Compressor operating outside of pressure stat cutout and cut in

[AGi]

Got a call for a restaurant line cooler that is not holding temp. The condensing unit is on a rack with 5 other units that run another line cooler, three walkin coolers and a walkin freezer. All are water cooled, by an ill maintained cooling tower. The unit in question runs r-401a, the compressor is less than a yr old with a new all range pressure stat. It is running at 220 psi on the high side and is cycling between 45 (cutout) and 65 (cutin). The pressure stat is set at 27 psi (cutout) and 42 psi (cutin). The stat swich opens when exposed to atmospheric pressure. I only have vague info on what service was done on it recently as the customer is no longer on speaking terms with the previous outfit and paperwork is non existent. Before I get too into this.......
What would cause the compressor to run at pressures beyond the cutout and cutin settings?

[monkey spanners]

I'm not sure from the description if its tripping on hp or lp, could you explain it again.

Generally if it a hp trip its due to not enough heat rejection in the condenser and if its an lp trip its due to not enough heat absorbtion in the evaporator.

Jon :)

[Brian_UK]

Also, is the compressor being controlled by the pressure switch that you think is controlling it or is it connected elsewhere?

[mikeref]

AGI. If it's a mechanical pressure switch than indicator on front of switch does not tell you accurate readings. It's only an indicator. You have to set pressures using gauges. set cutout then adjust differential only to have cutout setpoint move higher or lower:rolleyes:.

[AGi]

@Brian_UK, the compressor is definitely wired thru the pressure switch and their are no other controls on the system.
@mikeref, I've always known that the indicator on the pressure switch is not accurate, but I assumed that the operating pressures would be close to what the indicator is set at. This compressor is operating completely outside of the set range. From my experience, that was unusual.
More history on this system:
The cooling tower is ill maintained. The fill is calcified and the water is from a city well and is untreated. On this particular condensing unit, the water regulating valve has been bypassed. The mechanical room is not climate controlled and when the OAT is 85, the room ambient is 95. Obviously, heat transfer is an issue.

[AGi]

@monkeyspanners, with the gauges on, the compressor cuts in at 65psi and cuts out at 45psi. This is way outside the setpoints of 27psi cutout and 42psi cut in. I always thought with a mechanical pressure switch that the cutout and cutin would be close to what is set. If the pressure is too high on the low side (above the 42psi cutin) shouldn't the compressor continue to run until it reaches something close to the cutout. this compressor is cutting out at pressures higher than the 42psi cutin.

[monkey spanners]

Sounds like the lp switch is worn out and its time for replacement. It might adjust so that it is switching when you want though the scale will be out, just to get it working temporarily, but from experience they only get worse so i'd change it out.

Jon :)

[AGi]

Update:
Turns out the high pressures were caused by a dirty condensor coil. Initially I thought the water flowing through the coil was adequate. After I blew some compressed air through the coil and cleared it of scale buildup, the pressures settled down.

Thanks for the help.