Dunham Bush Water Chiller Oil Fault

[Tayters]

Gentlemen I need some advice with a Dunham Bush water chiller. Not the type of thing I've had much experience on so here goes....

System in basement, part wound recip compressor, 3 heads, 1 unloader, shell/tube evaporator, condenser on roof up 4 stories, R22, 1989 vintage.

I repaired a gas leak a few weeks back and recharged, now there is an oil pressure fault.
The compressor sight glass seems empty (or full but the glass looks clear not yellowey so I think empty), system will run (suction 60psig, oil pressure about 80psig. discharge 200psig) then oil pressure wobbles before reaching suction pressure then out on oil pressure switch after a couple of minutes.
I have attatched a schematic of the oil return system and a couple of low grade photos because this is the bit I'm stuck on.

Refering to the diagram, the oil sight glass is full, sol. valve energised (valve checked and is open), 30psig at 1/4" stub, rotalok open but this 30psig appears too low to get oil pass the NRV into the hot gas line. When the system was running previously the hot gas pipe at the shell/tube was very hot all the time indicating discharge gas of some pressure constantly there. No ports to check so not sure what pressure though.

So I'm not sure if the pressure regulating valve from the discharge pipe (looks like an EPR or CPR but no markings on it) is at too high a pressure. Even if it was I'd have thought there would be more pressure on the oil return line but I'm not sure why the oil line (it's a 1/2" pipe) goes into this hot gas line anyway.

It seems the low pressure on the oil line is the fault but why it's that low I don't know. If the float was stuck in the oil sep. then why would the sight glass still be full. Maybe the oil sep is empty I suppose and oil has just collected at the sight glass as it is on a lower piece of pipe. I've given the sep a clout but no difference in the pressure at the stub - assuming here also there is a float in the oil sep. It's a small can - just bigger than a tin of beans so unsure exactly of it's construction and no details on the Danfoss site.

Riser End.jpg
Compressor End.jpg
Pipe Schematic.jpg
There are 2 refrigerated systems making the pictures a bit hard to decipher hence the drawing. You've guessed right, the other system leaked out years back. Not sure if there was anything else wrong with it. At the riser end, the inlet to the oil sep T's directly from the elbow on the riser - no traps, it's just made up with a 1 5/8" T piece and one end reduced to 1/2"

Hmm, as I read that back with a calmer head it seems oil sep empty so oil outlet closed therefore accounting for my 30psig. So oil has got stuck somewhere in the system. Still unsure of the hot gas and oil return systems.

I'm reluctant to just go poking oil in it as there are no massive oil stains anywhere and this thing has been running to some degree since 1989 (another 30 years and Jon will no doubt be working on it!)

Any advice would be much apprieciated folks.

Cheers,
Andy.

[monkey spanners]

That oil seperator looks way to small for total system flow and looks like its piped just to catch any oil thats sitting just before the riser.

Does it feed oil into the evap just after the tev? I wonder if its t'ed into the line that feeds discharge gas into the evap should the suction pressure get too low to keep the evap pressure above freezing.

Is the filter in the oil line ok? If the line goes into the evap the oil pressure would need to be over 60psi to flow out of the oil line.

I suppose dumping the oil in the evap is insurance against any liquid refrigerant condensing in the hot gas riser and filling the seperator, if piped to return back to the sump it could be nasty!

I'd be thinking either not enough oil in the seperator, float stuck or filter blocked. But i don't work on stuff like this so....

If i couldn't get it to work i'd be tempted to fit a proper sized oil sep with a check valve on the outlet and a heater and just pipe it back to the sump as normal (and watch it nervously for a while when i turned it back on ), but i'd wait for the opinions of some experts before listening to monkeys.

Jon

[mikeref]

Oil line return pressure is 30 psi while suction is 60? No forward flow there. Maybe oil from seperator is gravity feed but not familiar with oil return to anything except to the compressor.. No flow out of seperator could be a sunk float and not likely out of oil. Compressor sump volume should be way more than this seperator can hold so i would think some oil will be returning via suction, however, maybe compressor oil level was at a minimum to start with and now running critical without seperator function.. Mike.

