Different Pressure Streams Mixing 3

[scMechE]

Hello Everybody!

At my job i`m designing the flue and chimney system for two different boilers (one is 125 BHP and the other one is 50 BHP). We want it to be a common system for both boilers. The problem is that, since the boilers are different, the flue gas for each one has different pressure at outlet.

The two flue gas streams are going to mix in a 45º wye and then the resulting stream will flow through the stack. In the attached file you can see the details.

So, my question is, can these two streams mix? Is it possible? Because i guess that the highest pressure stream will exert a back pressure over the lowest pressure stream, and they won't mix.

A workmate affirms that these mixture is possible because the high pressure stream enters to the common pipe with an angle, so it won't produce a back pressure over the low pressure stream, intead of it, a vaccum will be created over this stream an will pull it, creating of this way the mixture.

I would like to know if mixing this two flows is possible. If so, how can i calculate the resulting presure?

Thanking you in advance, Best Regards!!

 

[GHartmann]

You must start with the destination pressure (exit stack?).

Then calculate the pressure drop to your "Y" connection.

This will be the outlet pressure for both your boilers.

Remember the pressure is set by the downstream pressure and friction (dP) in the ductwork and nto necessarily by the boilers theirselves.

Good luck!

 

[scMechE]

Thanks GHartmann for your answer, i really appreciate it.

I understand what you mean about the downstream pressure and the pressure drop: this will determine the fluid's pressure right after it passes through the wye.

Since the boilers are packaged units, their blowers will introduce the combustion air into the furnace and push the flue gas out.

Now my question is, Does this implies that both boiler's blowers will have the capacity to push the flue gases of each boiler with the same pressure, making the mixture of the two streams possible and so the resulting stream pressure would be the destination pressure minus the pressure drop along the fluid's path?

Best Regards!

 

[prex]

The short answer to your question is: yes, of course they can mix and go out of the chimney.
To calculate the pressure distribution in the wye you should calculate the downstream pressure as suggested by GHartmann , then assume that:
-each stream will lose (acquire) the dynamic head corresponding to its change in speed (the entrance speed being unknown, you'll go through this by tentative and iteration)
-each stream will also lose, as a net head loss, some 70% of its inlet dynamic head, due to the sudden change in direction (the 70% figure is my guess, if the wye was a tee with center outlet, the loss would be 100%)
This will give you (after some iterations) the inlet pressure for each stream and will allow to check the ability of the blowers to supply the required flow rate.

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[zdas04]

The thing that both previous posters touched on but didn't quite explain is that the discharge pressure listed for the two boilers is the maximum that they can operate at, not the pressure they will operate at. The final exhaust pressure plus losses in the pipe control the pressure they will operate at. The pressure at the entrance to the lateral connection will be the same for each stream, so the only difference in boiler exhaust pressure will be the differences caused by friction in the pipe leading up to the fitting. Since the "high pressure" stream has a much larger mass flow rate, if the pipes are the same size then the high pressure boiler will experience a higher backpressure, but I expect it will not be much higher.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[racookpe1978]

One caution to all of the above.

A siphon effect (the vacuum-like "pull" of the lower pressure fluid flow into the higher pressure flow), or any such mixing of two flows, MUST have "flow" in both pipes or exhaust stacks to be assumed relevant.

Two or more STATIC systems connected by a wye WILL overpressurize the dead-end pipes (either the high flow or low flow pipes and boilers!) regardless of what happens at high boiler load or low boiler heat load. If a natural draft cannot be guarrantted under all circumstances for the off-service boiler (the fire zone of the off-duty boiler actually) then you WILL get deadly exhaust gasses building up inside the boiler and leaking out into the work and serrvice area around the burners.

Wind direction? Hot days, cool days, cool nights, shutdown conditions?

With this, can you always assume the furnace gasses will be "naturally drawn" out safely up and out of both furnaces.

 

[scMechE]

Thanks Everybody!

Where can i find the maximum discharge pressure at the flue gases outlet? Is it calculated or the manufacturer provides this data? I have already reviewed some boiler's manuals but i haven't found this kind of data.

 

[GHartmann]

You will probably need to find the data sheet for the forced draft fan. (But it will probably include the dP through the burner's register). Then you should account for the dP as the gas flows past the tubes, through the economizer (if you have one), and out the boiler exhaust.

Somewhere there should be the normal operating pressure (inches of H2O) inside the boiler.

You should also have a maximum operating pressure for the boiler "membrane" or "casing".

The prior poster did bring up a good point of designing the system to avoid gases from one boiler entering the 2nd boiler when it is non-operational.

