air ingress calculations
TRANSCRIPT
Lecture 26
Contents:
Exercise‐1
Exercise‐2
Exercise‐3
Key Words: Fluid flow, Macroscopic Balance, Frictional Losses, Turbulent Flow, Venturimeter, Orifice Meter, Pitot Tube , Stack, Chimney, Draft, Natural draft
Exercise ‐1
Calculate velocity and flow rate of air leaking through an opening of rectangular cross section in a furnace wall (as shown below in the figure) from the following data:
Figure 1: Air leakage through furnace opening
Cross section of the opening 0 .10 m 0.15 m
mm water
.52 m
Assume turbulent flow and f 0.0064
Draft across the opening 1.5
Wall thickness 0
Friction losses due to contraction and expansion are 0.5 and 1 respectively.
N:
rgy balance at plane 1 and 2
Δ z 0 tal
M 0
V V
Air is at 1.0133 10 N/m pressure and 298 K temperature
SOLUTIO
Applying ene
P P (Atmospheric pressure)
Since opening is horizon
No fan
0 at both planes 1 and 2 0.
Velocity
F (1)
d 0 d
Or
ρ F (2)
V 2f LD
ef ef (3)
is velocity of air in duct. De is equivalent diameter
De . .
V
.0.12
d 1.5 9.860 10 ρ air 1.19 kg /m
Substituting the values in eq 3
12.43 0.805 V
Air flow rate 0 5 9 m /s.
Now we can show that the flow turbulent.
Re D V µ
V 3.93 m/s .
.0
is
0.3 10 ; the w is turbulent.
A brick chimney 3.5m insi 45m high is to handle flue gases (average molecular weight 30) at 603K. The atmospheric pressure outside the chimney is 734mm Hg and outside air is at
y be assumed that the gases do not cool as they rise in the chimney. Make the necessary calculations and prepare the following graphs:
flo
Exercise‐2
de diameter (round) and
300K. It ma
a) Draft at the bottom of the chimney vs. flow rate of waste gases and
b) Horsepower equivalent of the flow energy available for draft at the bottom of the stack vs. florate of flue gases, Ignore the losses due
w to contraction and expanses of gases.
chimney flow rates from 0
to the flow rate at which the available draft at the bottom of the chimney is nil
ere T is in K.
Solu
a) M
For both the plots an a and b, the graph should cover the entire range of
Use f 0.0455 Re . and Viscosity of gas 19.3 10 T . g cm s wh
tion
echanical energy balance for flow of gases
g z z z z g D F 0.
ρ 0.586 kg m and ρ air 1.178 kg m
Substituting values and after simplification
Draft 261.33 ρ F
F 2f LD
V 2 55 V Dµ
0.04. L
DV
Putting V Q D
and µ 3.23 10 kg m s and other values of variables we get.
Draft 261.33 – 1.27 10 Q .
We note at Q 0, draft 261.33.
895 draft 0
b) Flow energy in W 261.33Q – .27 10 Q .
This equation shows that flow energy ill be maximum at Q 505.95 m /s. and zero at Q
895.119 .
And at Q m s
1
w
Exercise‐3
A brick flue must be designed to discharge 425 m /min and 1 atm) of flue gas from furnace to nd the four sharp 9 ds (L/D for one
sharp bens is 20). e flue is rectangular in cross section with a 2:1 ratio of height to width. The average temperature of the flue gas is 350 degree C.
e following:
(300K astack. The flue is horizontal with a total length of 100 m 0 degree ben
Th
Calculate th
a Pressure drop in mm water to be ex e internal cross section of the flue were 120cm 60 cm,
b) Energy consumed by friction in the flue (watts) c) What would be the minimum cross‐ sectional
pected if th
dimensional of the flue if the pressure drop is limited to 2.5 mm of waste gases.
e following values:
ular weight s 29, e 0.4 and e 1.0
Uni 455 Re .
Viscosit here T is in K, 1 N/m 0.102 mm of water
e gives
Use th
Molec of flue ga
versal gas constant 8314kJ/kg mol K and f 0.0
y 19.3x 10 T . g cm sec , w
SOLUTION:
a) Mechanical energy balanc
P P ρ V ef 4f LD
e
DL L
D4 L
D D 80 cm.
.80 205
V 20.4 2 and f 0.00371
ρ 0.563 kg m
Substituting the values we get
P P 521.4 N m⁄ 53.18 mm H O
b) Energy consumed by friction W F.
Where F V
2
ef 4f LD
ef .
Energy 0.563 . . 4.4422
7670W.
c) Let the height of the rectangular cross section is h Width h/2.
De
Mechanical energy balance
P P 2 LD
f V .
Substituti
..
ng the values
. 2 0.0455 . . 0.7
h 2.46 m
Cross section of flue 2.46 1.23m
ASSIGNMENT:
ity and flow rate of air leaking through an opening of rectangular cross section in a furnace wa shown below in the figure) from the following data:
Figure 1: Air leak ge through furnace opening
Cross section of the opening 0 10 m x 0.15, Draft across the opening 1.5 mm water
Wall thickness 0.52 m, Assume tur ulent flow and f 0.0064
Friction losses due to contraction and expansion are 0.5 and 1 respectively.
5 m /min (300K and 1 atm) of flue gas from furnace to stack. The flue is horizontal with a total length of 100 m and the four sharp 90 degree bends (L/D for one sharp bens is 20). The flue is rectangular in cross section with a 2:1 ratio of height to width. Th average temperature of the flue ga
Pressure drop cross section of the flue were 120cm 60 cm,
e (watts) f) What would be the minimum cross‐ sectional dimensional of the flue if the pressure drop is
of flue gas 29, e 0.4 and e 1.0, Universal gas J
Solving, we get,
width 1.23 m
1) Calculate velocll (as
a
.
b
Air is at 1.0133 10 N/m pressure and 298 K temperature
2) A brick flue must be designed to discharge 42
es is 350 degree C.
Calculate the following:
d in mm water to be expected if the internal
e) Energy consumed by friction in the flu
limited to 2.5 mm of waste gases.
Use the following values: Molecular weight
constant mol K and f 0.0455 Re . , Viscosity 19.3x 10 T . g cm sec , where T is
in kelwin, 1 N/m 0.102 mm of water.