MechEng2005
Mechanical
- Oct 5, 2007
- 387
I have a combustion chamber and would like to design duct work for capturing exhaust. I have the volume flow rate (in CFM) of the exhaust gases, size of the exit from the chamber, and temperature of the gases. I am assuming that the gases have the same properties as air (i.e. ideal gas, gas constant). I would like to know how to determine the behavior of the gas after it exits the chamber. The design includes a canopy hood with the bottom about 5 feet above the exhaust from the chamber. The hood has a duct on top which goes straight to the roof to exhaust outside.
Can I just use Bernoulli's equation to the maximum height of the exhaust duct that the gas will still travel through?
P1 + .5*D1*(V1^2) + SG1*H1 = P2 + .5*D2*(V2^2) + SG2*H2
P = Pressure
D = Density of gas
V = Velocity
SG = Specific gravity
H = Height
So if point "1" is at the exit stack, I know all the variables. Then if I say point "2" is outside, and that the gas is now at standard pressure, temperature, density, SG, and static (V2 = 0), then I can solve for the height.
I realize this doesn't take into account pressure losses going into the duct, so perhaps I could determine those based on the above equation with point "2" being the duct entry, determine velocity, which allows me to determine the entry pressure loss. Then I could subtract this from side "1" of my above equation to take this into account.
I am not sure about this approach, as the gas will be traveling through interior (basically static, no draft) air from the chamber exit to the hood. The equation doesn't take into account any changes that my occur there (i.e. mixing). Any thoughts how to at least approximate so I can determine a good design and if an exhaust fan is necessary?
- MechEng2005
Can I just use Bernoulli's equation to the maximum height of the exhaust duct that the gas will still travel through?
P1 + .5*D1*(V1^2) + SG1*H1 = P2 + .5*D2*(V2^2) + SG2*H2
P = Pressure
D = Density of gas
V = Velocity
SG = Specific gravity
H = Height
So if point "1" is at the exit stack, I know all the variables. Then if I say point "2" is outside, and that the gas is now at standard pressure, temperature, density, SG, and static (V2 = 0), then I can solve for the height.
I realize this doesn't take into account pressure losses going into the duct, so perhaps I could determine those based on the above equation with point "2" being the duct entry, determine velocity, which allows me to determine the entry pressure loss. Then I could subtract this from side "1" of my above equation to take this into account.
I am not sure about this approach, as the gas will be traveling through interior (basically static, no draft) air from the chamber exit to the hood. The equation doesn't take into account any changes that my occur there (i.e. mixing). Any thoughts how to at least approximate so I can determine a good design and if an exhaust fan is necessary?
- MechEng2005
