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Using Heat & Nitrogen In Vacuum
In these tips and on the podcast, we talk a lot about the importance of deep vacuum, and deep vacuum using proper processes and tools is the best. It's really the ONLY way to ensure that a system is clean, dry, and tight.
Just for review, that means:
Nitrogen purge, flow and pressurize > Cores removed > Vacuum gauge on the system > Large hoses > Vacuum pump tested > Pull to below 500 > perform an isolation “decay” test
This is all great and works just as advertised, but sometimes, techs can still get frustrated when pulling on systems that were previously in service or that are abnormally wet. This is when nitrogen sweeping and extra heat can come in handy.
Understand the Micron Gauge
A micron is one-millionth of a meter of mercury column. In other words, it's a REALLY tiny amount of pressure, and it requires a very accurate and precise instrument to measure.
A vacuum or “micron” gauge is a thermistor sensor that relies on heat transfer from the thermistor to measure the vacuum level. Because vacuum is a poor conductor of heat, the deeper the vacuum, the less heat transferred, which allows it to measure the vacuum level.
The gauge is calibrated to nitrogen or air, NOT refrigerant, so any small pockets of refrigerant in the system or the oil can interfere with the measurement.
When you are working on a system that previously had refrigerant in it, and you recover the charge down to atmospheric pressure, there is still a lot of refrigerant in the system. The result can be an erratic reading on the gauge and “stalling” of the vacuum as refrigerant slowly escapes from the oil.
Sweeping the system with a high-velocity flow of nitrogen can help displace the refrigerant and agitate the oil and help to free it up and remove it. (You can read more about sweeping with nitrogen with input from Jim Bergmann HERE.)
The same is also true with abnormally wet systems. While the nitrogen isn't a magical sponge soaking up water, flowing some nitrogen can help get the wet system moving.
When running into these issues during low-temperature conditions or on systems like freezers where components are still in the cold box, it can be helpful to use a heat gun to warm cold areas or parts that hold oil, such as the compressor or accumulator.
You can use heat during a deep vacuum and then break with nitrogen to help the process along.
This does not replace the deep vacuum process but can certainly help it along when you find yourself getting stuck.