Jim Wheeler: The science of a proper vacuum
Not many people think of the HVACR industry or of service technicians in general as being scientists.
Yet, the entire refrigeration process works by thermodynamic principles and laws that few laypeople understand. And when it comes to pulling a vacuum in an enclosed compression-cycle system, even technical people who have worked in the industry for many years may not fully grasp what is going on or what they are trying to accomplish. In fact, for this reason, many are trying to do the job without the proper tools and by following processes that work poorly or not at all.
Why is it necessary to pull a deep vacuum on an air-conditioning or refrigeration system that has been opened to the atmosphere or in many cases just serviced? Because the greatest efficiency robber and compressor killer of refrigerating systems is air and moisture that is introduced into the lines by being opened just momentarily, or by a refrigerant leak that has put the system in a vacuum, or by air that is introduced by service gauges. It is the most common cause of compressor failures.
Understand that all air contains some moisture. And when even small amounts of humidity get into an enclosed cooling system, it contaminates the compressor lubricant, which decreases its lubricating capacity and leads to shorted motor windings and locked rotors.
Also, even air itself, since it can’t condense, leaves bubbles in the enclosed system, which takes the place of the cooling refrigerant, which, in turn, decreases the cooling capacity while causing the compressor to draw more electricity. So, all the air and moisture must be removed from a cooling system for it to operate efficiently and to last a long time.
The problem is any system that was installed without pulling a deep vacuum or that has been serviced several times where air may have been introduced into the closed loop, probably has some air and some moisture in the lubricant. Also, even where a vacuum was pulled, in many cases, it was probably done wrong and with the wrong tools.
What is a service tech trying to do when he or she is pulling a vacuum? The point is to remove all the air and the moisture. And if the moisture has been absorbed into the lubricant or is in the form of droplets, the person must pull a vacuum so deep that it causes the moisture to literally “boil” away, and this takes some time and the proper equipment. What equipment?
- It takes a vacuum pump that has been properly serviced (oil changed), which can pull below 500 microns.
- It takes a 6-knob manifold gauge set.
- It takes an oversized vacuum hose and good service hoses that won’t collapse or leak in a deep vacuum.
- It takes a micron-reading vacuum gauge (no, pressure gauges don’t work).
Why does a service tech need a micron-reading vacuum gauge? Well, this is where the science begins. When moisture starts to be removed from the compressor lubricant or when there’s a leak in the system, the vacuum process slows down significantly, which is an indication that either there is a problem that requires finding and fixing the leak or pulling a vacuum until all the moisture is gone. Which is it?
Well, a system leak will show up consistently, while evaporating moisture will show a slow but steady pressure drop.
I might mention that recently I saw a new type of vacuum gauge called BluVac that sends a Bluetooth signal to a service tech’s cellphone where an app the company supplies gives a visual readout of the vacuum’s progress, and it also suggests the cause for any slowdown in the vacuum process.