Natural Draft is thermal draft. It occurs when heated gases expand. Since hot combustion gases weigh less per volume than room air or outdoor air, they tend to rise. The rising is contained and increased by enclosing the gases in a tall chimney.

Chimney Draft The pressure difference or chimney draft can be expressed as: dpch = h (ρo – ρi) g (1) where dpch = pressure draft in the chimney (Pa) h = height of chimney (m) ρo = density of outside air (kg/m3) ρi = density of inside air (kg/m3) g = 9.81 – acceleration of gravity (m/s2) (1) can alternatively be …

Thus natural draught is created at the cost of thermal efficiency. Then extra heat carried away by flue gases due to higher temperature required for producing natural draught = Cp (T � T2) for one kg of flue gases.

Natural draught allows natural circulation of air through the boiler system. The natural draught mainly depends upon the height of the chimney. Here “V” is the volume of the air or gas, “m” is the mass of the gas or air, “T” is the temperature measured in kelvin scale and “R” is the gas constant.

Demerits of natural draught Flue gases have to be discharged at high temperature since draught increases with the increase in temperature of flue gases. 3. Heat cannot be extracted from the flue gases for economizer, superheater, air pre-heater, etc.

Explanation: The only disadvantage of natural draught is that the available draught decreases with increasing the outside temperature. Explanation: The natural draught in the steam generator depends upon the air condition outside chimney & the temperature of the exhaust gases.

LIMITATIONS : (1) The maximum pressure available for producing natural draught by chimney is hardly 10 to 20 mm of water under the normal atmospheric and flue gas temperatures. Draught is the pressure difference which causes a flow of air or gases from one point to another point in the boiler system.

Natural draught system employs a tall chimney as shown in figure. The chimney is a vertical tubular masonry structure or reinforced concrete. It is constructed for enclosing a column of exhaust gases to produce the draught. It discharges the gases high enough to prevent air pollution.

Forced-draft fans focus the pressure and volume at the outlet of the fan to push air through a system, creating positive air pressure. Industrial induced draft fans operate by focusing the pressure and volume at the inlet of the fan to pull air through a system.

Explanation: In force draught system an air blower is installed near the boiler which blows into the furnace. The pressure of air throughout its path is above atmospheric pressure. This system also uses chimney to discharge hot flue gases into the atmosphere.

Drafts are produced by the rising combustion gases in the stack, flue, or by mechanical means. For example, a blower can be put into four categories: natural, induced, balanced, and forced. Natural draft: When air or flue gases flow due to the difference in density of the hot flue gases and cooler ambient gases.

Draught is defined as the difference between absolute gas pressure at any point in a gas flow passage and the ambient (same elevation) atmospheric pressure.

What are the purpose of Draught? To supply required amount of air to the furnace for the combustion of fuel. The amount of fuel can be burnt per square foot of grate depends upon the quantity of air circulated through fuel bed. To remove the gaseous products of combustion.

The mechanical draught is used for central power stations and steam jet draught is used for small installations like locomotives. In an induced steam jet draught, exhausted steam from the boiler is introduced by a steam jet into the smokebox from where they are discharged to atmosphere chimney.

2. DEFINITION  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.

Steam jet draught is a straight forward form of draught system in a boiler. In steam jet draught system a small portion of generated steam exhausts through a nozzle and the kinetic energy of the jet of high-velocity steam drags the air or flue gases in the boiler system.

In a mechanical draught system, the movement of air is due to the action of a fan. In this system of draught the air pressure throughout the system is above the atmospheric pressure and air is forced to flow through the system (furnace, economiser, air pre-heater and chimney).

Advantages of Artificial or Mechanical Draught It increases the rate of combustion by which low-grade fuel can also be used. It reduces the smoke level and increases the heat transfer co-efficient no flue gases side thus increases the thermal efficiency of the boiler.

The draught is one of the most essential systems of thermal power plant which supplies required quantity of air for combustion and removes the burnt products from the system. Draught can be obtained by use of chimney, fan, steam or air jet or combination of these.