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Proper evacuation after initial installation or after service where the
system has been opened to atmosphere is critical to proper operation of
an air conditioning system. Evacuation is a two step process of
degassing and dehydration. Degassing removes non condensibles which
cause increased head pressures and increased operating cost.
Where high temperatures are frequent, non condensibles combined with moisture will also cause oil
failure, decreased capacity and increased compressor wear and potential
failure. Losses associated with improper evacuation can be very high.
Moisture is the second issue. Moisture brakes down POE oil in HFC
systems, (like R410a) causing premature failure of the oil. Because POE breaks down
into its fundamental components, it can clog the metering device and
contaminate line sets. This could result in the need for complete system
replacement. Moisture refrigerant and mineral oils form acids that will
cause system failure due to copper plating and damage to compressor
A vacuum gauge is used to determine the level of atmosphere (degassing and dehydration) in the system. Quick and deep evacuation of an air conditioner or refrigeration system simply comes down to correct practices including proper installation and assembly, keeping out the moisture during fabrication and of course the right tools hoses and gauges to measure the level of degassing and dehydration. When moisture (liquid) enters a system or condenses the only way it can be removed is in a vapor. When it comes to system evacuation only small amounts of moisture are practical to remove this way. "It is not practical to remove large amounts of water with a vacuum pump as boiling water produces large amounts of water vapor. One pound of water (about 1 pint) produces about 867ft3 of water vapor at 70ºF." (1) Therefore in the words of David Boyd at Appion, "Keep it clean dry and tight".
After the lines and the various components have been installed, it is necessary to make sure that there is flow through the entire system by purging with a dry gas such as dry nitrogen from the liquid line to the suction side of the system. Purging will not only carry out small drops of water (if present) but it will also pick up some of the system moisture.
A standing pressure test is used to check for leaks by using a dry gas again like dry nitrogen. We never hope to find leaks while in a vacuum. (Although it does happen.) When air leaks in, moisture comes along for the ride which can take hours to remove if the amount is excessive. A temperature compensated pressure test like that available in the Testo series of digital manifolds will make the process fast and efficient. However if you are using a digital manifold like the Digi-Cool AK900, leaks will also be apparent simply due to the high resolution of the pressure sensors. If installing a typical residential system, the test can be performed and verified in about 15 minutes. Performing this test will again pick up some additional moisture that will not have to be removed during the evacuation process. When releasing this high pressure gas do not relieve the pressure all the way to atmospheric. Take it down to about 1 psig. so air cannot get back into the system.
Attach the micron gauge directly to the vacuum pump via the 1/4" connection and verify that the pump is capable of achieving a vacuum level of 100 microns or less. A good quality pump will easily achieve levels or below 50 microns. Pump blank offs are notorious for leaking, so do not depend on one for isolating the vacuum pump. Usr core tools to isolate the pump and the hoses thus minimizing any chance of gas permeation through the hoses. Remember even the best vacuum rated hoses will leak and that is why isolation is a necessity. If your pump cannot achieve 100 microns or less, change the oil with a high quality, low vapor pressure oil like Appion Tezom. Many times several oil changes are required to remove significant amounts of moisture from a wet pump. Compared to system breakdown, oil is cheap change it often. If the pump will still not achieve a deep vacuum, it may be time for replacement or service.
Water can only be removed from a system in vapor form. If the atmosphere that you are removing from the refrigeration system is moisture laden, as that moisture enters the pump it is in vapor form, it is in a state of equilibrium with the air in the system. This state of equilibrium is what is meant by the term ballast. (something that gives stability)The ballast, when it is open introduces free air into the pump during the discharge stroke to keep this moisture in equilibrium. If the gas ballast is closed, the pressure created in the discharge stroke will condense the water vapor and drop the moisture out into the oil. Having the ballast open during the initial pull down of a wet system will help to prevent condensation within the pump. (keep it open until you are at 15,000-10,000 microns,) Moisture is what kills the vacuum pump oil. When oil is wet, the vapor pressure increases to a point where a deep vacuum can not be created. (wet oil is white oil) If the oil is wet, it is cheaper and faster to change the oil then to let the gas ballast work it out. That moisture will also damage your pump if left in so always change the oil if you are work on a wet system. The reason I recommend you always change the oil is it is hard to see how cloudy is is through a small unlit sight glass. An open gas ballast prevents the pump from reaching its ultimate vacuum levels and should be closed after you reach 15,000-10,000 microns. The gas ballast used used only during the roughing period and only needed when there is moisture in the system. One of the most important things you can do is to always nitrogen sweep or purge a system before performing an evacuation. This means push the nitrogen through the system, one side to the next WITHOUT significantly raising the system pressure. This will push out the moisture vapors with out dropping them out into the system in liquid form. If you purge during assembly, and sweep the system with nitrogen prior to evacuation, you will likely not need to use the gas ballast at all. The gas ballast is only effective in removing small amounts of moisture, so a very wet system will require frequent oil changes if you want to make fast work of getting the job done.
