Evacuation

Hi guys
over the week i was working on evacuating the system. new installation.
1 .manufacturer want 250 microns of vacuum.
2. well first i hooked up the vacuum pump and the Vac stat digital gauge.
3. it pulled about 850 microns. i left the vacuum pump over night running but when i went again over there in the morning someone tripped the circuit breaker. so i lost all the vacc only for 15 mins.
4. i again put nitrogen in and after half an hour evacuated the system. left the vac running over night. in the morning down to 550 microns.closed the gauges and see what vac stat says. it is slowly rising. like in the first 15 minutes from 550 to 600 and then in other 15 minutes 615.
5. so thought might be a leak . put nitrogen in again to see if there is any leak. 5 hour 380 psi no movement. left nitrogen in over the weekend. will check on Monday.
6. Remote condenser , air cooled. server room air conditioner. 25 meters of pipe run.5/8 and 7/8 pipe size.
Question. how many micron rise over a period determine moisture or leak in the system. i know some people say slow rise means moisture and fast rise means leak. but what number can we call slow and what no of microns rise is call fast. i guess depend on the ambient temp as well.

Thanks

I prefer doing it the other way. I switch on my vacuum pump with only the vacmeter attached on it. I then can see if 1. my pump is pumping low enough and as low as the last time and 2. if my vacmeter isn't faulty. I don't thrust these vacmeters and believe me, I used at east 20 different ones.And the measuring difference between 5 identical vacmeters attached to the same collector on a vacpump is huge. This step takes max 1 minute. I know that I my pump must go as low as 45 to 50 Pa.
In the meantime I take my manifold and I attach it to the system with closed service valves
I repeat the test with only my manifold. I then can see if 1. my valves on my manifold are not leaking and 2. if my hoses are not leaking. The hoses with rubber inserts may not be firmly screwed or you squeeze the hole blocked and the inserts with tefflon needs to be screwed very tight. You can always see the difference of the insert on the brass grip: rubber is rough knurling and teflon is fine knurling.
You will see that this step will takes +/- 2 minutes and you will see that reaching 45 Pa will be very difficult. I'm satisfied with the double, 90 Pa. I switch off the vacpump and see if pressure isn't rising. Even with a not completely tightened hose, you will go lower than 100 Pa. So reaching that level doesn't guarantee a good manifold.
Rising within 1 minute to 125 Pa and staying there is for me a normal rise.
If pressure doesn't rise 50 Pa within 1 minute, I open the service valves and strat pumping.
Going as low as 250 Pa isn't low enough in my opinion. A good pump placed on a system with a small leak will go lower then 250 microns. Try to go as low as the system went with the manifiold attached, 125 Pa.
If you can go that low, your system is vac tight enough.
If you switch off your pump and pressure rises steadily within minutes and continues rising, then you have a leak. You also can re-vacuum after a 5 minutes rise and see if the pump goes as low as the first time and then redo the test. Perhaps the rise will not go that fast now and this means mostly moisture.
You have to learn listen to your pump and learn how fast a system pulls down to a reasonable vac level. I can see and hear after 2 to 4 minutes on a cold room system if I have a serious leak in my system. You hear the pup-pupping sound not changing and if you see continuous smoke coming out the outlet of your pump, then there's a bigger leak somewhere. Or you can close the outlet with your thumb for some seconds an see if you have a big pressure build up by the pump.

I would of given you some repoints for such a good answer Peter.
Only it would appear that I have to give out loads nowadays.
Before I can repeat reward Anyone!
Either that or I have my favourites?
Grizzly

Thanks a lot Peter. some very good tricks you have given to me. i have less exp so i just hook the gauges and start vaccing. will not do from now onward. let see what happens tomm will post the results. another one which one of these have greater moisture ability nitrogen or refrigerant. again thanks a lots mate.

after vaccing, your supposed to shut off the vac pump and moniter the torr gauge for any backdrop, you will get some, but then if no moisture then it will stop. energising compressor sump heaters to warm the oil to aid vaccing, also removing schraeder cores helps

On larger systems, I do it 'the old fashioned way'. Vacuum till +/- 200 Pa, break the vacuum with...nitrogen.. let it stay for some minutes and then start again the vacuum process. Monitoring your vacuum is a second good way to see if system is gastight. The 1st one is this http://www.refrigeration-engineer.co...ting-technique

found this bit of info
STEP 1 - Evacuate until the micron gauge reads
no higher than 350 microns, then close off the
valve to the vacuum pump.
STEP 2 - Observe the micron gauge. Evacuation
is complete if the micron gauge does not rise
above 500 microns in one (1) minute.

350 microns = 0.35 mm Hg = 46 Pa, 500 microns = 0.50 mm Hg = 66 Pa. See they use slightly different numbers.
350 microns is for many pumps their lower limit I guess. Pulling a complete installation to that level will almost be impossible I guess.
Anyhow, vapor pressure of water at 0°C is 600 Pa.

I would not say that 350 microns is good vacuum pump limit.
My Refco 15 years old-never rebuilded 2 stage pump pumps down to 100 microns with new oil.
I worked recently with reasonably priced vacuum pumps of 1 micron end vacuum.
20 micron is standard pump rating today for low budget pumps.

If we use correct tools, end vacuum in system could be as low as 50 microns.
I have achieved 67 microns in system recently, with vacuum pump of 20 microns end vacuum after 48 hours of vacuuming at -20°C outside temperature.

Pressure rise in system, after isolation of vacuum pump, greatly depend on system volume, but I use rule of thumb of 100 microns per 1 hour mean that I should continue with vacuum drying or purge with dry nitrogen,or recheck if system is tight.

Only when I get rise slower than 100 micron per 1 hour, I go to end vacuum test.

P.S. to OP:

Check my other posts about vacuum with search function of this forum.

Try using 1/4" copper in place of the rubber gauge hoses. A lot easier to guarantee a good seal with flares than with the gauge hose for larger and new systems. Also try and remove schrader cores and core depressers in the hoses if you use them.

I built my own custom rig for vacuuming, the pump uses all 1/2" copper to a cut off solenoid should power be lost, the vac gauge placed nearest to the device being vacuumed and a quality ball valve to isolate the pump, then using those nice SS Vac lines,(I think Robin Air made them or yellow jacket) never let me down, eventually for doing Ultra-cold and cascade systems I will be adding a cold trap to it to allow better pull downs.

I keep the 1/4" down to an absolute minimum, and all ways remove the valve cores, or if I can I braze to the line set and then use a pinch off tool to separate depending how critical it is. Deeper the vac the more confident I am of the systems performance.

Now that I am back working in the trade I'll be making a new vacuuming rig, when I do I will post pictures or a schematic for you all. won't be too soon mind you.