Summary

Draught

1. Natural: Chimney draught (No energy required)2. Artificial (25-350mm of water column)

i. Mechanical or fan Induced: near the base of chimney (when boiler is provided with

economiser and air preheaters.) Balanced (Both) Forced: base of boiler grate(2.5cm – 7.5cm of water column)

ii. Steam jet Induced Forced

Draught: the small pressure difference which causes the continuous flow of gases inside the boiler. (air outside the boiler and gases within the furnace or chimney)

Functions of Chimney

1. It forces a sufficient quantity of air into the furnace (grate) for proper combustion of fuel.

2. To remove product of combustion3. It circulates the hot flue gases through the flue tubes, superheater, economiser, air

preheater etc.4. It discharges the hot flue gases to the atmosphere through the chimney.

Chimney: natural draught: vertical long cylindrical structure, (brick, masonary (stone brick mortar), reinforced concrete or steel) (reinforced: strengthen or support (an object or substance), especially with additional material.

Advantages: 1. Easy to construct2. No power required for producing the draught3. Long life of chimney4. No maintenance is required

Disadvantages:1. Tall chimney is required2. Poor efficiency (efficiency decreases with increase in outside temperature)3. No flexibility to create more draught to take peak load

Sr. No. Forced Draught Induced Draught

1Fan or blower is placed before grate or situated within the oil burner assembly in case of oil fired boiler.

The fan or blower is placed after the grate or after the flue passages and before chimney.

2 Pressure inside the flue passages is slightly more than atmospheric pressure.

Pressure inside the flue passages is slightly less than atmospheric pressure.

3Due to above reason, there is serious danger of fire to come out and injure someone if any leakage in boiler flue passage take place.

Due to above reason, there is no danger of fire to come out from boiler flue passage in case of a leakage. Hence Induced draught is more safe.

4

It forces fresh air into the combustion chamber, which helps in burning of fuel and production of hot combustion gases. Due to this force, the hot flue gases are further pushed through the flue passages in boiler.

It sucks the hot flue gases from the combustion chamber through flue passages and then passes on these to economizer, air pre heater and chimney. Due to this suction, fresh air is also sucked in to combustion chamber.

5Forced draught fan require less power because it has to handle cold and dense air, the volume per unit mass of which is less.

Induced draught fan requires more power because it has to handle hot flue gases, the volume per unit mass of which is more.

6 Flow of flue gases through boiler is more uniform. Flow of flue gases is less uniform

Draught

In order to maintain the continuous flow of fresh air into the combustion chambers, it is necessary to exhaust the products of combustion from the combustion chamber of boiler. A pressure difference has to be maintained to accelerate the product of combustion to their final velocity and to overcome the pressure losses in the flow system. This pressure difference so maintained is known as "Draught"

Methods of Producing Draught

The method of production of draught may generally be classified as:-

i) Natural Draught: Is obtained by the use of a chimney. A chimney is a vertical tubular structure a brick, masonry, steel or reinforced concrete or then reinforced plastic, built for the purpose of enclosing a column of hot gas, to produce the draught. The effective draught is reduced in a brick chimney due to the leakage through the cracks and imperfect bonding of bricks. This leakage loss can be eliminated by the use of a concrete or steel chimney. Steel chimney also requires less space and has less weight and is less costly, but the life of steel chimney only about 15 years compared to the 50 year life of brick or concrete chimney. Lining of the steel chimney can increase its life. The concrete chimney is costlier and takes longer time to install but it has long life as compared to other types of chimneys. The draught produced by the chimney is due to the density difference between the column of the hot gases inside the chimney and the cold air outside. A diagrammatic arrangement of a chimney of height H metres above the grate is shown in figure.

ii) Artificial draught: - In most of the modern power plant, the draught used must be independent of atmospheric conditions, and it must have a greater flexibility to take the fluctuating loads on the plant. It would be impossible to run to days large steam power plants requiring tens of thousands of tons of steam per hour without the aid of draught fans. The draught required in actual power plant is high (300 mm ) and to meet this high draught requirements artificial draught system should be used. If the draught is produced by a fan, it is known as fan draught and if the draught is produced by a steam jet, the draught is known as steam jet draught.

1. Forced draught: - In a forced draught system, a blower is installed near the base of the boiler and air is forced to pass through the furnace, flues, economiser, air preheater and to the stack. This draught system is known as positive draught system or forced draught system because the pressure of air throughout the system is above atmospheric pressure and air is forced to flow through the system. The arrangement of the system is shown in Fig. 1.24. A stack or chimney is also used in this system as shown in figure but its function is to discharge gases high into the atmosphere for better dispersion of ash particles and pollutants. As it is not designed for producing draught, the height of chimney may not the large.

Fig. Forced Draught

2. Induced draught: - In this system the blower is located near the base of the chimney. The air is sucked into the system by reducing the pressure through the system below the atmospheric pressure. The induced draught fan sucks the gases from the furnace and the pressure inside the furnace is reduced below that of the atmosphere thus inducing the atmospheric air to flow through the furnace. The action of the induced draught is similar to the action of the chimney. As the draught produced is independent of the temperature of the hot gases, the gases may be discharged as cold as possible after recovering as much heat as possible in the economiser and the air preheater. The arrangement of the system is shown in figure. This draught is used generally when an economiser and an air preheater are incorporated in the system. The fan should be located at such a place that the temperature of the gas handled by the fan is lowest.

Fig. Induced Draught

3. Balanced Draught: - It is always preferable to use a combination of forced draught and induced draught instead of using forced or induced draught alone. If the only forced draught is used. The furnace cannot be opened either for firing or inspection because the high pressure air inside the furnace will blow out suddenly. If only the induced draught is used, the furnace cannot be opened either for firing or inspection because the cold air will try to rush into the furnace as the pressure inside the furnace is below atmospheric pressure. This reduced effective draught and dilutes the combustion gases. To overcome the both the difficulties mentioned above, a balanced draught is always preferred. The draught is a combination of forced and induced draught.

Fig. Balanced Draught

4. Steam Jet Draught: It is above an artificial draught. It may be forced type or induced type. A steam nozzle located near the smoke box induces the flow of gases through the tubes, ash pit grate and flues. The arrangement of induced type steam jet draught is shown in figure This type of arrangement is generally used in locomotive, boiler, where the exhaust steam from the steam engine is passed through the nozzle to induce the air to flow through the fuel bed, furnace, flues and smoke box creating a pressure below that of the atmospheric in the smoke box.

Fig. Steam Jet Draught