Synthetic polyolester

[Mhobson]

I'm new to this forum. We build ice vending machines and control the condensing unit with our own controls.

Here is our issue/concern. We are using R-404 with Polyolester oil. our condensing unit has 2 fans that are controlled to operate at 2 pressures. One at 210psi and the other kicks on at 250psi. Our high pressure cut out is 350 psi. We have more than 100 units out running this way without issues.
One of the fans failed and the unit ran for an unknown perion of time without reaching 350 and faulting at the 350psi cut out. We believe that the unit ran at a much higher than normal pressure for a long period of time. Could running at, say 310psi for days, cause a problem with the oil or something else? The sight glass on the compressor is brownish/grey when normally it is a clear oil. We are not experts in refrigeration but are fairly good with controls. If we need to have a secondary or another safeguard we can monitor the high pressure and if over a certain number, 300, and it is there for a certain length of time then we can issue a fault then.

Thanks in advance for any help!

[aramis]

Supposing the system was clean at startup:

Working at 310 psi is not a problem on its own but combined with high superheat and low suction pressures, can produce discharge temperatures high enough to start chemical reactions between the refrigerant and the oil and/or additives it may have.

Increasing the condensing pressure has two opposing effects: Decrease compressor performance and higher injection from the expansion valve. If the first effect dominates then your suction pressure lowers.

Chemical reactions are worse if you add contaminants to the system like humidity.

The filter dryer can absorb some contaminants and keep the chemistry of the system under control until it saturates and cannot absorb more acids.

Once the acids become available they attack the varnish that covers the motor wires. This can change the oil color and eventually all colors turns black when you burn the motor.

Make and acid test to the oil to check if it is ok and/or do a preventive maintenance: change the oil and filter dryer with one that absorbs acids. Better still if you add (temporarily) an anti-acid suction filter though performance will decrease. If you have slime in the compressor you should clean it and change refrigerant charge also.

[aramis]

... by the way, welcome to RE forums!

[install monkey]

cant you have a single fan instead then if it fails it will trip the hp switch more quicky- having 2 smaller fans prooves that 1 failed fan can cause the system to run at a higher head presure but not causing the hp switch to disable the system

[monkey spanners]

Fit a discharge temperature safety switch to cut the control circuit should the temperature get too high.

Somethin like this, or even a simpler bi metalic device,

http://www.youtube.com/watch?v=VP_aIGvvldw

[Brian_UK]

If you want to keep your fan set-up why not fit a 1st-stage HP safety pressure switch set above the fan switch but below the final HP switch.

Either use it to shut down the system or to raise an alarm, or both.

[mikeref]

Could look at the relationship between current draw on compressor when running at 250 psi and ,say 320 psi. Set the contactor overload to manual reset on the higher amps.
Should also look at the result of a clogged condenser with both condenser fans running. Easy enough to do with a piece of cardboard covering condenser to simulate that condition. Is 350 psi really too high for your HP safety? Liquid temperature at 54 degrees C?