Results 1 to 28 of 28
Thread: Triple Evacuation with Nitrogen
Hybrid View
-
14-11-2010, 04:31 PM #1
Triple Evacuation with Nitrogen
Why do we triple evacuate a system with nitrogen prior to charging?
-
14-11-2010, 04:46 PM #2
Re: Triple Evacuation with Nitrogen
nitrogen or OFN is free of oxygen which is a content of water / moisture
In lay mans terms think of swiping a dry sponge through a bath of water
it ends up wet and its easier to remove wet OFN which is moist than is is to remove pure water as the OFN carries the water
-
14-11-2010, 04:56 PM #3
Re: Triple Evacuation with Nitrogen
So the nitrogen being free of oxygen has some imortance because oxygen is a component of the water molecule? I wonder how this has importance?
Does the OFN operate as a sponge or as a broom?
Why did we use, say, R22 on R22 plant, back a few decades ago, to perform a tripple vac - why did we not use OFN back then already?
-
14-11-2010, 05:01 PM #4
Re: Triple Evacuation with Nitrogen
I can not comment as I have never used R22 as purge gas
-
14-11-2010, 05:23 PM #5
-
14-11-2010, 06:06 PM #6
Re: Triple Evacuation with Nitrogen
-
14-11-2010, 06:41 PM #7
-
15-11-2010, 09:02 PM #8
Re: Triple Evacuation with Nitrogen
A long time ago all the refrigerants were Cheap in todays terms.
Also the triple evacuation was also described as a dilution method for large systems.
You must appreciate that Vac pump are so much better than they were years ago.
You might not appreciate this but when most commercial plant was open drive we did not have vac pumps, we used the system compressor to evacuate the system.
On large systems it was not possible to reduce moisture to the level we now consider acceptable and the triple or dilution method was the industry norm.
It meant reducing the system to the lowest vacuum you could achieve and adding refrigerant to absorb non condensible and then evacuate again the dilute the residual non condensible constituents.
Don't forget we did not know then that we were environment criminals, it was we all did.
It was the norm to vent service cylinders to reduce the pressure so you could recharge them.
How time change.
-
14-11-2010, 08:45 PM #9
-
14-11-2010, 09:41 PM #10
Re: Triple Evacuation with Nitrogen
Pure nitrogen and OFN, are the same thing, you purchase OFN as a method of purity.
Triple evacuation, if you have free moisture (water droplets), when you go into a deep vac, you go below the triple point. Some of the water vapourize and some will turn into ICE. Continue vacuum, the ice will sublime (like dry ice "solid CO2") The density of the vapour can be many 1000M3 per Kg, so your vac pump has little or no effect. You now add nitrogen basically to add energy to the system, raising the boiling point above the tripple point.
-
14-11-2010, 09:51 PM #11
Re: Triple Evacuation with Nitrogen
The specific heat of nitrogen is 1kJ/kg.K
The heat required to vapourise, or sublime if necessary - if you have been vacuuming for too long before the trip-vac process, water is 2.5kJ/gram.
So if the nitrogen is 1K warmer than the free water and all of that 1kg comes into contact with the free water then it would vapourise less than half a gram.
The problem is the nitrogen expands into the system causing a temperature there actually colder than the water or ice is.
So how does it all work?
-
14-11-2010, 10:09 PM #12
Re: Triple Evacuation with Nitrogen
The expansion of the nitrogen is the joule thompson effect,
How ever whilst the nitrogen is being introduced it is picking energy up from the thermal mass of the system. thus adding energy to the system.
If you are adding 1Kg of nitrogen, approx 0.8m3, you are looking a an system thermal mass of in excess of 100kg, so the nitrogen would warm close to the mass temperature, which inturn would add energy to the free H2O. On a system like this we should be talking about removing grams of water, not KGs of water. (that then becomes a different kettle of fish), then I would use heat and velocity pressure to remove large excesses
-
14-11-2010, 10:19 PM #13
Re: Triple Evacuation with Nitrogen
Okay, say the 1kg of nitrogen warms up to 20°C being the ambient temperature. And the free water in the system is at -11°C (frozen) and is 1 gram in weight.
The 1kg of nitrogen would be able to pass 31kJ of heat to that gram of moisture. But how much actually comes in contact with the 1 gram of moisture? How much does the ice actually subcool on account of the rise in system pressure? Does the heat cause more vaporisation than the pressure causes subcooling?
-
14-11-2010, 10:28 PM #14
-
15-11-2010, 08:29 AM #15
Re: Triple Evacuation with Nitrogen
Hi there,
I found that with triple evacuation:
1. the vacuuming time is shortened
2. the vac-gauge reading after the vac-pump is turned off is a lot more stable, rises less
Mitsubishi City Multi is asking for triple evacuation on their commissioning sheet.
-
15-11-2010, 06:06 PM #16
Re: Triple Evacuation with Nitrogen
As I understand it, the advantage to triple evacuation is the progressive dilution and removal of the original contaminants.
This effect can be amplified by breaking vacuum at the discharge port, which pushes contaminants through the loop to the suction port, where the next evacuation is then started.
The advantage to breaking the vacuum with OFN as opposed to refrigerant is that the OFN is not absorbed by the oil in the compressor, which makes the next evacuation much quicker and easier.
-
15-11-2010, 11:02 PM #17
Re: Triple Evacuation with Nitrogen
Vacuum after OFN pressure test to remove all gasses, though after first vac, residual moisture may still be present, so break vac with OFN again as this will mix with moisture at raised pressure, making removal on second vac more efficient. As it has been said before, vacs were usually broken with refrigerant(as refrigerant was dry and did OFN's job),however, on a system known to be contaminated, OFN/vac proceedure will not achieve desired result as moisture in the oil will not be removed. A drop of H2O covered by oil will remain in place after evacuation, so oil change is also necessary.
-
25-11-2010, 02:26 PM #18
Re: Triple Evacuation with Nitrogen
I let my vacuum gauge determine the length and number of times I vac a system. From experience I have learned that if you are slow in reaching 500 microns the first vac cycle, break it with N2 and it will go faster.
-
25-11-2010, 04:56 PM #19
Similar Threads
-
Triple evacuation
By kpapa in forum FundamentalsReplies: 32Last Post: 22-02-2010, 10:35 PM -
Nitrogen Condensing by Temperarture
By Reflexive in forum Technical DiscussionsReplies: 0Last Post: 21-12-2008, 12:23 AM -
measuring vacuum
By nadeem majeed in forum FundamentalsReplies: 12Last Post: 09-11-2008, 11:03 AM -
Triple evacuation on split system
By batfink in forum Technical DiscussionsReplies: 10Last Post: 05-10-2008, 01:11 PM -
EPA 608 Cert. Evacuation and Confusion on EPA Exam
By hvactech in forum TrainingReplies: 3Last Post: 12-09-2008, 04:07 AM