Hi I’m looking into building an orifice style bench. The heads I will be working on will flow about 220cfm when finished, so I have no need to flow over 300cfm. I do have some space constraints so I will be making my own design. It will be based off the standard three chamber design and hopefully be capable of reverse flow.
Currently I have two motors that flow 112cfm a piece. I plan on using them and adding a third or fourth.
1)My question is what size orifice will I need? And do I need multiple?
2)Can I use a digital hand held manometer that’s capable of reading differential pressure up to 60” w.c.? I don’t mind doing some simple math.
3)Will I need an air bypass port with a motor speed control?
4)If I’m failing to see or understand something,
Please let me know
Thinking of making an orifice metering bench
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Tony
- Posts: 1445
- Joined: Wed Mar 03, 2010 3:40 pm
- Location: Melbourne, Australia
Re: Thinking of making an orifice metering bench
1) Orifice size is really a judgement call, there is no one correct answer.
Too small and there will be too much pressure drop across the orifice at max flow, limiting the available max flow test pressure.
Too large, and at low flow there will hardly be any pressure drop to measure at the low flow end.
Forum members after years of experience, testing and discussion have settled on 16 inches of pressure drop across the measurement orifice at full maximum bench flow, and 28 inches of test pressure. That works out pretty well with electronic pressure measurement and one orifice should cover the full range.
If using water manometers, each orifice has a practical flow range of only about 4:1 (16:1 pressure readings). Beyond that its just too difficult to by eye accurately measure the very small increments of manometer rise at the low end.
With electronics you can resolve right down to the last final digit on the display with speed, clarity and assurance.
2) Yes you can use a store bought digital manometer, but the amount of extra work with pencil and calculator really slows down testing, and you will eventually decide to get the Forum digital manometer and software. The software also reduces the possibility of math and rounding errors creeping in. Another advantage is the Forum software allows for the adjustment of smoothing the pressure readings. An off the shelf manometer may give either bouncy jumpy readings if it too fast responding, or be painfully slow to settle to a final stable steady reading if its too heavily damped. The Forum manometer software allows tweaking the damping to some optimum value for fast clear readings.
3) With full motor speed control you don't need an air bypass. If all the motors are controlled together you do not need the one way air flow valves to prevent reverse flow through any motors that are switched off.
I suggest you get a copy of the Forum flow bench plans anyway, even though you plan to design your own bench.
There are a lot of really great ideas that you might like to incorporate in your own bench design. That will also give you access to a part of the Forum that you cannot now see, and you get to see and discuss other peoples projects, methods of construction, tools, materials, etc.
Its well worth it !
Too small and there will be too much pressure drop across the orifice at max flow, limiting the available max flow test pressure.
Too large, and at low flow there will hardly be any pressure drop to measure at the low flow end.
Forum members after years of experience, testing and discussion have settled on 16 inches of pressure drop across the measurement orifice at full maximum bench flow, and 28 inches of test pressure. That works out pretty well with electronic pressure measurement and one orifice should cover the full range.
If using water manometers, each orifice has a practical flow range of only about 4:1 (16:1 pressure readings). Beyond that its just too difficult to by eye accurately measure the very small increments of manometer rise at the low end.
With electronics you can resolve right down to the last final digit on the display with speed, clarity and assurance.
2) Yes you can use a store bought digital manometer, but the amount of extra work with pencil and calculator really slows down testing, and you will eventually decide to get the Forum digital manometer and software. The software also reduces the possibility of math and rounding errors creeping in. Another advantage is the Forum software allows for the adjustment of smoothing the pressure readings. An off the shelf manometer may give either bouncy jumpy readings if it too fast responding, or be painfully slow to settle to a final stable steady reading if its too heavily damped. The Forum manometer software allows tweaking the damping to some optimum value for fast clear readings.
3) With full motor speed control you don't need an air bypass. If all the motors are controlled together you do not need the one way air flow valves to prevent reverse flow through any motors that are switched off.
I suggest you get a copy of the Forum flow bench plans anyway, even though you plan to design your own bench.
There are a lot of really great ideas that you might like to incorporate in your own bench design. That will also give you access to a part of the Forum that you cannot now see, and you get to see and discuss other peoples projects, methods of construction, tools, materials, etc.
Its well worth it !
Also known as the infamous "Warpspeed" on some other Forums.