I originally posted this question on sci.engr.mech but this might be a more appropriate newsgroup.
INITIAL POST
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 return line from the heat exchanger outlet to the
>chiller.
>
>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 without a
significant
pressure drop across the bypass 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.
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