View Full Version : Moisture removal from R744

19-09-2014, 09:00 PM

Been having problems with moisture in several supermarket systems 16ppm in the gas and 370ppm in the oil.
Replaced the liquid line filters several times and retested found moisture level had dropped below 10 ppm.
I know the bench mark for the gas is 10ppm does anyone know what the acceptable level of moisture is in the oil.
Carried out a complete oil change but moisture level still high in the oil I would expect it to be higher but unsure of what the acceptable level would be.


Rob White
19-09-2014, 09:40 PM

Don't know if this helps?




20-09-2014, 11:11 AM
You have ratio between humidity in refrigerant and humidity in used oil depend of physic laws (henry law etc.), water vapor partial pressure in system and used oils chemical properties.

Ie hygroscopic oil bind more humidity than mineral oil on same partial pressure water vapor over liquid surface, and water partial pressure is also couple to humidity level on liquid part of used refrigerant and have same physical and chemical rules as oil for how much humidity absorb to liquid body refrigerant on specific water vapor pressure in system.

Example liquid R134a have also hygroscopic force on water vapor (and ammonia lot stronger binding force...) and need strong molecular sieve desiccant to remover water from liquid if want 10 ppm humidity level, but R12 or R290 (propane) have not strong forces to bind water and can use 'simple' deccicant as silicagel to remove humidity from liquid refrigerant and still hold 10 ppm humidity in refrigerant liquid body

If you take down humidity in refrigerant from 16 to 10 ppm - you also take down humidity in oil same degree after while via 'head space dehumidification' (ie. dry refrigerant via drier filter and dried refrigerant dry oil in circulation in system)

ie. 16ppm/10ppm = 1.6 and if you start with 370 ppm on oil and 16 ppm in refrigerant you should lower humidity in oil to 370 / 1.6 = 231 ppm after some time if you hold refrigerant level on 10 ppm humidity.

And !, you cannot expect new oil have low humidity level from manufacturer and in many case they also calculate customer systems drier-filter take down water to proper level in oil.

hmm - R744 is carbon dioxide and high pressure...

Vapour density on saturate point of CO2 as 20 degree C is 194.2 kg/m^3 and liquid have 773.4kg/m^3

Expand ratio is 4.024 between vapour and liquid volume on 20 degree C and 5.73 MPa absolute pressure

1 kg CO2 and 10 ppm water is same as 10 mg water per kg CO2 or in volume have 7.73 mg water per litre volume,
and expand liquid CO2 to vapour form with 4.024 bigger space, give 1.922 mg water per litre volume.

Partial pressure for water as 1.922 mg/litre give saturate point at -11.3 degree C or 2.3 mBar partial pressure

I calculate only of gaseous form (ie status suction gases after compress) depend liquid CO2 is very hygroscopic (same as R134a, ammonia and many other HCF) and bind and hidden water in liquid body as desiccant and result lower partial pressure of water vapour above liquid surface, but if refrigerant boil of for example in evaporator - refrigerant cannot longer hide water or water vapor and below 0 degree C, can start make ice...

Your need to calculate itself for your system, temperature and pressure but if CO2 have 10 ppm water and work with almost 10 time higer pressure and 10 time higher water partial pressure compare to example R134a, is no wonder you have higher level of humidity in your oil compare to a ordinary R134a system...

Depend of high pressure in CO2 system and give high mass per volume (and also give high mass of water per volume), you should have only 1-2 ppm humidity on CO2 if you want same 'dryness' as ordinary R134a-system and expected < 50 ppm humidity in oil...

Higher working pressure -> lower humidity level on refrigerant if you want same humidity level on oil in compressor - water partial pressure (and temperature) is everything if you hunt humidity level in liquid body of refrigerant or oil ...

28-09-2014, 12:37 AM
Thankyou very much for the replies much appreciated.