oil failure

[hem057]

walk in freezer, semi hermetic, with outdoor condenser, head pressure control, we are having colder than usual weather, and oil failure during these cold days will trip, when weather warms up unit is fine, then if temp drops again unit will trip, oil level and pressure is fine i.e. 40 psi of oil pressure; any thoughts as to what could cause this

[skotup]

Is the sump heater/crank case heater circuit breaker tripped? if so- reset it. can you get a bigger sump heater? or short the normaly closed sump heater contact on the compressor contactor? change the oil? oil seperator fcut? oil filter clogged? take it with a grain of salt, im just a sparky.

[tjc76]

what type of head pressure control- fan cycling or condenser flooding? if condenser flooding, do you have enough winter charge in unit? possible low suction gas velocity inhibiting oil return, also, is the oil failure sentronic or mechanical? if mechanical, is the cover in place?

[hem057]

I should have mentioned that this unit has sentronic, and yes is flooded condenser, sight glass in full, head pressure 150, suction 15, 408a refrig, and oil pressure is 40 (55 - 15)

[Dowadudda]

Did you do a blow by test? Did you see if the crankcase heater is working. Is the machine staying pumped down during temp satisfied or defrost. Is it short cycling? Is there oil visible in the sump when reset.

I said blow by first cause this one always gets overlooked even by me. I had found one the other day. I'll share it with you guys as a way to add to the general noggins of al of us. The unit ran okay. It would pump down on temp or for defrost. The crankcase worked good. I had oil in the sump upon reset. So I am doing the net oil psi thing. REMEMBER. Net oil is to be measure at the crankcase, not suction service valve. It was okay. So.

I front seated the suction service valve to isolate pump so I could tap the crankcase to measure net oil. So I pumped down, I front seated. But then the damn pressure kept rising. not fast but also not slow. It would short cycle on low pressure control about every 10 to 15 seconds. well the gas is bleeding back by from the high side, through the rings, and into suction.

But it was so sublte. With out trying to isolate the compressor I would have never picked up on it. If you pumped it down at the liquid line solenoid valve, it still would bleed back through but, it would not show up on the gauges for awhile, and the rise much slower, thinking that it just may be gas off out of the oil something. In a pump down, that is a large suction area, from compressor to the loads that would need to fill with vapor bypassing from high side to really see a rise in say 2 or 3 minutes. It does not show up like a Liquid Line Solenoid Valve leaking by is what I am getting at.

What in reality was going on. The defrost for some, someone set for a pretty hefty length. like an hour and 10 minutes. So it would pump down, then air defrost. The whole time suction side filling up with vapor and then condensing. Now we go to start the pump, and we got liquid flooded start. That gas comes into the crankcase, starts boiling off violently, filling the scaveneger pick up for the oil pump up with vapor, not oil, loss of prime on pump, boom, we go out on oil pressure saftey.

That is one of them where if your just doing the routine stuff, you may not catch that first go round.

lately, I have had a slew of oil issues. From bad piping systems to any all things. I am creating myself a list, not a gay list like you see on the internet. But a methodical list. Oil problems kick everyones but. I want to shortne the gap.

[k-fridge]

head pressure 150,
Head too low. Most of the headmaster valves are set a 180 or higher. You may have a bad valve or improperly charged.

Also, make sure the crankcase heater is working properly.

[tjc76]

150 head seems a little low, what type of flood control are you using? head master(3 port)? ori(holdback valve on cond outlet)with ord(differential check on hot gas bypass line to receiver)? also do you ever witness the oil trip, or does it run for a good amount of time before tripping again?

[Dowadudda]

I challeneg everyone to explain the reason for their help. Help a guy qualify the issue.

K fridge. Head pressure low. Okay, I agree, but what would that do? Why would that be the root cause of an oil failure?

A lack of adequate liquid pressure, or even flash gas, to the inlet of an exapasion valve, will not alow the expansion valve to properly feed the evaporator, and load it correctly with he delicate vapor liquid mix. In doing so. This creates lower suction pressure, lower suction velocity. Suction velocity is to "sweep" the oil back to the compressor.

