Thermal Engineering

Md. Mohiuddin

Lecturer

Department of Mechanical Engineering

ME 1105

Boiler Accessories

Boiler Accessories

• The devices which are used as integral parts of a boiler, and help in running efficiently.

• Some important boiler accessories are
• Economiser
• Air preheater
• Superheater
• Feed pump    

Schematic of Boiler Plant

Economiser

Function:

• A device used to pre heat feed water during supplying it to the boiler by utilizing the heat in the exhaust flue gases before leaving through the chimney.
• The economizer improves the economy of the steam boiler.

Location:

• It is placed in the path of the flue gases in between the exit from the boiler and entry into air preheater/chimney.

Why this device is called Economiser instead of Feed Water Pre-heater?

Economiser

Construction:

• The Green’s economizer used with the stationary boilers is shown in the Figure.
• It consists of a group of vertical cast iron tubes which are fitted between top and bottom headers.
• For safety against high pressure, a safety valve is mounted on the top of the header.
• Like a boiler, it has a blow-down valve fitted at the bottom to discharge sediments collected at the bottom of the economizer. 
• Scrapers are provided on the tubes to remove the soot of flue gas deposited on the tubes of the economizer.
• The soot thus removed from the tubes is collected in a chamber provided at the bottom of the economizer.
• The two stop valves, one at the bottom header and the other at the top header are provided to stop or allow water into and out of the economizer, respectively.

Economiser

Working:

• The feed water from the feed pump first enters into the bottom header of the economizer before it enters into the boiler.
• From the bottom header the water then passes through the vertical tubes and reaches into the top header, from where it finally leads into the boiler.
• At the same time the flue gas moves around the tubes and gives off their heat to the water flowing inside the vertical tubes and water is thereby heated in the economizer.
• While the economizer is in operation, the scrapers provided on the tubes are moved up and down continuously with the help of chain and gear arrangement so that soot deposited on the pipes may be removed and maximum efficiency of the economizer may be achieved as the soot deposit on pipes reduces the heat transfer to water.

Economiser

Advantages:

1. Increase in thermal efficiency of the boiler plant by utilizing waste heat, saving in fuel.
2. There is about 15 to 20% of coal saving.
3. Hot feed water causes an increase in steam generating capacity because it shortens the time required to convert water into steam.
4. Longer life of the boiler as this reduces the temperature difference between different parts of the boiler, thus reducing thermal stresses.
5. It prevents the formation of scale in boiler water tubes because the scale now forms in the economizer tubes, which can be cleaned easily.
6. Dissolved gases such as air or CO2 may also be removed by pre-heating the feed water, reducing corrosion and pitting.

Disadvantage:

1. It reduces the natural draft as it obstructs the passage of flue gases.

Air Preheater

Function:

• An air preheater also recovers some portion of the waste heat of the flue/chimney gases and utilizes it for preheating of air supplied to the combustion chamber of the boiler.

Location:

• Air preheater is usually placed after the economiser and before the gases enters the chimney.

Advantages:

• Waste heat from the flue gases is recovered for heating air and causes boiler efficiency is increased about 2% for each 35-40°C rise in air temperature.
• Lower grade coal can be burnt efficiently with preheated air.
• Comparatively less air is required to burn low grade fuel and thus cost of producing draught will be less.
• Combustion can be more efficient and an intense flame can be achieved in the furnace. This increases the evaporation rate of the boiler.
• It results in better combustion with less soot, smoke and ash.

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Disadvantages:

• Increase in the capital and running cost of the preheater as induced draft fan for removing gases and forced fan for forcing cold air through the air preheater are used.

Steam Superheater

Function:

• The wet or saturated dry steam is superheated by increasing steam temperature above its saturation temperature.

Location:

• The superheater is installed in the path of flue gases after the furnace. Sometimes, for bigger boilers, the superheater may be placed in an independent fire furnace.

Classification:

• Based on mode of heat reception
1. Convective superheaters: In the convective superheaters, the superheaters are placed between or near the water tubes where the superheater tubes receive heat by convection from combustion gases.
2. Radiant type superheaters: In the radiation superheaters, the superheaters are placed in the walls of the furnace of a steam boiler where the superheater tubes receive heat by direct radiation from fire and re-radiation from refractory walls.
3. Combination type: In combined superheaters, the steam first enters the radiant superheater and then the convective superheater. In this type, heat of combustion is transferred to the superheater tubes by radiation and then convection.

Classification (Cont’d):

• Based on the movement of gases and steam
1. Parallel flow superheater:   The hot flue gases and wet steam flows in the same direction.
2. Counter flow superheater: The hot flue gases and wet steam flows in opposite direction.
3. Combined flow superheater: The hot flue gases and wet steam first flows in opposite direction and then in the same direction.
4. From the above classification, the counter flow superheater is commonly used because of its smaller size, lighter in weight and  maximum efficiency.
• Based on the arrangement of the superheater tubes:
• Overdeck: This is placed in the space over the water tubes .
• Interdeck: This is placed between the water tubes which are located near the furnace.
• Intertubes: This is placed inside the bank or row of water tubes.

Steam Superheater

Advantages:

1. It decreases the specific steam consumption (flow of steam required to produce 1 kW net work output) of steam engines or turbines.
2. It decreases the condensation losses in the steam pipes and steam engine cylinder.
3. It eliminates the erosion of the turbine blades.
4. The efficiency of the steam plant increases.

Steam Superheater

Feed Pump

Function:

• The function of feed pump is to pump the feed water into the boiler against the pressure of boiler.

Classification:

1. Rotary pumps: This type of pump is used when a large amount of feed water is to be supplied to the boiler. These are generally high speed centrifugal type driven by either electric motor or small steam turbine which is run by the steam from the same boiler to which water is to be fed.
2. Reciprocating pumps: The Duplex feed pump is a common type of reciprocating pump which is used for medium size boilers.
3. Injector: The injector is another type of feed pump in which the energy of steam jet is used for feeding water into the boiler. It is suitable of small boilers or where space is not available for the installation of a feed of feed pump as in the case of Locomotive boiler. Its maintenance cost is less and it is thermally very efficient as the steam required to operate the injector is returned to the boiler.

Feed Pump

This is the figure of a duplex feed pump, commonly used for medium size boilers.

Thank You