What do you understand by “Draught”? Briefly explain with sketch; Forced, Induced, and balanced draught.
DRAUGHT SYSTEM
The draught is one of the most essential systems of the thermal power plants which supplies the required quantity of air for combustion and removes the burnt products from the system. To move the air through the fuel bed and to produce a flow of hot gases through the boiler, economizer, preheater, and chimney require a difference in pressure. This difference in pressure to maintain the constant flow of air and discharge the gases through the chimney to the atmosphere is known as draught. Draught can be obtained by use of chimney, fan, steam or air jet, or a combination of these. When the draught is produced with the help of a chimney only, it is known as Natural Draught and when the draught is produced by any other means except chimney it is known as artificial draught.
Classification of Boiler Draught: Natural and Artificial Draught
Fuel Bed Resistance (hb): The fuel bed resistance depends on fuel size, bed thickness, and combustion rate. The effect of combustion rate on resistance for different types of stokers is shown in Fig. The resistance of the spreader stoker because much of the coal is burned in suspension. The drought resistance of spreader stoker may be taken as 6 cm of water head
Head Loss in Equipment (he): The manufacturers generally supply data for equipment resistance like air heaters, economizers, boiler passes, superheaters, etc.
Measurement of Draught: The draught losses in different parts of the boiler plant are measured in mm of water with the help of manometers. This pressure may be above atmospheric pressure or below atmospheric pressure. For very accurate measurement, the inclined type manometer is used. The typical draught at different points of the boiler plant is measured by a U-tube manometer. The measurement of draught serves not only to find out the resistance to the air and gas flow but also indicates the rate of flow.
ADVANTAGES :
(1) It does not require any external power for producing the draught.
(2) The capital investment is less. The maintenance cost is nil as there is no mechanical part.
(3) Chimney keeps the flue gases at a high place in the atmosphere which prevents the contamination of the atmosphere.
(4) It has a long life.
LIMITATIONS
(1) The maximum pressure available for producing natural draught by the chimney is hardly 10 to 20 mm of water under the normal atmospheric and flue gas temperatures.
(2) The available draught decreases with an increase in outside air temperature and for producing sufficient draught, the flue gases have to be discharged at comparatively high temperatures resulting in the loss of overall plant efficiency. And thus maximum utilization of Heat is not possible.
(3) As there is no thorough mixing of air and fuel in the combustion chamber due to the low velocity of air therefore combustion is very poor. This increases the specific fuel consumption.
(4) The chimney has no flexibility to create more draught under peak load conditions because the draught available is constant for a particular height of the chimney and the draught can be increased by allowing the flue gases to leave the combustion chamber at higher temperatures. This reduces the overall efficiency of the plant.
Nearly 20% of heat released by the fuel is lost to the flue gases. The chimney draught is only used for very small boilers. Nowadays the chimney is never used for creating draught in thermal power plants as i t has no flexibility, and the total draught produced is insufficient for high generating capacity. The chimney is used in all power plants only to discharge the flu e gases high in the atmosphere to maintain the cleanliness of the surrounding atmospheric air.
Artificial Draught
Because of insufficient head and lack of flexibility, The use of natural draught is limited to small capacity boilers only. The draught required in the actual power plant is sufficiently high (300 mm of water) and to meet high draught requirements. We can achieve artificial draught, mainly with two methods. One gets produced by a steam jet, and forced air produces another. By introducing artificial draught one can significantly reduce the height of the chimney to fulfill the same purpose of removing flue gasses from the atmosphere. The artificial draught is more economical when the required draught is above 40 mm of water.
It has been seen that the draught produced by chimneys is affected by atmospheric conditions. It has no flexibility, poor efficiency, and a tall chimney is required. In most modern power plants, the draught used must be independent of atmospheric conditions, and it must have greater flexibility (control) to take the fluctuating loads on the plant.
What is the purpose of Draught? To supply the required amount of air to the furnace for the combustion of fuel. The amount of fuel that can be burnt per square foot of grate depends upon the quantity of air circulated through the fuel bed. To remove the gaseous products of combustion.
Forced Draught
In a forced draught system, a blower is installed near the base of the boiler, and the air is forced to pass through the furnace, flues, economizer, air preheater, and to the stack. This draught system is known as a positive draught system or forced draught system because the pressure and air is forced to flow through the system.
