Key Takeaways
- Modern evacuation requires specialized tools: Large-diameter hoses, core removal tools, and dedicated vacuum setups dramatically reduce evacuation time compared to traditional manifold gauge methods
- POE oil changes everything: Hygroscopic POE oil absorbs moisture readily, requiring deeper vacuum levels and longer evacuation times than systems with mineral oil
- Cold weather creates unique challenges: Moisture can freeze in systems during winter evacuations, requiring heaters and extended pull-down times to achieve proper micron levels
- Proper setup eliminates restrictions: Removing Schrader cores and using 3/8″ to 3/4″ vacuum-rated hoses instead of 1/4″ charging hoses can cut evacuation time by hours
The Science of AC Evacuation and On-Site Pull Down
The science of how we pull an evacuation on air conditioning and refrigeration systems has changed dramatically over the years. Back in the day, technicians would purge refrigerant through the system to remove air, moisture, and other contaminants. Today’s systems demand much more precision. The introduction of POE oil has definitely changed the dynamic of evacuation procedures. POE is hygroscopic, meaning it absorbs moisture from the air like a sponge.
This article focuses on a particular job that was completed on a compressor trio (3 compressors piped in parallel to make up one circuit) on a McQuay air-cooled chiller. This real-world example demonstrates both the challenges and solutions for complex evacuation scenarios.
The Modern Evacuation Setup
We can eliminate restrictions by employing large-diameter hoses, removing Schrader cores, and eliminating manifold gauges entirely. Using a core removal tool allows techs to remove cores and re-install them when needed without breaking the vacuum. The core removal tool is a complete assembly that includes a core remover stem, a ball valve for system isolation, and sometimes an auxiliary tee for additional connections.
The ball valve allows you to isolate the system from the pump. This has many advantages, including performing decay or rise tests without disconnecting equipment, removing or installing cores under pressure or vacuum, and protecting your vacuum pump from system contamination.
Vacuum Hose Selection and Setup
A surefire way to eliminate a manifold is to run vacuum-rated hoses directly from the system to the pump. Many vacuum pumps are manufactured with several ports built into them, usually in different sizes as well. Vacuum-rated hoses range in size, but the most common are 3/8″ diameter (good baseline), 1/2″ diameter (better flow rate), and 3/4″ TruBlu hoses by Accutools (maximum flow rate).
You will need to use a micron gauge placed somewhere on the system side, not on the pump side, to monitor evacuation performance. It’s advised to keep the gauge upright to avoid any system contaminants from entering the sensor.
Step-by-Step Evacuation Procedure
After the pressure test is complete and you’ve confirmed system integrity, it is vital to ensure your vacuum pump oil is fresh and free of contaminants. It’s advised to periodically perform an ultimate vacuum test on your pump prior to evacuation by placing your micron gauge directly onto the pump itself.
The evacuation steps include: Install core removal assemblies to the system service ports and remove the Schrader cores. Attach large diameter vacuum hoses to the core removal assemblies and run them to the vacuum pump. Place a micron gauge on the system side to monitor evacuation performance. Start the pump and open up the core remover ball valves. Monitor the evacuation until reaching 500 microns or below. Perform a rise test by closing off the ball valves on the core tools and monitoring the micron gauge.
Job Site Evacuation: Real-World Challenges
Real-world job site conditions don’t always match textbook procedures. On a complex chiller job, after a 20-hour evacuation, the system was just under 1000 microns. This was far from acceptable. The pump ran overnight with temperatures below freezing. An evacuation can struggle in colder temperatures as moisture can freeze within a system. Small ice droplets can form and significantly slow down the process. Sublimation will take place under these conditions but can dramatically increase evacuation times.
To address the slow pull-down, several strategies were implemented: Performed a nitrogen sweep to help carry out moisture, Changed the vacuum pump oil for better moisture-carrying capacity, and Activated system heaters to raise internal system temperatures and vaporize trapped moisture. The heaters are typically used in extreme cold temperatures to keep system pressures up during operation.
The combination of these strategies worked. The heaters raised system internal temperatures above freezing. The new oil grabbed more contaminants effectively. The nitrogen potentially melted formed ice droplets. The pump ran for another 48 hours and achieved 335 microns – well below the target of 500 microns.
Summary
Proper evacuation has evolved from simple purging to a precise science requiring specialized tools and techniques. Implementation of large-diameter hoses, removing restrictions, and eliminating potential leak points will ensure your evacuation is as efficient as possible. Understanding when and how to employ additional techniques like nitrogen sweeps, oil changes, and system heating can mean the difference between a successful evacuation and a callback.
