View Full Version : Low pressure transducer alarm?

Daz man
24-05-2013, 02:56 PM
Afternoon folks,

I had an AL012 alarm flag up on an RV1802 Aermec chiller last week.

AL012 = Low pressure transducer alarm. On circuit 2.

Circuit 1 chugging along fine with no faults recorded.

Carried out the obvious and reclaimed the existing R407c charge to start with to find circuit 2 was 15kg short of charge. Data plate claiming it should have a 72kg charge. pulled out I got 56.7kg.

After an extensive leak test with Helium/Nitrogen mix (fabulous stuff by the way) I found a couple of leaks around the TEV.

Vacced out/down to roughly 3 torr.

Re-charged with correct charge 72kg of R407c and left operating.

worked fine for two days but yep, you guessed it, the same alarm reared its ugly head again.

Any ideas? Was thinking maybe a ****ie transducer? (Pic attached) are these on schraders or has the gas gotta come out again if it is iffy?

Help would be appreciated.

24-05-2013, 03:45 PM
According to alco tech documents it has a core depresser so hopefully it has been fitted to a Schrader valve. With thanks to Google. ;)

Daz man
24-05-2013, 03:57 PM
Thanks Brian. Do transducers fail or are they reliable pieces of kit?

24-05-2013, 04:12 PM
Hi Daz man.
I may be wrong but I think that is a low pressure switch and not a transducer.
Obviously a continuity check across would confirm.
I will see what I can dig up for you.

24-05-2013, 04:42 PM

Hi Daz Man.
Yep, I was correct and so is Brian!

The attached does indeed refer to a schraeder connection.
Only hope it is not like some of the York units I have looked at in the past, where the depressor is in place but no schraeder in the fitting.

Daz man
24-05-2013, 04:46 PM
Cheers Grizzly. The actual fault which displays on the Carel interface states its a transducer but that could just be a default text?


Just fired the chiller up again by turning off circuit (stage 1) and left stage 2 running it appeared fine. Carel B probes were reading as follows 2.9 bar suction 16.4 head!

Just seems strange how it will operate for a while then just fault out on lp?

On Monday I will try operate compressor stage 2 on its own all day to see if it stays in. It only seems to fault out at the moment when both stages are initiated.

What would happen if the water in the plate heat exchanger was getting to cold? Just a thought?

Daz man
24-05-2013, 04:48 PM
Will treat with caution! If in doubt out comes the gas!

24-05-2013, 06:26 PM
What would happen if the water in the plate heat exchanger was getting to cold? Just a thought?

If you have a poor / low water flow then the control circuit could overshoot and one of the resultant issues can be LP.

You could always isolate the comp / suction line and reclaim into the evap / liquid line / condenser / receiver..
Don't know your chiller.

But I have pulled down / pumped down, a discharge / suction line and as the internal pressure just rises to positive whipped off a faulty unit and on with the new.
If the internal schraeder is missing?
Because of the slight positive pressure you don't introduce any non condenseables.
Job done.

Not ideal, but a practical solution to a sometime huge job for such a small switch.

mr cool
24-05-2013, 09:08 PM
I have had these switches fail on me on many an occasion! Some times they can wander off from the pre set value, on some there is an adjustment via an Allen screw underneath a rubber cap, but if its been correctly set once, ran for time and then starts playing up I would just rip it out and replace, and more often than not with anything else than another alco one! (I am not a great fan of them)

Daz man
24-05-2013, 09:18 PM
Thanks Mr Cool! Appreciate that.

Daz man
24-05-2013, 09:22 PM
The only obvious things in the circuit which could cause this is -

isolation valves - checked to find open, liquid line core driers - replaced, solenoid valve - proved by putting a lifter on (still faults out on lp), TEV - appears o.k.

Gotta be an iffy transducer!

Daz man
24-05-2013, 09:39 PM
Also may have no relevance but as part of ongoing maintenance works the water strainer has been removed to check for any obstructions to find o.k. In clean condition

25-05-2013, 08:50 AM
I have had a major issue with a Trane scroll chiller once, where it was tripping on LP because of exactly what Grizzly suggests. In our case it was the CHW bypass valve operating too slowly, so the flow switch didn't cut out but the water temp dropped like a stone, causing the transducer & control module to 'think' there was an LP fault.
That was in warranty, but it's not impossible you have a similar issue here - only the chiller had been replaced in our case. Of course it was all our fault, until we eventually had the entire system data logged & proved our suspicion about the bypass valve.
Try changing the transducer & if that doesn't work, look carefully (data log maybe) your water & refrigerant conditions.

25-05-2013, 08:57 AM
plz. check the low side pressure with pressure gauge if less then you charge the refrigerant. if refrigerant are OK then you change the low pressure transducer.

25-05-2013, 09:03 AM
Hi Daz.
Strainers, Process usage, primary and secondary chilled water circuit pumps and pumps being exchanged for larger or smaller all have a potential influence.

But hey!
Back to basics first, replace the switch.
( as you suggested in the first place!)

It would be good if you have binder points to plot the flow through the cooler?

Good luck.

mr cool
25-05-2013, 05:09 PM
Is the soli a pulsing electronic expansion valve? I have had the seat on one of them stick occasionally causing really annoying and intermittent lp tripping, changed the valve and it ran sweet as a nut for about 18 months, then started doing it again, same fault! That was on a ammonia plant though and the ptfe seals were scored, hope this helps. But flow rates and water temp are really important too!

Daz man
29-05-2013, 10:26 AM
Hi blokes - appreciate all the feedback.

It sure was an iffy pressure transducer, also there was no Schrader in the suction line connection to it!
So all be warned if in doubt suck it out! (I did as I wasn't sure)

Very simple super heat & sub cooling question for you all now if that's ok? Just to make sure I'm right

The scenario first R407c refrigerant on a basic air/water chiller with small plate heat exchanger.

Super Heat - Suction gauge line connected to the Schrader connection just before the screw compressor. Pressure 3 bar (converted I make that -4 degrees)
My Fluke thermometer is reading a suction line pipe temperature of 9.5 degrees (its not an infrared, K couple)

Sum = 9.5 - -4 is 13.5 degrees of super heat.

Sub Cooling - High side gauge line connected to the Schrader connection just before the expansion valve after the liquid inline core drier. Pressure 16 bar (converted I make that 40 degrees)
thermometer is reading a liquid line pipe temperature of 26 degrees (measured just before where my line is connected)

Sum = 40 - 26 is 14 degrees of sub cooling.

Seems to me I have high sub cooling and low super heat?

R407c refrigerant charge has been charged to data plate (72kg) so can't be low on charge, can it?

Hope this makes sense, is this the correct way to work out the equation?