Chapter (3)

DRAUGHT SYSTEM

Steam is an important medium of producing mechanical energy. Steam has the advantage

that it can be raised from water which is available in nature; it does not react much with the

materials of the parts of power plant. Stem power stations are the important means for

generating electrical power.

3.1 Main Parts of a Steam Power Plant

An ordinary power plant consists of the following equipment:

(i) A furnace has burners or grates to which fuel is supplied and where combustion is

initiated. Space above and around burners or grate is called combustion chamber. In

the furnace, heat energy is released as a result of reaction between air and fuel. This

energy heats up a mass of combustion gases, which flow through the boiler unit.

(ii) Steam generator or boiler comprises all evaporator and heater units which are

arranged in such a way that hot gases can flow through these units, transferring

most of their heat energy to the water, which by this evaporated and converted into

a high energy steam.

(iii) Main power unit such as an engine or turbine to use the heat energy of the steam

and perform work.

(iv) Piping system to convey steam and water.

Figure (3.1) shows a schematic arrangement of equipment of a steam power station.

Fuel is Supplied from outside in form of coal, oil or gas. Heat produced due to burning of

fuel is utilized in converting water contained water drum into saturated steam. The

steam generated is passed to the super heater and heated up to the desired

temperature. Superheated steam then flows to the turbine. Low pressure steam leaving

the turbine passes to the condenser. Water from the condenser is entering water heater

which heats up the water to a certain temperature.

Figure 3.1: Arrangement of steam power plant

Bled Steam taken from the turbine at suitable extraction points is sent to low

pressure and high pressure water heaters.

Air Taken from atmosphere is first passed through air pre-heaters, where it is

heated by flue gases. The hot air then passes through the furnace. The flow gases after

passing over the boiler and super heater tubes. Flow through water heaters

(economizer), air pre-heater and finally they are exhausted to the atmosphere through

the chimney.

3.2 Draught systems

The purpose of draught system is to supply required amount of air for the

combustion and remove the products of combustion from the plant. To move the air

through the furnace and to cause the hot gases flowing through the boiler unit,

economizer, air pre-heater and chimney requires a difference of pressure to that

necessary to accelerate the burn gases to their final velocity and to overcome the

pressure losses equivalent to pressure head. This difference of pressure required to

maintain the constant flow of air and to discharge the gases through the chimney to

atmosphere is known draught system.

When the draught system is produced using the chimney only, it is known as

Natural draught and when the draught is used by any other means except the

chimney is known as Artificial draught

The various types of draught systems are:

i. Natural draught

ii. Mechanical draught

iii. Steam jet draught

(i) Natural Draught

Natural Draught System is used in boilers of small capacities. This system does not play

an important role in modern high capacity thermal power plants. Natural draught is produced

by the difference in weight of a column of cold external air and that of a similar column of hot

gases in the chimney. This system is dependent on the height of humidity and average

temperature of the gases in the chimney. Figure 3.2 shows natural draught system.

Figure 3.2: Natural draught system

(ii) Mechanical draught

In boiler of large capacities, fans are used to produce the necessary draught in order to

reduce the height of chimney , to obtain brought that is independent of atmospheric condition

and to control draught easily in case of fluctuating of loads on the plant. Mechanical draught

might be induced, forced or balanced

Figure (3.3) show the induced draught system in which fan is located near the base of

the chimney. In this system the air is sucked in the boiler by reducing the pressure through the

boiler below atmosphere. The fan sucks the burned gases from the boiler side and discharges it

to the chimney.

In a forced draught system, a fan is located near the base of the boiler unit and air

forced to pass through the boiler unit and the chimney. Most high rating combustion

equipment employs forced draught fan for supplying air to the furnace. The arrangement of

forced draught system is shown in figure (3.4)

If the forced draught is used alone, then the furnace cannot be opened either for firing

or inspection because the high pressure of air inside the furnace will try to blow out suddenly

and the furnace stops.

If induced system is used alone, then also the furnace cannot be opened either for firing

or inspection because cold air will try to rush into the furnace. This reduces effective draught

and weakens the combustion. To overcome both difficulties mentioned above, a balance

draught is always preferred. In other words, it is desirable to use combustion of forced draught,

and induced draught instead of using a one of the alone.

Balanced draught system is a combination of induced and forced draught systems the forced

draught fan forces the air through the fuel bed on the top of grate and the induced draught fan

sucks in gases from the boiler side and discharges them to the chimney.

