Cochran Boiler || It’s Important Concept with 10 FAQs

Cochran Boiler

Content

History of Cochran boiler

Cochran boiler was initially bought by Thompson-Cochran group as package boiler technology to South Africa and became an international leader in boiler making after joining with Rolls-Royce group.

It was mainly used in ships to produce steam for a different purpose. It can use oil/coal or heat recovery from the exhaust of diesel engine, to produce steam. These boiler’s were also known as composite boilers.

Cochran boiler definition

It is a vertical drum axis, fire tube boiler which has many horizontal tubes to increase heating surface area. It is categorized as a natural draft, natural circulation, low-pressure boiler.It has better efficiency than the simple vertical boiler. Any type of fuel such as coal or oil can be used with it. 

Cochran Thermax Boiler

It is a vertical drum axis, fire tube boiler which has many horizontal tubes to increase heating surface area. It is categorized as a natural draft, natural circulation, low-pressure boiler. It has better efficiency than the simple vertical boiler. It requires a minimum floor area.  Different kind of fuel such as coal and oil can be used inside this boiler.

Cochran boiler construction and working

A Cochran Boiler consists of the following parts:

1. Shell: 

It is the main body of boiler which in-closes both steam and water.

2. Grate of the boiler

It is part of the boiler where solid fuel is stored and designed for easy airflow through it and simple removal of ashes.

3. Combustion Chamber of the boiler

Part of the boiler where fuel is burnt to produce high – temperature flue gas. The inner surface is lined with fire bricks to avoid overheating of the boiler body.

4. Fire tubes

These are horizontal tubes connected in a bunch whose one end is attached to the furnace and the other to the chimney to increase the contact area of the heating surface.

5. Fire hole

A small hole at the bottom of the combustion chamber is used to position fuel in the boiler.

6. Firebox (Furnace)

The mediator between fire tubes and the combustion chamber is known as fire box.

7. Chimney

It is an exhaust pipe through which flue gases are released to the atmosphere.

8. Man Hole

A manhole is a small opening for maintenance and inspection of the interior part of the boiler.

9. Fire Brick Lining

It is a typical type of insulation made of clay and provided in the interior of Cochran Boiler to reduce convection heat transfer to the outer surface.

10. Ash Pit

Ashes are stored in Ash Pit, which is located below the Grate.

11. Smoke Box Door

It gives access to clean smoke deposit from Smoke Box.

12. Anti Priming Pipe

It is used to prevent water droplets from getting carried away with steam.

13. Crown

It is the place where the burning of fuel inside boiler takes place.

14. Pressure Gauge

It is used to measures steam pressure.

15. Safety Valve of the boiler

 It is a safety accessories mounted on the boiler to release extra steam when pressure inside boiler exceeds safe limit.

16. Water Level Indicator

It is a safety device and uses to inspect the level of water inside the boiler and prevent boiler operation at a low water level.

17. Water Level Gauge

To check the level of water inside the boiler, a glass tube gauge is fitted on the outer surface of the boiler is known as water level gauge.

18. Fusible Plug

A safety mounting on the boiler to prevent any damage due to overheating of boiler. When the temperature of boiler water exceeds the safe zone, the fusible plug will melt and water will flow into boiler’s furnace and extinguish the fire.

19. Stop Valve

It is a safety device mounted on the boiler body to stop the steam flow into the mainline. It is a normally closed valve.

Working of Cochran Boiler

The working principle of Cochran Boiler is similar to that of a vertical fire tube boiler. At the grate, fuel is supplied through the fire hole. Fuel is burnt, and hot gases formed are used to transfer heat to water through fire tube. Water gains thermal energy and gets converted to steam. 

Cochran vertical boiler

A vertical boiler is categorized as a natural draft, natural circulation, low-pressure boiler.

It is a multi-tubular boiler which results in increased surface area of heat transfer.

It is generally used to generate steam for small machinery. These boilers are especially used on ships as auxiliary boilers.

Cochran Boiler
Fig 1: Vertical Boiler (Pic. credit: Wikipedia)

Application of Cochran boiler

The Cochran boiler is used in:

  • Paper and pulp industry.
  • Chemical processing plant.
  • Refining units.
  • Various process application industries.

Cochran boiler diagram

Fig 2: Cochran Boiler (Pic credit: ecoursesonline)

Cochran boiler is which type of boiler

It is an upgraded form of simple vertical boiler where heating surface area is increased by the use of multi-tubular fire tubes.

