Will my bypass circuit work with my heat exchanger?

I am pumping chilled water thru a heat exchanger and need to control the
flow rate. My flow circuit has a modulating three-way diverting valve
that can direct all the flow thru the heat exchanger, all the flow thru
a bypass around the heat exchanger or it can divide the flow between
both the bypass and the heat exchanger. I am concerned that the way I
have plumbed it might not provide good flow control.
The chiller supply line enters the 3-way valve at inlet port A. Valve
outlet port B runs to the heat exchanger inlet and valve outlet port C
tee's into the line which runs from the heat exchanger outlet to the
chiller return line.
I think I may need a needle valve or orifice in the bypass line to
ensure that the pressure drop thru the bypass line is always greater
than that thru the heat exchanger. I am fearfull that when the 3-way
valve is splitting flow between the heat exchanger and bypass that the
pressure will equalize across the heat exchanger and I will lose flow
thru it if there is little or no pressure drop in bypass line. This is
because the bypass connects to a common return line rather than using a
separate return line. I don't want to add a second return line but wish
to find a way for this general arrangement to function.
Any advice would be greatly appreciated.
Dave Miller
Reply to
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You write as though stopping the flow through the heat exchanger is a bad thing. This is after all, the basis of a bang-bang controller. Still if the valve cannot succesfully modulate the flow as is, providing an impedance in the bypass will give the valve a better chance to spread the flow two ways, I suppose.
If it will work as is, keep it simple, naturally.
Brian W
Reply to
Brian Whatcott
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.
Thank you for your response. I'll search the web for info on "bang-bang controller" wrt temperature control.
Reply to
Dear D.Miller:
This refers to the sound of a solenoid turning off and on. You should get a pretty good temperature control this way, with a couple of degrees of swing off of nominal, depending on loading.
There was a fellow that passed through here about three months ago that had his heat exchanger "silt" up because (among other things) his through flow was too low. If your water quality is good, then you may not have the same problems by metering the flow down.
David A. Smith
Reply to
N:dlzc D:aol T:com (dlzc)
Dear D.Miller:
If you've got the controller, and presumably the valve to match, controlling it in on-off (bang-bang) fashion would wear it out quickly.
PWM is not going to work with a 4-20mA controlled valve. PWM works by controlling the position of an on-off valve "spool" by metering its position directly
Just implement it as you have the parts for, or return them. Use a check valve with a 10 psig (or whatever) spring on your bypass, to give you enough differential pressure. Just try and implement a 20 mA pulse every so often, to keep things stirred up. No reason you should have the same problems as the other guy, because you likely have a different system.
source -> tee -> control valve -> exchanger -> tee -> out then from tee to tee on bypass (my viewer does proportional fonts, so I don't do ASCII diagrams)... tee -> check valve with arrow (->) -> tee
If you add a flow control valve to *this* bypass circuit, be sure to put it *downstream* of the check valve. The vorticity induces by the flow control action will wear out the check "poppet" if you put it upstream.
David A. Smith
Reply to
N:dlzc D:aol T:com (dlzc)
This is a common system. If the heat exchanger pressure drop on the cooling water side is more than a few psi it might help to have a manual balancing valve in the bypass line. This will force the 3-way valve to operate over a wider range of stroke.
Reply to
Bill Sc
I don't like valves to balance bypasses, they tend to get "adjusted" accidentally unless they are locked off. From the heat exchangers data sheet it should be possible to calculate an equivalent " hole " and a simple plate orifice can then be fitted into the bypass.
Many diversion valves have a large central zone where they present little restriction to either flow path, you may well find you have a central zone giving no real control, with the two end zones being very sensitive.
Pulse width modulation from a PID controller works well for slow reacting systems, but may not be suitable for continuos processes.
-- Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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Reply to
Jonathan Barnes

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