Controling flow with a 3-way valve

I originally posted this question on sci.engr.mech but this might be a more appropriate newsgroup.
INITIAL POST

the
significant
FOLLOW UP POST I am omitting the response I received from sci.engr.mech as I not sure it it is considered proper to copy another's post across different newsgroups. I added the following info to my original post:
A needle valve in the bypass may be a cheap and easy way to ensure I can "tune" the system. The equipment is being installed next week after which I will have 2-3 day to get it up and running. I want to avoid any obvious problems and be prepared for those which are less likely, but still possible.
The current design calls for regulating a continuous flow thru the heat exchanger rather than starting and stopping flow. I don't expect the valve to act perfectly linearly but I want to avoid having the last 10% of valve travel controling 80+ % of the flow range thru the heat exchanger.
One of my application uses a manual 3-way valve which will be set and seldom adjusted. I am not overly concerned with the performance of that system. However I have another similar application where I seek to control process water temperature running thru the other side of the heat exchanger. In this second application the 3-way proportional valve will be actuated thru a 4-20 ma signal from a controller. Here I am concerned that the process may be unstable if the valve is forced to operate over a small fraction of its design range and at one extreme of its travel.
Comment and advice would be of great assistance and much appreciated.
Thanks,
Dave Miller
--
D. Miller
dvm001atrochester.rr.com
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One thing you might consider is a riser on the bypass open to the atmosphere at the top, which has a height equivalent to the anticipated pressure drop through the heat exchanger. This will give a constant pressure on the heat exchanger valve, even at nearly full bypass flow. An orifice or needle valve might work well, but the pressure drop is going to be proportional to the velocity through them, which in turn varies with the bypass valve position.
Your concern that if the bypass was opened far enough that virtually all the water would go that route is founded. By making sure that there is a constant pressure on the valve, you will get better control of the flow through it.
Michael

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Dave,
I don't think you have a problem at all. What you are describing is common practice with one difference: It is better to put the 3-way at the outlet of the exchanger. In this way the exchanger is at a slightly higher pressure and is slightly better at avoiding problems associated with boiling.
Your concern that there is no resistance on the bypass side is unfounded. The three way valve, when partially open, itself adds the necessary restriction to each side. Another common approach is to put a control valve only on the bypass line. In that case it is often necessary to put a manual throttling valve on the exchanger side to provide some driving pressure to the bypass. A 3-way valve combines these two valves into the same body.
You might find the article "Controlling Shell and Tube Exchangers" found at www.driedger.ca interesting although it is aimed at larger equipment where 3-way valves are usually not practical. The use of two, linked valves is exactly the same.
As far as I know, newsgroup posts are essentially public. I, myself, have never hesitated in transferring stuff wherever I wanted to.
Walter.

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