Balanced Draught
It is always preferable to use a combination of forced draught and induced draught instead of forced or induced draught alone. If the forced draught is used alone, then the furnace cannot be opened either for firing or inspection because the high-pressure air inside the furnace will try to blow out suddenly and there is every chance of blowing out the fire completely and furnace stops. If the induced draught is used alone, then also 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 reduces the effective draught and dilutes the combustion.
Induced Draught
In this system, the blower is located near the base of the chimney instead of near the grate. The air is sucked into the system by reducing Induced Draught instead of near the grate. The air is sucked into the system by reducing the pressure through the system below the atmosphere. The action of the induced draught is similar to the action of the chimney. The draught produced is independent of the temperature of the hot gases therefore the gases may be discharged as cold as possible after recovering as much heat as possible in the air-preheater and economizer.
This draught is used generally when an economizer and air-preheater are incorporated into the system. The fan should be located in such a place that the temperature of the gas handled by the fan is the lowest. The chimney is also used in this system and its function is similar to that mentioned in a forced draught but the total draught produced in the induced draught system is the sum of the draughts produced by the fan and chimney. The arrangement of the system is shown in Figure.
Steam Jet Draught
A simple and easy method of producing artificial draught is the steam jet draught. It may also be of the forced draught type or induced draught type. When the jet of steam is directed into the smoke box or in the chimney it induces the draught and the air is drawn through the ashpit, furl bed, and boiler flue tubes. When the jet is installed in the ashpit, the draught becomes of the forced type, and air is forced through the fuel bed, boiler flue tubes, and the chimney.
In the case of locomotives, the exhaust from the non-condensing steam engine is directed into the smoke box. With this arrangement, the draught is automatically adjusted to suit the requirements of the boiler. The steam jet draught system requires very little attention and is economical when cheap and plenty of low-grade fuels are employed. It has one disadvantage it cannot be started until steam pressure is available. Steam passing into the furnace will carry away heat in the same manner as moisture in the fuel.
Natural Draught
The natural draught system employs a tall chimney as shown in the 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. The draught is produced by this tall chimney due to the temperature difference between hot gases in the chimney and cold external air outside the chimney.
Air Pre-heaters
•They are simply heaters that heat the air before it enters the combustor, thence resulting in fuel consumption and increasing thermal efficiency.
•The fuel savings are nearly directly proportional to the air temperature rise in the pre-heater. Typical savings are 4% for a 2000F air temperature rise and about 11% for a 5000F temperature rise in the pre-heater.
•Air pre-heater is also a requirement for the operation of pulverized-coal furnaces to dry that fuel.to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process.
They may be used alone to replace a recuperative heat system or to replace a steam coil. In particular, this article describes the combustion air preheaters used in large boilers found in thermal power stations producing electric power from e.g. fossil fuels, biomass, or waste.
Advantages of mechanical draught over natural draught.
1. The artificial mechanical draught is better in control and more economical than the natural draught.
2. The rate of combustion is high as the available draught is more. The better distribution and mixing of air with fuel is possible therefore the quantity of air required per kg of fuel is less.
3. The airflow can be regulated according to the requirement by changing the draught pressure.
4. The chimney draught is produced at the cost of thermal efficiency of the plant because it is necessary to exhaust the gases at high temperatures to produce the draught. In mechanical draught, the exhaust gases can be cooled to the lowest possible temperature before exhaust and improving the overall thermal efficiency of the plant.
5. The height of the chimney used in mechanical draught can be reduced sufficiently as the function of the chimney is only to exhaust the gases high in the atmosphere to prevent contamination.
6. The efficiency of the artificial draught is nearly 7% whereas the efficiency of the chimney draught is hardly 1%.
7. The fuel consumption per kW due to artificial draught is 15% less than the natural draught.
8. The fuel burning capacity of the grate is 200 to 300 kg/m 2 in area of the grate per hour with mechanical draught whereas it is hardly 50 kg/m 2-hr with natural draught.
9. It prevents the formation of smoke as complete combustion is possible even with less excess air. The major disadvantage of the artificial draught is the high capital cost required and the high running and maintenance costs of the fans used.
The advantages of forced draught fan over induced draught fan:
(i) Fan size and power required for the same draught are 1/3 to 1/2 of that required from an induced draught fan installation because a forced draught fan handles cold air.
(ii) Forced draught fan does not require water-cooled bearings.
(iii) Tendency of air leak into the boiler furnace is reduced.
(iv) There is no heat loss due to the inrush of cold air through the furnace doors when they are opened for firing and cleaning fires.