Evacuating Refrigeration Systems

Key Takeaways

• Evacuation Principle: Reducing system pressure lowers water’s boiling point, allowing moisture to evaporate at room temperature for effective dehydration
• Micron Targets: Aim for 200-500 microns for most systems, with manufacturers often requiring below 300 microns to maintain warranty
• Decay Test Critical: After reaching target vacuum, system must hold with less than 100-500 micron rise in 15 minutes to verify no leaks or excess moisture
• Triple Evacuation: Alternating between vacuum and nitrogen sweeps removes stubborn moisture more effectively than straight evacuation alone
• Proper Setup: Use short, large-diameter vacuum-rated hoses, remove Schrader cores, and evacuate from multiple points for fastest results

Evacuation is Unique…

To Refrigeration and Air Conditioning Systems, when compared to the Commissioning Practices for most other types of Pressure Piping Systems. Evacuation is critical before Charging a Refrigeration system to prevent early equipment failure and ensure proper system operation. Note: this article is two of three in my series on Pressure Testing, Evacuation, and Charging.

This topic was once smaller and simpler when only aiming to pull what appeared to be a Perfect Vacuum (“30” Inches of Mercury Vacuum) on an Analog Compound Gauge. Now that Digital Micron Gauge use is common practice during Evacuation, this has driven the sophistication of both Evacuation tools and practices to a new level of accuracy and skill. Additionally, today’s manufacturers of new Refrigeration and Air Conditioning (AC) Equipment regularly specify very low Vacuum requirements before Charging to maintain OEM (Original Equipment Manufacturer) Warranty. More stringent Vacuum specifications require a higher level of skill and efficiency when carrying out Evacuation.

H2O (Water) boils at 212° Fahrenheit (°F) or 100° Celsius (°C) at Atmospheric Pressure (14.7 Pounds Per Square Inch Absolute at Sea Level). If we reduce the pressure inside a Refrigeration System, we also reduce the temperature at which water boils. This is the principle we leverage when performing system Dehydration with a Vacuum Pump – this is Evacuation. For a deeper understanding of how heat transfer works in refrigeration systems, including the relationship between pressure and temperature, see our guide on sensible vs. latent heat in HVAC systems.

Understanding “Negative” Pressure

The image above shows a Compound Gauge. This gauge has the ability to read Positive Pressure and Vacuum (Negative) Pressure, hence its name (Compound Gauge Micron Range). In the Refrigeration and AC trade, Compound Gauges are generally used on Manifold Gauges, or installed into systems that may run their Suction Pressure in a Vacuum. The increments marked on the image are:

• Inches of Mercury Vacuum (“Hg vac.) are used to roughly scale “Negative Pressures” (more accurately, Microns are used).
• Inches of Mercury (“Hg) commonly express Atmospheric Pressure (Note: there are 29.92″Hg at sea level).
• Pounds Per Square Inch Absolute (PSIA) are used to show Atmospheric Pressure or its absence in Negative Pressures. Note: the Absolute Scale starts at 0, in a Perfect Vacuum.
• Pounds Per Square Inch Gauge (PSIG) is an “adjusted” scale that shows “zero pressure” with an empty piping system, or when the gauge is held open to the atmosphere.

The markers on the above image’s Gauge in PSIG are:

• 10 PSIG – Green Line (Positive Pressure)
• 0 PSIG – Red Line (“No” Pressure, or “Flat”)
• -7.35 PSIG – Blue Line (Negative Pressure)
• -14.7 PSIG – Purple Line (Negative Pressure – Perfect Vacuum)

If the colored lines are followed to their left, the equivalent values in the other three increments are realized. This gives reference to equal values in the four scales expressed.

Micron Scale

I am not sure where the Refrigeration and AC trade would be today without the trustworthy Micron Scale. At Atmospheric Pressure (0 PSIG) there are 760,000 Microns. In a Perfect Vacuum, (29.92″Hg Vac.) there are 0 Microns.

Note: You cannot achieve a Perfect Vacuum – it is a theoretical state. These values can be seen in the below image along with their respective H2O Boiling Points.

Required Micron Values

To begin, ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) will often reference a requirement of obtaining a minimum 500-1000 Microns Vacuum before Charging a system with Refrigerant. This is to be followed by a Decay Test, to again prove that the system is leak-free and that you are not just maintaining the Negative Pressure by running the Vacuum Pump. We will detail the Decay Test near the article’s end.

Here are some common Micron Targets for Evacuation, and what scenarios they are typically used for:

• 500-1000 Microns: This is the minimum acceptable range. Very Large Systems with Auto-Purger Units (the moisture will automatically be extracted during operation), or retrofit/repair applications (oil trapped inside a Heat Exchanger that has H2O absorbed into it which slowly boils off, a closed valve may be leaking pressure by, a Compressor Shaft Seal may leak only while under Vacuum) would utilize this target.

