Evacuation-Fact vs Fiction


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Evacuation defined

Evacuation is the process of dehydrating. What we do when we evacuate a refrigeration system is to lower the pressure within the system to a point where water will boil at a lower temperature.This is a degassing process.The water vapor is removed from the refrigeration system as a gas.

Water boils at 212°F at sea level. However, if we pull a 500-micron vacuum on a sealed refrigeration system, the water inside of that system will boil at -12°F. Of course, the system must be sealed,or no vacuum can be established.

I am using the term “water” here very loosely. There is no amount of evacuation that will remove actual liquid water from a refrigeration system. What we are talking about here is moisture. The moisture that exists everywhere in the atmosphere.

What evacuation cannot do

As previously stated,evacuation cannot remove water from a system, nor can it remove particulate matter and acids. Particulate matter,like grains of sand or oxidized copper from an improperly brazed fitting,cannot be removed by evacuation.

Remember, when we evacuate a system, we are lowering the pressure within the system so that free moisture can be pulled off.Evacuation can no more remove particulate matter than it can remove gravel from a refrigeration system.Furthermore, acid removal is a chemical process that involves solvents and procedures not associated with evacuation.

Evacuation has no respect for time

There is no correct answer to the question, “How long should you evacuate a system?” Time has nothing to do with evacuation. In fact, if someone starts telling you how long you should evacuate a system, stop listening to them. Evacuation is a function of pressure, not time.It might take 10 minutes or it might take 10 hours to establish the desired degree of pressure.

The magic number in the industry is 500 microns.As stated earlier, water will boil at any temperature above -12°F in a 500-micron vacuum.So what is a micron? A micron is usually defined as a length, as a micrometer or one millionth of a meter and represented by the Greek letter mu (μ). However, in physics it is a very small unit of pressure, equal to that exerted by a column of mercury 1μ high.

Loose the low side gauge

There is a prevalent myth in our industry that you can use a low side gauge (compound gauge) to evacuate a refrigeration system. The fact is, it is impossible to read 500 microns on a low side gauge. The width of the needle on that gauge is 25,400 microns wide!You’ve got really good eyes if you can read 500 microns on that gauge.

A compound gauge reads pressures above atmospheric in pounds per square inch gauge(psig),and also pressures below atmospheric pressure in inches of mercury (“Hg). The only reason that units of pressure below atmospheric are shown at all is if you’re working on a chiller.The low side barrel will be in a vacuum. Also,when you’re pumping down a system,you would then need to know when your system pressure was below atmospheric pressure.

That 30” Hg is supposed to represent the weight of the atmosphere. In other words, if you were to extend a one square inch tube straight up from sea level to a height of 120 miles(the extent of the Earth’s atmosphere), the air inside of that tube would weigh 14.696 pounds(often rounded up to 14.7 pounds) which is equal in weight to a column of mercury 29.92 inches tall.

Gauge pressure ignores the weight of the atmosphere. When you zero out your low side gauge, you are ignoring 14.7 pounds of atmospheric pressure. Absolute pressure includes atmospheric pressure. Therefore, 0 psig is equal to 14.7 psia. Anytime you are given a gauge pressure and asked to convert it to absolute pressure, just add 14.7 to the gauge pressure to get the absolute pressure. For example, 10 psig = 24.7 psia. Gauge pressures are converted to absolute pressures to calculate compression ratios of compressors.

Necessary tools

You will not only need a micron gauge but a good, two stage vacuum pump. Single stage pumps can only evacuate to 28” Hg.Be sure to use 3-foot-long(vs 6 foot), 3/8” diameter hoses (vs 1/4”)and remove the cores from the Schrader valves using a core removal tool.This will speed up evacuation time by a factor of eight. Afterall, time is money.


Posted In: Technical Tips

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