Cochran boiler specification

Following are the specification of the Cochran Boiler 

  • Steam Capacity: 3500 kg/hr
  • Working Pressure: 6.5 – 7 bar (Rated pressure 15 bar)
  • Heating surface area: 120 m2  
  • Height: 5.79 m
  • Shell diameter: 2.75 m
  • Tube diameter: 6 cm
  • Efficiency: 70 to 75%

Cochran boiler working

The working principle of Cochran Boiler is similar to that of a fire tube boiler. Different types of fuel such as coal or oil are transferred at the boiler’s grate through the fire hole. Fuel is ignited through the fire hole in boiler, and the natural flow of air takes place into the combustion chamber from the atmosphere.
The high-temperature flue gases formed during the combustion of provided fuel, flow through the bunch of horizontal fire tubes. The heat is convected from the fire tube (inside high temperature flue gas is flowing) to the water. The enthalpy of water is increased steam formation takes place. The exhaust gas is released into the atmosphere.

Cochran fire tube boiler

It is a type of boiler in which flue gas flows inside the fire tube, and water is surrounded to these tubes. Convection heat transfer takes place from hot gas inside the tube to the surrounding water to convert it into steam.

Cochran steam boiler

With high efficiency and reduced fuel required, Cochran economizers are cost-effective and used as waste heat recovery to generate steam.

Accessories attached to the Cochran boiler

Attachments on the Cochran Boiler

1. Water level indicator

It indicates water level inside the boiler which help us to maintain water level between high and low level. 

2. Pressure gauge

The steam pressure inside the boiler is measured with the help of an instrument known as a pressure gauge. 

3. Safety valve

 A quick-release valve, mounted on the boiler’s body to protect the boiler from bursting due to excessive pressure is known as safety valve.

When internal pressure reaches the set value of the safety valve, it automatically gets opened, and the high-pressure steam is released.

4. Stop valve

The main function of a stop valve is to stop the operation of the boiler when required and also control flow within the boiler. 

5. Blow off valve

It is used to remove sediment and scale deposits at the bottom of the boiler’s drum while it is in operation and is also used to empty the boiler for cleaning or inspection.

  • Feed check valve mounted on the boiler.

Water flow from feed pimp to boiler is controlled by feed check valve

7. Fusible Plug

It is used to cut-off the fire of boiler in furnace, when the level of water is below the safe zone to avoid damage to the boiler.

Major advantages of the Cochran boiler

Major advantage of Cochran Boiler are listed below

  • Lower cost of initial installation.
  • Floor area requirement is less.
  • Easy to handle and operate.
  • Different types of fuel can be used.
  • It is portable and handy.

Classification of Cochran boiler

Cochran boiler can be classified on the basis of different criteria as Vertical, Multi-tubes, Fire tube, Internally fired, and Natural circulated boiler.

Cochran boiler advantages and disadvantages

Advantages of Cochran boiler 

  • Initial installation cost is less.
  • Required less floor area.
  • Easy to handle and operate.
  • Different types of fuel can be used.

Disadvantages

  • Steam generation rate is low.
  • Carrying out maintenance and inspection work is difficult.
  • Limited pressure range is available.
  • Large area is required for installation because of its vertical design.

Cochran boiler capacity

Boiler capacity is defined as steam production rate at full firing condition and usually expressed on a weight basis. Cochran boiler capacity is in the range of 500 kg/s at 16 bar.

 Cochran boiler design

The main cylindrical body of boiler which in-house water and steam is known as shell of a boiler. The top the boiler have hemispherical dome shaped structure to provide space for steam generated.This shape is provided to to have higher area to volume ratio.It has a compact structure and occupies less floor area. It is mostly used for the low capacity requirement. 

Cochran boiler dimensions

  • Shell diameter: 2.75 m
  • Height of boiler: 5.79 m
  • Heat exchanger tube diameter: 6cm
  • Heating surface 120 m2

Cochran boiler Economizer

An economizer can be used to reduce fuel consumption by up to 6%. It utilizes waste heat from the main engine of the ship to generate steam.

Cochran boiler horizontal

It is mainly used in locomotives and is a multi-tubular, internally fired, fire tube boiler with natural circulation. It is designed to meet sudden fluctuation demands of steam.

Cochran boiler maintenance

Following are the list of some routine maintenance activities carried out on the boiler:

  • Water quality testing and treatment by using chemicals. 
  • Chemical cleaning of soot deposit on economizer by using high-pressure air.
  • Regular blowdown to reduce scale deposits.
  • Maintaining records of testing and inspection log.
  • Overall visual inspection.
  • Lubrication of components.
  • Daily inspection of motor condition.
  • Cleaning of filters and inspection of pilot and burner assemblies.
  • Regular inspection of gasket condition of the steam line and replacement of damaged gaskets.
  • Important to maintaining the required water level inside the boiler’s drum.

