Oil Pressure Switch

[Milk man]

Going to look at a blast freezer tomarrow. I think everyone and their brother has been out to give it a try.

It's an old R-12 system that has been converted to maybe R-409a. The OPS needs to be reset daily. It is in a small meat packing plant. The differental was 50psig last time someone check it.

What might I look for with an intermitant OPS trip.

[absrbrtek]

See if its a 3 wire or 4 wire setup, seen the 3 wire setup feedback and think the compressor is running when its actualy off causing the trip. Next would be oil return, is there oil in the compressor when they reset it? Bad switch would be my last venture amongst other things.

[icemeister]

An R12 blast freezer?.........that beast has to be at least forty years old!

Typical problems may involve something that's causing liquid floodback and diluting the oil enough to cause a loss of oil pressure. This can happen obviously if the metering device (TXV) is overfeeding. Another common problem would be flooded starts where liquid is slugged back to the compressor at startup or after defrost (more likely) and washing out the oil. If this has been a chronic problem with this system, it may be in the piping where there is situation preventing proper oil return.

I'd check the TXV, make sure this system is on a pumpdown cycle control and check out the suction piping from the evaporator to the compressor for any glaring traps or sags. Since this system has been converted to a blend, check what kind of oil is in the system. It should be at least 50% AB oil if you want any decent oil return at all.

Another area to check would be the contactor. Assuming this system is 3-phase, you may be intermittently failing to start the compressor due to a burned contact and if the oil pressure control is on the other two legs, you'll get an oil failure every time.

Good luck.

[refer dude 2479]

you have a net oil pressure of 50 psi. What kind of a compressor is it. This is high for most the the normal compressors. Even open drive carriers are rarely this high even new with warm oil.

[Milk man]

Thanks for the replies. I'll print everything out in the morning before I leave.

This may have been a R-502 to begin with. I know nothing about it excep what I've been told.

[cynic]

Another thing watch it run for awhile had one last month that after about a half hour+ of runtime oil pressure would slowly start to drop off. All I did was switch the compressor rotation and it hasn't tripped since.

[Freezeking2000]

Lynn the most common reason for op safety tripps if net op is good is the unit goes off on internal ol and times out on oil. I will bet you have high amperage and high suction superheat, make sure the suction superheat at comp is lower then 40 degrees 20 is best!

[basser]

Supply the model number. I agree with some of the replies. If you have a line break protector the oil control will catch protector trips. Verify the amp draw using the compressor data sheet, not RLA. Also agree that a compressor probably will not have 50psi differential oil pressure.

Compare oil pressure from oil pump to crankcase pressure (not suction pressure). You could have a pressurized crankcase. Example follows:

Oil pump pressure 50psi
suction pressure 5psi
differential is 45psi, but this is not true oil pressure.
Crankcase pressure is 40psi. Differential between is now 10psi. This is your net oil pressure. This is a gross example of a pressuized crankcase, but I think you can understand my point.

I'm interested in what you find.

[R12rules]

First break this diagnosis down into categories.

Is there oil in the crankcase when you go to reset the ops ?
If no, you got an oil return problem
This could include, but is not limited to; slugging on start up and or post defrost.

If yes, you got either an oil screen probelm or electrical problem like a bad contactor or possibly internals tripping.

Your most common problems, which I have encountered is bad contactor and dirty oil screen.

Wouldnt be a bad idea to change the oil in the unit now anyway. While you got that plug out, check the screen and clean it.

And it wouldnt hurt to check that crankcase pressure versus your suction pressure too. Make sure they are like you want them to be.

[doctorbob]

check the contactor first, if you have bad contacts its a shure bet the compressor fails to start and the switch times out. Funny they always start when you push the reset button. A good mechanic used to preach contactors to me all the time and he was right. Good luck

[dualtemp]

When I get an oil fail call and no obvious reasons for it, first thing I do is change the contactor.

