Evacuating an A/C system - vacuum vs temperature

[Barryng]

We recently replaced an air cooled 25 ton condensing unit. Although at the same elevation, the evaporator is indoors and the condensing is outdoors. There is circa 70 feet of pipe between the condensing unit and the evaporator with about 50 feet outdoors. The refrigerant lines are welded stainless steel with only a small section (<10')consisting of brazed thick wall copper. We also replaced the entire outdoor run of vapor line. None of the new pipe was dehydrated. The system was open to the atmosphere for approximately a week.

We evacuated the system prior to reinsulating the new vapor line. We had a very difficult time achieving less than 500 microns because we could not maintain a constant decreasing pressure trend. Currently evening lows have been circa 60 (with some heavy rain) and daytime highs approach 80. During the day, pressure would steadily decrease as anticipated. During the evenings pressure would significantly increase. I also noticed that pressure decreased very dramatically when the noon sun hit the vapor line. This behavior is counter intuitive. We observed this behavior over the three day period we evacuated the system.

Suspecting a leak, we comprehensively checked the entire system for leaks snooping with 130 psig N2 and then with 40 psig of refrigerant and an electronic detector but found no problems.

I can think of no explanation that would explain this behavior. It was suggested that lower evening temperatures meant we were condensing out whatever water vapor remained in the system. It seems to me that this would cause the pressure to drop, not rise, because water droplets take up significantly less volume than if in gaseous form. This also seems to imply that acceptable vacuum criteria is temperature dependent.

I would like to better undestand the behavior of air and water vapor at very low pressures. Any explanation to explain this behavior would be welcome. We were finally able to get acceptably close to our 500 micron goal when the sun warmed the lines. I have two more redundant systems to deal with in the next month and would like to better predict and control the vacuuming process. This is especially important because our facility can incur time dependent restrictions that cannot be violated when any of these units are not in service.

 

[TXiceman]

Evacuating and dehydrating a system that has been openis pretty much open ended. One thing is to presure test everything with dry nitrogen and get al of the leaks first. Next, blow down al of the low points from your high pressure leak test.

Do not use too large of a vacuum pump as it can freeze any water in the piping. Once the water thaws, the pressure will come back up. So a steady and slow pull down is better.

The best thing to do is to blow out all of the open lines, and tape them closed each day as soon as you are completed with that section. Do not leave any line open to the elements.

If the system is completely dry, you will see a slight rise in vacuum pressure as the ambient increases.

To give you an idea on a long dry out, we had a customer (refiner) hydro test (yes, with water) all of the refrigerant piping before start up (500 HP low temp chiller). Before we would start the system, it had to be dried and it was too wet to attach to a vacuum pump. They blew heated nitrogen through the system for 2 weeks (24/7). When we were able to attach a vacuum pump, we had to run with a cold trap (about 3 days) as the system was still so wet that the vacuum pump oil as load with moisture in a matter of an hour. Then it as about 2 more days on the vacuum pump to get it dry. In other words, it was almost 3 weeks to dry the system.

Ken

TXiceman

http://www.rae-corp.com

 

[imok2]

I see no reason why a high-volume vacuum pump could not be used to evacuate a smaller system.
However, in that case — as in any vacuum pump application — there are three basic rules:
First, you should use only an electronic vacuum gauge that reads down to a few hundred microns.
Second, the gauge should be connected directly to the system being evacuated.
Third, there should be a shut-off valve between the pump and the system.
The presence of a leak would be indicated not by a given number of microns rise in pressure, but by a continuous rise over a period of time. An immediate significant pressure rise after the shut-off valve is closed might simply indicate some residual refrigerant or air dissolved in the oil, or possibly some moisture remaining in the system.

In that case, open the shut-off and continue the evacuation. When the pressure reaches 500 microns and the valve is shut and the pressure rises only to 600 or 650 microns and then stops rising, I would say the evacuation is complete. If the pressure continues rising, even slowly, I would suspect a leak.When ever you evacuate a wet system you need to change the vacuum pump oil often and a cold trap will help to remove the moisture before it contanminates the oil and it also helps to lower the vapor pressure in the system

 

[spector]

Sorry Gents,

Sometimes when we chase our tails we get confused, right?

