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 Is there a formula to calculate flowbench plenum size? 
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Joined: Mon Feb 23, 2015 6:10 am
Posts: 269
Location: Northern NSW, on the wrong side of the Great Divide.
Is there a formula to calculate flowbench plenum size?. For example the PTS-8 (600 cfm) has a plenum area (on either side of the orifice plates) of xx cubic foot, someone (in this case just say me) wants to build a bench to flow 1200 cfm. In order for the plenum to not be a restriction it will need to be a certain size (yy cubic foot) so I am wondering if there is a formula to calculate this or is it all trial and error?

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Wed Oct 18, 2017 8:56 pm
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Joined: Wed Aug 25, 2010 7:54 am
Posts: 428
Location: Riverina Australia
No formula that I know of. I don't think it's critical as long as both chambers are similar size ( beta ratio ). But personally I think the pts 12 is big enough. If you couldn't get 1200@28 to flow through one, it wouldn't be because of the plenum sizes on the orifice side.

Jim

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Wed Oct 18, 2017 9:46 pm
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Joined: Fri Jan 08, 2010 10:36 pm
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Location: Maryland
Plenum and settling chambers require some thought, the only formula you need to keep in mind is Beta Ratio (orifice ID to outer wall) if this goes above 25% then it can effect linearity in the flow numbers and need to be included in the flow math.

When you are talking about flowing Large volumes of air, orifice size plays a role as the smaller the orifice the more velocity the air around it has and the the more effect things in the path of flow around the orifice can disrupt this flow and accuracy of readings.

If i was going to build a bench that i would be flowing in the 1000 CFM range on a regular basis i would have Bruce machine me special orifice plates that would be based on a larger bolt pattern than what we use in the 8 motor bench. I would want to be able to use a 5.25 to 5.5 or even 6 inch internal plate so i am not maxing out my manometer scale thus keeping the velocity down and reducing my chances for errant turbulence that could effect readings.

Last think about what you are going to flow, how is it going to attach to the bench? is the bench opening going to affect this flow? is the output of say a muffler going to have violent velocity at 1000 cfm pointing into the chamber. You are not limited to the 5" 6 x 6 bolt pattern on the top of the bench you can design an aluminum mounting plate that be used to adapt anything you want, Holly flange etc. but still have a standard adapter to allow use of head mounts etc.

Just trying to think outside the BOX ;) here. Remember the goal of the PTS Project was to put a bench together for reasonable cost that would perform as good if not better than any commercial bench on the market and i think today it is quickly becoming the standard. The large bench was an attempt to satisfy a hand full of people who wanted more, and it was put to paper but was never built and tested as the 8 motor bench has been.

Rick


Thu Oct 19, 2017 8:30 am
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Joined: Mon Feb 23, 2015 6:10 am
Posts: 269
Location: Northern NSW, on the wrong side of the Great Divide.
Thanks Jim & Rick

FYI I asked because I am trying to understand the science of bench design more.

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Thu Oct 19, 2017 3:14 pm
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Joined: Fri Jan 08, 2010 10:36 pm
Posts: 1171
Location: Maryland
Let me say this so everyone understands, the PTS bench is a good bench and fits 90% of the people that come to the forum, it is often pushed on members and visitors and it should not be, that said we all do this because we have been through the countless threads of guys building benches out of scrap who knows what? trash cans, old desks, job site ply wood with concrete still on it and countless hours helping to finally see them throw in the towel. My hat is off to these guys trying to make something from nothing and we often loose site of this as it was the foundation to the forum.

So to keep in the spirit of the forum the questions should be asked and the designs and ideas challenged, i personally do not have all the answers i just understand the basic principles so i can quickly cut to the concept and issues that may not be seen in the excitement of building a bench. If you look in the old forum you may find my posts of a 16 motor 1000 cfm Rube Goldberg bench i was planning when the schooling started. I will always remember a comment "Rick why do you want to make it so complicated?" so we all learn and we all grow even Bruce if you don't know he was once a DIE HARD Pito tube bench guy! WTF was he thinking LOL...

Ask the question, push the boundaries, challenge the establishment, But if you come here impatient to learn and looking for the answers to your homework you will get the standard answer "Stop waistling your time; build the PTS bench!" it will save you time and money and frustration.

I still want my 16 motor bench and now i know i can build it if i can find some room in the shop to put it.


Rick


Thu Oct 19, 2017 9:24 pm
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Joined: Thu Feb 25, 2010 3:39 pm
Posts: 715
Quote:
guys building benches out of scrap who knows what? trash cans, old desks, job site ply wood

I resemble that comment! :mrgreen:

Quote:
So to keep in the spirit of the forum the questions should be asked and the designs and ideas challenged,


I agree this PTS bench Has evolved and should continue to do so.
It started as a Pito bench to Ratiometric, changing the round hole at the bottom too a larger rectangle. Bruces new blower access door gasket, too new ideas on the slider plate assembly. Option from Water gauges, too Digital, too full on Computer Control Bench, and probably a lot more changes that i have no clue about.

