Classification of Boiler Draught: Natural, Artificial, Steam Jet and Mechanical.
Draught may be defined as the small pressure difference which causes a flow of gas to take place. In case of a boiler the function of the draught is to force air to the fire and through a boiler furnace and flue, and to discharge the products of combustion to atmosphere via stack or chimney.
Proper combustion in a boiler furnace can only occur when sufficient quantity of air is supplied to the burning fuel. If the supply of air is insufficient, then the combustion will be sluggish and insufficient even with superfine furnace construction and the most skilful stoking.
The classification of draught is as below:
The natural draught is produced by chimney or stack and artificial draught is produced by artificial mean such as:
(a) Steam Jet
Classification # 1. Natural Draught:
In case of a natural draught if we consider a condition when fire is not lighted, the pressure at the grate level surface is the same at all points on the surface i.e., at the chimney base as well as on the grate and we assume its value to be P1.
The pressure at all points at horizontal surface ‘b’ passing from the top of the chimney in fig. 7-2 is also the same and we take its value to be P2. The value of P2 is less than P1 because the surface ‘b’ is higher than the surface ‘a’ by H, the height of the chimney. When the fire is lighted up on the grate, the hot gases will fill up the chimney as well as flue gas passages.
Now, though, the pressure at the chimney top is P2 the pressure at the grate level below the grate is P1 but at the base of the chimney it will be P2 + pressure due to the hot gas column of height H. This pressure will be less than P1 because P1 = P2 + the pressure due to the cold air column of height H.
Now, a cold air column of the same height is heavier than a hot gas column and so the pressure difference at the base of the chimney corresponding to grate pressure is the pressure difference in weight of the column of height H of cold air and that of hot gases, and this is known as total static draught or theoretical maximum static draught.
Thus total static draught is the total pressure difference which results owing to the difference in the weights of the column of the hot flue gas inside the chimney and a column of the outside air of the same area and height.
When carbon burns the reaction is represented by C + O2 = CO2. This equation shows that the volume of carbon dioxide produced is equal to the volume of oxygen required for complete combustion of fuel. If air is supplied to the fuel, the nitrogen does not take part in combustion and therefore, the volume of chimney gases produced due to combustion of carbon is equal to the volume of air supplied, the volumes being measured at the same temperature.
When hydrogen burns the reaction is represented by the equation 2H2 + O2 = 2H2O which shows that the volume of steam produced is twice the volume of oxygen consumed, but since most of the fuels contain very small percentage of hydrogen, the volume of steam in the products of combustion is a small fraction of the total volume of the gases in the chimney.
Therefore, we can safely assume that the volume of flue gases in the chimney is equal to the volume of air supplied, both volumes being measured at the same temperature. It should be clearly understood that though volumes of the gases and air supplied are the same their masses are different.
If m kg be the mass of air supplied per kg of fuel, then (m + 1) kg will be the mass of flue gases.
Let T0 be the absolute temperature corresponding to 0°C, T1 be the absolute temperature of cold air outside the chimney and T be the absolute mean temperature of the chimney gases,
The difference between the pressures exerted by the columns of cold air and hot combustion products, the height of the latter being equal to the height of the chimney H metre causes the natural draught.
One kg of air occupies 0.7734 cu metre at NTP. Let H metre be the height of a chimney measured from grate level and h mm be column of water which will be balanced by a draught.
Mass of products of combustion per kg of fuel will be (m + 1) kg. Volume of air per kg of fuel burnt will be 0.7734 m cubic metre at T K. It is also equal to the volume of flue gases per kg of fuel at T K.
The total static draught is the total pressure difference P N/m2, which results owing to the difference in the weights of hot flue gases inside the chimney which is H metre high and acting on A m2 area and a column of the outside air of the same area and height.
Thus, we see that the height of a chimney determines the draught which it will -develop under the given conditions of atmospheric air and flue gas temperature. The static draught developed will be the same for a given height regardless of the number of boiler it serves.
But the area of cross-section of a chimney should be increased if it serves more than one boiler because more coal or fuel will be consumed and so the discharge of flue gases during a given period should be increased.
The theoretical amount of draught is seldom obtained with a chimney, and the actual draught may be 0.8 of the theoretical draught possible. If we denote H’a as the actual draught produced by a chimney in terms of the height of a column of hot gases in metre, then the theoretical velocity, C of gas flow, is given by-
Under actual conditions of operation, the velocity of gas flow may be only 30% to 50% of the theoretical velocity owing to roughness of the interior surfaces of the chimney.
The cross sectional area of the chimney can be obtained when the mass flow of flue gases and the coefficient of velocity are known. If Q is the volume of flue gases handled in m3 per second, then-
The value of Kv may be taken from 0.3 to 0.5.
The chimney draught is most effective when the maximum mass of hot gases is discharged in a given time, and it will be shown that this occurs when the absolute temperature of the chimney gases bears a certain relation to the absolute temperature of the outside air.