I'll offer this answer, and the odds are you will agree after considering it.

You are confused because increasing temps should cause a decrease in pressure and vice versa. And because you are concerned about your ability to remove all moisture due to conditions and the fact that you fear the piping was exposed to the elements, you have talked yourself into thinking you have a problem.

Stop and consider this:

The unit is tight and at least mostly evacuated and to me, the events you described prove it....Because when the equipment heats up it expands it increases the internal volume of the piping and vessels...And as it expands the pressure inside drops, right?

Then when it cools, the internal volume decreases and the pressure inside increases.

It behaves just the opposite of how it would were it to contain air and moisture.

 

[spector]

P.S.

Now that you see what is realy happening, I would do the final evacuation at night when the unit is coldest and the internal pressure highest so the pump doesn't have to work as hard.

 

[Barryng]

Spector, thanks for the response. I had not previously considered the effects of the internal system volume changing with temperature.

Even at 1000 microns or less, the system stills contains a finite amount of air and moisture. I believe you are saying that the pipes contract at colder temperatures and "compress" the internal volume, raising the internal pressure. I am assuming the effects of "compression" are greater than the effect of contraction. Why were the two 7 CFM vacuum pumps not able to keep up with this seemingly small change?

 

[spector]

Maybe the pumps actually do keep up with the change and your system is tight. Let's find out if they do.

The best way to find out is to pin down what at first seems to be a moving target....How do you do that?...You graph it.

Grab some large format graph paper and plot out the trends. That will freeze things for you and visually depict what is actually happening inside the system...It will also show your progress at identifying/repairing leaks and tell you when the system is finally tight.

You already know that the temps/pressure will rise and fall as temps change, right?....Yep, so when the temps/pressure graph out parallel then you have reached your max evacuation ability and proved the system is tight.

All you do to get there is continue to pump and graph temps/pressure until pressure stops falling in relation to temps....Then stop the pumps and lock the system tight and track both trends to see if they parallel each other as temps change....If pressure increases and narrows toward temp trend, then you got a leak...If they stay parallel as temp changes, you are tight.

If then max evacuation proves insufficient, you need better evacuation pumps.

What you want to see is two parallel lines below the desired target.

 

[TXiceman]

If you use a high volume pump on a small system with some free water in it, the pressuer will decrease so quickly that the water will freeze before you can vaporize it. You think you have a deep vacuum and you shut off and valve out the pump. As the ice warms and melts, the water will vaporize due to the low presure. The presure will increase more than the normally accepted few microns.

So you are much better off to use a smaller pump and take longer to evacuate and dehydrate a refrigeration system.

Ken

TXiceman

http://www.rae-corp.com

 

[spector]

Txiceman,
I agree except in this case the pressure DECREASES as the vessel warms up and expands....Then it increases as it cools and shrinks.

I'm thinking he's evacuated about as far as he can reach...Now he needs to see if he's tight.

 

[cjw81]

Some nice responses on this post. A decrease in pressure is entirely possible with the above conditions mentioned by spector and to add to that you may be boiling off refrigerant in the system during the day when the pipe gets heated, this will cause a corresponding pressure drop. I do not use evacuation as a definitive leak test for that reason (refrigerant or vapors boiling off in the crank case or other part of the system including the refrigerant drier boiling off trapped refrigerant)and have seen this condition occur many times in the field. Have also seen it drive guys nuts when they couldn't find the "[leak]."(that didn't exist) You have followed proper accepted procedure with the pressurized leak test. If you want to get a deeper vacuum, make up a pipe in a pipe and fill it with dry ice and pull the vacuum over the dry ice in addition to that charge the system again with nitrogen and pull a vacuum again. It will dramatically increase the vacuum.