You may be to call your vendor to obtain the information you are looking for.

Good luck.

 

[scMechE]

I was thinking about what racookpe1978 says. His point is really important, so should i design the chimney and breeching just on the basis of the natural draft, without taking on count the flue gas discharge pressure at boiler's outlet?

 

[MortenA]

I tried to read through all the posts and didnt feel that they covered my remarks, forgive me if its already been said:

Two fundamental rules govern flow:

1) Flow is towards the lowest pressure
2) You CANT have "different" pressure at the same place (or node when modelling the thing)

So in the smoke stach issue:

Where the two stream meet they will have the same pressure!

Pressure wont flow toward the "low pressure source" because its still higher than the atmosperic pressure at the end of the stack (assuming that the total ambient head in the chimney is not higher that the shut-in pressure of the low P source (fan or whatever).

Unless! the pressure loss in the chimney (including ambient positive og negative head) is higher for the higp P source on its own than the shut in pressure of the low P sorce! This may be the case in start up where the air in the stack is cold (chimeys are not my field im speaking from a general flow knowledge)

BUT your flow rates are the "dependant variable" in this calculation (although its the one you want to control).

This is seen from a 1D perspective. There may be certain "local phenomena" due to actual construction of the chimey etc. where the flow may move against my stated principles. With regards to siphon effct i think this is limited since it requires momentum and the momentum of the smoke moving at a relatively low speed i dont think that can create a low of vacuum.

I tried a simple drawing below. So in general at Tee the pressure is the same. If your dP from Tee to Exit is lower than P2 then your flow will be towards Exit - but your P1 flow may reduce since the fan operates against a higher pressure (moving up its fan curve)

P1 ->-------Tee-------Exit
/
/
P2 ->----

 

[MortenA]

Bummer my drawing came out all wrong

 

[scMechE]

Thank you all once again.

After some days, i finally got the exhaust gases pressure for both boilers. I don't want to publish the values, since someone might use them wrongly, for a case where these values don't apply. So lets just suppose that the small boiler will have a discharge pressure equal to A" W.C. and the large boiler will have 3A" W.C. (3 times the pressure for the smaller boiler).

This shows that the small boiler's blower won't rise the pressure enough, to match the pressure of the exhaust gases of the large boiler. So, should a fan be installed in the low pressure line to rise the gases pressure and achieve the same pressure in both streams, making the mixture possible? Or the so called "Siphon Effect" could do the job of mixing the streams?

Personally, i think that if the fan isn't installed, the low pressure stream will "crash" against the "high pressure" one (which will continue flowing), making the small boiler exhaust gases start accumulating on its discharge duct and after a moment, inside the same boiler. If my explanation isn't clear, perhaps the attached file could do better. I don't thing that this siphon effect would happen, since there is no pressure drop anywhere in the junction that could cause a vacuum, and create the siphon effect. What makes me doubt is that an engineer that works with me, and who has a far larger experience, affirms that this siphon effect is possible just because the "high pressure stream" is introduced in the main duct with an angle, so it will pull the other stream.

Am i right, or not? Thank you!

 

[GHartmann]

The boiler manufacturer cannot determine what the outlet pressure of his boiler will be without knowing the exhaust duct design. He can indicate what pressure his outlet gas can overcome. I believe this is what he is trying to convey.

What most of the posts were trying to point out is that the pressure is set by the outlet ducting pressure drop.

Lets suppose you design your ductwork (very large) such that the pressure drop is A" to your "Y" connection. Then both boilers will be able to vent into this system.

What I would check, rather than adding an additional fan is see if the small boilers fan can utilize a VFD to get a slight speed increase to raise the boiler flue gas pressure to your 3A"

Again in your skecth, the pressure at the "Y" is set by the outlet duct pressure drop. It is not set by the pressure of the larger boiler. The larger boilers pressure can only be at the pressure set by the dP through the duct.

Perhaps this is what your more experienced engineer is trying to say just differently.

You will still need isolation of the boilers from each other to avoid backflow when one of the boilers is not operating.

 

[prex]

There is a fundamental concept that you didn't grasp up to now: the pressure head supplied by any pumping device is associated with a flow rate on a curve. So what will happen is that the lower head blower will raise the delivered head and decrease the flow rate, and the inverse will occur for the other blower, till the same outlet pressure is reached for both flows.
The limiting case where the larger blower will force the smaller one to turn in the reverse direction, so that a counter flow will occur, is in principle possible but quite unlikely, and easy to determine when you understand how to calculate this system.
Please reread the above posts, what you need to understand has already been discussed.

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