Mechanical draught

Gas, oil and coal fired boilers are used extensively to provide heating, hot water and steam for process applications, and in the generation of electrical power. In all cases where a fuel is being burnt, fans are used to provide combustion air, to trans­port exhaust gases and in some cases used to transport and deliver the fuel (coal) into the furnace.

Fans are used to deliver the air used for combustion into the fur­nace, and are known as forced draught (FD) fans. In smaller boiler units FD may be the only fan in the system. In larger units additional fans are also installed which transport the exhaust gases out of the boiler unit. These are known as induced

 

Figure 21.51 Callendar dryer

 

Mechanical draught

Mechanical draught

Cortn« FD Fan

Figure 21.56 The position of various fans in a typical water-tube boiler plant Courtesy of R Mulholland

Draught (ID) fans and normally handle hot gas at around 140°C. On larger coal fired plant additional fans, called primary air (PA) fans are used to transport the powdered coal, typically the con­stituency of talcum powder, into the furnace. In older plants, the alternative was mill exhausters, down stream of the pulverising mill. These were modified paddle bladed centrifugal fans with heavy duty casings and impellers.

For industrial boilers the absorbed power of an individual fan would be in the range of 100 to 10,000 kW depending on the size of the plant. The fans, which are used to transport the air and exhaust gas through the boiler furnace and ducts, are sized not only to cater for the flow requirements, but also to overcome the pressure drop through the system. Figure 21.56 shows the position of the various fans in a typical water-tube boiler plant.

The three main types of fans are as follows:

Forced draught fan (FD)

This fan draws atmospheric air and delivers it, within a ducting system, to the combustion furnace. In larger plants the air is supplied into a heat exchanger where the air is preheated, nor­mally to around 300°C, before entering the furnace. Both cen­trifugal and axial fans can be used.

To cater for changes in the boiler loads, in the case of centrifu­gal units the fan output is varied by the use of variable geometry inlet vanes or variable speed. On axial fans variable pitch, in motion, blade adjustment is used to regulate the airflow.

Induced draught fan (ID)

This fan is situated at the opposite end of the boiler ducting sys­tem from the FD fans and handles combustion gases, normally at around 140°C. The fans are specially designed to cope with the higher gas temperatures and in some cases must be able to cope with erosive dust.

Again the fan output must be adjustable to cater for the variable output of the boiler unit. Because of the lower density ID fans are larger than the FD fans but in many cases look similar. Both centrifugal and axial fans can be used, however when dust par­ticles are present the axial fan blades require a more elaborate erosion protection system.

Primary air fan (PA)

This type of fan is only found on a coal-fired plant and its main function is to transport powdered coal into the furnace. It draws atmospheric air and supplies it to a heat exchanger where tem­perature is normally raised to around 300°C. The hot air then passes through the coal grinding plant where it picks up the coal dust and transports it on to the furnace.

Although the fans are significantly smaller than the FD and ID fans, the requirement to provide a much higher air pressure means that they still absorb significant power. As a result of the higher pressure requirement, the duty is more suited to a centrifugal fan.

Combustion

The most economical use of fuel has received the attention of manufacturers, designers and government agencies for many years. Older readers will perhaps remember those essential books oftheiryouth — The Efficient Use of Fuel and TheEfficient Use of Steam. Since that time, of course, the cost of fuel has in­creased enormously, whilst the need to reduce carbon dioxide emissions is now seen as an aid to self-preservation.

Defined in simple terms, combustion is the chemical composi­tion of oxygen with combustible material such as carbon, hydro­gen and, if unavoidable, sulphur. Oxygen is of course a constit­uent of the air around us. Under normal ambient conditions, air contains about 21% oxygen by weight. The remaining 79% however is almost entirely composed of nitrogen which to all intents and purposes is inert.

