Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: electrically operated valves
Summary: A point about digital flow control using valves...
Message-ID: <3171@kitty.UUCP>
Date: 21 May 89 02:26:24 GMT
References: <23526@shemp.CS.UCLA.EDU> <11170007@hpfcdj.HP.COM> <631@corpane.UUCP> <137@enuxha.eas.asu.edu>
Organization: Recognition Research Corp., Clarence, NY
Lines: 32

In article <137@enuxha.eas.asu.edu>, kluksdah@enuxha.eas.asu.edu (Norman C. Kluksdahl) writes:
> It works like this-- take 3 or 4 (or more) electrical ON-OFF valves, and
> connect them in parallel to a LARGE inlet manifold.  The key is to have 
> very little pressure drop from the first valve to the n'th valve, and to have
> lots of capability for flow.  Follow each valve with a manual valve, which
> you then adjust for a given flow.  The first manual valve is adjusted for
> n gallons per minute, the second for 2n, the third for 4n, etc.
> By selecting which valves are on, you can control the total flow.  The output
> of the manual valves is then fed to an output manifold, again large.

	For the above scheme to work on an _accurate_ repeatable basis
without the use of a flow rate feedback sensor, it is necessary that the
pressure on the inlet valve manifold be regulated.  In addition, error
will be introduced if the "backpressure" presented to the outlet side
of the valve manifold changes - as a result of increased flow and/or
other conditions caused by delivery of the fluid.

	The point to bear in mind is that flow rate across an orifice
varies as the square root of the differential pressure across the orifice.
You can calibrate an orifice (i.e., an adjustable valve) to deliver a
given flow rate ONLY for a given differential pressure; if this differential
pressure changes due to inlet pressure and/or outlet backpressure conditions,
so will the flow rate.

	In industrial/chemical process applications, flow rate is never
set solely through a valve without feedback either from a flowmeter, or from
an ultimate process variable (like temperature, in a mixing application).

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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