The theoretical velocity, C, of the gases produced by a total static draught is given by the equation–
C2 = 2gh’
where h’ is the height of a column of flue gases corresponding to the draught pressure.
Thus, when maximum discharge takes place, the height of a column of hot gas which would produce a draught which will be equal to the height of the chimney.
In case of natural draught, the temperature of the flue gases leaving a chimney is higher than that of flue gases leaving it in case of artificial draught system because a certain minimum temperature is required to produce a given draught with the given height of the chimney.
Due to the higher flue gas temperature, the heat carried away by flue gases is more in case of natural draught system and this heat is lost as far as steam generation is concerned. Thus, we see that the draught is created at the cost of thermal efficiency of the boiler plant installation because a portion of the heat carried away by the flue gases could have been utilized either for heating feed water going to the boiler or heating the air going to the furnace. Both these effects would have improved the efficiency of the installation.
Let t°C be the mean temperature of the flue gases leaving the chimney in the natural draught system and t2°C be the mean temperature of the flue gases leaving the chimney in the artificial draught system. The corresponding absolute temperatures will be denoted by T and T2, respectively.
If Cp is the mean specific heat of flue gases then extra heat carried away by one kg of flue gas due to higher temperature required to produce the natural draught = 1 x Cp (t – t2) kJ.
The draught pressure produced by the natural draught system in height of hot gases column-
The efficiency of a chimney is proportional to the height but even for a very tall chimney the efficiency will be less than 1 % and thus we see that the chimney is very inefficient as an instrument for creating draught.
If m be the amount of air supplied per unit mass of fuel, the mass of flue gases formed by the combustion of unit mass of fuel will be (m + 1).
Extra heat carried away by flue gases in the natural draught system from heat supplied by unit mass of fuel
Qe = (m + 1) × Cp (t – t2)
If CVnet be the net calorific value of the fuel, then heat spent on draught expressed as % of heat supplied
When the ashpit door is closed and there is no flue gas flow. Under this condition a draught gauge located at the grate level at the base of the chimney will read the total static’ draught which the chimney develops. When flue gas is flowing the grate level draught gauge reading will be less than the total static draught and the difference in two readings gives the drop in draught.
The drop in draught is caused by:
(i) Frictional resistance offered by the flues and gas passages to the flow of flue gases
(ii) Flow of flue gases at various bends in the gas circuit, and
(iii) Imparting of velocity to the flue gases.
Due to the drop or loss in draught we get another term known as available draught. The available draught at any point along the flue gas circuit is the draught which is read by means of a draught gauge located at the point under consideration. Thus, we get the relation available draught = total static draught – total draught drop.
The draught necessary for a given boiler plant depends on:
(i) To overcome friction in flue,
(ii) To overcome the resistance offered to the flow of gases by the baffle walls and boiler tubes,
(iii) To force the flue gas through economizer, air pre-heater, etc.,
(iv) To force the air through the fuel bed on the grate,
(v) To overcome the friction loss in the chimney, and
(vi) To impart velocity to gas in the chimney.
The draught required to overcome friction in flue is 10 mm of water column for 100 metre length and the drop in draught at each right angle bend is 2 mm of water column. The draught required to overcome the resistance offered to flow of gases in the boiler tubes and baffles depends on the kind of boiler used and type of baffles, and on mass flow. It varies from 5 to 40 mm of water column.
The drop in draught in some boilers and their components are as under, though it varies over a considerable range.
The drop in draught in fuel bed is one of the most important factors in determining the total static draught. This depends on the kind and the size of coal, amount of coal, amount of coal burnt per hour and the design of a furnace. The drop in draught is zero for pulverized fuel, oil or gas.
The loss in draught in a chimney is 20% of the total draught produced by it. The loss in draught in a damper can be neglected.
Classification # 2. Artificial Draught:
In present day boiler installations, the total static draught required varies from 25 to 350 mm of water column because loss in draught takes place in boiler tubes, superheater elements, baffles, economizer and air pre-heater. Besides, resistance to air flow is also offered by combustion equipment.
It will not be practical to build a stack high enough to produce a draught of such a large magnitude. Natural draught is dependent on climatic conditions and is less when the outside air temperature is higher and thus it is necessary to obtain a draught which is independent of weather conditions.
To meet the desired pressure requirement which is independent of climatic conditions, an artificial draught is created. It may be a mechanical draught or a steam jet draught is used while for central power stations and many other boiler installations a mechanical draught is preferred.
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 chimney it induces the draught and the air is drawn through 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 case of locomotives, the exhaust from 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 that 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.
The mechanical draught is the draught created by use of mechanical equipment to create a artificial draught.
The Advantages of Mechanical Draught are:
(i) Increase in Evaporative Power of a Boiler:
By installing the mechanical draught system, the quantity of fuel burnt, per square metre of grate area, is increased. Generally with natural draught system the average fuel consumption per square metre of grate area varies from 80 to 170 kg depending upon the height of the chimney.
