Moisture Evacuation In HVAC Systems

Deep Vacuum Requirements

Let’s start with the vacuum pump itself. Vacuum pumps are rated in CFM (cubic feet per minute), and bigger does not always mean better or faster. Unlike a pressure pump, when we increase the CFM of a vacuum pump we don’t necessarily increase the evacuation speed.  While in a vacuum, the restrictions in the system being evacuated (Schrader cores, Schrader adaptors, and hose diameter) dictate the speed at which you can evacuate more than the pump size itself. According to J/B, a 7 CFM vacuum pump is good for up to 49 tons, while a 3 CFM pump is good for up to 9 tons.

Connecting The Pump

When connecting the vacuum pump, consider using a 3/8” evacuation hose. According to vacuum pump manufacturers, a 1/4” hose will only accommodate 1 CFM of vacuum. By increasing the manifold evacuation hose to 3/8”, the evacuation speed will also be increased.  It is crucial to always pull the vacuum from both high and low-side service ports. Then remove the Schrader cores from the service ports during evacuation by using an evacuation service tool core remover. This tool has a valve on it that can be used during the vacuum hold test.

Oil Change

It is necessary to change the oil before each evacuation. That may seem like a lot, but much of the moisture you remove from a system will condense in oil. A vacuum pump with wet dirty oil will not perform as well as the same pump with clean dry oil. You should always use a vacuum pump with a gas ballast, which must be opened at the beginning of the evacuation. This will help prevent the heavy moisture at the beginning of the evacuation from condensing in the pump and contaminating the oil. Be aware that many cheaper pumps don’t have a gas ballast.  On larger systems or very wet systems, oil may need to be changed during the evacuation.

Measuring the Vacuum

A compound low-side gauge is designed to read pressure, not vacuum. It reads 30” hg (inches of mercury) and is not an accurate measure of vacuum. A micron gauge measures in microns and is much more accurate. 1” hg is equal to 25,400 microns, while 30’’ hg is equal to 762,000 microns. A micron gauge should never be connected through your service hoses, but instead connected directly to the system service ports. The evacuation tool or service hose ball valves can then be used to isolate your hoses from the system being evacuated during vacuum hold test. Be aware, service hoses and gaskets can leak appearing like as system leak.

Vacuum Test

Once you have achieved a 500 micron or lower vacuum, isolate the system from the service hoses, manifold and vacuum pump at the evacuation tool valves or hose ball valves. Vacuum should stabilize and hold below 500 microns. If the vacuum rises slowly and stops, there is moisture in the system, and evacuation should be continued. If the vacuum rises to atmospheric pressure, you have a leak that needs to be repaired. If you can hold a 500 micron or lower vacuum, then you know you have a clean, dry and leak-free air conditioning or refrigeration system.

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