Questions answers & calculation on Boiler draught & Chimney

1-What do you mean by the term draught?

It is the pressure difference, which causes flow of gases to take places.

2-What is the function of draught in Boiler?
  • To supply the combustion air required for proper combustion
  • To evacuate the combustion products from the furnace or combustion chamber
  • To evacuate the combustion products to the atmosphere
  • So draught is the pressure difference between the inside the boiler and outside air. Inside the boiler pressure is due to combustion products (Flue gas) & outside pressure is due to fresh atmospheric air.
3-What are the different types of draughts used in power plant boilers?
  • There are mainly two types of draughts.
  • Natural draught & artificial draught
  • Natural draught is generally obtained by Chimney
  • Artificial draughts have main two types
  • Steam jet: It is again classified into Induced & forced draught
  • Mechanical draught: This is classified into Induced draught, forced draught & Natural draught

4-What do you mean by the Natural draught?

Natural draft is obtained by the use of Chimney. It produces the draft where the air and gas are forced through the fuel bed, furnace, and boiler passes.

5-How do you produce the mechanical induce draft?

It is produced by Induced draft fan

6-How do you produce the forced mechanical draft?

It is produced by Forced draught fan & secondary air fans.

7-How do you produce the balanced mechanical draft in Boilers?

Balanced draught is produced by ID & fans.

8-What are the advantages of mechanical draught?

  • Reduced chimney height
  • Easy control of combustion & evaporation
  • Improvement in the efficiency
  • Prevents smoke
  • Can consume low grade fuel
  • The fuel consumption per HP of mechanical draught boilers is 15% less than that of artificial boiler
  • Grate required for fuel burning is less

9-What do you mean by static draught?

The difference of pressure causing the flow of gases is known as static draught. Its value is very small
10-What do you mean by Static pressure?

It is the potential energy put into the system by the fan. It is given up to the friction in the ducts and at the duct inlet as it is converted to velocity pressure.

11-What do you mean by velocity pressure?

It is the pressure measured along the line of the flow that results from the air flowing through the duct.

12-What do you mean by total pressure?

It is the sum of static pressure & velocity pressure

13-How do you calculate the air flow in the duct?

Volumetric air flow Q m3/sec = Duct area in M2 X Velocity in m/sec

14-How much draft is maintained in the boiler furnace zone?

It is -3 to + 3 MMWC for proper combustion

15-What is the FD air draught loss in Air Pre heater?

It is 50 to 60 MMWC

16-What is FD air draught loss in SCAPH (Steam coil air pre heater)?

It is 20 to 30 MMWC

17-What is the draught loss in FD air in flow element?

It is around 10 to 20 MMWC

18-What is the draught loss of flue gas in Economizers?

Around 70-90 MMWC

19-What is the draught loss of flue gas in APH?

Around 50-60 MMWC

20-What is the draught loss of flue gas in ESP?

It is 20 to 30 MMWC

21-What is the draught measured at ID fan outlet?

Draught measured at 2 to 4 meter from ID fan discharge nozzle is up to +5 MMWC & as you go nearer to Chimney the draught goes into negative

22-How do you calculate the differential pressure of a ID fan?

Differential pressure of a ID fan =Pressure (draught) at suction side – Pressure at the discharge side

23-How do you calculate the differential pressure of a FD fan?

Differential pressure of a FD fan =Pressure (draught) at discharge side + Pressure at the suction side

24-What are the reasons for more positive draught in Boiler furnace?

It is due to:
  • More FD air flow
  • Sudden load drop on Boiler
  • Improper spreading of the fuel
  • Variation of fuel moisture

25-What are the reasons for more negative draught in Boiler furnace?

It is due to:
  • Higher ID fan speed & lower FD fan speed
  • Low load on Boiler
26-Why the FD fan size and power rating is less than ID fan?

Because FD fan is used to handle cold air & ID fan for hot gases (FD air + Fuel). Hot gases have lower density than cold air, hence size & capacity of the to expel the gases is more.

27-What are the reasons for draught losses?
  • Frictional resistance offered by the flues and gas passages to the flow of the flue gases
  • Loss near the bends in the gas flow circuit
  • Loss due to friction head in equipments like grate, economizer, super heater etc
  • Loss due to imparting velocity to the flue gases
28-Why do prefer artificial draught over natural draught?
  • To reduce chimney height
  • To get more draught
  • To reduce fuel consumption
29-What do you mean by the term Chimney?