Figure 3.3: Induced draught system

Figure 3.4: Forced draught system

Figure (3.5) shows the arrangement of the balanced draught system and the pressure

distribution through the system shown in figure (3.6). It is obvious from the figure (3.6) that the

pressure inside the furnace is near the atmospheric therefore there is no danger of blow-out of

flames or there is no danger of inrushing the air into the furnace when the doors are opened

for inspection.

Figure 3.5: Balance draught system.

Figure 3.6: Pressure distribution through the plant for balanced draught system.

iii. Steam Jet Draught

Steam Jet Draught may be induced or forced draught depending upon the location of

steam Jet producing the draught.

Induced draught produced by steam Jet is shown in figure (3.7). This system is used in

locomotive boilers. Exhaust steam from the engine enters the smoke box through a nozzle to

create draught. The air is induced through the fuels, the grate and the box.

Figure 3.7: Steam jet induced draught system.

3.2.1 Advantages of the mechanical draught over natural draught:

The artificial draught is better in control and more economical than natural draught. The

advantages of mechanical draught over natural draught are listed below:

1. 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. This further reduces the mass of the flue gases formed and heat carried by

exhaust gases.

2. The air flow can be regulated according to the requirement by changing the draught

pressure.

3. The mechanical draught is independent of the atmospheric temperature whereas the

chimney draught is seriously affected by atmospheric temperature.

4. Low grade fuel can be used in combustion chamber as the intensity of artificial draught

is high.

5. The chimney draught is produced at the cost of thermal efficiency of the plant because

it is necessary to exhaust the gases at high temperature to produce the draught. In

mechanical draught, the exhaust gases can be cooled to lowest possible temperature

before exhaust and improves the overall thermal efficiency of the plant.

6. 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

the contamination.

7. The efficiency of the artificial draught is nearly 7% whereas the efficiency of the chimney

draught is hardly 1%.

8. The fuel consumption per unit power due to artificial draught is less than the natural

draught.

9. The fuel burning capacity of the grate is 200 to 300 kg/m2 in area of the grate per hour

with mechanical draught where as it is hardly 50 kg/m2 with natural draught.

10. It is prevent 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 an high

running and maintenance costs of the fan used. The running and maintenance cost of the

natural draught is practically nil.

3.3 loses in the Air-Gas loop system and its measurements

The total draught required to produce the current of air and to discharge the hot gases

to the atmosphere is the arithmetical sum of all draught losses in the series circuit.

The total draught loss in the air and gas loop system are given by

ht = hv + hb + he + hd

Where ht = Total draught loss in mm of water

hv = Velocity head (velocity of gas exit from the chimney).

hb = Fuel bed resistence equivelent to mm of water head.

he = Head loss in the equipment.

hd = Head loss in ducts and chimney

The details of each loss are given below:

1. 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 a very

important factor. Moreover, the type of coal used is affecting the resistance of the

draught.

2. Heat Loss in Equipments (he)

The manufactures generally supply data for equipment resistance like air heater,

economizer, boiler passes, superheater, etc...

A survey of best data indicates that the draught losses follow a parabolic law. The data

given by the manufactures is supplied at one rating of the complete unit; therefore the

loss at another rating can be calculated by using the following equation

he2 = he1 (s2s1

)

Where n is usually 1.8 to 2

he is the draught loss at the steam generating rate of s .

The draught losses through the boiler setting are accountable for the gases

sweeping over the steam generating tubes, superheater elements that must be made at

the end of each pass. The draught loss varies as the square of velocity of gas flow or

square of the ratio of boiler rating.

3. Velocity Head loss (hv)

The velocity head loss is always equal to v2/2g . Where v is the velocity at the

exit of the chimney. The draught system is designed to give minimum v2/2g loss but it

must be sufficient to diffuse and mix with the surrounding atmospheric air. Its value also

depends upon the natural air velocity at chimney height. Higher velocity head is

required if the natural air velocity is higher. No general data can be given for such loss. It

is decided as per site of the power plant, air temperature and natural air flow condition.

4. Head Loss in Ducts and Chimney (hd )

The draught loss due to friction in air and gas ducts and chimney can be

calculated by Fanning equation (details of this equation can be found in fluid mechanics

book). The value of hd depends on the smoothness of the duct and Reynold number of

the fluid flowing.

Measurement of Draught

The draught losses in different parts of the boiler plant are measured in mm of

water with of manometer. This pressure may be above or below atmospheric pressure.

The typical draught at different points of the boiler plant measured by U-tube

manometer is shown in figure (3.8).

The measurement of draught is not only to find out the resistance to the air and

gas flow but it also indicates the rate of flow.

Figure 3.8: Measurements of draught

3.4 CHIMNEY DESIGN

Chimneys are made up of steal or brick and concrete. Chimney generally denotes brick concrete

construction whereas stack means steel construction. Under the heading of chimney design,

the height of the chimney, as well as its diameter will be found.