Cochran boiler manufacturing process

A numerically controlled plasma cutting machine is used to cut a flat plate. This flat plate is rolled for the required diameter with the help of hydraulic pressure. Longitudinal submerged arc welding is carried out for seamless joining.

Inspection and Quality assurance is carried out throughout the manufacturing process. X-ray technology is used to inspect critical welds. For precise tube-hole alignment, CNC milling is used.

Tubes are manually welded in alignment with tube holes. Furnace, combustion chamber, and front tube shell are fitted to the boiler body.

Pressure testing is done by using a non-destructive method.

Cochran boiler parts

  • Shell
  • Grate
  • Combustion Chamber
  • Fire Tubes
  • Fire Hole
  • Furnace
  • Chimney
  • Fire Brick Lining
  • Manhole
  • Flue Pipe

Cochran boiler principle

Convection heat transfer takes place from flue gases to the water by the fire tubes to form steam.

Cochran waste heat boiler

It is a boiler that produces steam by utilizing waste heat from the exhaust gases of the main propulsion unit of a ship or else use fuel at the port. It is also known as a waste heat recovery boiler.

Cochran water tube boiler

It is a water tube boiler in which water is present in the tube and high-temperature flue gas is present in the surroundings of the tube to produce steam.

Difference between Cochran and Babcock boiler

A Cochran boiler is a vertical fire tube boiler in which flue gas flows through the fire tube. Heat energy is transferred mainly in the form of convection from the hot fire tube to the surrounding water to generate steam.

Babcock boiler is a water tube boiler in which water flows inside the tube, surrounded by hot flue gas. Heat energy is transferred mainly in the form of convection from hot gases to the water inside the tube to generate steam. This type of boiler is used to produce high-pressure steam.

Difference between Cochran boiler and Babcock & Wilcox boiler

A Cochran boiler is a vertical fire tube boiler in which flue gas flows through the fire tube. Heat is transferred from the hot fire tube to the water surrounding the tube to generate steam.

Babcock & Wilcox boiler is a water tube boiler in which water flows inside the tube, surrounded by hot flue gas. The heat is convected from tube to water to increase enthalpy of water and convert it to steam. The high-pressure steam is produced by this boiler. It has a longitudinal drum and horizontal, inclined tube for generating high-pressure steam. The angle of inclination of tubes is about 15° or more with horizontal.

Difference between Cochran boiler and Lancashire boiler

Cochran boiler is a vertical fire tube boiler in which flue gas flow through multiple fire tube. It is used to generate low-pressure steam and has an internally located furnace.

Lancashire is defined as a horizontal fire tube type, internally fired boiler. The length of the Lancashire boiler is approximate 7 – 9 meters, and the diameter is in the range of 2 -3 meters.

The definition of Cochran boiler’s efficiency

The efficiency of the Cochran boiler is defined as the ratio of heat actually required to produce steam in certain time span to heat liberated in furnace during the give time period.

Where,

m1 = is mass of water

hf = specific enthalpy of water at the saturated liquid curve

h1 = specific enthalpy of water in sub-cool region

x = dryness fraction

hfg = difference of enthalpies between saturated vapour and saturated liquid curve at constant pressure.

C = heat capacity of water

History of Cochran boiler

Cochran boiler was initially bought by Thompson-Cochran group as package boiler technology to South Africa and became an international leader in boiler making after joining with Rolls-Royce group.

It was mainly used in ships to produce steam for a different purpose. It can use oil/coal or heat recovery from the exhaust of diesel engine, to produce steam. These boiler’s were also known as composite boilers.

Following are the Limitations of Cochran boiler

  • The steam generation rate is low.
  • Pressure handling capacity is limited.
  • Not suitable for high steam generation rate.
  • Because of its compact size, it is difficult to inspect and maintain.

Problem 1: Feed water supplied per hour 690 kg at 28℃, steam produced 0.97 dry at 8 bar, coal fired per hour 91 kg of calorific value 27,200 kJ/kg, ash and unburnt coal collected from beneath the fire bars 7.5 kg/hour of calorific value 2760 kJ/kg, mass of flue gases per kg of coal burnt 17.3 kg, temperature of flue gases 325℃, room temperature 17℃, and the specific heat of the flue gases 1.026 kJ/kg K.