A/C & Refrigeration systems are designed to operate with only oil and
refrigerant flowing through them. When a typical system is installed and/or serviced, air and moisture enter the system. Oxygen, nitrogen and
moisture (all make up our air or atmosphere) are detrimental to system operation. Removal of the air and other non-condensibles is called degassing and removal of the moisture dehydration. Removal of both is typically referred to as evacuation.
Assuming the valve cores are removed, connect large diameter vacuum rated hoses to the back of the core tools (do not use side ports of the core tool for evacuation) at both the high and low side of the system so that both sides can be pulled down simultaneously. While at first it may seem counter intuitive to use large diameter hoses the value rapidly becomes apparent after starting the evacuation. 1/2" hoses will decrease the time required for evacuation by a factor of 16 times over the typical 1/4" hoses used by most of the industry. Larger hoses reduce friction and therefore increase conductance speed. The conductance speed of 1/4" hose is so small it should never be used for evacuation. If you can, avoid 1/4" hoses for evacuation as they are too time consuming and costly to be effective. Connect the hoses directly to the vacuum pump with a brass flare tee or with a vacuum rated manifold. Do not use manifolds that are not equipped with o-ring seals as packing often holds under pressure but leaks in a vacuum. Keep connections to a minimum and points of access to the maximum. In other words, connect to as many places as you can on the system but eliminate unneeded hoses or fittings. If only two points of access are available, connect directly to the vacuum pump eliminating the need for a manifold.
Install a high quality vacuum gauge with a copper line or brass connector directly to the core installed on the suction line. This will allow the evacuation rig (hoses and fittings) to be completely isolated from the system during the "standing pressure tests" where the quality of the vacuum will be measured.
Start with fresh and dry vacuum pump oil. Vacuum pump oil is extremely hygroscopic (moisture absorbing) so starting fresh will make things go a lot faster. If your pump is equipped with a gas ballast, open the ballast until a level of 1000 microns is reached. Within narrow limits, the purpose of the vacuum ballast is to prevent water vapor from condensing in the pump during the discharge stroke of action. Generally speaking it is better and faster to change the oil rather than to wait for the gas ballast to remove excess moisture from the oil during pump operation. Moisture destroys vacuum pump oil by increasing its vapor pressure so much that a high level of vacuum cannot be created. The pump cannot develop a higher vacuum then the vapor pressure of its sealant. If in doubt change it out!
Pull a vacuum until a level of 1000 microns is reached, (if using large diameter hoses and core tools, evacuation of the line set and evaporator coil will take less than 15 minutes for a typical residential system of up to 5 tons). Isolate the vacuum with the core tools allowing the pump to continue to run and record the leak rate (after about a 5 minute stabilization period) indicated by the vacuum gauge if equipped. Leak rate is simply derived from a drop in vacuum over a unit of time, typically displayed in microns per second. A rise in the pressure after a short stabilization period indicates there is still moisture in the system or the presence of a small system leak.
Open the core tools and allow the system to continue the evacuation process until the vacuum level is 500 microns or less. Then repeat the "standing test" to determine of there is a decrease in leak rate after the stabilization of the vacuum. If there is no leak, the 2nd leak rate in the system the leak rate should be considerably less than the first indicating progress in the job of dehydration.
If the leak rate has not decreased, two things may be happening:
1) The system is still contaminated with moisture. (Possibly trapped under the compressor oil.)
2) The system has a small leak that was not detected by the initial high pressure test. (Some leaks are more apparent under vacuum than pressure.)
A high quality, high resolution vacuum gauge like the Accutools BluVac can indicate a leak much faster than a pressure gauge due to the sensitivity of the instrument. While the micron gauge is quite capable, testing for a leak in a vacuum is not acceptable practice over a standing pressure test as moisture is drawn into the system during the evacuation process. If you find you have a leak under vacuum, break the vacuum with dry nitrogen and try to find it under a pressure. DO NOT open the system to atmosphere under a vacuum! Doing so undermines all your time and effort to this point.