3.4.1 Height of the chimney

The draught produced is the difference between the hot gas column inside the chimney of

height H meters and the cool air column outside the chimney.

Consider the height of the chimney above the grate level H as shown in figure (3.9). The

pressure acting on the grate from the chimney side is given by

HWPP ga 1

And the pressure acting on the grate from atmospheric side is given by

HWPP ga 2

Figure 3.9: Diagram of natural draught.

Where pa is the atmospheric pressure,

wa is the density of atmospheric air.

wg is the density of hot gases passing through the chimney.

The net acting pressure on the grate of the combustion chamber due to the

pressure exerted by gas columns and air columns is given by

P = P2 - P1 (3.1)

as wa > wg

P = (Pa + wa H) (Pa + wg H)

P = H(wa wg ) kgf /m2 (3.2)

The natural draught will be the more effective the higher the chimney and the greater

the difference between the temperatures of the external air and the flue gases.

The densities wa and wg can be calculated by ideal gas laws as follows:

Pv = mRT

Taking m as unity,

t0 = 0 or T = 273ok and P0 = 1.03 kgf /cm

2 ( R=29.27 kgf. /kg.o k )

Then =29.27 273

1.03 104= 0.773 m3/kg

is the specific volume of air at P0 = 1.03 kgf /cm2and t0 = 0 .

The specific volume of air at Tok and P= 1.03 kgf /cm2 is given by

= 0.773 T

273 m3/kg

So volume of m kg of air at Tok

Va= 0.773 m T

273 m3

Where m is the mass of air required to burn 1kg of fuel. Then the weight of chimney

gas per kg of fuel is (m+1) kg .

It is assumed that the volume of flue gases coming out of the chimney is the volume of

air required for the combustion of fuel at the temperature of flue gases (i.e V = Vg at tg ).

Therefore

Vg = Va= 0.773 m Tg

273 m3

And density of air

wa =1

0.773

273

Ta kg/m3

or wa = 1.293273

Ta kg/m3

Also density of flues gases

wg =mg

Vg =

(ma+1)

Vg

= (ma+1) 273

0.773 ma Tg

wg = 1.293(ma+1

ma)

273

Tg kg/m3

Substituting wa = 1.293273

Ta and wg = 1.293(

ma+1

ma)

273

Tg into equation (3.2), the pressure P

causing the draught is given by

P = 1.293 273 H [1

Ta (

ma+1

ma)

1

Tg] kgf /m

3

If this draught is h mm of water as measured by U-tube water manometer, it is given by

h = 353 H [1

Ta (

ma+1

ma)

1

Tg] mm of water

as 1 kgf /m3 = 1 mm of water

If the pressure difference is known, the height of chimney in meters can be calculated.

The actual chimney draught (required chimney draught) is the sum of all friction losses external

to it (i.e losses through boiler, superheater, economizer, air pre-heater, duct work, sharp bends

and chimney), less the effective draught produced by fans in case of artificial draught.

N.B.: Equation for calculation h has been derived for standard atmospheric pressure

(1.03 kgf /cm2). Therefore, densities corrections to both wa and wg must be made

according to the barometer reading. i.e h = 353 H [1

Ta (

ma+1

ma)

1

Tg] is valid at P =1.03

kgf /cm2. If the barometer reading is different from the standard pressure, the

following equation must be used h = H (wa wg ).

3.4.2 Diameter of the Chimney

The velocity of flue gases passing through the chimney is given by

f.g = 29 (H Hf)

Where H, is the height of column of hot gases in metere and Hf is the fractional losses of

height of column of hot gases.

So, velocity f.g = 4.43 (H Hf) = K H

Where K is constant and is given by the following equation K = 4.431 Hf

H

The value of K can be found experimentally.

Mass flow of the gases

Mg = A uf.g Density

So

A = Mg

f.g. g =

Mg

4.43 (H Hf) . g =

From which A, the area of the base of the chimney can be found by:

A =

4 D2 m2

D = 4

Mg

K H . g m

Where D is the diameter of the chimney at the base.

The value of H is given as follow:

H = P

g

Where P = 1.293 273 H [1

Ta (

ma+1

ma)

1

Tg]

And g = 1.293 273 (ma+1

ma)

1

Tg

Thus

H =

1.293 273 H [1Ta

(ma + 1

ma)

1Tg

]

1.293 273 (ma + 1

ma)

1Tg

H = H [(ma

ma+1)

Tg

Ta 1] m

3.5 Power Required for Draught Fans

The draught produced by the fan is hf (= htotal hchimney)mm of water and the volume is

V then the work done by the gas is given by

= hf (kgf

m2) v (m3)

= hf . v kgf . m

Thus

I.HP = hf .v

4500 hp

Where hf is the draught produced by the fan in mm of water and v is the rate of volume

discharged by fan (m3/ min).

