Evacuating Refrigeration Systems | HVAC Know It All

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.

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 coloured 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.

The image’s Micron Reading of 18,144 equates to an H2O boiling point of 69°F. This reference to Room Temperature (68-72°F) shows the required level of Vacuum to allow H2O to evaporate from a Refrigeration Piping System surrounded by an Ambient Temperature of 69°F.

If the Ambient Temperature is reduced, a lower Vacuum is required to cause evaporation of H2O in the system. If the Ambient Temperature is maintained, the lower the Vacuum reached, the faster Evaporation of H2O will occur in the system. For more information on how temperature affects system operation, see our article on Non-Condensables in Refrigeration Systems.

Simply put, the lower the Micron reading you achieve during your Vacuum, the less moisture you maintain in the system. Some moisture will always remain in the system, but we aim to get this reduced as low as possible.

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. For more information on proper equipment setup, see our guide on The Science of AC Evacuation and On-Site Pull Down.

Non-Portable Vacuum Pumps (like the Leybold Model pictured below) are intended to be installed permanently due to their large size, weight, and cost.

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:

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.

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.
• 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.

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.

Note: all hoses’ O-rings/fittings should be inspected for good condition before use.

• 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 will be damaged, while some Digital Gauges can be used both for Vacuum or Pressure.
• Evacuate from two locations on the system if possible. A Tee or Y-Fitting will help you join your hoses to the Pump in this manner. This is shown below in the diagram with one connection point on the Suction Line, and one on the Liquid Line. This will allow you to remove moisture from two spots in the system, speeding up your Evacuation Process. Preferably, these two locations are far apart from each other/separated by system components. Note: two separate Vacuum Pumps are commonly used at once on a Large System, or if you are looking to get to Charging quickly.
• You can use Manifold Gauges to pull your Vacuum through (see final image) but know this is not the most efficient. The non-vacuum hoses (yellow, red, blue) will collapse slowing Evacuation, and the Manifold has a large additional number of leak points. You may have time on your side (other tasks to do on the job site, or you are returning to the site the next day), or be working on a quite small and/or new systems with no additional contaminants. Avoid pulling a Vacuum through Manifold gauges for maximum efficiency.
• Nylog Blue may be used to shore up any fitting you would like in your Vacuum Pump/Hose assembly. At least having this on hand to assist with any connection points you may encounter problems with is advisable.
• Install the Micron Gauge as far away from where you are pulling the Vacuum as possible. This will ensure you are not getting a “False Reading” from the Gauge of the system’s Micron Level.

4. Begin Evacuating

• If you are blowing down Nitrogen (Element N2) from a Pressure Test or Holding Charge, blow down the system to 1-2 PSIG (more pressure than this will push/spray the oil out of your pump). You do not want the system Flat before Evacuation, as moisture would make its way back into the system.
• Ensure Gas Ballasts are open and turn on your Vacuum Pump(s). Open their isolation valves to begin Evacuating the system. An eye should be kept on your Micron Gauge to ensure the system begins pulling down from 760,000 Microns. It is a good time now to double-check that all required valves are open, and your connections are tight. If a leak is expected during Vacuum, the Inficon Whisper is a helpful Leak Detector due to its Ultrasonic capabilities (you can hear the leak with its headset). Note: sometimes things leak under Vacuum which had no issue holding Positive Pressure.
• As moisture is removed from the system over hours or days, changing Vacuum Pump Oil in your Vacuum Pump may be necessary. You may again validate your Vacuum Pump while giving it a break from Evacuation to be certain. It is however common to simply change the oil after an overnight pulldown if you are not yet at the desired Micron Target. Ensure you are careful to close the isolation valve from the system before turning off your pump for oil change/validation. If applicable, close the Gas Ballast at a reading of 2000 Microns. Note: if you need to leave for the day and finish Evacuation the following day, it is acceptable to close the Gas Ballast at 2000-5000 Microns if you have reached that point. In many cases, this will allow Evacuation the best chance of being completed overnight.
• Adding heat to the system will reduce the Vacuum Requirement for moisture to evaporate. An example of this would be using a heat gun to warm a Receiver or Accumulator. This can speed up your vacuum if applicable.
• Note: evacuating systems that are fully or partially outdoors in low ambient temperatures will make evacuation take longer to complete due to water’s lower propensity to evaporate. Any H2O below the freezing point may also have turned to ice and would need to Sublimate. This can be sped up through Triple Evacuation or adding heat.

5. Completing Evacuation

• Once you have achieved your Target Micron Range you are ready to start the process of Completing Evacuation. We will use a Target of 200 Microns in this example. Say that you have come back to your Vacuum at 7:00 am and read the Micron Gauge at 89 Microns (see the image above the “Conclusion” Paragraph).
• Perform a Decay Test on your system by closing the isolation valve on your Vacuum Pump, and then turning your Pump off. (The pump is turned off to save power, avoid wear on the pump, and prevent noise. It is not harmful to keep the pump running.)
• Observe the Micron Gauge over 15 minutes. You aim to see a rise of no more than 100 Microns preferably, but a rise of up to 500 Microns in 15 minutes is acceptable. A rise greater than this indicates that you have an unacceptable quantity of moisture in your system, and you are not done Evacuating (sweeping with Nitrogen may be an option at this point). A rapid Micron rise well out of this range indicates that you have a leak.
• The time is now 7:15 am and 15 minutes have passed – you read your Micron Gauge at 139 Microns (rise of 50 Microns in 15 minutes). You have passed the Decay Test, and are ready to Charge.
• Note: on large-volume systems, it can be useful to do an Extended Decay Test up to 1 hour for a Final Leak Check. If the Micron Value does not stop rising in this time frame, a leak likely exists. Without this test, you would find this leak when you went to charge refrigerant. If you can make the time for it, carrying out an Extended Decay Test may ultimately save you time later for large-volume systems.

Triple Evacuation (Nitrogen Sweeping)

If you find that you are having challenges in removing moisture with straight Evacuation, 1-2 “Sweeps” of Nitrogen through your system may be most helpful in completing your Vacuum quickly. This would be more of a reactive instance of carrying out a Nitrogen Sweep.

Proactively, Triple Evacuation can be done as a planned process. This could be done as practice on all systems, or you may want to do this if you are aiming for a very low Micron Range below 100 Microns, or if you anticipate needing to remove considerable moisture from a system. Here are the steps to Triple Evacuation:

1. Evacuate to 1000 Microns.
2. Purge 5-10 PSIG of Nitrogen through the system for 5 minutes. Ensure that you are pushing the Nitrogen through the entire system, and releasing it at an opposite point. This is so that the Nitrogen has the maximum ability to Entrain H2O with it.
3. Blow the system down to 1-2 PSIG and Evacuate to 500 Microns.
4. Purge 5-10 PSIG of Nitrogen through the system for 5 minutes.
5. Blow the system down to 1-2 PSIG, and Evacuate to your Final Target Micron Range and perform a Decay Test.

Triple Evacuation works because you go back and forth using two different methods of moisture removal on the system (evacuation and purging with a dry gas). You will notice a faster Vacuum pull- down after each Sweep. I have used it with great success on systems that were stubborn during Evacuation.

Conclusion

Evacuation is one of the most important steps in Commissioning Refrigeration and AC Systems. With some planning and attention to detail, Evacuation can be carried out with confidence to promptly prepare your system for Charging. I will cover Charging next in the final article of this series.