[wannafreeze]

Originally posted by icemeister
Another area to check would be the contactor. Assuming this system is 3-phase, you may be intermittently failing to start the compressor due to a burned contact and if the oil pressure control is on the other two legs, you'll get an oil failure every time.
[/B]

icemeister,can you explain this one little more?

[Milk man]

Originally posted by altan

Originally posted by icemeister
Another area to check would be the contactor. Assuming this system is 3-phase, you may be intermittently failing to start the compressor due to a burned contact and if the oil pressure control is on the other two legs, you'll get an oil failure every time.

icemeister,can you explain this one little more? [/B]

Oil pressure switches work off time and differental.
Not my strong area, but this is what I think I know.

OPS is a differental switch. Measured from the oil pump discharge and a fitting on the low side of the compressor. My switch needs to see 9# differential. Once it does not see the 9# it energizes a heater that will warp a bimetal NC switch and kill the power to the contactor.

The heater will take somewhere around 90 seconds to warp the switch. This is so the compressor will have time to start and build oil pressure

Now if the contactor has a set of bad contacts and the OPS is wired off the other two good contacts then the copressor will not start but the OPS will energize and time out. Then the OPS manual reset will have to be pushed and the compressor might start if the bad contacts pass current.

[Milk man]

I didn't get out there. One of the senior guys went. I haven't talked to him to see what he found yet.

[icemeister]

Originally posted by lynn rodenmayer

Oil pressure switches work off time and differental.
Not my strong area, but this is what I think I know.

OPS is a differental switch. Measured from the oil pump discharge and a fitting on the low side of the compressor. My switch needs to see 9# differential. Once it does not see the 9# it energizes a heater that will warp a bimetal NC switch and kill the power to the contactor.

The heater will take somewhere around 90 seconds to warp the switch. This is so the compressor will have time to start and build oil pressure

Now if the contactor has a set of bad contacts and the OPS is wired off the other two good contacts then the copressor will not start but the OPS will energize and time out. Then the OPS manual reset will have to be pushed and the compressor might start if the bad contacts pass current.

Yup. That's what I was going to say.

[wannafreeze]

[QUOTE
Now if the contactor has a set of bad contacts and the OPS is wired off the other two good contacts then the copressor will not start but the OPS will energize and time out. Then the OPS manual reset will have to be pushed and the compressor might start if the bad contacts pass current. [/B][/QUOTE]

I can understand that compressor may not start because of a bad cont.contact thus in the absense of oil pressure the OPS will time out and would need to be reset.but I can`t understand the OPS being wired off the other two good contacts..after or before???if its before cont.contacts have nothing to do with OPS operation.

[Milk man]

The OPS's heater is in parallel with two contacts. The OPS has two opposing pressure switches. The high pressure switch overpowers the low pressure switch. This opens a NC switch. The switch is opened due to a pressure difference. If the pressure differental is below the set point the NC contacts close and one leg off the contactor (L1) will feed the heater and the other line off the contactor will acts as L2. The bimetal switch will warp over and open the circuit to the contacor coil.

[wannafreeze]

Originally posted by lynn rodenmayer
The OPS's heater is in parallel with two contacts. The OPS has two opposing pressure switches. The high pressure switch overpowers the low pressure switch. This opens a NC switch. The switch is opened due to a pressure difference. If the pressure differental is below the set point the NC contacts close and one leg off the contactor (L1) will feed the heater and the other line off the contactor will acts as L2. The bimetal switch will warp over and open the circuit to the contacor coil.

yes I can see why the heater is supposed to be wired on the load side of contactor.if it was on the line side ,on the off time of the compressor there would be no pressure difference to open switch that cuts power to the heater.thus the heater would warp the bimetal on the off time..

[Milk man]

I talked to the mechanic today. He said it was short cycling on it's low pressure switch. Short refrigerant.

[Freezeking2000]

When that happens you dont have good oil return, thats wht you had no pressure!