View Full Version : A few facts about AC

06-02-2007, 08:56 AM
Ok for the record I am a 20 plus year tech and certified AC tech.
Most people just dont understand the basics of the system and how/why it works and what its supposed to do.
The big misunderstanding is that it produces 40 degrees regardless of the out side temps,,,WRONG.
Ok a few basics.AC uses a liquid medium under pressure that boils BELOW the low ambient temps outside.
I dont have the actual spec but R12/R134A boil at like 20 below or so (if I remember my spec correctly) Now just like a rad if you preasurize it the boiling point raises to a point.
Ok so we have something that at 80 degrees is trying to boil away without being under pressure.
Now how it works. the cooling medium is underpressure in the system the suction side of the compressor pulls the refrigerant in and compresses it and inturn the highside goes through a process called surperheat (if you compress anything it gets hot) this is usually a gas holding heat. so it goes to the condenser and gets heat removed (as its hotter that outside ambient temps) and then condenses to a liquid, high pressure fluid that is directed to the control device which is either an orrifice tube(precise metering hole) or an expansion block ,this is basicly a device that controls the flow so as to NOT flood the evaporator.
What you want is the high pressure liquid to spray into the low pressure side of the evaporator.
Now when the refrigerant makes the change from liquid to true gas (not the spray going in) it is abosorbing the heat coming from the air moved across the evap core. This heat saturated gas is then sucked up by the compressor compressed (superheat cycle) then goes to the condenser to start all over again.
Now if you dont have air flow over the condenser or its plugged up in some way your system wont function properly.
so here are some actual readings you can use to assure a system is working properly.
The inlet temp to outlet temp of the condenser should be a 40 degree drop or your not going to get good cooling regardless. This means if 200 degrees goes in then 160 should come out (not true specs) you just need the 40 degree drop.
Now if you use a temp gun place a piece of masking tape on the target points to get a true read or use a contact type thermocouple.
also the metering device needs to be small enough so as to NOT flood the evap core but still provide enough flow to keep it working without freezing.
This also means it needs to have proper airflow too.
I would never use a system that trys to cooloutside air and you and your car and dehunidify all of you and the outside air.And cool it too.
Also the inside air should give your roughly a 30 degree drop.Several companies sell these analyzers and I use it all the time (http://buy1.snapon.com/catalog/item.asp?P65=&tool=all&item_ID=69058&group_ID=1526&store=snapon-store&dir=catalog)
and it gives you a check mark if the system is fuctioning acceptible.
It reads ambient temp and outlet temp.
If your not getting 38 degree air from an AC system, its not supposed to. Your getting heat removal, as the car cools and all of its contents are cooled and it removes moisture (dehumidifies) your system will start working eaasier and only have to maintain for the radiated heat coming in or in the car (the warm bodies) as it enters.
One of the simples things I have done was found an insulating underlayment at local hardware store to install under the carpet. Remeber heat goes up and hot exhaust and asphalt radiate as much as 45 percent of the felt heat the system has to remove.
Anything you cna do to insulate works to your advantage.
Also NEVER run an AC system on regualr air, this is cooling outside air, dehumidify it , cool you and the humidity off you.
The best way to make your system work better is use max ac/fan for a few minutes then turn the fan down one notch, this will give the air more time to cool down or in actuallity let the heat time to move to the evap core/refrig.
Another thing to look for is a shop with very precise charging equipment. I personally like RTI unts as they let the tech actually read a graduated cylinder. Another thing is on the aftermarket systems is they have very low volume so you have to figure in a system with 1 lb or less 1 or 2 oz makes a very big difference and most service systems run 2 to 6 oz per hose so if the tech isnt thinking when charging then its low(or if he guesses wrong)I used to use an older RTI machine and we had discussions about this problem and several people actually believed me when they still had cars cycling like they were low.
I like on small volume systems to use the check valve fittings on the hoses and precharge them from a vacuum. THEN install them and mark the amount and charge.
Some systems like the one my present employer uses does so by scales and precharging is hard. Dont get me wrong I love using weights to charge but some systems are just so big of a pain in the A$$.
Think of this in a system of 16 oz(1lb) 10 percent is only 1.6 oz and if the hoses ona machine hold 2 to 3 oz each your only getting maybe 12 or 13 oz in the system(remember you release pressure the liquid would gassify so your not holding true liquid after opening the hose valves but it still looses some volume.
on a 16 oz system then at 13 oz its 20 PERCENT LOW!.
forthe most part small volume systems cant handle morethan 5% low or high and some wont even tolerate 2.5 %.
Another thing is being able to properly circulate refrigerant at a speed allowing it to absorb proper amounts of heat.
I personally have actually help some VA setups by adding larger condensers and upping the system volume.
Also a larger dryer/accumulator helps as the systems ability to absorb moisture is also related to performance. IF you buy a system and the drier is open to air its junk,,,, period.
I have actually helped systems by replacing the drier and recharging.
A system WILL absorb moisture int a pressurized system. Most dont believeit but I charged a setup for a guy 3 yrs ago and it finally got low enough to cycle nas with the identifier hooked up it had 8 percent oxygen! And had never been open since my work and had been pulled down to a vacuum at intstallation.
also if you have a CCOT (Cycling Clutch Orifice Tube) system in hot areas an AAOT (Automatic Adjusting Orifice Tube) conversion may help.
A fella came in with a 57 chevy and had an un known brand CCOT system and wouldnt cool properly, so we pulled it down (I figured out it had used a Honda drier)replaced the drier and aftermatching the orifice tube color to chart we swapped in an AAOT (also called a self adjusting orifice tube) a spare filter before it as they can get jambed by trash and recharged and I use the specific number realtionship (ie most R134A systems run in the 28 to 32 low side with around 250 on the high side with cooling fans on pulling, max ac fan on high or next step down. I look for a steady 27-28 then rev the system and look for it pulling down too much which is indicative of missing volume.
I try to keep them out of the on off on (as long as the sytem functions and makes proper duct temp cycling is ok, you generally try to keep it less than 5 degree on to off,, like 40.5 to 42.5)if possible.
Dont get me wrong I am glad there are aftermarket companies making systems but after repairing AC for years i do see many systems that are good but need some help sometimes.
Now excuse me but a system that issimilar in size to a honda Civic isnt going to cool a black on black 57 Chevy BelAir that good.
My sugestion is OVERKILL on the AC. The big trick with the old under dash setups wasthat they had huge condensers, super sized compressors and lots of fluid volume so this is why they worked so good.
And they never had to cool outside air either, its always was in MAX AC set up.
I actually like rebuilding some of the OE systems to work with R134A.
Also if you retrofit clean the crap out of it. I use straight methanol based air brake antifreeze first then go through it with the NAPA Dura 141 flush last.As long as you have it squeaky clean (very hard) use the newer oild rated for the compressor you will use. I personally like using ROC oil over PAG or POE synthetics,, it simply works better period and this has been in over 100 systems I converted but now I have problems sourcing it.
The trick is to get the oil ,in proper quantity, a fresh drier installed and vacuumed down in less than an hour. It shouldnt loose any vacuum if at all possible in 10 minutes. Some times the system needs to be charged to completely seat the seals/orings and using only green orings if possible, blue are ok but I have best luck with green.
And of course the big thing is proper volume or refrigerant.
Now again I use proper amounts UNLESS I see that the system is undercharged. The difference in techs is one that evacs,recharges to spec and sends you down the road if its "cool".
The good tech will make sure its operating properly, read the temps in and out of the condenser, use the dual temp guage to verify.
And test drive it to make sure its is stabile.
Anyone wants to ask questions give me a shoutIll try to help.
Lee Abel