If the system has a leak the vacuum gauge will continue to rise until atmospheric pressure has been reached. However if the system is vacuum tight but still contains moisture the rise will level off when the vapor pressure equalizes in the system typically between 20,000 and 25,000 microns between 72º and 80º F. At that point that vacuum reading will become stable. (Note: a system that continues to level off at 3500-4500 microns may have turned system moisture to ice. Should this occur, the system temperature may have to be raised by an external heat source to get the moisture out of the system.)
If the system indicates moisture, a multiple evacuation with a nitrogen sweep will significantly reduce the amount of moisture in the system. To preform this procedure, reduce the system pressure to between 1000 and 2500 microns. Isolate the vacuum pump with the core tools and disconnect the vacuum hose from the low side of the system. Break the system vacuum with nitrogen introduced at the side port of the core tool. Break the vacuum with nitrogen to that equivalent to atmospheric pressure (760,000 microns) then purge nitrogen through the system at 1-3 psig. from the high to the low side letting it vent out the open port of the core tool. Do not pressurize the system as this will not remove moisture. There is no need to pressurize the system unless you are performing a leak check. Increasing the system pressure will actually cause the water to drop out of the nitrogen similar to that of compressed air in an air compressor. Nitrogen does not absorb water, but entrains it and helps it move along out of the system, allowing the liquid water to warm, evaporate, and increase the water vapor pressure without introducing additional moisture into the system. If the system is drying out you will notice that deeper levels of vacuum are quickly achieved indicating progress in the job of dehydration. If desired or required repeat this process until the moisture is removed. Typically no more than a triple evacuation with sweep are required. If marked progress is not achieved during this process, repeat the nitrogen purge to remove liquid moisture that may exist. If a leak is indicated, it must be repaired before the evacuation can be completed.
After the second drop test check the condition of the vacuum pump oil. Oil that is milky contains moisture and will not allow a finishing vacuum to be achieved due to the increase of vapor pressure and loss of sealing caused by the moisture in the oil. If the oil is wet, change it with clean dry oil. If in doubt change it out!
After the second standing test, allow the vacuum pump to run until the system is preferably below 200 microns. (With a good pump 50-100 microns is easily achievable.) Isolate the vacuum rig with the core tools and allow the system to stand for 15 to 30 minutes. If the micron level does not rise above 500 microns the evacuation is complete. If the pressure rises above 500, open the core tools again and allow the evacuation to continue. Experience and or a high resolution micron gauge will allow for shorter times of evaluation.
After the evacuation is complete, if you are working on a new installation, keep the pump isolated and open (crack) the liquid line service letting a small amount of refrigerant into the system bringing the system slowly into a positive pressure. (Note: When the vacuum gauge indicates "high pressure" you are above 20,000 microns but still in negative pressure.) As the gauge can handle up to 500 psig, you need not be concerned with damaging the micron gauge by over pressurization. Once the liquid line is completely open, open the suction service valve, re install the valve cores and remove the vacuum gauge and core tools. (Note: Refrigerant may make the vacuum sensor act if under a vacuum or erratic after removal until the refrigerant vapor is out of the sensor. The sensor is calibrated for air and a refrigerant atmosphere will affect the readings.) After the cores have been installed and core tools removed, purge your manifold hoses and install gauges to finish commissioning the system.
If servicing and existing installation, break the vacuum with the required system refrigerant prior to removing the core tools then continue the commissioning procedure as required by the manufacture.
We recommend the Accutool BluVac for several reasons. It has several advantages of all other vacuum gauges. Problems with oil contamination, field calibration, and work flow have all been addressed. At 0.1 micron resolution you can easily see if the vacuum pump is gaining ground, if the vacuum pump oil needs changed, and when the gauge is isolated the decay of the vacuum and the ultimate system pressure. Because of the resolution of the BluVac, we highly recommend you use vacuum rated core tools and hoses. All hoses leak, and at .1 micron resolution that will be very apparent.
To properly perform evacuation also consider the RapidEvac kit from TruTech Tools. Used as shown, it will reduce evacuation times by a factor of 16 over 1/4" hoses. The labor savings using this kit are very substantial and will reduce manpower requirements and down time of the equipment being services.
Looking for the perfect vacuum rig? Look no further we have it. VACUUM RIG
(1) Refrigeration Air Conditioning Technology 5th Edition Copyright 2005
(2) Review of Vacuum for Service Engineers 1988