oil return in flooded evaporators with R507

[Roberto Cancio]

Hi all,

Does anyone have any literature on how to make oil return systems flooded and how to calculate .


Thaks,

[chemi-cool]

I had the same problem some time ago, The system runs now with double amount of oil and I added 2% propane to help carry the oil. no problems since.

[Roberto Cancio]

Thanks for your reply,

But my doubt is how is the sizing of oil rectifier for system flooded with R507 .

Could you help me with this problem?

[Roberto Cancio]

9407please see sketch

[cricri]

hi roberto,
what type of compressor do you use (piston, screw)?
what are the suction and discharge pressure?
what type of oil and viscosity?

[Magoo]

With 507 the oil will be floating on top of refrigerant in surge vessel. Do you have drain ports at liquid control level in surge vessel.

[Roberto Cancio]

Hi Cricri,
-I used three compressor piston Bitzer - S6F-30.2Y;
-Capacity total - 70Kw;
-Evaporing and condensing (-45°C/+40°C);
-I have used a sintetic oil Bitzer BSE 32;

Thanks for your atention-

[Roberto Cancio]

Hi Mr. Magoo,9424
Thank for your atention, in Brazil is commum three points of capitation oil (atachad croqui).
My doubt is how make calculation the evaporator (shell and tube) for returning oil.

[Magoo]

Hi Roberto
the problem you will have recovering oil from system at -40'C is that the oil will have the consistancy of thick honey and won't flow easily. Next any HX at the surge vessel will add load to system and not recover any oil. I have installed a hot well rectifying vessel outside of coldstore area then manually allowed vessel to fill partially then isolated an warmed contents vented to suction isolated again and drained oil, the system was on R22 at -40'C

[Roberto Cancio]

Dear Magoo ,

You could send me a flowchart rectifier used ?

thank you

[cricri]

Hi Roberto,
you must stay below the maximum point of oil concentration before phase separation between R507 and oil. I don't know this point for BSE 32. You can make the calculation if you know:
- the amount of oil carried over buy the oil separator (PPM)
- if you admit for exemple 2% of oil in the refrigerant
- you know the capacity of the compressors
then you can determine the rectifier.
usualy we use an ejector powered with hot gas,
we pick up an amount of the mixture from the LP vessel to the rectifier and then to the oil tank. it's better to fit a temperature sensor to prevent liquid return.
the rectifier can be a PHE fitted to the liquid line, we use generaly a rectifier from our design fitted to the discharge line but then you waste capacity.

[Tycho]

Don't have any calculations, but I have a design that is widely used on ***** systems, in norway, to retrieve oil from the LP side.

9436

it's a tube heat exchanger, approx 6 inches across and 3-4 feet high.
inside is X amount of 5/8 tubes. the design is completely welded shut, so there are no inspection possibilities, tho I have seen one make with a huge flange on the bottom for service.

it is fed from the bottom with liquid from the ***** pump discharge, and there should be a filter before the tex (not shown in my sketch), it will flood the inside of the pipes with *****/oil, on the outside of the pipes, outside the pipes we run liquid from the condenser/pilot receiver, so that every time the main expansion valve on the system opens, warm liquid will cause the cold liquid/oil mix to boil and lift it up into the compressor suction.

there is a regulation valve on the line to the compressor suction, ideally this should be full open, but depending on the design of the suction line, but in some cases it is desirable to be able to adjust the flow :)

There is another design of this system, but it is more cumbersome and will require it to be placed at a certain height on the liquid separator and also require a balancing pipe to keep the level constant.

The shown design however can be placed anywhere, horizontal or vertical. on systems with more than one compressor it's easy to use one for each compressor by fitting a solenoid on the liquid feed before the tex.

Also, I always poke the return pipe through the suction line as shown in the next sketch, so that in case it leaks through the valves while the compressor is not running, it will go back in the main suction and run back to the liquid separator.
9437

[Tycho]

should also add, the only weakness I have seen with these rectifiers are that the ones mounted vertically may become "swamped" with oil.

if there is too much oil in the system, it will eventually fill itself up with "pure" oil, so if you have to mount them standing, it may be wise to add a service valve between the rectifier and the reg valve where you can connect a hose directly to the compressor suction, so you can manually "dribble" the oil back, or simply a drain valve on the bottom feed so you can "reset" it :)