Module 12. Boiler draught

Lesson 28

PRODUCTION OF DRAUGHT AND ITS LOSSES

28.1  Introduction

Boiler draught may be defined as the small difference between the pressure of outside air and that of gases within a furnace or chimney at the grate level, which causes flow of air/hot flue gases to take place through boiler.

The draught is necessary to force air through the fuel bed/ grate to aid in proper combustion of fuel and to remove the products of combustion i.e. flue gases to the atmosphere after they have given their heat to water being evaporated in boiler. Draught also provides velocity to flue gases and so increases the heat transfer co-efficient in the boiler. Thus draught is essentially required in a boiler and can be produced by a number of methods discussed in the next article.

28.2  Methods of Production of Draught

The draught may be classified on the basis of how it is produced and is illustrated as

Fig. 28.1 Classification of the draught

28.2.1 Natural draught-chimney

Natural draught is obtained naturally by the use of a chimney. Chimney is a conical shape vertical tubular steel or masonry or concrete structure having a large height as shown in fig 28.2. The flue gases after transferring their heat in the boiler are guided by chimney to a considerable height in the atmosphere. As chimney has a large height and is the only outlet from boiler, it remains filled by hot flue gases. These exhaust gases, however have given their heat to water in the boiler, are still hotter than the boiler room air. Due to this, the exhaust gases are lighter in weight than outside air and so lifts up naturally in the chimney and finally escape out in the atmosphere from top of the chimney. As the flue gases lift up in the chimney, more gases from fuel bed flow towards chimney to take their place. During this proces, they first flow through the boiler and do the job of heating of water and produce steam. As flue gases flow through boiler and then to chimney, fresh air from outside naturally enters the boiler and help in burning of fuel and production of hot flue gases. So, in this way, a small pressure difference is naturally created between the base of chimney and the air inlet point of boiler because of density difference between hot flue gases inside the chimney and fresh colder air outside the chimney. This pressure difference is called natural draught, because it is produced naturally or automatically by chimney without any external aid. However the amount of draught produced by chimney is very small and is used in small boilers only, Chimney is also required otherwise to safely remove the harmful poisonous flue gases in the atmosphere at a safe height, where they may not harm the freshness of air on earth.

In this way, a chimney performs one or more of the following functions:

i)     It produces draught due to the density difference between the column of hot gases inside the chimney and the cold air outside the chimney.

ii)   It carries the products of combustion to such a height in the atmosphere, before discharging that they will not be objectionable or hazardous to surroundings.

Fig. 28.2 Natural draught system

28.2.2  Artificial draught

When the draught is produced by some external agency i.e. mechanical fan/blower or by steam jet itself, it is called artificial draught. Artificial Draught is generally required in modern boiler installations, which require a total static draught of 30 to 350 mm, which is impossible to produce by installing a chimney because the maximum draught which can be practically produced in chimney is 12 mm of water. In modern commercial boilers, more value of draught is required to increase the heat transfer co-efficient and hence the thermal efficiency. So, artificial draught is must to use to overcome the flow resistance offered by large flue passages through the boiler and also a number of other accessories pre-heater, economizer, super heater etc.

The Artificial Draught may be either mechanical draught (which is produced by fan/blower) or steam jet draught (which is produced by using a high velocity jet of steam). Mechanical Draught is used in small installations or in Locomotives. The artificial draught is further classified as

            (a) Forced (b) Induced (c) Balanced

28.2.2.1  Forced draught

It is a positive pressure draught in which fan is installed at the base of the boiler before grate which forces the outside air through fuel bed, furnace and air pre-heater and then flue gases through flue passage, economizer etc. The enclosure for the furnace has to be very tightly sealed so that gases from the furnace do not leak out in the boiler house.

28.2.2.2  Induced draught

In this system a fan or blower is located at or near the base of the chimney which creates a partial vacuum in the furnace and flue passage and thus the air and flue gases are drawn through the boiler due to comparatively higher pressure of outside air. It is convenient to produce induced draught and like in forced draught, any type of fan/blower may be used.

