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?
Advantages:
- 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
Given
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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