A vacuum pump doesn't actually vacuum out the system, but rather removes moisture by lowering the pressure within the system and reducing the boiling point of water at normal temperatures, thus vaporizing the moisture and air out of the system.
For vaporization to occur, you need to expose the system to 1.066 psia at a temperature of 104 F (the boiling point of water is 27.75 inches of vacuum at 104 F). The lower the internal pressure the lower the boiling point of the moisture. So the lower you bring the system, the less heat you need. Once all the moisture has been vaporized, both the pressure and the heat dissipation rate will decrease, and the measurement on the gauges will decrease.
The more liquid there is in a system the longer it will take to pull a vacuum. If a system has a leak, the vacuum will not be deep and the needle will rise when the valves are closed and the leak or the moisture in the system will not allow you to produce a deep vacuum.
There are two main types of vacuum pumps: single-stage and two-stage. Single-stage pumps are used when we do a triple evacuation style. The two-stage pumps are used when we need to pull a deep vacuum. Most two-stage vacuum pumps can pull down to about 50 to 80 microns.
No pump will remove all the air. If your pump can produce 28 inches Hg, it will remove only about 94 percent of the air. You will need to heat the moisture to about 100° F or higher, then the moisture can be pumped out as vapor.
Some service techs evacuate a system for eight hours at a high temperature or some will do it even longer at a lower temperature. Often the evacuation procedure needs to be repeated a few times to ensure that it is free of moisture, but only by the use of a micron meter can we determine the level of vacuum achieved.
If a manifold is used, then it must be degassed for a few days prior to evacuation. This is done to remove all the vapors in the manifold and
to ensure correct readings. Then the system must be evacuated from the high side and the low side. Use as large a diameter and as short a line as possible.
You can attach a lamp in series with a test cord to the compressor and common terminals to act as a resistor, allowing a small amount of voltage to go through the compressor windings, warming them. This allows the moisture trapped in the system to vaporize.
The actual procedure is starting with a low-vacuum and then switching to a deep-vacuum pump. You can repeat this process until 300 microns is indicated on the gauges. A technician then will know that the system being worked on has been thoroughly evacuated and that it is dry.
The best way to determine if there is moisture in the system or if the system has a leak is to use a deep-vacuum gauge. The correct way to check the system is to stop the vacuum pump and close the valve to the pump. Watch the deep-vacuum gauge. If it rises, there is still moisture in the system.
A regular vacuum gauge cannot be used to measure deep vacuum because it will not be accurate enough. You need to use a solid-state thermistor vacuum gauge or an electronic vacuum gauge in this situation. Heat and pressure work together in either of the aforementioned instruments to give accurate readings. When working with a normal high-pressure gauge the dial is expanded to allow for easier reading.
For triple evacuation, you should pressure test the system for several hours (using dry carbon dioxide or dry nitrogen). Pressure is put into the system to check that there is no pressure loss. The idea is to then repeat the evacuation procedure three times. Triple evacuation does not remove all the moisture from the system; only deep vacuum and heat will completely vaporize the moisture and other solvents.
There are two excellent ways for a technician to eliminate moisture from a refrigerant system using the boiling process:
• Apply heat to the system, causing the moisture in it to boil.
• You can use a deep-vacuum pump to reduce the pressure and the boiling point of water in the system.
Errol Becker is a technical support specialist at Advanced Test Products.