BOILER ACCESSORIES:

These are the additional auxiliary components installed to increase the efficiency of steam power plant. They improve the working conditions and help in maximum utilization of heat energy from the flue gases. Here is a list of six major accessories of a boiler.

1. Economiser

2. Air preheater

3. Superheater

4. Steam Trap

5. Steam Separator

6. Feed pump

1. Economiser :

Scrapers (S) provided on the vertical C.I pipes are operated by chains. They scrape off soot formed on pipes during their up and down movement.

Fig. shows a Green's Economiser. It consists of a large number of vertical tubes connected to horizontal pipes, one at the top and the other at the bottom. Feed water from source enters the bottom pipe and flows upto top pipe. Meanwhile, boiler exhaust gases are allowed to pass over these tubes. Feed water absorbs heat of these gas and get heated. Hot water from top pipe is fed to boiler.

Advantages :

• Fuel economy; 15 to 20% saving in fuel burnt in the boiler furnace

• Higher utility of heat from flue gases

• Increase in thermal efficiency of the plant. Efficiency increases by 1% for every 6°C rise in temperature of feed water Increase in evaporative capacity of boiler

• Saving of time in evaporation

• Increases boiler life

• Reduces thermal stresses in the boiler components

2. Air Preheater :

Air preheater or simple air heater is a waste heat recovery apparatus. It is placed between the economiser and chimney.

Advantages :

• Efficiency of boiler increases roughly by 2% for every 35-45°C rise of air temperature.

• Evaporation rate of the boiler increases

• Improved combustion with less smoke, soot and ash

• Low grade fuels can be used

3. Superheater:

Superheater is mainly used to superheat the steam. In general superheaters are part and parcel of a boiler plant and are called integral superheaters. The steam generated by a simple boiler is usually wet or at the most dry and saturated. But highly superheated steam is necessary for steam engines and turbines. Superheater, therefore, is used to raise the temperature of saturated steam, at constant pressure.

Figure shows a Sudgen's hairpin or U-type superheater. It is used in low pressure boilers like Cornish, Lancashire, and Babcock and Wilcox Boilers.

Steam from the boiler drum, flows through a steam pipe (S) and enters the superheater tubes through its header H1. Superheater (SH) has number of U-tubes through which steam remains flowing from header H1 to header H2. Meanwhile, hot flue gases from furnace are allowed to pass over these tubes, on their way to chimney. Thus, heat exchange takes place between hot exhaust gases and saturated steam. The steam, which gets superheated flows through header H2 into the main steam pipe (MS); stop valves are shown fitted in the pipes.

A damper D acts as a gate between the passage of flue gases and superheater. It controls the quantity of flue gases entering the superheater or completely shuts off by passing the flue gases to chimney. This prevents overheating of superheater tubes.

Superheaters are classified as follows :

(i) Convective Superheaters In these type of superheaters, heat of flue gases is transferred to the surface of superheater tubes by convection.

(ii) Radiation Superheater Here heat from the furnace is transferred by means of radiation to the superheater.

(iii) Combination type Superheaters In this case steam first enters the radiant superheater and then the convection one.

4. Steam Trap :

It is used to collect and automatically drain away the water resulting from the partial condensation of steam. It does not allow steam to escape with the condensed water. The steam condensed in the steam pipe flows to the steam trap by gravity. When water level in the trap rises to a sufficient height, a valve (V) opens a passage and water is forced out of it. After the water has been discharged, the passage is closed, thus preventing steam from escaping along with water.

5. Steam Separator :

Parson's steam separator or drier is shown in Figure. It removes the water particles available in steam pipe between the boiler and the engine or turbine. Water particles formed due to partial condensation of steam and other corrosive elements are also removed. The steam is led to pass through a chamber in which it strikes the baffle plate. It gets deflected and changes its direction as shown. The comparatively heavier water particles get separated and drop down into the bowl which can be drained off periodically.

6. Feed Pump :

Feed pump is used to deliver water to boiler. It is desirable to supply same quantity of water as is evaporated in a given time. It is a double acting feed pump which derives its driving power from prime mover (engine or turbine). The pump shown is called Duplex direct acting steam pump. On each side of the plunger for piston) of pump, there are suction and delivery valves (A and B). Because both sides of plunger are active i.e., water is admitted on either side and the discharge is continuous. During suction stroke water is drawn from source into chamber C. From here water enters the water cylinder. During delivery stroke water on the other side of piston is discharged out.