View Full Version : Air conditioning Vacuum pumps...
07-10-2005, 10:25 PM
What do they do exactly? The kind I'm talking about are the ones they hookup to automotive AC systems, to do, whatever it is they do. Do they remove the entire contents of the refrigerant? And where does it go from there? Is it all contained in the pump, or does it get released into the atmosphere?
07-10-2005, 10:47 PM
They evacuate the system and remove the refrigerent in preparation for recharging. I think most states now require that the refrigerant be captured and it is then sent to a recycling center where it can be cleaned and reused.
When filling the system the air needs to be removed by the vacuum pump so that a full charge of refrigerant can be put in. If the system has a lot of air combined with the refrigerant it just doesn't work well at all.
Meaty Beaty Big and Bouncy
07-10-2005, 10:51 PM
You use a vacuum pump to create a vacuum in a new system or one you've repaired. You need to get all the miosture out. You draw the vacuum down to 28 inches of mercury, close it off, make sure it holds it the vacuum, then charge the system with refrigerant. The used refrigerant, if any, can be pumped into a cylinder & disposed of properly. It's illegal to pump refrigerant to the atmosphere.
Meaty Beaty Big and Bouncy
07-10-2005, 10:56 PM
07-10-2005, 11:24 PM
Two different types of pumps exist. One is a recovery unit which takes existing refrigerant out of the system and pumps it into a recovery cylinder. Assuming that you have to repair a system owing to a leak, you aren't going to run a recovery pump into vacuum, because then you're potentially drawing in moisture laden air, which contaminates the recovered refrigerant. If a working system is simply going to be disposed of, then yes, you pull it down as far as you can go.
A simple refrigerant vacuum pump is used to evacuate a system prior to recharging, and it has no storage capacity. To make the job go faster, typically I'll repair a system, then purge with dry nitrogen so the vacuum pump isn't working so hard to pull out moisture, which is a noncompressible and can wreak havoc with some refrigerants.
28" of Hg used to be the standard, but newer near azeotropic blends such as R-410A strongly urge evacuation to 500 microns, which requires an electronic gauge.
07-11-2005, 12:12 AM
Virtually all of the refrigerant is removed by the recovery machine. In pre-recovery days, the refrigerant was released into the atmosphere.
Generally the system will be opened in the process of repair, which typically involves replacing a faulty part. Air and the moisture it contains are thus introduced into the system. This air and moisture must be removed for proper performance. The vacuum pump removes most of the air and any trace refrigerant in a fairly short time.
Moisture takes longer to remove. The system is left under evacuation for typically at least 30 minutes to achieve this. In the low pressure environment of the vacuum, the boiling point of water is lowered, allowing more of it to evaporate and be withdrawn by the pump.
Some shops have combination units with recovery machine and vacuum pump in one device, others use separate pieces of equipment. Recovery machines are almost always recycling devices which separate oil out of the refrigerant and filter out moisture on-site, allowing the recovered refrigerant to be re-used.
Re-using the refrigerant is fine if it's pure. The industry standard is 98% pure R-12 or R-134a. The only way to determine this is with a refrigerant identifier. The better shops use an identifier to make sure that they don't revover and re-use contaminated refrigerant. Contaminated refrigerant can be recovered in a machine dedicated to that purpose, then sent to a special processing plant for proper disposal (for a significant fee).
Many shops don't use identifiers, and thus don't know for sure what they're recovering and subsequently installing into cars. Some do-it-yourselfers and unscrupulous repair facilities use various refrigerant blends* (typically sold as substitutes for R-12) or the wrong stuff -- e.g. R-134a for an R-12 system, R-22 (fine for home window units, eats the seals and hoses in automotive units), even propane ! (works great, explodes great). Even new sealed jugs of refrigerant are sometimes contaminated. If you want to be certain you're getting pure refrigerant put into your car, make sure the shop has and uses a refrigerant identifier. Otherwise they can't and don't know what they're dispensing.
*The blends may be EPA approved, but that doesn't mean they work properly or don't harm the system. Using anything other than pure refrigerant of the right type will void any warranty from the car manufacturer and the parts manufacturer.
Two different types of pumps exist. One is a recovery unit which takes existing refrigerant out of the system and pumps it into a recovery cylinder. Assuming that you have to repair a system owing to a leak, you aren't going to run a recovery pump into vacuum, because then you're potentially drawing in moisture laden air, which contaminates the recovered refrigerant.
Minor nit to pick.
The recovery pump does in fact pull the system into a slight vacuum, about 15" worth.
The instructions for using the recovery unit is to run it till it shuts off (at 15" of vac) then wait two minutes, if the system is still in vacuum, you are free to open the system. However, if the system has returned to a postive pressure, repeat the recovery procedure. Lather rinse repeat until the system will remain a vacuum for 2 minutes after the recovery pump has shut off.
OK, I know what you are thinking, how could the system return to a postive pressure if you pulled a 15" vacuum on it. The answer is that the refrigerant under pressure will disolve into the oil in the system, the same way CO2 disolves into a coke, or nitrogen gas disolves into a SCUBA diver's blood. If the system is cold it might take two or three passes with the recovery unit to get all the refigerant out. Warm it might only take one.
There is no danger of pulling mositure laden air into the recovery tank, since if the system were open to the air, it would be at zero pressure. zero pressure = no recovery.
07-11-2005, 03:49 AM
Thanks all for the info, very informative.
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