Chimney is a vertical tubular structure built either of masonry, concrete or steel. Chimneys found application in power plants, textile plants, steel industries, Diesel plant etc. For smaller boilers up to 50 TPH steel chimneys are used and the boilers of capacity more than 50 TPH concrete chimneys are used

30-What are the functions of Chimney?
  • To vent the flue gases produced in the Boiler
  • To produce draught
  • To reduce the load on Boiler ID fans
  • To protect environment by venting the flue gas at suitable height
31-Explain the construction of concrete Chimney?

Chimney is normally cylindrical in construction. It is exposed to hot gases inside & cold air at outside. Inside of the chimney is lined with refractory bricks to protect concrete part from high temperature & erosion by flue gas. Chimney is specified based on its inside, outside diameters & height.

32-How does the draught produce in Chimney?

Draught in the chimney is due to density difference between the columns of the hot flue gases in chimney & cold air outside.

33-How do you specify the Chimney?

Chimney is specified based on its Outer diameter, inside diameter and height.

34-How do you calculate the draught produced in Chimney (Hw)?

Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)
H = Chimney height in meters
Ta = Atmospheric temperature in K
Tg = Flue gas temperature in K
Ma = Mass of air & Mass of flue gas = Ma+1

35-How do you calculate the velocity of the flue gas in the Chimney?

Velocity of flue gas V = √(2XgXH1)
H1 = Head in terms of gas column
Calculated as H1 = H ((Ma/(Ma+1)) X Tg/Ta)-1)

36-How do you calculate the mass of flue gas flowing through the Chimney?

Mg (kg/sec)= Density of gas (kg/m3) X Area of Chimney (m2) X Velocity of flue gas in Chimney (m/sec)

37-In which type of draught flue gas temperature leaving the Boiler is higher?

Flue gas temperature in Natural draught chimney is more as compared to artificial draught

38-Calculate the height of the Chimney required to produce a draught equivalent to 20 mmwc if the temperature of the FG  is 180 deg c and ambient temperature is 25 Deg C.And air required for complete combustion per kg of fuel is 7 kg

We have formula,
Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)
Ta = 273 + 25 = 298 K
Tg = 273+180 = 453 K
Ma = 7 kg
Ma+1 = 7 + 1 =8 Kg
Hw = 20 mmwc
20 = 353 X H (1/298 – 1/453 (8/7))
H = 43.17 meter

39-Calculate the mass of flue gas flowing through the Chimney when the draught produced is equal to 18 mmwc. Temperature of flue gas is 250 Deg C and ambient temperature is equal to 30 deg c. The flue gas formed per kg of fuel burnt are 15 kg.Take diameter of chimney 2 meter

Hw = 18mmwc
Tg = 250 + 273 = 523 K
Ta = 30 + 273 = 303 K
Mass of flue gas Ma + 1 =15 kg
Chimney diameter D= 2 meters
Let us calculate Chimney height H
Hw = 353 X H (1/Ta – 1/Tg (Ma+ 1)/Ma)
18 = 353 X H ((1/303 – 1/523 (15/14))
H = 34.28 m
We have H1 = H ((Ma/(Ma+1)) X Tg/Ta)-1)
H1 = 353 X 34.28 (((14/15) X (523/303))-1)
H1 = 20.94 meter
Velocity of flue gas inside the chimney V = = √(2XgXH1) = √(2x9.81x20.94) = 20.2 m/sec
Mass of dry flue gas Mg = A X V X Density of gas
Density of gas at temperature 250 deg c = 273 X1.293 / 523  = 0.67 kg/m3
Area of chimney A = 3.142 X 22/4 = 3.142 M2
Mg = 3.142 X 20.2 X 0.67 = 42.52 kg/sec

39-Calculate the draught produced in Chimney of height 50 m, if density of the flue gas is 0.8 kg/m3 and atmospheric temperature is 25 deg c.

Density of atmospheric air = 273 X 1.293 / (273+25) = 1.18 kg/m3
Draught produced in Chimney is given by
Hw = H (Density of cold air-Density of flue gas) = 50 X (1.18-0.8) = 19 mmwc

40-How do you increase the draught produced in Chimney?
  • By increasing the height of Chimney
  • By increasing the flue gas temperature
41-How do you calculate the minimum height required for Chimney?

Minimum height of the chimney is calculated based on Sulphur dioxide emission.
H = 14 X Q0.3
Q = Sulphur emission rate in kg/hr

42-How do you calculate the draught pressure for maximum discharge?

It is given by P = 176.5 X H / Ta
Hw = Chimney height in meters
Ta = Absolute atmospheric temperature in Kelvin
Hw = Draught in mmwc

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