Rick are you talking about the diameter of the hole of the orfice plate or that you would need more space(wall farther apart from the plate.) or are you talking about the baffle its self?
Quote:
If i was going to build a bench that i would be flowing in the 1000 CFM range on a regular basis i would have Bruce machine me special orifice plates that would be based on a larger bolt pattern than what we use in the 8 motor bench. I would want to be able to use a 5.25 to 5.5 or even 6 inch internal plate so i am not maxing out my manometer scale thus keeping the velocity down and reducing my chances for errant turbulence that could effect readings.


No i have no intention of trying to make this 8 motor bench into a 1000 cfm bench but anything i can understand better will help. Even if it don't apply to this bench or not.

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Gordon


Thu Oct 19, 2017 10:19 pm
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Joined: Mon Feb 23, 2015 6:10 am
Posts: 269
Location: Northern NSW, on the wrong side of the Great Divide.
1960FL wrote:
Let me say this so everyone understands, the PTS bench is a good bench and fits 90% of the people that come to the forum, it is often pushed on members and visitors and it should not be, that said we all do this because we have been through the countless threads of guys building benches out of scrap who knows what? trash cans, old desks, job site ply wood with concrete still on it and countless hours helping to finally see them throw in the towel. My hat is off to these guys trying to make something from nothing and we often loose site of this as it was the foundation to the forum.

So to keep in the spirit of the forum the questions should be asked and the designs and ideas challenged, i personally do not have all the answers i just understand the basic principles so i can quickly cut to the concept and issues that may not be seen in the excitement of building a bench. If you look in the old forum you may find my posts of a 16 motor 1000 cfm Rube Goldberg bench i was planning when the schooling started. I will always remember a comment "Rick why do you want to make it so complicated?" so we all learn and we all grow even Bruce if you don't know he was once a DIE HARD Pito tube bench guy! WTF was he thinking LOL...

Ask the question, push the boundaries, challenge the establishment, But if you come here impatient to learn and looking for the answers to your homework you will get the standard answer "Stop waistling your time; build the PTS bench!" it will save you time and money and frustration.

I still want my 16 motor bench and now i know i can build it if i can find some room in the shop to put it.


Rick
All I asked was "is there a formula to calculate flowbench plenum size?" and as already mentioned in my last post I asked it because I am trying to understand the science more. I never asked anyone specifically and never expected anyone to know everything. I'm an educator by profession and the one thing I know that kills inquisitiveness is a reply that doesn't actually reply to the question. I'm more than happy to receive replies that say something like Jim's, and your other, reply but I am wondering why you made this reply. If you feel pressured by my asking the question I asked please accept my most humble apology because it was never my intent to put you, or anyone else, under pressure. I'm not impatient, I'm not pushy, but I am inquisitive. I tend to ask straight forward questions or at least I think I ask straight forward questions and I do that so things don't go off beam. I have spent my teaching career telling kids that no individual person knows everything about every topic or even about one topic. I tell them it is ok to ask questions. I'm getting the feeling that is not the case here.

Please feel free to reply but I'm done and dusted, I think I'll just lurk for a while now.

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Fri Oct 20, 2017 1:21 am
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Joined: Thu Feb 25, 2010 3:39 pm
Posts: 715
To me it was an interesting post and don't see how you got offended?
Haven't had my first cup of coffee yet thow :?

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Gordon


Fri Oct 20, 2017 8:36 am
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Joined: Fri Jan 08, 2010 10:36 pm
Posts: 1171
Location: Maryland
Storm

Quote:
Please feel free to reply but I'm done and dusted, I think I'll just lurk for a while now.


Wow, Not sure where that is coming from but Oh well I'll keep my thoughts to myself from now on when it comes to your bench build.

Quote:
All I asked was "is there a formula to calculate flowbench plenum size?"
The answer is NO! because it all relates to Velocity and the path the air travels to and form the measurement orifice

The most import is Beta Ratio also know as Beta Factor if you have ready any of the threads on orifice in a tube or pipe there is plenty of Educational material there.

Last and Final words, My response was to all that were reading not just you, but you were the one that moved your personal agenda topic from Malvins thread closed forum to here so i chose to respond here so all would know where i was coming from. That is, the form needs to be more open minded to others bench designs as that is the foundation of where we came from i was trying to encourage your questioning not push it off but i guess since you are in academia and i did not provide a direct answer to your homework you were offended and for that I apologize.

Still looking forward to more on your thread of your bench build

Rick


Fri Oct 20, 2017 1:04 pm
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Joined: Fri Jan 08, 2010 10:36 pm
Posts: 1171
Location: Maryland
Quote:
Quote:
guys building benches out of scrap who knows what? trash cans, old desks, job site ply wood

I resemble that comment! :mrgreen:


Gordon that one was for you.... I always enjoyed that thread.

Rick


Fri Oct 20, 2017 1:05 pm
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