Atmospheric air will enter a refrigeration system any time it is opened for service or any time a new system is being assembled. Atmospheric air contains nitrogen, oxygen, and water vapor, all of which are harmful to a refrigeration system.
Nitrogen is a noncondensable. It will not condense in the condenser, but it will occupy space that is needed to condense refrigerant. This will cause an increase in the high-side pressure, resulting in higher discharge temperatures and higher compression ratios at the compressor. The oxygen and water vapor can cause a chemical reaction that will produce acids in the system. These acids cause deterioration of the system’s parts, copper plating, and a breakdown of the motor’s insulation, leading to premature compressor failure.
Removing air from a system is called degassing, and removing water vapor from a system is called dehydration. Together the process is called evacuation: degassing + dehydration = evacuation. Using a vacuum pump, a technician must reduce a system’s pressure below the atmospheric pressure of air surrounding the system to an acceptable (vacuum) level to ensure the system is properly evacuated.
Vacuum levels are commonly measured in terms of in. Hg (inches of mercury), mm Hg (Torr), and microns. These values are derived from an experiment using a bell jar, a clear tube with one end sealed and the other end open, a bowl, and some mercury (see Figure 1 - top). Mercury is poured into the bowl and the tube, filling the tube to the top. The open end of the tube is temporarily sealed and then inverted and placed into the bowl of mercury. When the temporary seal is removed, the height of the mercury in the tube will rise to represent the pressure of the atmospheric air pressure pushing down on the bowl of mercury.
At standard atmospheric pressure, sea level, and 59ºF, the mercury in the tube will rise up the tube 29.92 in. (760 mm). If the tube and bowl were placed within a ball jar, at first the height of the mercury would remain unchanged, since the atmospheric pressure surrounding the bowl and tube also remained unchanged. However, if a vacuum pump were hooked up to the bell jar, as the atmospheric pressure within the bell jar was reduced, the height of the mercury in the tube would begin to drop, representing the reduction in atmospheric pressure within the bell jar.
If the perfect vacuum were pulled on the bell jar, the mercury would not be able to rise up the tube and would be at a zero level, again representing a perfect vacuum within the bell jar, since there is no pressure left to cause the mercury to rise up the tube. This relationship is used to express the pressures below atmospheric pressure and the degree of which a vessel is under a vacuum.
When speaking in terms of a vacuum, the term in. Hg. vacuum is used, also referred to as in. Hg. gauge. The values are inverted so that the height of a column with 29.92 in. Hg. vacuum represents a perfect vacuum and 0 in. Hg. vacuum represents no vacuum present.
A micron is another unit of measurement when discussing vacuum levels, and today, the most often used measurement by refrigeration technicians. One micron equals 1/25,400 of 1 inch; 25,400 microns equal 1 inch. So using this vacuum scale, 759,968 (760,000) microns represents atmospheric pressure, and 0 microns represent a perfect vacuum. Refrigeration systems need to be exposed to vacuums well below 29 inches Hg vacuum. Reading vacuum pressure below this measurement is very difficult, and using microns allows for a more precise unit of vacuum. Since a vacuum level of 25,400 microns equals 28.92 inches Hg vacuum, very low vacuum levels can be easily read. A vacuum level of 500 microns would represent a system under a very deep vacuum.
Regardless of the measurement used, technicians should pull the necessary vacuum to ensure the system they are working on is properly evacuated.