With mechanical draught, any type of mechanical stoker can be adopted and any desired rate of fuel consumption upto 200 kg/m2 of grate area can be maintained in any type of boiler. With tubular boilers the fuel can be burnt at a better rate. As the combustion rate of fuel has increased, the steam raising capacity of the plant is increased.
(ii) Capability of Consuming Low Grade Fuel:
With mechanical draught installation many kinds of low grade fuels can be used for steam raising purposes.
(iii) Easy Control of Combustion and Evaporation:
Greater economy in working can be obtained because complete combustion is possible with less excess air. The rate of air supply can be regulated to suit the irate of fuel consumption which in turn regulates the evaporation of steam.
(iv) Prevention of Smoke:
Smoke prevention depends upon the quantity of air supplied and the condition of firing. With mechanical draught and mechanical stokers high temperatures and efficient combustion can be obtained with less difficulty.
(v) Improved Efficiency of the Plant:
As the temperature of flue gases can be lowered with mechanical draught system heat recovery devices are incorporated in the boiler plant. By installing the economizer the temperature of feed water entering boiler is raised, while the air pre-heater heats the combustion air. Both these devices improve the thermal efficiency of the boiler plant.
(vi) Reduction of Chimney Height:
With the mechanical draught system the function of a chimney is only to discharge flue gases t a convenient height to suit the surroundings and comply with local by-laws.
The disadvantages of mechanical draught are:
(ii) Upkeep of machinery
(iii) Increased wear and repairs which are expected owing to the increased duty which the boilers are called upon to perform.
In an induced draught system, the pressure over the fuel bed is reduced below that of the atmosphere by means, of a fan located at or near the base of the chimney. The induced draught fan is driven either by steam or by electricity.
By creating a partial vacuum in the furnace and flues, the products of combustion are drawn from the main flue and they pass up the chimney. This draught is used usually when economizers and air pre-heaters are incorporated. This draught is similar in action to the natural draught.
The induced draught from fan or jet should be placed such that the temperature of gas handled is lowest possible. This clearly suggests that it should be located after the air pre-heater. As the fan has to handle large volumes of hot gases, the bearing adjacent to the fan should be water jacketed.
It is used alone for gas, oil or pulverized coal firing. When there is an existing chimney, induced draught should be used to supplement the natural draught. The chimney is retained and comparatively small fan is employed to discharge the gases into its base. This reduces the initial and running costs as compared with simple induced draught.
With induced draught system the entire air and gas path from furnace to chimney base being at a pressure lower than atmospheric, the tendency of air leak is the maximum.
Forced draught often (but not necessarily) maintains a pressure above atmospheric, in the boiler setting, therefore, the flue gases may be forced out into the boiler room through cracks or leaks where boilers are hand-fired, flame may shoot out of firing doors if the draught is not cut out before the doors are opened.
Forced draught is commonly used with under feed stokers, as considerable pressure is needed to force the air required for combustion up through the deep fuel bed. In general, all types of large boilers operated at high ratings are normally equipped with forced draught.
Forced draught fans may have their air supply from the boiler room or from air pre-heaters, the latter being widely used with large boiler units.
It is a combination of the forced and induced draught system. The forced draught fan supplies air at a sufficient pressure for air to pass through the fuel bed either direct or through air heater. Induced draught fan draws the flue gases through the boiler flues, economizer and pre-heater and then sends them to the chimney.
If the air pressure from the forced draught fan is just sufficient to overcome the resistance offered by the fuel bed and the suction of the induced draught fan is just sufficient to draw the flue gases from the furnace the pressure in the furnace will be balanced or in equilibrium.
In actual practice the induced draught fan maintains a draught of 0.2 to 0.5 mm of water in the furnace so that the leakage of furnace gas into the boiler room is prevented. Thus, in a balanced draught system the forced draught fan overcomes the resistance in the air pre-heater and chain grate stoker while the induced draught fan overcomes draught losses through boiler, economizer, air pre-heater and connecting flues.
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 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 inrush of cold air through the furnace doors when they are opened for firing and cleaning fires.
If h be the draught in mm of water and V be the volume of air in m3 or gas handled by the fan per second, the work done on air or gas = hV kJ/sec (kW). If η be the efficiency of the fan, then
Let m be the quantity of air supplied per kg of fuel and N kg be the amount of fuel burnt per second.
With an induced draught fan, the amount of flue gas formed will be (m + 1) kg per kg of fuel burnt. Therefore when N kg of coal is consumed per second, the amount of flue gases handled by fan per second will be equal to N (m + 1) kg.
We assume that the density of flue gases is equal to that of air. This assumption differs from the assumption made in equation (7-5), but it is made because it gives result on safer side.
When both the fans produce the same draught and have the same efficiencies.
As in induced draught system the vacuum exists in the furnace, air will leak into the system and as a result more volume would have to be handled by the fan and therefore the power requirement of induced draught fan will increase. With ordinary brickwork and no special precautions the power will increase by 20%.