Before combustion actually takes place, a solid fuel must be heated to ignition temperature. The volatile gases in combina­tion with the oxygen in the air supply then burn, and by increas­ing the temperature of the remaining material, ignite the fixed carbon. This is converted into carbon monoxide or carbon diox­ide, according to the amount of oxygen present. Any non-com­bustible material remains as ash. It should be noted that pulver­ised coal generally burns firstly, by the formation of carbon monoxide (and other volatile distillates) and then further to carbon dioxide.

For liquid fuels the combustion process is simpler. They are soon converted into gaseous compounds, which burn very much as gases proper.

Gaseous fuels burn immediately and do not have the severe problems of an ash residue. They may however produce signifi­cant quantities of moisture in the form of water vapour.

When fuels are burned, the whole of the heat produced cannot be used. Apart from furnace radiation losses, some of the heat is taken up by the products of combustion.

Perhaps most importantly there will be a considerable loss due to the amount of excess air used in an endeavour to obtain complete combustion. For this reason alone the use of me­chanical draught is now almost universal.

The theory of combustion is comparatively simple. It is much more difficult to apply in practice, however. Properly mixing air in the correct proportions with fuels and combustible gases to obtain complete combustion is not easy. Often the quantity of air delivered to the furnace is far in excess of that theoretically required. Although excess air means a loss of boiler efficiency, it is often necessary to ensure complete combustion, the amount depending on the quality, quantity and size of the fuel burnt. When fan draught is used, the air supply can be closely regulated and controlled.

Operating advantages

Whilst the numbers of boiler plant continuing to use natural draught alone is now very small, it is as well to remember the advantages that are obtained with the use of mechanical draught fans:

• increased boiler output and reduced heat losses via the chimney

• lower grade and less costly fuel may be used

• exact adjustment of draught to boiler load requirements

• improved combustion obtainable which with proper firing will reduce smoke emissions

• permits the addition of heat recuperating equipment such as economisers and air pre-heaters to reduce exit gas tem­peratures and therefore heat losses.

Determining the correct fan duty

The carbon dioxide percentage in the flue gases at exit from the boiler is a measure of the excess air admitted for combustion. It is dependent on the average maximum theoretical C02 % of the particular fuel being burnt and the method of firing. Table

21.7 gives the general range of boiler operating conditions.

It may be assumed that the lower C02 % corresponds to “good” combustion whilst the higher figure is ‘very good’. Medium per­centages would be 8 to 10% for coal, whilst 5 to 8% would be considered “poor”. It will be noted that the figures for oil are closer to their theoretical maximum, reflecting increased ease of obtaining good combustion with this fuel.

Type of boiler

Fuel and firing

Boiler efficiency

%

Operating C02 range

%

Average maximum theoretical CO2

%

Water-tube

Pulverised fuel

85 to 88

12.5 to 15

18.5

Coal — stoker

77 to 84

11 to 13.5

Oil

82 to 86

11.5 to 12.5

15.4

Wood

75 to 82

11 to 14.5

20.2

Shell

Coal — hand

60 to 68

9 to 11

18.5

Coal — stoker

68 to 75

10 to 13

Oil

70 to 77

11 to 12

15.4

Table 21.7 General range of boiler operating conditions

For more detailed information concerning particular boiler types and other fuels you should consult the boiler manufactur­ers.

Mechanical draught

Figure 21.57 Percentage of C02 at various gas temperatures

V-

V-

|-

X

7^

V

—^

-s’

0,

(-

=

Per Cent [CO + CHJ

0 12 3 4

1 i 1 1 1

Per Cant CO,

Figure 21.58 The effect of varying percentages of C02 with flue gases

Note: With modern boiler types such as the condensing boiler, efficiencies greater than those specified above are possible.

Neglecting the losses due to radiation and the unburnt fuel in the ash, the effect of an increase in the percentage of C02 at various gas temperatures can be seen from Figure 21.57.

Figure 21.58 shows the effect of varying percentages of C02 with flue gases at a temperature of about 200°C, a usual figure where economisers are in use. This figure also indicates the rapid increase in heat loss when the combustion is incomplete, as indicated by the presence of carbon monoxide and hydro­carbons. The air supply should not therefore be reduced to such an extent that any appreciable amount of CO is present, so even under the best conditions, the percentage of C02 can­not easily exceed about 14% with coal.