• Below 500 Microns: This is the most utilized range that I have seen across many applications of new larger installations, and retrofit/repair applications that have no issues (no leaking valves/shaft seals, all new oil in system or oil-free system).

• Below 200 or 300 Microns: Today this is quite a standard specification from manufacturers of equipment such as Ductless Splits. This ensures a very dry system before Charging.

• Below 100 Microns: I have achieved this level of Vacuum in Compressor Test Stands and Compressor Production Lines. In my experience, this level of dehydration is reserved for equipment that must have its operating data recorded very accurately. Although obtainable, it can be more time-consuming, and often Triple Evacuation is required. If you can obtain this level on any Vacuum you are pulling, this is optimal. It would not be the most time-consuming to do this on a smaller system such as a Residential Split AC under good conditions. Note: in low vacuum/lab applications “Torr” or “Millitorr” may be used instead of Microns for accuracy.

Vacuum Pumps

The most apparent tool in an Evacuation Procedure is the Vacuum Pump. There are different types of Vacuum Pumps, which all function on the premise of reducing system pressure to a level where moisture can evaporate within the piping system, and then be drawn out in the vapor state by the pump. See Leybold’s Website for a nice video animation showing Vacuum Pump operation. There is also detail on Gas Ballasts, which I will cover here next.

Gas Ballasts

Gas Ballasts are a great feature on some of the better Vacuum Pumps. They effectively allow moisture to be pushed out of the Vacuum Pump during the first part/start of the Evacuation, then at a system Micron reading of (usually) 2000 Microns, a Manual Gas Ballast is closed. At this relatively low Vacuum Level, you can ask your oil to absorb some moisture and “do its job” in getting Evacuation completed. Think of it as saving the oil until you really need it to absorb moisture so that it does not become saturated prematurely. This improves Evacuation speed/efficiency and will make for less required oil changes, as the oil maintains its capability to absorb moisture for longer.

Types of Vacuum Pumps

There are Portable and Non-Portable Vacuum Pumps. Portable Vacuum Pumps are the more common type in the HVAC/R industry. They are usually powered by 120-volt power, and there are many Battery Powered Options today. These pumps have a range of capacity of 1-23 CFM (Cubic Feet Per Minute) and have varying degrees of portability between job sites. These Vacuum Pumps may have no Gas Ballast, or they may be manual or automatic.

Non-Portable Vacuum Pumps (like the Leybold Model pictured below) are intended to be installed permanently due to their large size, weight, and cost. These are typically used in industrial refrigeration applications, particularly those with screw compressors that require precise oil management to prevent oil loss issues.

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Pulling the Vacuum

Following a successful pressure test, the next step before charging is to dehydrate the system by Pulling a Vacuum. Here are the steps to take to pull your vacuum:

Step 1: Validate Your Vacuum Pump

• Attach your Micron Gauge directly to your Vacuum Pump, and turn the pump on. Your pump should pull down to a very low micron value in a matter of seconds. You will perhaps see 5-30 microns within 3 seconds if there are no issues. This proves that your pump is indeed capable of pulling the micron value that you require.
• If this is not the case, change your Vacuum Pump Oil (ensure to use OEM oil if the pump’s manufacturer requests) and repeat the test to find an acceptable result.
• If you can still not validate your pump after the oil change, there may be a leaking fitting or a mechanical issue to be resolved with the pump. Check all hose seals and gaskets as worn seals are a common cause of vacuum issues.

Step 2: Ensure System Restrictions are Eliminated

• If accessing the system through a Schrader Valve, use a Schrader Core Removal Tool to remove the Schrader Core during evacuation. For systems without service valves, you may need to install permanent service valves for proper access.
• Manually or electronically drive open the system’s valves. If you cannot power a Solenoid Valve to mechanically open it, utilize a Solenoid Coil Magnet. Understanding expansion valve operation is crucial here, as some valves may restrict flow during evacuation.

Step 3: Hook Up Your Pump and Hoses

• It is preferable to use hoses that are large, short, and “Vacuum Rated“. This TruBlu Kit has a hose which does not collapse under negative pressure. Standard Charging Hoses are made for positive pressure, and their Internal Diameter is lessened during vacuum, which slows down the operation. Using a 3/8″ or 1/2″ Vacuum Hose is always preferable to using a 1/4″ hose. It is best to attach to the largest system access valve(s) available, such as a 3/8″ “Charging Valve” on a Chiller.
• Hook up your hoses for Vacuum with Charging in mind. Ideally, when you get to Charging you will not have to remove or modify anything, at least until you get the system into a slight positive pressure. This would avoid any possibility of compromising your Vacuum, so you do not have to move hoses/fittings around before Charging. Note: some Micron Gauges cannot see any or much Positive Pressure or they