System evacuation is just good service common sense. You need to remove any water vapor, nitrogen, and oxygen contained in the air before you can add refrigerant. If you leave air in the system, it will collect in the condenser, taking up space and causing the high-side pressure to be higher than normal.
Any water vapor contained in the air may freeze and cause a restriction, normally at the outlet of the metering device. Also, the water vapor and oxygen can mix and, if sufficiently heated, may create acids in the system.
Completely evacuating the system using a good-quality vacuum pump and micron gauge can prevent this from occurring. Many refrigeration manufacturers recommend evacuating a system to a 500-micron level. This normally ensures that the system is completely degassed and dehydrated.
Vacuum pumps should have their oil changed on a regular basis according to the manufacturer's recommendation. The sensors used in many micron gauges may need to be cleaned.
Again, always follow the manufacturer's recommended procedures for cleaning these sensors. Many manufacturers recommend cleaning the sensors with alcohol. This can be done by adding a few drops to the 1/4-inch flare connection, then gently shaking the unit and letting it air dry.
Along with regular maintenance, a technician should always check the operation of both vacuum pump and micron gauge before using them on a system. It's better to discover a problem with either of these devices before you use them on a system and potentially waste time on the job.
When you are setting up the vacuum pump and micron gauge, make sure there is a valve installed between the system and the vacuum pump/micron gauge setup. Normally you can use the refrigeration manifold gauges for this purpose.
There should also be a valve between the micron gauge and the vacuum pump (Figure 1). This allows a technician to perform a standing vacuum test on the system once a 500-micron level is achieved.
Before you open the vacuum pump and micron gauge to the system, shut the valve between the system and the vacuum pump/micron gauge setup and run the vacuum pump. This will only pull a vacuum on the vacuum hose and micron gauge.
The micron gauge should register a 500-micron level in a relatively short amount of time. If it doesn't, there is a problem with the micron gauge, vacuum pump, or interconnecting hose. You need to identify which of these devices is causing the problem and fix it before opening the vacuum pump to the system.
Joe Marchese is the owner of Coldtronics, Pittsburgh. He may be reached at 412-734-4433, www.coldtronics.com, or email@example.com.
Publication date: 11/01/2004