Considering the transmission efficiency between the prime mover and fan and mechanical

efficiency of the fan, the break horse power is given by:

B.HP = hf .v

tran. .4500 hp

3.5.1 B.HP of the Prime mover required to run forced Draught Fan

B.HP required to run forced Draught Fan is calculated as follows:

ma is the mass of air supplied per kg of fuel

Mf is the fuel consumption per minute (kg / min )

Mass of air supplied per minute (Ma) is given by

Ma= ma Mf kg / min

Volume of air supplied per minute ( Va) is given by

Va = Ma

a m3/min

= Mf ma

a m3/min

Where a is the density of air ( a= 1.293273

Ta kg / m3 )

Hence Va = Mf ma

353Ta m

3/min

Therefore,

B.HP = hf Mf.ma.Ta

3534500tran. hp

3.5.2 B.HP of the Prime mover required to run the induced draught fan is calculated as follow

hf is the draught produced by the fan in mm of water. Mass of gas handled by the induced

fan per minute Mg) is given by

Mg = Mf (ma+1)

Volume of flue gases handled by the induced fan per minute is given by

Vg = Mg

g m3/min

Where g is the density of flue gases g = 1.293 (ma+1

ma)

273

Tg

Vg = Mf (ma+1)

1.293273 (ma+1

ma)

1

Tg

= Mf ma

353Tg m

3/min

Therefore

B.HP = hf.I .Vg

trans. .4500

B. HP I.F = hf.I Mf .ma.Tg

3534500tran. hp

Example 3.1

A chimney is 28 meters high and the temperature of hot gases inside the chimney is320.

The temperature of outside air is 23 and the furnace is supplied with 15 Kg of air per 1 Kg of

fuel burnt. Calculate:

a) Draught in mm of water.

b) Draught head in meters of hot gases.

Solution:

a) The draught in mm of water is given by :

= 353 [1

(

+ 1

)

1

]

= 353 28 [1

23 + 273 (

15 + 1

15)

1

320 + 273]

= 15.1 mm of water.

b) Draught head in meters of hot gases is given by:

= [(

+ 1) (

) 1]

= 28 [15

15 + 1

593

300 1]

= 23.88 meters of hot gases.

Example 3.2

Determine the height of chimney to produce a draught of 22 mm of water if the flue gases

temperature in the chimney is 290 and ambient temperature in boiler house is20 . The gas

constant (R) for air is 29.26 . . and for flue gases is26.2 . . . Assume

barometer reading is 760 mm of mercury.

Solution:

Let :

= Absolute pressure of gas ( 2 ).

= Volume of gas (3).

= Gas constant.

= Absolute temperature of gas (K).

57

Now, =

Or = then, =

3

Difference of pressure ()

= Height of chimney (H) ( )

= 22 mm of water

= 22 2 (As 1 2 = 1 mm of water )

= 1

=

=

=1.033 104

29.26 (273 + 20)

= 1.2 3

( = = 760 = 1.033 2 )

. =1

.=

=

=1.033 104

26.2 (273 + 290)

= 0.69 3

(Since the draught is usually very slight it is possible to use = )

Height of chimney (H)

=

=22

1.2 0.69

= 43.1 meter.

Example 3.3

Compute the head that I.D fan must develop for the following conditions: Maximum boiler

load equal to 75 , draught loss in duct work equal to 7.5 mmof water head at

75 rating, air pre-heater draught loss equal to 5 cm of water head at 60

rating, boiler draught loss equal to 2.75 cm of water head at 10.5 rating.

Assume that losses vary as 1.8 power of the steam flow rate.

Solution:

Let:

= Draught head develop by the I.D fan (mm).

= Head loss in duct work at the maximum boiler head (mm).

= Head loss in air pre-heater at the maximum boiler head (mm).

= Head loss in boiler at maximum boiler load (mm).

Now, = (

)

= 7.5 of water

= (

)

1.8

= 50 (75

60)

1.8

= 74.715 of water

= (

)

1.8

= 27.5 (75

100)

1.8

= 16.38 of water

Therefore,

= + +

= 7.5 + 74.715 + 16.38

= 98.595 of water.