Oil will travel with liquid fine. but when refrigerant is in a vapor state, the oil falls out of the refrigerant vapor and separates, gravity cause it to fall. It's heavier than the vapor. But if the vapor is of adequate velocity and pressure, the oil gets swept along with the flow of vapor. If that velocity is reduced, oil tends to drag and settle.

[Dowadudda]

a crankcase heater is for to prevent migration. Refrigerant will migrate to the coldest place. If your pump sits in a cold ambient, colder than the coldest load on it, vapor will migrate to pump. If the compressor is off for an extended period of time, then you have vapor migrating to pump, then condensing. On start up, you have a scenario of gassing off that liquid in the crankcase. The reason it gasses of, or rather boils, is because on start, that crankcase immediately and quickly, the pressure goes lower. This creates the environment for that condensed migrated liquid to boil off very fast. That cause the oil pump to start scavenging vapor gas, instead of oil.

[skotup]

a crankcase heater is for to prevent migration. Refrigerant will migrate to the coldest place. If your pump sits in a cold ambient, colder than the coldest load on it, vapor will migrate to pump. If the compressor is off for an extended period of time, then you have vapor migrating to pump, then condensing. On start up, you have a scenario of gassing off that liquid in the crankcase. The reason it gasses of, or rather boils, is because on start, that crankcase immediately and quickly, the pressure goes lower. This creates the environment for that condensed migrated liquid to boil off very fast. That cause the oil pump to start scavenging vapor gas, instead of oil.


.....so how would you fix that?

[Dowadudda]

is the unit equipped with a crankcase heater? If so, is it working? If your equipment is operating in a cold ambient, a crankcase heater is a must.

[hem057]

just came from the site, so will give a bit of an update, head pressure is 160, there is no cold control, so unit is either in defrost or is running all the time, I think this could be part of the problem, though, every time I have been there oil bulls eye shows 1/3 to 1/4 full, and have never been able to catch it when it trips.......have even put it through a defrost cycle....so even in defrost the way it is set up it does not keep cycling on and off, with defrost contact open no power is passed on to the pressure control, (this is an odd arrangement, but that is the way it is, I usually set them up with pressure control, solenoid valve and thermostat, put down system)

[rocket]

net oil pressure is not "pump out - suction"
it is "pump out - crankcase"
the crankcase pressure may be different than suction (blowby)
measure it right please

[yotaman16]

Fill the oil sight glass to half and install and set up low ambient controls and see how it runs

[Dowadudda]

net oil pressure is not "pump out - suction"
it is "pump out - crankcase"
the crankcase pressure may be different than suction (blowby)
measure it right please

and who exactly stated it incorrect?

[icemeister]

.......there is no cold control, so unit is either in defrost or is running all the time, I think this could be part of the problem..................(this is an odd arrangement, but that is the way it is, I usually set them up with pressure control, solenoid valve and thermostat, put down system)

So this system isn't set up for pumpdown. I too think this may well be part of the problem.

I can imagine when the system goes into defrost with the compressor off, the defrost heaters on and a very low ambient temperature at the compressor, the crankcase heater (currently your sole line of defense to prevent refrigerant migration to the crankcase) possibly can't keep the oil sump temperature higher than the evaporator temperature. With the defrosting evaporator eventually heating up to around 50ºF, if the sump temperature is lower, migration can occur.

I suggest adding a LLS and set it up for pumpdown during defrost. That way, the oil sump temperature would have to be below your cut-in on the LPC for migration to happen. It becomes your second line of defense against migration.

[hem057]

I appreciate you comments about blow by, my experiece has been that with blow by on the cylinder walls that there will be alot of disruption in the oil sump, noticeable even in the bulls eye by churning and frothing of the oil, but won't as you suggested observing the rising pressure in the crank case require both bad pistons and bad high side valves that would then allow high pressure to seep back into the crank? also amp draw of the compressor would start to fall of dramatically
when I see this oil churning condition then I could see a difference between suction and crank pressure, but my oil in the sump does not show excess movement, that is why I am using suction pressure at the suction valve on the compressor as my reading, at that point I just wanted to make sure that the oil pump is able to do what it is supposed to do.....

