7 replies to this topic

[Terry T]

If a gas passing through a 10" pipe has a pressure of 15 psig enters a stack that is 36" in diameter. The stack is 80 ft long and is open to the atmosphere. Would the gas as it enters the stack still have a pressure? or would the pressure at the base of the stack be atmospheric.

The 15 psig was calculated right before the gas entered the stack. Can PV=nRT be used to calculate the pressure in the stack after it has been expanded? Also, can the transition from the pipe to the stack be treated as a sudden expansion from 10" to 36" or can it be assumed that the pressure will go to atm as it enters the stack?

I am using the sudden expansion formula from the attached website and it is only giving me a 70% reduction in pressure due to the sudden expansion. So, at base of the stack I am getting 4.5 psig.

However, if i work back from the top of the stack it does not converge. The pressure drop across the 36" stack is only 0.2 psi, then the pressure based on that theory is 0.2 psig.

http://www.thermexce...ourc/pdcgas.htm

Inlet pressure of gas is 95 psig and temperature is 1150 F.

The pipe is made up of a combination of 8" and 10" pipe. The pipe starts as 8" turns into 10" and then ties into a 36" stack.
Here are some details of the piping

Length and Fittings of 8" Pipe
Length - 55 ft
90 deg bends - 3
Valve - butterfly valve
Expansion from 8" to 10"
Expansion joint - 1

Length and fittings of 10" pipe
Length - 50 ft
90 deg bends - 2
45 deg bends - 2
Expansion from 10" to 36"
Expansion joint - 1
T-piece - 1

Based on the above info i have calculated the pressure just before it enters the stack to be ~15 psig.

[Terry T]

The flow of the gas is 135000 lbs/hr and the MW is 27.5

[sheiko]

If a gas passing through a 10" pipe has a pressure of 15 psig enters a stack that is 36" in diameter. The stack is 80 ft long and is open to the atmosphere. Would the gas as it enters the stack still have a pressure? or would the pressure at the base of the stack be atmospheric.

The 15 psig was calculated right before the gas entered the stack. Can PV=nRT be used to calculate the pressure in the stack after it has been expanded? Also, can the transition from the pipe to the stack be treated as a sudden expansion from 10" to 36" or can it be assumed that the pressure will go to atm as it enters the stack?

A quick reply:

I believe you can consider the transition from 10" to 36" as a sudden expansion using the appropriate K-value. But Bernoulli also tells you that, as the velocity decrease in the pipe when the diameter increase, the pressure will also increase accordingly. So you will have to calculate a net pressure drop equal to the difference between the pressure loss (sudden expansion) and the pressure increase (due to decrease in velocity).

Thus, the pressure at the base of the stack will be equal to the pressure drop in the stack + exit pressure loss (K-value = 1).
The pressure before the transition from 10" to 36" would be equal to the pressure at the base of the stack + the net pressure drop I have mentionned above.

This is how I see it just before going to bed.

Edited by sheiko, 02 July 2011 - 04:51 PM.

[EyMiller245]

My take is that compared to buoyancy, the pressure characteristics of the gas entering the stack are sort of a moot point.

I don't really deal with stack gases very much, but if that gas is 1100F and open to the atmosphere...its gonna rise mostly due to buoyancy, regardless of pressure drop from the stack or what pressure it enters at. Usually the only design contsraints I can think of with stacks are making sure the temperature is high enough to eliminate condensation issues and making sure the stack is high enough so the gases dont perturb the surroundings.



Anyone with more experience correct me if I'm wrong, but thats my take on it.

Edited by EyMiller245, 01 July 2011 - 09:11 AM.

[kkala]

Attached "stack.xls" and its graph makes a try to clarify pressure and draft along the mentioned stack, subject to comments (first time doing such calculation).
Pressure drop due to friction along the stack was taken as 0.2 psi. Pressure drop of 4.5 psi along the stack looks too high.
Case was simplified.
Hopefully it is useful, solving some of the queries.

Attached Files

•  stack.xls   22KB   238 downloads

[kkala]

In the previous post I failed to notice that flue gas rate and Molecular weight was given, so gas velocity at stack could be calculated (it was supposed as 15 m/s).
Data is : flue gas rate=135000 lb/h, MW=27.5, temperature=1150 oF, stack diameter 36" (0.9144 m).
So flue gas velocity at stack is u~69 m/s, according to my calculations.
Could you please check? This velocity looks high. I think sound effects in chimney can occur at u~45 m/s, or higher (Gas purification for air pollution control, by Gordon Nonhebel). However it could be acceptable in case of ample available ΔP for the stack, like the case of this thread, accompanied with an acoustic study (prediction of noise to be developed).
Apart from higher ΔP at stack exit (was 0.17 in H2O), calculations of "stack.xls" (attached in previous post) are not practically affected. Frictional ΔP along stack continues to be assumed as 0.2 psi.

Edited by kkala, 07 July 2011 - 01:09 AM.

[sheiko]

Terry,

Same question here...: http://www.eng-tips....d=301657&page=2

[kkala]

Terry, Same question here...: http://www.eng-tips....d=301657&page=2

It is worth while reading the thread referenced above, giving a lot of additional information on the subject.
It is noted that according to calculation there, flue gas temperature (compressible flow) falls from 1150 oF in the 8" pipe to 445 oF in the 36" stack (pressure falls from 85 psig to 5.86 psig in the calculation). Advise on this (high) temperature drop would be useful.
I believe stack base pressure must be in the order of 0.2 psig (140 mm H2O g), not much higher, as calculated starting from stack exit.
Attached "stack.xls" of the post on Jul 05 has to be adjusted, if stack gas temperature is different to 1150 oF.

Edited by kkala, 08 July 2011 - 12:55 AM.