Example 3.4

A 15 of air supplied per of fuel burnt to the combustion chamber of a boiler using

fuel at 3900 . The temperatures of flue gases and ambient air are 273 and 32. If

the minimum draught required to start the fire is 9.5 of water and the boiler is to operate at

maximum artificial draught of 7 of water, find out the followings:

a) Height of the chimney to produce a natural draught.

b) The reduction in the height of the chimney by using an artificial draught.

c) Diameter of the chimney if the flue gases velocity inside the chimney is 5 .

Solution:

a) The draught is given by:

= 353 [1

(

+1

)

1

]

or = 353 [1

305 (

15+1

15)

1

510]

From which = 226.68 meters

The height of the chimney to produce a natural draught is 226.68 meters.

b) =

= 95 70

= 25 of water.

= 353 [1

(

+1

)

1

]

25 = 353 [1

305 (

15 + 1

15)

1

510]

From which = 59.8 meters

Therefore, the reduction in the height of the chimney as a result of using an artificial draught

can be determined as;

= () () = 226.68 59.8 = 166.88 meters.

c) Now :

=

= ( + 1)

=

42

= 1.293 (+1

)

273

Or, ( + 1) = 1.293 (+1

)

273

42

Or, = 4

353

= 4390015510

60603535= 2.445 meters.

Example 3.5

Specify the diameter and height of chimney for the following service:

- gas per hour at 20000 capacity 32

- Boiler draught loss at 25000 capacity 4

- Duct draught loss 1

- Temperature of air 25

- Gas constant of air 28.95

- Temperature of gas to chimney 340

- Gas constant of flue gases 30.3

- Allowable gas velocity through chimney 10

- Barometer reading 750 .

Solution:

a) Height of the chimney is calculated as follows:

= [ ]

=

= 1.033750

760= 1.0191 2

(As 760 of = 1.033 2 )

= 30.3 . .

= 340 + 273 = 613

Then, =

=

1.0194104

30.3613= 0.5488 3

Also density of air

=

=

1.0194104

28.95(25+273)= 1.1816 3

The draught head is calculated as follows

. = +

= (

)

(Assuming n=2)

= 40 (20000

25000)

2= 25.6 of water

Hence, = 25.6 + 10 = 35.6 of water

35.6 = [1.1816 0.5488]

From which, = 56.258 meters

Hence height of chimney =56.258 meters

b) Diameter of the chimney is calculated as follows

= =

42

Or, = 4

=

432000

60600.548810= 1.436 meters

Hence diameter of chimney = 1.436 meters

Example 3.6

A coal fired boiler uses fuel of the following composition by volume:

Hydrogen = 50%, = 40%, 2= 6% and 2= 4%. The analysis of the products as determined

by an Orast apparatus is as follows:

2 = 15.33%, = 0.7%, 2 = 5.22% 2 = 78.75%.

The rate of fuel burnt is 3200 . The temperatures of flue gases and ambient air are

227 and 27 respectively. The maximum load of the plant is 30 ton of steam per hr. The

allowable height of the chimney is 40 m. The draught losses in the air-gas loop system are as

follows:

- Draught loss in fuel bed 6 of water

- Draught loss in ducts and chimney 0.3 of water

- Draught loss through boiler at 20 0.9 of water

- Draught loss through air pre-heater at 40 5 of water

Determine the induced draught fan B.H if the mechanical efficiency is 70%.

Solution:

Consider 100 of fuel burnt or 100 of products, the relevant combustion equation

is:

[502 + 40 + 62 + 42] + 2 + 79

212

15.332 + 0.7 + 5.222 + 78.752 + 2

Carbon balance:

40 + 6 = 15.33 + 0.7

= 0.34848

Hydrogen balance:

50 =

= 17.424

Oxygen balance:

40

2 + 6 + = 15.33 +

0.7

2+ 5.22 +

2

From which = 20.55 100

The mass of air required to burn 1 of fuel is given as follows:

=.2.100

100..23

Where, = Molecular weight of fuel ( )

= (where = ).

= 22 + + 22 + 22

= 0.5 2 + 0.4 28 + 0.06 44 + 0.04 28 = 15.96

Hence, =20.5532100

1000.3484815.9623= 5.14

The draught head produced by the chimney ()

= 353 [1

(

+1

)

1

]

= 353 40 [1

273+27 (

5.14+1

5.14)

1

273+227] = 13.334 of water

The total draught required

= + + +

= + + (

)

2

+ (

)

2

= 60 + 3 + 9 (30

20)

2+ 50 (

30

40)

2= 111.375 of water.

The draught head produced by the fan ()

= = 111.375 13.334 = 98.041 of water

B.H of induced draught fan =...

3534500

= 32005.1450098.041

6035345000.7= 12.1 P