[D.Evans]

The hot gas injection is designed to increase the suction superheat thereby encouraging the expansion valve to open wider to allow more refrigerant to the evaporator. It would appear that it also acts as a venturi drawing oil return from the oil separator back to the compressor via the evaporator. As this hot gas injection is normally set at around 58psi then if your suction pressure is 60psi then I would think that this oil return is having little effect. Is there any movement in the oil return sight glass in your drawing.

[mikeref]

Hot gas into suction line is to prevent evaporator running too cold, attempts to compensate compressor when load is minimal rather than shutting on/off compressor. Still, this doesn't solve oil return problem. Still think oil seperator is dead.

[Tayters]

Thankyou for your replies.

I've added oil to the compressor and now net oil pressure stays at 40psi. I drilled a stratigic 4" hole in the electric panel so the compressor oil level can now been seen without mirror and torchlight. Now with the system running longer I've seen the oil sight glass empty and fill and the comp. oil level can be monitored.
Didn't get a chance to measure the pressure in the oil line BUT suction pressure would drop to about 30psig for a few minutes then rise to 55psig for a minute before repeating the cycle. This is a new one. Temperature and pressure readings indicate that the system is short of charge again.

It wont give up without a fight!

Cheers,
Andy.

[HVACRsaurus]

Just watch out when you add refrigerant, you might drag a quantity of oil back with additional charge.

[mikeref]

Tayters, 30 to 55 psi cycle, have to keep an open mind on this. Compressor unloading on low pressure maybe due to low load also hot gas opening on low pressure could be low load again or compressor not unloading. Multiple expansion valves? All feeding? Evap inlet and outlet temperatures? Years of service and sketchy history, now this chiller is depending on you to make it sing again..Mike.

[Tayters]

Been a while back but just to update.

System was short of charge again. Was getting the detector to go off when the top of the buffer tank was removed. Pressure tested the heat exchanger. RIP fella.
That would explain where the oil (and refrigerant)went to but nobody can explain the oil return setup. The boss passed my sketch onto an old friend of his. Formally a site manager and most of his working life looking after chillers and he hadn't come across it either.

Thanks for your input chaps.

Cheers,
Andy.

[simon@parker]

there wouldnt be 4 plants in a row ? 120ft down a hole ? plant mostly white and cream would there ? have worked on the set up you have described the discharge line in to the or just after tev is to maintain evap pressure its a danfoss regulator and they dont make them no more if yr comp going out on oil and its same i worked on go for body filter first if thats clear take heads off you may have fooked gasket onews i worked on oil seps always blocking had to replace sep completly it was a helical one fitted filter in line before it as i kept digging out old valve reeds lol



doh just read other posts lol

[r.bartlett]

Last edited by Tayters; 17-07-2011 at 06:47 PM. Reason: Grammer

Sorry, it just made me giggle

[Tayters]

Sorry, it just made me giggle

Glad someone noticed my subtley!

Not sure you giggled as much as I did after seeing the link to those album covers you once posted though.
'The Braillettes'!!!

Cheers,
Andy.

[Tayters]

there wouldnt be 4 plants in a row ? 120ft down a hole ? plant mostly white and cream would there ? have worked on the set up you have described the discharge line in to the or just after tev is to maintain evap pressure its a danfoss regulator and they dont make them no more if yr comp going out on oil and its same i worked on go for body filter first if thats clear take heads off you may have fooked gasket onews i worked on oil seps always blocking had to replace sep completly it was a helical one fitted filter in line before it as i kept digging out old valve reeds lol



doh just read other posts lol

Hi Simon,

That sounds like the type!
Knew what the hot gas line was for but didn't know, infact still don't know, why the oil return went into this pipe and not the compressor. Must have worked though all those years.

Cheers,
Andy.

[r.bartlett]

Glad someone noticed my subtley!

Not sure you giggled as much as I did after seeing the link to those album covers you once posted though.
'The Braillettes'!!!

Cheers,
Andy.

braillettes.jpg
To be fair, I was crying at those album covers :-)

Lets revisit that shall we:

http://stevecarter.com/albumcovers.htm