Find

1. The boiler efficiency

2. The percentage heat carried away by the flue gases

3. The percentage heat loss in ashes

4. The percentage heat loss unaccounted

5. Explain what may have actually happened to the heat included under unaccounted losses.

Solution:

Heat supplied to the boiler per hour= mass of fuel per hour x calorific value of fuel

                                    = 91 x 27,200 kJ/hr

                                    = 24,75,200 kJ/hr

At 8 bar, 

Enthalpy of water at saturated liquid curve (hf) = 721.1 kJ/kg (Data from steam table)

Enthalpy of steam at saturated vapour curve (hg) = 2769.1 kJ/kg

Latent heat of vaporization (hgf) = hg – hf

                                 = 2769.1 – 721.1 kJ/kg

                                 = 2048 kJ/kg

Enthalpy of wet steam = {hf + (X*hfg)}

Where, X = dryness factor

Enthalpy of wet steam = {721.1 + (0.97*2048)}

                                  = 2707.67kJ/kg

Enthalpy of feed water at 28℃ = C x T

Where,

C = heat capacity of water

T = Temperature of water in ℃

Enthalpy of feed water at 28 ℃ = 28 x 4.187

                                  = 117.24 kJ/kg

Heat utilised in steam production per hour

  = mass of steam produced per hour(m) x difference of enthalpy of wet steam and feed water.

  = 690*(2707.67 – 117.24) = 1787396 kJ/kg

1. Boiler efficiency: Ratio of heat utilized in steam formation per hour to heat supplied to the boiler per hour.

= 1787396/2475200

= 0.7221 bar

= 72.21% efficient

2. The percentage heat carried away by the flue gases = mg x Kp (tg – tf)

  Where,

mg = is mass of flue gases = 17.3 kg/kg of coal fired,

Kp = specific heat of flue gases = 1.026 kJ/kg K

tg = temperature of flue gases = 325℃

tf = room temperature = 17℃

Total heat energy carried away by the flue gases

 = 17.3 x 1.026(325 – 17)

 = 5467 kJ/kg of coal.

Percentage heat carried away by the flue gases per kg of coal fired

 = (5467/27200) x 100

 = 20.1%

 3. Percentage heat loss in ashes is the ratio of heating value of ash in kJ/hr to heat supplied to the boiler in kJ/hr.

 = {(7.5 x2760)/2475200} x 100

 = 0.836%

 4. Unaccounted heat loss percentage is calculated by 

 = 100 – (72.21 + 20.1 + 0.836)

 = 6.854%

 5. The heat included under unaccounted losses are those due to radiation, fuel that are not fully burnt, loss of heat with hot ashes etc.

FAQ/SHORT NOTES

Ques.1: Where is Cochran boiler used?

Ans: Cochran boiler is used in following sector:

  • Paper industries
  • Refining industries
  • Chemical industries
  • Different process applications

Ques.2: What are the major three different types of boilers?

Ans:Three different type of boiler is Combi Boiler, Heat(regular) Boiler, System boiler.

  • The Combi Boiler

The boiler having single unit and used to provide hot water for domestic purpose at home.

  • Heat only (regular) boiler

 A hot water cylinder is attached in it.

  • System boiler

A system boiler is mostly similar to Combi boiler other than hot water production. A steel hot water cylinder is attached to it.

Ques.3: What is the area of the heating surface of a Cochran boiler

Ans: With the efficiency of around 70 – 75 percent, the Heating Surface area of Cochran boiler is 120 m2. Working of this boiler is 7 bar and rated for 15 bar.

Que 4: What is Units of break specific steam consumption?

       A. kg/kW-hr          B. kJ/kg-K

       C. kJ/kg              D. kg/kW

Ans: A

Que 5: Maximum thermal energy loss in a boiler is due to 

       A. Flue Gas           B. Ash content

       C. Radiation losses   D. Incomplete combustion

Ans: A

Que 6: Which instrument is used to measured temperature of the flue gases most accurately?

       A. Thermometer      B. Thermocouple

       C. Pyrometer         D. Wheat-stone bridge

Ans: C

Que 7: Cochran boiler is a type of

       A. A horizontal fire tube boiler

       B. A vertical fire tube boiler

       C. A horizontal water tube boiler

       D. A vertical water tube boiler

Ans: D

Que 8: Orientation of water tubes in a simple vertical boiler are

       A. Horizontal

       B. Inclined

       C. Vertical

       D. All of the above

Ans: B

Que 9: The ratio of diameter of internal flue tubes of a Lancashire boiler to diameter of its shell is

       A. One-fourth

       B. One-third

       C. Two-fifth

       D. One-half

Ans: C

Que 10: The process of heating dry steam at constant pressure, above saturation temperature is known as

      A. Isentropic

      B. Super heating

      C. Sub cooling

      D. Isothermal

Ans: B

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About Krishna Singh

I am Krishna K Singh working as a project scientist. Holding 5 years of industrial experience in Cross Mechanical domain. Presently, I am working on Deep sea mining projects which are of international importance.I am glad to be a part of Lambdageeks family and want to help students with in-depth understanding of Mechanical and Thermal.
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