This is the third of three columns focused on the process of evacuating a refrigeration system and focuses on the test element of that process, including a look at how the vacuum fits within the rest of the test regime.
In last month’s column I noted that the objective of pulling a vacuum is not the same from job to job. In a new build field-erected system a strength integrity will be required for all the site-installed pipework, and often this will extend to some of the factory built equipment, too, if it cannot reasonably be isolated from the test. The site strength test will typically use a gas as the test medium because of the difficulty of drying out the system after a hydraulic test. This is a dangerous procedure—probably one of the most hazardous things that we do—because the newly installed pipework is being raised to a pressure it has never seen before. Usually oxygen-free nitrogen is used as the strength test medium. Oxygen under pressure would be highly dangerous and air is nearly as bad.
Once the strength test has been passed the pressure can be slightly reduced and a tightness test can be conducted. This involves getting close to the unit and hunting down any possible leaks that would result in refrigerant loss if the plant was charged. The strength test does not need to be maintained for long, but a valuable part of the tightness test is leaving the pressure high for several hours and seeing whether it varies. Variation, of course, might be due to temperature changes as well as leaks, but as a colleague said to me recently, “It’s a funny thing but nobody ever reports the tightness test pressure going up due to temperature change.”
These test procedures highlight the differences in mental attitude to the task in hand. Some people view the exercise as a box to be ticked on the way to project completion. They will run the tightness test at as low a pressure as they can get away with and their heart will not really be in it. Then they will tick the box and move on.Others see it as their mission to find a leak. Their mental attitude is that the leak exists and the challenge is to nail it. It is far easier to be motivated to find a needle in a haystack if you really believe in the needle. The second type of person will aim to run the tightness test at as high a pressure as possible. That makes needle finding easier.
Once the tightness test has been completed and the nitrogen has been safely vented to atmosphere, it is necessary to get the vacuum pump connected and persuade the rest of the test gas to leave the scene. For the dedicated needle hunters this is a second bite at the cherry, if I may mix my metaphors. Some systems will be tight under pressure but will leak under vacuum. For example, if there is an inclusion in a weld that blocks the gap when pressurized but is pushed in by atmospheric pressure when the system is under vacuum, leaving a pinhole which then leaks refrigerant when the system is charged.
In other circumstances the vacuum process is trickier. When the refrigerant is being changed, for example a conversion from R-22 to an HFC blend like R-422D, the O-ring seals in the plant may have been leak-free before the process of conversion started, but once they have been pulled out of shape by the vacuum then pushed back again by the new refrigerant, they will leak badly. The leak is not caused by the new refrigerant, but by the journey to get from old to new. It is quicker and easier to recover the old refrigerant, open up the system while it’s empty, change all the seals, box it up, pull a vacuum and then charge the new gas than it is to mess about trying to change some of the seals once the new refrigerant is already in the system and is already leaking.