Posted by: lumpidydumpy ® 04/21/2008, 19:01:30 Author Profile eMail author Edit

I am trying to control the flow of air through a 1.5" schedule 80 pipe. I need to be able to control from .5 cubic feet per hour (cfh) to ~1000 cfh. My company has some control valves that we use for this application, but they are only good for higher flow rates. I was thinking that maybe I could use the help of the binary number system and create a bank of orifice plates that have an orifice to control flow for the following values: .5, 1, 2, 4, 8, 16, 32, 64, 128, 256 cfh. Then I could open and close valves strategically to set a flow rate. The question Im having in my head is if I set up the system the way I am thinking (see picture) will my pressure change too much? The orifices are sized by the pressure differential, and if it changes when other orifice plates are opened, then I couldn't predict my flow, right? Does anyone see any flaws in my thinking? Does anyone have any suggestions as to how I might be able to use this concept properly? Thanks for all of your help. * The 2.5 inches water is the pressure inside the pipe...I forgot to type column. It should read 2.5 inches water column. Thanks.   Modified by lumpidydumpy at Mon, Apr 21, 2008, 19:02:59

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: Controlling Flowrates with orifice bank
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Posted by: FerretMK4 ® 05/15/2008, 15:54:47 Author Profile eMail author Edit

Since you have a low head, consider using a "hot-wire" type mass air flow sensor (Omega has one that I used in a cylinder head flow bench). The Omega one has a head that's 1/4" dia and mounts in a 1/4" tube fitting on the side of the pipe. There will be some blockage in a 1.5" pipe so you'd need to put the sensor in a larger section to get a more accurate and linear response, unless you can deal with the fact that at the higher flow rates your sensor will read higher flows than are actually present (based on area). You also have to deal with turbulent vs laminar flow over this large flow range, and may have to fit honeycomb flow-straighteners and for sure must not have any joints or elbows within at least 5 diameters upstream or downstream of the sensor. These sensors have electronic meters or signal conditioners that supply the power and interpret and linearize the signal and provide an output that can be used to control bypass valves or restrictor valves to regulate the flow. Be aware that most air flow measuring systems will have some time delays both in the sensor itself and in the response of the valve(s) and ductwork. If you want fast response you have a challenge.

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Posted by: zekeman ® 04/22/2008, 08:49:26 Author Profile eMail author Edit

I agree with Randy. However you could use your scheme in an open loop where you would need valves in each of the digital lines. You could implement it electronically using an analog to digital converter where you dial in the analog value required and the digital output would go to drivers that actuate the valving.

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: : Controlling Flowrates with orifice bank
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Posted by: lumpidydumpy ® 04/22/2008, 10:23:40 Author Profile eMail author Edit

Exactly!! I was planning on using a simple actuated on/off valve and let software choose the valves based on my set flowrate.

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Posted by: randykimball ® 04/21/2008, 23:03:43 Author Profile eMail author Edit

Hello.. Is it possible you could step into the current century and use a variable speed fan controled by a mass-air flow sensor? These are the simple devices used in many of the car engines to determine how much air is flowing through the intake so the computer can compare other data to it and determine fuel pulse width for the injectors. Then you can select what ever CFM you want. Variable speed motors and controlers are simple, inexpensive, and easy to program these days. Many mass-air flow sensors come ready to provide the 0 to 5 volt out put requested by many motor controls. I haven't looked in a few years but I'm sure there are more and far better sensors with digital readouts and I/O for controling what ever you wish than there were then. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Mon, Apr 21, 2008, 23:10:38

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Posted by: lumpidydumpy ® 04/22/2008, 10:17:18 Author Profile eMail author Edit

I am trying to control the rate of decent of a bell prover with this system. The decent rate is directly related to the flowrate, so I need to be precise. Especially in the low flow rates (.5-25 cfh). So do you know if these fans can control the flowrate? I am tring to find info on them, but it seems like there is no control of air flowing through them. Am I incorrect on this?

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Posted by: randykimball ® 04/22/2008, 16:17:07 Author Profile eMail author Edit

No...but.. Your flow rate is measured at the mass-air flow sensor and it feeds a device that controls the motor speed. So you have a closed loop... meaning your flow is maintained at the setting you set.... automatically. This is because it increases or decreases the RPM acordingly to results at the sensor. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

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