Why do we triple evacuate a system with nitrogen prior to charging?
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Why do we triple evacuate a system with nitrogen prior to charging?
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:)
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?Quote:
Originally Posted by Quality
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?
I can not comment as I have never used R22 as purge gas
The importance of N2 being oxygen free is that oxygen can react violently with oil and cause explosionQuote:
Originally Posted by DTLarca
Sure - but that statement seems to come from a position that assumes there is a real purpose and even a real need for tripple evacuation. I'm sort of asking why do so many people assume that tripple evacuation with nitrogen is even necessary.Quote:
Originally Posted by james10
Sure, but what proof do you have that so many people belive this to be necessary?Quote:
Originally Posted by DTLarca
I think it is left over from when single stage vac pumps were more common ;)
Jon :)
You're right - I don't have inductive evidence but rather only abductive evidence - the sort Sherlocke Holmes used as opposed to the sort an election exit pollster would use :)Quote:
Originally Posted by monkey spanners
But that seems a good point regarding single stage pumps.
But then why would they rely of triple-vac when using single stage pumps?
It never make sense to me. I never use this method.Quote:
Originally Posted by DTLarca
A deep vacuum with a good vacuum gauge, live it for at least one hour [depends on the system size].
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.
The specific heat of nitrogen is 1kJ/kg.KQuote:
Originally Posted by mad fridgie
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?
I think they used to break the vacuum with refrigerant. That way if the pump only gets out 98% on each vac by the third one there is very little air left in. I think the moisture thing is a bit of a red herring.Quote:
Originally Posted by DTLarca
People also tend to think if one of something is good then more must be better.
There may be a case of people not wanting to appear lax in their working practices so they claim to do what they think or have heard is best practice. I for example never ever braze without ofn...
I have found on systems that are taking a long time to vacuuming that putting a few bar of ofn in and starting again helps speed things along, i suspect the slowness is due to refrigerant disolved in the oil and the ofn going in stirs the oil up a bit (often movement can be seen in the comp sight glass for example)
Jon :)
The expansion of the nitrogen is the joule thompson effect,Quote:
Originally Posted by DTLarca
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
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.Quote:
Originally Posted by mad fridgie
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?
Indeed - the idea was that the first vac would leave 4% contaminants. Then the next 4% of 4% which is 0.16% and then the next was 4% of 0.16% which gives basically 0%.Quote:
Originally Posted by monkey spanners
But I'm not sure whether it would mean the next vacuum was quicker after stopping to introduce a little nitrogen. Did you ever stop to introduce a little air and see if it was also quicker second time :)
The nitrogen is acting as a heat transfer medium, as only radiation is transfered in a vacuum. Therefore, it is not just the energy of the nitrogen but the continual energy pick up from the system due to convection/conduction.Quote:
Originally Posted by DTLarca
And how much heat does it transfer :)Quote:
Originally Posted by mad fridgie
It transfers enough energy until equalibrium is reached in both temperature and vapour pressure.Quote:
Originally Posted by DTLarca
Your next question should now really involve "Time"
Yes, time is a part of the question - but a fundamental part of the question. We know that the vacuum pump on it's own - with multiple oil changes and/or a cold-trap will work well - given time - but we want it all speeded up - so we come along with this nitrogen idea. The question is "Is it more effective in removing free water than is simply vacuuming with heat and time?" :)Quote:
Originally Posted by mad fridgie
My conclusions from recent experiences is that simply time and heat are required and not any interference with nitrogen.
On larger systems blowing hot air through the the unit works best.
Here's a picture from my trip to the Carrier factory in Lyon.
You have use the right system for the right application. I suspect that the vessel has been hydroliclly tested with water, so heaps of free water in side. Heat and velocity pressure (as stated earlier), then final vac (there should be heaps of thermal mass to vapourize what ever is left).Quote:
Originally Posted by DTLarca
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.
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.
A long time ago all the refrigerants were Cheap in todays terms.Quote:
Originally Posted by DTLarca
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.
Not intentionally....Quote:
Originally Posted by DTLarca
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.
Hey Jon, burning the midnight oil tonight, err.. last night.. whatever:confused::DQuote:
Originally Posted by monkey spanners
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.
I have seen this also.Quote:
Originally Posted by michaele
Jon :)