Figure 21.59 shows approximately the relation between draught and the rate of combustion for various types of fuels burnt on ordinary grates. Higher values of draught are re-

Mechanical draught

Rate of combustion kg/m3

Mechanical draught

Figure 21.60 Height of chimney required to give natural draught

Quired for chain grates, fluidized beds etc. Figure 21.60 shows the height of chimney required theoretically to give natural draught up to 325 Pa for various flue gas temperatures at nor­mal altitudes.

From these two diagrams it will be seen that the highest rates of combustion are impossible without chimneys of considerable height or with high gas temperatures. It will be seen from Figure 21.57 that this would cause an excessive loss of useful heat.

Mechanical draught

Figure 21.61 Nomogram for combustion air and gas volumetric flowrate

подпись: figure 21.61 nomogram for combustion air and gas volumetric flowrateAllowances must be made for infiltration into the boiler and flue system and the increase in volume of flue gases due to mois­ture in the fuel. Air and gas volume flowrates so obtained do not include margins for the infiltration and fuel moisture mentioned in Table 21.8.

Thus when calculating the flue gas flowrate to be handled by an induced draught fan, it is customary to increase the flow by 15% to allow for overload. This is the design duty.

Allowance for Infiltration

% addition

Allowance for moisture

% addition

Poor brick flues

15

Bituminous coal

3

Good brick flues

5

Fuel oil

4.5

Average steel flues

3.5

Dry wood

10

Rotary air heater

7.5

Grit Collector

Table 21.8 Margins for infiltration and fuel moisture

21.6.2 Combustion air and flue gases

21.6.5.1 Volumetric flowrates

The nomogram in Figure 21.61 provides an easy means of ob­taining a close approximation of the weight and volume flowrate of air required for combustion of the fuel and the volumetric flowrate of flue gases.

If the design efficiency of the boiler and the average operating percentage can be obtained with reasonable accuracy, the flow of gases to be handled by the induced draught fan may be de­termined sufficiently accurately for sizing purposes.

In estimating these quantities it may be possible to obtain from site the data required. If, however, the information regarding operating conditions is unreliable or scanty, it is possible to use the appropriate data from Table 21.7.

The method of using the nomogram is detailed below, but for il­lustration an example of a particular installation has been cho­sen, this being shown on the nomogram by appropriate lines. It uses the data, as in Table 21.9.

Boiler efficiency

80%

Boiler evaporation at maximum continuous rating

18116 kg/hr

Operating C02

11.5%

Maximum theoretical C02 (for average coal)

18.5%

Temperature of air entering boiler

16°C

Temperature of flue gases

177°C

Table 21.9 Boiler installation details

Using the nomogram in Figure 21.61:

1) Join point on A (80%) to point on B (18,116 kg/hr) extend­ing line to cut C (Boiler Input — 14654 kW).

2) Join point on G (11.5%)to point on F (18.5%) extending to cut E (Excess Air — 60%, see note below).

3) Join intersection on C to intersection on E cutting D. (Weight of air required for combustion — 485 kg/s).

Then to obtain volume of air required for combustion:

4) Join intersection on D to point on J (16 °C), extending line to cut H giving volume of (6.61 m3/s).

Next, to obtain volume of flue gases (dry products):

4a) Join same intersection on D to point J (177 °C), extending

Line to cut H giving volume of flue gases (10.2 m3/s).

Note: In the example the excess air amounts to 60% and it will be noted that if the percentage of excess air is known it is unnecessary to plot the operating and theoretical C02 points.

Given the boiler to be coal-fired and furnished with brick flues in moderately good condition, the allowances to be made are:

Infiltration into flues, say 10%,

Moisture in coal 3.0%,

Plus allowances for overload etc. 15%, thus:

The total volume of the gas becomes:

10.2×1.1×1.03×1.15 = 13.3 m3/s

Posted in Fans Ventilation A Practical Guide


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