[rocket]

if your suction and crankcase pressure is not close then you have blowby
net presuure is alway crankcase subtracted from oil out,
your suction is 15, what is your crankcase pressure?

the oil failure reacts to oil out - crankcase pressure
remember the cap tube oil failure, it went to crankcase not suction

[Dowadudda]

I appreciate you comments about blow by, my experiece has been that with blow by on the cylinder walls that there will be alot of disruption in the oil sump, noticeable even in the bulls eye by churning and frothing of the oil, but won't as you suggested observing the rising pressure in the crank case require both bad pistons and bad high side valves that would then allow high pressure to seep back into the crank? also amp draw of the compressor would start to fall of dramatically
when I see this oil churning condition then I could see a difference between suction and crank pressure, but my oil in the sump does not show excess movement, that is why I am using suction pressure at the suction valve on the compressor as my reading, at that point I just wanted to make sure that the oil pump is able to do what it is supposed to do.....

The reeds most likely were bad too. I never pulled the head. I should have been very clear here. I was trying to give an example of how a subtle subtle blow by can cause some issues.

This blow by was occuring during an off cycle, with pump at rest, and would fill the sump and suction with vapor which would condense and then have a flooded start. I never got that far to do net oil though. Remember. I was isolating the compressor so that I could tap the port of crancase. As I front seated the suction service valve, after pumping it down, the pump would fill with vapor. From the seeping back through the discharge reeds, through the rings to the crankcase. If the suction service valve was open, the vapor would go down the suction line.

It sits there then and condenses. Kapish

Why do I get the impression some of you simply want to argue.

Net oil is based on crankcase presure versus discharge oil pump pressure. That is the right way.

[Dowadudda]

I don;t think it would be good practice to assume that if you do not see frothy oil, that your to not check net oil pressure at the crankcase.

[markettech]

walk in freezer, semi hermetic, with outdoor condenser, head pressure control, we are having colder than usual weather, and oil failure during these cold days will trip, when weather warms up unit is fine, then if temp drops again unit will trip, oil level and pressure is fine i.e. 40 psi of oil pressure; any thoughts as to what could cause this

Set me staight - is this a complete outdoor condensing unit, or is it an indoor compressor with a roof condenser?

[hem057]

dowadudda, I feel bad if you think I or (we) are just interested in arguing, I am sure that you have been in the trade for a while, and then we know that sometimes trouble shooting can be difficult and require patience, sometimes one problem will mimic another, like on this job we could have started changing sentronic sensor, etc, etc, and the the problem persists, and then tbe customer sees his bill with parts, and starts to loose confidence in us, so I DO VALUE the comments that are made because we all have encountered different situations and I find the forum is tremendously helpful, but we also have a huge range of experience that we are getting feed back from, so we have to sift through the comments. So I says thank you to those that share their experience.

[hem057]

I should have made this point clear at the beginning, I thought I did by saying the condenser is on the roof (as opposed to saying condensing unit is on the roof) so the compressor is in a small room inside the building down below, in the DQ I am working at, the condenser only is on the roof, my employee tells me that it is a head master control ( I didn't physically go up yesterday, we were in the middle of a bad winter storm here in southern ontario) sorry; I should have been more explicit, I know we are providing words details so must be clear.

[markettech]

Just a scientific wild a$$ guess, but since the head is low do you think there may be a chance the system is undercharged and compressor is short cycling to the point of overload trip due to liquid stacking in condenser?

Just a quick thought (as I'm walking out the door).

Be sure to keep posting - you've got my interest peaked on this now.

[hvacman77]

The reason I asked is from my experience in the Chicago area, many moons ago, I remember a few particular cold snaps say -25, were a headmaster or fan cyle head control will not be able to keep the head-suction ratio in proper range, oil failure was destined....we installed heat tape on the receivers with ambient temp controllers, to maintain proper sub-cooling, pressure ratio, thus superheat and net oil

[onetime]

Maybe the problem is the non-pump down, on start up after defrost the comp. could be overloading and tripping on internal overload and then with no differential the oil pressure switch trips.

[hvaccop]

If system is 3 phase I would check my contactor and make sure I am not single phasing at some point and tripping my oil pressure. The oil pressure switch is still energized looking for pressure diff. but the compressor tripped the overload from single phasing. You may have a pick up screen really dirty