High Vacuum/Deep Vacuum

As illustrated above, the purpose of a vacuum pump is to reduce the internal system pressure of a refrigeration/air conditioning system so moisture and other contaminants can be removed.

The term "high vacuum" describes a condition where the internal system pressure is extremely low, or close to a perfect vacuum. The higher the vacuum is in a system, the closer the micron reading is to 0 microns. Deep vacuum can be thought of in the same way. The deeper a vacuum is, the closer the micron reading is to 0 microns.

"High vacuum" and "deep vacuum" describe the same condition inside a closed system. For refrigeration/air conditioning service applications, high vacuum = good vacuum, or a low micron reading on the system.

Selecting High Vacuum Pumps

To make the following discussion easier to understand, it would be best from this point on to think of pressure (psi) as the amount of mercury it will support, e.g. atmospheric pressure at 29.92" Hg instead of 14.7 psi. This will permit us to use Chart II as a visual aid when determining the vacuum which must be attained to boil water under various ambient temperatures.

Chart II shows a vacuum pump capable of eliminating all but one inch of mercury. It is able to remove moisture at an ambient temperature of 80 degrees F or over. While any pump pulling within one inch of atmospheric pressure can eliminate moisture, it must also be capable of holding that vacuum throughout the dehydration process. In addition, it must pull that vacuum on the entire system and not simply at the intake of the pump.

Gas Ballast (Vented Exhaust) Pumps

The gas ballast or vented exhaust feature on some vacuum pumps permits relatively dry air from the atmosphere to enter the second stage of the pump. This air reduces the compression in the final stage, which helps to prevent moisture from condensing into a liquid and mixing with the vacuum pump oil. Here's a comparison to show how it works:

Imagine a damp towel being twisted until water drops out. This can be compared to a high vacuum pump which is not equipped with a gas ballast. Moisture being pulled from a wet refrigerant system is compressed internally in the vacuum pump and condenses into a liquid. Now imagine the same damp towel entwined with a dry towel and then twisted. It would take a considerable amount of twisting before any water would drop out. Thus the process of the gas ballast arrangement permits the moisture-laden air passing through the pump to mix with relatively dry air to such a degree that compression does not cause condensation.

The damp towel comparison also illustrates why this valving feature cannot handle large amounts of moisture. If the towel is almost saturated with water, even the introduction of the completely dry towel will not prevent some of the water from dropping out when the two are compressed. Because of this, some pumps are designed to run with both high internal temperatures (to reduce moisture condensation in the oil) and a gas ballast.

Due to the "severe" application of a vacuum pump when used to "boil water," it is necessary to select a quality two-stage model equipped with a gas ballast and high internal running temperature to achieve adequate performance over a long period of time. However,  even the best pump must have regular maintenance to perform optimally.

Frequent oil changes should be anticipated and considered as the single most important factor in a preventative maintenance program. Remember, even a pump equipped with a gas ballast cannot handle large amounts of moisture without some being condensed into the oil. If allowed to remain inside the pump, this moisture will result in lock ups or loss of efficiency and/or capacity. Normally, oil changes will not be required during a single dehydration job. But it would be well to change oil after each major pump down. This is especially critical when pumping down a system known to be wet or experiencing a compressor burn-out.