28.2.2.3  Balanced draught

It is a combination of forced and induced draught. In this system, the forced draught fan overcomes the resistance in air pre-heater and grate while the induced draught fan overcomes draught losses through boiler, economizer and connecting flue passages etc. Balanced draught is most effective and used in large boilers. Depending on the type of fuel burnt and type of boiler, the fan or blower used may be of any type as radial or axial etc. It is usually run by an electric motor. In case of oil burners, air blower is fitted in the burner assembly itself. Finally the flue gases pass on to chimney and escape to atmosphere because of pressure created by fan.

Advantages of Mechanical Draught

1)   It is more economical and its control is easy.

2)   Desired value of draught can be produced by mechanical means which cannot be produced by means of natural draught.

3)   It increases the rate of combustion by which low grade fuel can also be used.

4)   It reduces the smoke level and increases the heat transfer co-efficient on flue gases side thus increases the thermal efficiency of boiler.

5)   It saves the energy and the heat of flue gases can be best utilized by it.

6)   In this way, it reduces the fuel consumption and makes boiler operation cheaper.

7)   It reduces the height of chimney which now is only controlled by the requirement of pollution norms.

Disadvantages of Mechanical Draught

1)   Initial costs of mechanical draught system are high.

2)   Running cost is also high due to requirement of electricity but that is easily compensated by the savings in fuel consumption.

3)   Maintenance cost is also on higher side.

4)   Noise level of boiler is also high due to noisy fan/blower etc.

Table 28.1 Comparison between forced draught and induced draught

Forced Draught

Induced Draught

1) Fan 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.

3) Due 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.

5) Forced 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

Fig. 28.3 Induced steam jet draught

28.2.3  Steam jet draught

It is a very simple and easy method of producing artificial draught without the need of an electric motor. It may be forced or induced depending on where the steam jet is installed. Steam under pressure is available in the boiler. When a small portion of steam is passed through a jet or nozzle, pressure energy converts to kinetic energy and steam comes out with a high velocity. This high velocity steam carries, along with it, a large mass of air or flue gases and makes it to flow through boiler. Thus steam jet can be used to produce draught and it is a simple and cheap method. Actually the steam jet is directed towards a fix direction and carries all its energy in kinetic form. It creates some vacuum in its surroundings and so attracts the air of flue gases either by carrying along with it. Thus it has the capacity to make flow of the flue gases either by carrying or inducing towards chimney. It depends on position of steam jet.

28.2.3.1  Forced steam jet draught

Steam from the boiler after having been throttled to a gauge pressure of 1.5 to 2 bar is supplied to the jet or nozzles installed in ash pit. The steam emerging out of nozzles with a great velocity drags air along the fuel bed, furnace, flue passage and then to the chimney. Here steam jet is forcing the air and flue gases to flow through boiler hence it is forced steam jet draught.

28.2.3.2  Induced steam jet draught

The jet of steam is diverted into smoke box or chimney. The kinetic head of the steam is high but static head is low i.e. it creates a partial vaccum which draws the air through the grate, ash pit, flues and then to motor box and chimney. This type of arrangement is employed in locomotive boilers. Here steam jet is sucking the flue gases through boiler so it is Induced Steam Jet Draught.

Advantages

i)      It is quite simple and cheap.

ii)    It has the capability of using low grade fuels.

iii) It occupies very less space.

iv)  Initial cost is low.

v)   Maintenance cost is low.

vi) Exhaust steam from steam engine or turbine can be used easily in Steam Jet Draught.

Disadvantages

i)        It can operate only when some steam is generated.

ii)      Draught produced is very low.

28.3  Advantages of Artificial Draught over Natural Draught

1)      Easy control on combustion of fuel and evaporation of water.

2)      Significant increase in capacity or evaporation power of boiler.

3)      Increase in fuel burning capacity of grate.

4)      Fuel consumption decreases sufficiently.

5)      Improvement in efficiency of plant.

6)      Reduced chimney height.

7)      Prevention of smoke.

8)      Low grade fuel can be used.

28.4  Draught Losses

The draught is required to overcome the various draught losses in boiler as discussed below:

a)      Loss due to the frictional resistance offered by flue gas passage to the flow of flue gases.

b)      Loss due to bends in gas flow circuit, which also offer flow resistance.

c)      Loss due to friction head in grate, economizer, super heater etc.

d)     Loss due to flow resistance offered by chimney.

e)      Loss due to imparting some velocity to flue gases, which is required to increase heat transfer in boiler and also to throw away the flue gases from chimney.