Chiller pump down

[otrotabi]

We are servicing a couple of glycol chillers with R404A. One of them randomly pumps-down at start up. There´s a solenoid valve (Emerson 240RA9T7T 7/8") and a TCLE thermOstatic expansion valve.

All works well except sometimes during start up the chiller pumps down, actually due to the Low Pressure delay, the chiller stops when pressure goes down to 0 bar.

We are wondering what might be the cause. Is it ok to assume a malfunctioning thermostatic valve can not be the cause because pressure goes really low ? In that case, what may cause a random solenoid valve functioning ?

[redroge]

Is there a low pressure bypass timer at start up ?

[seanf]

Whats the ambient temp, gycol temp on start up, and is the thermostatic valve covered in ice when it faults on starting up?

[otrotabi]

Is there a low pressure bypass timer at start up ?

Yes. It is set it for about 20 seconds. This is why pressure goes so low sometimes.

[otrotabi]

It´s hot here now, about 35 to 40 Celsius. Chillers start when glycol temp is -3º and stop when they get to -5º.

No, there´s no ice on the thermostatic valve that I was able to notice.

However my main question is: is it possible for a thermostatic valve to close liquid line 100%, causing pump down ? I know electronic valves can do it, but mechanical valves are not meant to do that, but I wonder if soemthing goes wrong with the orifice that could be the case.

We have already replaced thermostatic element (basically because it is far easier to do than replacing the solenoid valve) but I am wondering whether we just lost some time doing that.

[otrotabi]

So, as a follow up for this topic, I found this article: https://blog.parker.com/site/usa/en-US/details-home-page/hvacr-tech-tip-how-to-correctly-size-solenoid-valves-us where it states that : Grossly oversized valve which will not have the minimum 1 psi pressure drop and its disc/piston/diaphragm will NOT return to its normal position.

However in this case, normal position is closed and the valve does close, the problem is it will not open. From an analysis of how these kind of valves work, an oversized valve would probably not close properly, but is should open.

Are we wrong in our undestanding ?

[redroge]

Is the solenoid valve energised ?
Is it energised when the compressor starts or before ?
Daft question is it in the right way round ?

[Grizzly]

Hi Otrotarbi.
I may be completely off track.
What model do you have please?
I would be checking the Glycol flow circuit for restrictions. if Not!
Is there anything else that could cause a Suction Pressure drop?
Basically i am intrigued to know what chiller System you have.
Good luck grizzly

[otrotabi]

Is the solenoid valve energised ?
Is it energised when the compressor starts or before ?
Daft question is it in the right way round ?

The chiller does work. Sometimes the symptoms I describe happen. Yes, I checked the electrical circuit and evetyting seems normal.

[otrotabi]

Hi Otrotarbi.
I may be completely off track.
What model do you have please?
I would be checking the Glycol flow circuit for restrictions. if Not!
Is there anything else that could cause a Suction Pressure drop?
Basically i am intrigued to know what chiller System you have.
Good luck grizzly

Yes, we did check glycol flow, there are two different flow switches on the circuit so no flow seems to be out of the question.

They are custom made industrial chillers with everything brand new. S&T heat exchanger, Copeland Discus compressor and it works 9 times out of 10 starts. It is a little puzzling the way the liquid solenoid valve behaves anyway. We will remove the solenoid piston next time, there is a big suction accumulator on the circuit so there is no actual risk of liquid floodback. Images allowed in this forum need to be small, but maybe you can get an idea of what it looks like.


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