R-290 and Oil Separation

Greetings. We will possibly be building an industrial refrigeration package that uses propane as the refrigerant. My background is with Ammonia, so propane is a little new to be, but I am learning.

My question is about oil separation. What I have seen in an ammonia system is an oil still pot just below the evaporator. The oil that builds up in the evaporator eventually migrates to the oil still pot. A screw plug (or flanged) immersion heater is in the side, that boils the ammonia off, leaving the oil behind. After enough cycles, the oil volume is significant enough to return to the compressor. The oil still pot has solenoid isolation valves to block it off from the evaporator, and high pressure gas from the receiver pushes the oil back to the compressor.

How is the oil separation typically done on a propane system? With an electric heater, or with a small shell and tube exchanger?

Any information would be greatly appreciated.

If you are using propane, the heater would have to be explosion proof of course. Heat exchangers work OK also.

The big difference with propane versus ammonia is where you drain the oil from. On ammonia systems you drain from the bottom of the vessel because the oil is more dense than the liquid ammonia. On propane systems you need to use a skimmer system, which is set up to drain the oil at the liquid level within the vessel. Propane and oil mix. This cause an oil rich layer to form very close to the operating liquid level in the vessel.

Thanks Iceman, I was aware of the SG difference. Other than being explosion proof, is there anything else I need to watch out for? I am hesitant to use an electric heater, if something fails, such as the thermostat, and it goes full-open, that seems to be potentially very dangerous. Assuming you have come across this in the past, what have you done?

If you are running screws, you can dump the liquid/oil mixture from the outlet of an educator nozzle into the compressor suction. As long as the flow is metered and controlled. If you are using recip.'s, I would use a heat exchanger in the compressor discharge line. One of those liquid -suction heat exchangers for subcooling liquid refrigerant.

I would not be too worried about the heater. USe a 460 volt heater element, but wired for 240 volt. The watt density is a lot lower, which helps to reduce the surface temperature of the element. Since there is no air in the system the propane won't explode.

It's the propane outside the system that's a potential problem.;)

Also, you will want to use a polyglycol based oil for propane refrigerants.

The oil and refrigerant is fairly immiscible so it is fairly well dispersed in the evaporator...not just at the top. I always just took the oil still feed from the bottom of the flooded evaporator.

Using the heater at 1/2 voltage and a lower watt density is a good idea as well. Once you get the oil distilled from the refrigerant, you will need to pump it to the discharge or pressure it back into the compressor suction slowly.

How low your Evap Temp? Are you recirculating liquid or is this a line-boost package or?

1) not real economical percing liquid out of oil-rich if the hot gas is only 40-degrees hotter than the Evap Temp...Big heat exchanger....

2) if the circuit requires pumping: Take a sample off the pump discharge, run it through centrifugal separator, remove oil from center, put refrigerant back into separation vessel...Can also be done vessel to vessel if you have more than one low side pressure....

3) High side to low side centrifuge works well too if you are on a balanced charge plant...

4) If scale of plant is very large but involves minor temperature lift: Consider high side thermosyphon central oil scrubber/ oil cooler.

Interesting Stuff Guys.
Thanks for the info.
Grizzly