View Full Version : Reducing diameter of rising suction pipe for oil return

30-10-2015, 05:57 PM

I am about to fit a domestic walk in dairy fridge set point 5c load about 1kw. Danfoss Optyma outside, Searle TEC Evaporator inside.

Pipe run is about 25M and the outdoor is above the indoor by about 1M, similar to a cellar.

Liquid 1/4 copper
Suction 3/8 copper

I have been advised by my supplier to reduce the suction for oil return to 1/4 where the pipework rises, this would be a couple of sections about 500mm.

Can anyone explain the thinking behind this theory, I've never come across this before and I am unsure about reducing a 3/8 suction to 1/4.


30-10-2015, 10:36 PM
I can see no reason to reduce from what is already a very small suction to 1/4. The reasoning would be incresed velocity to drive oil up the line but for a 500mm rise, bit overkill i think. I'd have gone 1/2 suction with a 3/8 liquid but depends on how small room is?

Rob White
31-10-2015, 09:10 AM

Who is your supplier?

Reducing the suction to 1/4" will create a pressure drop.
If you are worried about oil return put a oil trap top and bottom
of the riser.



31-10-2015, 03:32 PM
On 25 M run you will need to increase the diameter by two steps to maintain cooling and cooling the compressor.

Suction to 5\8 and liquid to 3\8.

Glenn Moore
01-11-2015, 09:12 AM
This is all to do with oil return. The small hermetic compressors in these units have only a few cc's of oil in them and it is easy for them to run out of oil and seize up unless the pipe work is designed to assist the oil return. 25 metres of pipe run is a lot for a small black pot , so the suction pipe work exiting the evaporator should have a shallow trap in it so that once to oil has exited the evaporator it can't run back into the evaporator. Then the suction run from the short riser from the evaporator should have a slow in incline back to the compressor to assist the oil to naturally run back towards the compressor . During the system evacuation I would then draw a couple of cc's of extra oil into the system so that the system has a sufficient amount of oil to keep the compressors oil level constant ie the amount of oil leaving the compressor should equal the amount of oil returning to the compressor. Failure to add a small amount of oil to the system , and the compressor then running dry and seizing will lead to a warranty rejection of the seized compressor.

02-11-2015, 07:57 AM
You can fit a small oil separator. 25M run is rather long for 1/4 and 3/8. Liquid line might not supply a full head to TX, or starve a capillary tube feed due to pressure drop. Definitely don't reduce suction to 1/4" on your riser.

06-11-2015, 05:24 PM
Thanks, but I can't say I'm any further forward. Would anyone else like to throw in a completely different and opposing view to all the others? ;)

It might be easier to have a fight.

06-11-2015, 07:11 PM
get the condensing unit much closer, 3m at the most and your problems are over.

Glenn Moore
08-11-2015, 09:38 PM
Hi Contactor
Send me your email address or speak to Joe or Ron at D&W Glasgow and ask for a copy of Glenn's WHY COMPRESSORS FAIL articles especially PART 5 this might help you in your probblem

08-11-2015, 11:22 PM
Hi .Contactor
google search " Saravel Refrigerant Pipe Sizing ", a useful program for selection pipe sizes.