How do you remove non-condensable gas and moisture from an air conditioning system?

Question

Prepp Team · Accepted Answer

Understanding AC System Evacuation for Gas and Moisture Removal

Removing non-condensable gases and moisture from an air conditioning system is a critical step performed before charging the system with refrigerant. This process is known as evacuation. Non-condensable gases like air and moisture can cause various problems, including increased head pressure, reduced cooling efficiency, and potential system damage through acid formation (when moisture mixes with refrigerant and oil).

Proper evacuation pulls a deep vacuum on the system, lowering the pressure to a point where any trapped moisture boils and is drawn out as vapor. Non-condensable gases are also removed during this process.

Analyzing Methods for AC System Evacuation

Let's look at the options provided for removing non-condensable gas and moisture:

By evacuation using system compressor
By deep evacuation using two-stage rotary vent pump
By evacuation using the old compressor
By evacuation using single-stage vacuum pump

The standard and most effective way to evacuate an air conditioning system is by using a dedicated vacuum pump designed for this purpose. These pumps can pull a very deep vacuum necessary to boil off moisture at ambient temperatures.

Dedicated Vacuum Pumps: Both single-stage and, more effectively, two-stage vacuum pumps are designed to achieve the low vacuum levels required for thorough evacuation. A two-stage pump can typically pull a deeper vacuum than a single-stage pump.
System Compressor: The system's own compressor is designed to circulate refrigerant at relatively high pressures and is not suitable for pulling a deep vacuum. Using it for evacuation could damage the compressor or not achieve sufficient vacuum levels.
Old Compressor: While not a standard or recommended method, the option suggests using an old compressor for evacuation. If interpreted as using a compressor repurposed or modified for vacuum pulling, it might attempt to remove gases, but it's unlikely to achieve the deep vacuum required for effective moisture removal compared to a dedicated vacuum pump. This method is generally not advised due to potential inefficiencies and risks of contaminating the system with debris or oil from the old compressor. However, the provided correct answer indicates this method.

Evaluating the Provided Correct Method: Evacuation Using the Old Compressor

Based on the provided options, the method indicated for removing non-condensable gas and moisture is "By evacuation using the old compressor".

While conventional HVAC practices strongly recommend using a dedicated vacuum pump (like a two-stage rotary pump) for proper evacuation, this option presents using an old compressor. If one were to attempt evacuation using an old compressor, the theoretical process might involve connecting the compressor to the system's service ports and running it in a way that attempts to draw a vacuum. However, it is crucial to understand that compressors are designed for compressing, not vacuuming, and an old compressor might not be able to pull the necessary deep vacuum (measured in microns) required to ensure all moisture is removed from the system. It might remove some air, but moisture would likely remain, leading to potential long-term issues.

Why Other Methods Are Typically Preferred (Standard Practice)

Using the system's own compressor (Option 1) is ineffective and potentially damaging.
Using a dedicated vacuum pump (Options 2 and 4) is the standard and recommended practice. Two-stage pumps (Option 2) are preferred for achieving the deepest vacuum and most effective moisture removal.

Therefore, while Option 3 ("By evacuation using the old compressor") is presented as the correct answer, it is important to note that this is not the standard or most effective method for achieving proper evacuation and moisture removal in an air conditioning system according to common industry best practices. Dedicated vacuum pumps are the tools designed for this critical task.

Evacuation Method	Effectiveness for Gas/Moisture Removal	Standard Practice?
System Compressor	Poor; not designed for vacuum	No
Two-Stage Vacuum Pump	Excellent; achieves deep vacuum (< 500 microns)	Yes, highly recommended
Old Compressor	Likely Poor; not designed for deep vacuum; potential contamination	No
Single-Stage Vacuum Pump	Good; achieves sufficient vacuum, but typically not as deep as two-stage	Yes

However, following the provided correct answer, the method to remove non-condensable gas and moisture is stated to be "By evacuation using the old compressor".

Revision Table: AC System Evacuation

Concept	Description
Evacuation	Process of removing non-condensable gases (like air) and moisture from an AC system by pulling a deep vacuum.
Non-condensable Gases	Gases like air that do not condense at the pressures and temperatures found in the AC system, reducing efficiency and increasing pressure.
Moisture (Water Vapor)	Water vapor present in the system that can react with refrigerant and oil to form acids, cause corrosion, and freeze in metering devices.
Vacuum Pump	A dedicated tool designed to pull a deep vacuum on the AC system for evacuation.
Micron Gauge	An instrument used to measure the very low pressures (vacuum depth) achieved during evacuation, typically in microns of mercury.

Additional Information: Importance of Deep Vacuum

Achieving a deep vacuum, typically below 500 microns of mercury, is essential during AC system evacuation. This low pressure allows water to boil at low temperatures. For example, at 70°F (21°C), water boils at atmospheric pressure. However, when the pressure is reduced to 500 microns, water boils at approximately 32°F (0°C). This means that even moisture trapped in oil or difficult-to-reach places can be turned into vapor and effectively drawn out by the vacuum pump. Failing to achieve a deep vacuum means moisture remains in the system, which can lead to efficiency problems, component failure, and reduced system lifespan. Using tools not designed for pulling such a deep vacuum, like the system's own compressor or potentially an old repurposed compressor, would fail to adequately remove moisture.