Boiler Types and Examples: Water Tube Boiler

Water Tube Boilers

• Water tube boilers are used when pressure above 10 bar and capacity in excess of 7000 kg of steam per hour is required.
• They have two types: Horizontal straight tube (Babcock and Wilcox) and Bent tube boiler.

Babcock and Wilcox Boiler

• It is an example of horizontal straight tube boiler.
• It consists of a drum connected to a series of front end and rear end headers by short riser tubes.
• To these headers are connected a series of inclined water tubes of solid drawn mild steel.
• A feed valve is provided to fill the drum and inclined tubes with water.
• The water flows through the inclined tubes via downtake header and goes back into the shell in the form of water and steam via uptake header.

Stirling Boiler

• It is an example of bent tube boiler.
• It consists of two upper drums known as steam drums and a lower drum known as mud or water drum.
• The steam drums are connected to mud drum by bank of bent tubes.
• The steam and water space of the steam drums are interconnected with each other, so that balance of water and steam may be obtained.
• Manhole at one end of each drum is provided for carrying out cleaning operation.

Fire Tube Boilers

• The various fire tube boilers are described as follows:
  • Simple vertical boiler
  • Cochran boiler
  • Cornish boiler
  • Lancashire boiler

Simple Vertical Boiler

• It consists of a cylindrical shell, the greater portion of which is full of water and remaining is the steam space.
• At the bottom of the fire box is grate on which fuel is burnt and the ash from it falls in the ash pit.
• The fire box is provided with two cross tubes.
• It is self-contained and can be transported easily.

Cochran Boiler

• It is one of the best types of vertical multi-tubular boiler.
• It consists of a cylindrical shell with a dome shaped top where the space is provided for steam.
• The rate of production in not exceed 2500 kg/h and pressure 7.5 to 10 bar.

Cornish Boiler

• It consists of a cylindrical shell with flat ends through which passes a smaller flue tube containing the furnace.
• The products of combustion pass from the fire grate forward over the brickwork bridge to the end of the furnace tube.
• It is still used in some industries.

Lancashire Boiler

• It is reliable, has simplicity of design, ease of operation and less operating and maintenance costs.
• It is commonly used in sugar-mills and textile industries where along with the power steam and steam for the process work is also needed.

Boiler Accessories

• Economiser: increases the temperature of the feed water, increases the evaporative capacity of the boiler, and increases the overall efficiency of the plant.
• Air Preheater: increases the temperature of air before it enters the furnace, accelerates the combustion and facilitates the burning of coal.
• Superheater: increases the temperature of the steam above its saturation point, reduces the steam consumption of the engine or turbine, and increases the efficiency of the steam plant.
• Steam Separator: separates the steam from the water and removes any impurities from the steam.### High Pressure Boilers
• In high pressure boilers, pumps are used to maintain forced circulation of water through the tubes of the boiler, ensuring positive circulation and increasing evaporative capacity.
• The use of small diameter tubes in large numbers and multiple circuits increases the efficiency of heat utilisation.
• Pressurised combustion is used to increase the rate of firing of fuel and heat release.
• Compactness of high pressure boilers reduces the required floor space.
• The high velocity of water through the tubes eliminates the tendency of scale formation.
• Uniform heating of all parts reduces the danger of overheating and thermal stress.

Lamont Boiler

• The Lamont boiler works on forced circulation, maintained by a centrifugal pump driven by a steam turbine using steam from the boiler.
• An electrically driven pump is also fitted for emergency situations.
• The boiler can generate 45 to 50 tonnes of superheated steam at a pressure of 130 bar and a temperature of 500°C.

Loeffler Boiler

• The Loeffler boiler solves the problem of salt and sediment deposition on the inner surfaces of the water tubes by preventing the flow of water into the boiler tubes.
• The boiler uses forced circulation and evaporates feed water by means of superheated steam from the superheater.
• The boiler can carry higher salt concentrations and is more compact than indirectly heated boilers with natural circulation.
• Loeffler boilers with a capacity of 100 tonnes/h and operating at 140 bar have been commissioned.

Boiler Mounting and Accessories

• Accessories are auxiliary plants required for steam boilers to operate efficiently.
• Feed pumps are used to deliver feed water to the boiler, with two types: reciprocating and rotary pumps.

Feed Pumps

• Reciprocating pumps consist of a pump cylinder and a piston, with two types: single-acting and double-acting.
• The reciprocating pump may not be suitable for high overloads due to overheating.
• Rotary pumps are of centrifugal type and are commonly run by a small steam turbine or an electric motor.

Scotch Boiler

• The Scotch type marine boiler is a popular boiler for steaming capacities up to 1000 kg/h and pressure of about 17 bar.
• It consists of a cylindrical shell with one to four cylindrical, corrugated steel furnaces.
• The furnaces are internally fired and surrounded by water, with a combustion chamber located at the back end.

High Pressure Boilers

• High pressure boilers are preferred for high pressure and high output applications.
• Unique features of high pressure boilers include:
  • Forced circulation of water using pumps.
  • Type of tubing, including partial systems and multiple sets.
  • Improved methods of heating, such as evaporation above critical pressure, mixing of superheated steam, and increasing water velocity.
• Advantages of high pressure boilers include:
  • Increased evaporative capacity.
  • Compactness.
  • Reduced danger of overheating.
  • Improved heat transfer.

Injector

• An injector is a device used to feed water into the boiler, commonly employed for vertical and locomotive boilers.
• It uses steam pressure to increase the pressure and velocity of the feed water.
• Advantages of an injector include:
  • Low initial cost.
  • Simplicity.
  • Compactness.
  • Absence of dynamic parts.
  • High thermal efficiency.
• Disadvantages of an injector include:
  • Low pumping efficiency.
  • Inability to force very hot water.
  • Irregularity of operation under extreme variation in steam pressure.

Economiser

• An economiser is a device that utilises the waste heat of the flue gases to heat the feed water.
• There are two types of economisers: independent and integral.
• The independent type is installed in a chamber apart from the boiler setting, while the integral type is part of the boiler heating surface.
• Advantages of an economiser include:
  • Increased efficiency.
  • Reduced fuel consumption.
  • Compactness.