency varies from fuel to fuel: With coal firing, it gives 70% thermal efficiency, whereas with oil firing, it gives about 75%.

Design and Construction

The main parts of the Cochran boiler with respect to its exclusive design are mentioned below, along with the dimensions of the main parts of a typical Cochran boiler.

complete schematic diagram of Cochran boiler

1. Shell

The shell of the Cochran boiler is cylindrical. Its crown is designed to have a hemispherical geometry. Such a shape is not without reason. It is designed to withstand the bulging effect caused by the high pressure of the generating steam.

2. Fire-Box

It has a crown of hemispherical shape, which also has the purpose of absorbing radiant heat from the furnace. It is worthwhile to mention that cavity geometries such as hemispheres have higher absorptivity as well as emissivity under the effect called the cavity effect.

This special quality does not come with flat surfaces. Thus, the cavity effect of the surfaces accounts for their capacity to absorb and emit thermal radiation. But why? It is because of the intrinsic radiative properties of such surfaces.

It is fabricated in one piece only, thereby having no joint.

3. Smoke-tube

These are the horizontal tubes that act as the convective surfaces. They are typically 150. They are in place by getting fixed in the vertical tube plates through expansion or else by screwing into the holes.

The former is the most commonly followed design practice compared to the latter. This setting is meant to mitigate the bulging effect caused by the high steam pressure.

4. Combustion Chamber

The combustion chamber, the floor, and the sides of the furnace are fitted with refractory material, which acts as a protective lining against abrasion, destructive heating, and corrosion.

combustion chamber of cochran boiler

5. Hand-Hole

For cleaning and inspection of the lower parts in the water space, a number of hand holes are provided in the lower section of the outer shell.

6. Gusset Stays

Rigidity is provided to the top of the combustion chamber by means of the supports known as gusset stays. It is shown in the figure. Their purpose is to transfer the stresses from the top of the combustion chamber to the boiler shell.

It is worth noting that the hemispherical tops of the boiler shell and the furnace, which forms the bottom of the pressure space, do not require any stays as such.

7. Ogee Ring

The bottom of the furnace is connected with the cylindrical shell through a special thick ring called an ogee ring. It is pressed out of its thicker plating in order to prevent grooving due to little circulation of water in this area and, thereby, sled deposition from the boiler water.

Ogee Ring of Cochran Boiler

Enough refractory material should be fitted here to prevent the radiant heat from falling on the ogee ring and thus keep it from overheating and distortion due to scaling and sedimentation.

8. Manhole

Internal access to the Cochran boiler is provided through the manhole drilled in the upper section of the outer shell.

2-D Sketches

All the three 2-D sketches of the Cochran boiler are shown below.

2-D Sketches of the Cochran Boiler

In its operation, the hot flue gases produced in the fire-box enter into a small pipe called a flue pipe. From the flue pipe, they travel through the combustion chamber, which is lined with refractory material, and strike the shell plate of the boiler at the back end of the combustion chamber.

The rear side of the combustion chamber is settled in fire bricks, which, when dismantled, allow room for the cleaning and maintenance of the smoke tubes. The back plate is specific in its operation: it is used to direct the gases into the smoke tubes, which are surrounded by water. The flue gases accumulate in the smoke box from smoke tubes. From there, they move towards the uptake or chimney for exit.

Merits

There are a number of industrial benefits to using the Cochran boiler, such as due to its:

  1. Steady steam output is required for steady loads.
  2. Small footprint.
  3. Compact, heavier, and sturdier design.
  4. Seamless (jointless) furnace design.
  5. Quicker steam generation due to its small design.
  6. Easy to install and readily integrate with the existing set-up: as an exhaust gas boiler by using the heat from the exhaust of the internal combustion engine to generate steam.
  7. Reliable operation with little design variations by numerous manufacturers.
  8. There is no requirement to maintain large feed-water reservoirs.
  9. Compatible with solid as well as liquid fuels, although oil-firing is common in sea-going vessels.

Demerits

There are indubitably some operational limitations of the Cochran boiler, which are articulated below:

  1. Relatively marginal evaporative capacity compared with modern industrial boilers.
  2. Comparatively higher thermal inertia, which makes it less suitable a choice for applications where rapid change in steam demand is required.
  3. Continuous maintenance of the smoke tubes to prevent soot deposits.
  4. More specific fuel consumption and, thereof elevated operational cost.
  5. Pronounced vulnerability to non-uniform structural stresses and strains due to its vertical design.

Applications

In industry, it can replace the Cornish boiler and Lancashire boiler with added benefits and higher thermal efficiency.

As stated previously for these boilers, Cochran boilers find their inevitable applications in a number of operations carried out in the pharmaceutical, textile, hospitality, wood and lumber, paper and pulp, food, and chemical industries.

They are easily integrated as an important component with HVAC systems for heating large buildings.