Controlling on/off valve

[desA]

Hi everyone,

I'd like to ask assistance in solving a technical/control issue for a system I'm working with. I'm a mechanical design engineer, new to the hvac industry & very inexperienced on the practical control side. So, please bear with me.

Concept:
Open the valve, when the compressor operates
Close the valve, when the compressor is off.

I've included two sketches below of my thoughts.

http://i39.tinypic.com/1zeywjd.jpg

http://i41.tinypic.com/2ljkyom.jpg

Technical questions:
I'm looking for a budget, simple way on how to do this.
What type of valve?
How to convert the compressor ON/OFF to a valve OPEN/CLOSE operation (controller, switch?)
Can the compressor activation signal (controller, or otherwise) be used to activate the valve? If so, how?

Help from the forum guru's would be ever so gratefully accepted. Thanks folks. :D

[Yuri B.]

In accordance with the information you have provided, switch the drive of the valve through an additional NO (normally open) contact on the compressor's contactor. Unless you describe the system the compressor and the valve are parts of, I suspect, nobody will give you a better advice.

[littleyapper]

yuri b is spot on .... end of thread

[desA]

In accordance with the information you have provided, switch the drive of the valve through an additional NO (normally open) contact on the compressor's contactor.

Thanks, Yuri.

So, can I take it that you're saying that you can drive a solenoid valve via an NO (normally-open) contactor from the compressor?

This would make good sense - nice & simple.

I'd been trying to work through having to use a lower-power control circuit etc, but a direct power contact from the compressor would seem to be very elegant.

[Yuri B.]

On simple refrigeration systems, working without pump down on stop, contactor, starting the compressor, energizes the liquid solenoid valve - on stopping, closes it. Let the valve in your system, no matter what intended for, be just controlled similarly.

[desA]

Thanks, Yuri... :)

[frank]

Using an additional contact on the compressor contactor will open the valve whenever the contactor is energised, even if the compressor is not running, for what ever reason.

A better way to do it to ensure that the compressor is actually running before opening the valve is to use a current sensing device on the compressor wiring

http://www.electrocontrols.co.uk/datasheets/Q01.pdf

[desA]

Thanks very much, Frank.

That is a very elegant solution, which will be sure to also be useful in a whole host of applications. Excellent.

[Yuri B.]

Well. That a compressor draws current - even its nominal one - still does not mean that it is working properly (leaky compression valves, phase changed on a screw comp, etc). Besides, current control is rather expensive: current transfoormer + microprocessor + relay output. More reliable and cheaper would be a simple mechanical thermostat on the discharge pipe, near the condenser input (set, let's say at 60 C). A closed circuit - NO contact on the comp contactor in sequence with the contact on the thermostat - would be a secure cofirmation that the compressor works (and makes compression).

[Magoo]

Looking at schematics reminds me of a "Carrier" liquid feed control to heat exchanger TEV. Simple liquid control and reduced cost of liquid solenoid valve. When compressor stops the pilot three way solenoid interruped the external equalizer line and injected HP liquid pressure on top of TEV and slammed TEV shut, simple shut off valve.
Old is not necessarily wrong.
magoo

[desA]

Well. That a compressor draws current - even its nominal one - still does not mean that it is working properly (leaky compression valves, phase changed on a screw comp, etc). Besides, current control is rather expensive: current transfoormer + microprocessor + relay output. More reliable and cheaper would be a simple mechanical thermostat on the discharge pipe, near the condenser input (set, let's say at 60 C). A closed circuit - NO contact on the comp contactor in sequence with the contact on the thermostat - would be a secure cofirmation that the compressor works (and makes compression).

Thanks again, Yuri.

The aim is to keep the solution as simple & inexpensive as possible. The series solution of NO compressor contact & thermostat is good, as it also brings in additional temperature control functionality - at a reasonable on-cost.

Excellent. :)

[desA]

^^ Thanks for your wisdom, Magoo.

So many ways to solve this case.

[bowral fridgy]

You're a bit wooly with the description but it seems you need a solenoid operated valve energised from the compressor control circuit

[desA]

Thanks 'bowral_fridgy'.

There have been some excellent suggestions & I'm now fairly comfortable in a useful solution.

I purposefully left the description a little 'fuzzy' as it could apply to a number of devices.