06-03-2007, 04:24 AM
Lee, Nice post!

It's a little hard to read though, so you may want to edit it, adding some spaces between the informative lines of text. Please don't think I am trying to insult you or your post, I just want to help people to understand this A/C stuff. I was an ASE cert. Master Tech for years, and A/C seems to be one of the least understood systems in a modern car!

Key points you make that most people don't get:

1) A/C removes heat, it does not cool the air!

2) On average, it will only remove X degrees of heat. Thus, if the outside air is 80 degrees, and you are removing 40 degrees on average, the outlet temps would be 40 degrees. Further, if the outside air reaches 100 degrees, the outlet temps will be 60 degrees.

3) The three most important specs of an A/C system are:
1a) Size of condensor
1b) Size of evaporator
1C) Size of compressor

4) If any of the above parts are undersized, the system will not cool properly.

Thank you very much for typing up your post! You point out the very reasons my father an I installed a modified GM evaporator case in his '56 BelAir instead of an aftermarket system. We wanted cold air, and lots of it to keep that big car comfortable inside!



Same goes for my '72 442, I kept the factory evaporator case, using an evaporator from a '79 Trans Am (to convert it to an orrifice tube).


Shiny Side Up!

06-03-2007, 09:33 PM
I would just like to add that using a powerful electric fan can really aid in pulling a lot of heat out of the condensor, especially at low speed. Good tips, nice read.

06-04-2007, 06:57 PM
#1) Do longer plumbing runs hurt anything? I'm thinking about routing the A/C and heater lines under the passenger-side fender and shaving the firewall. Assuming the lines are hard-plumbed and well protected from rocks, do you see any long-term harm to the A/C system from this practice?

#2) What pipe would you recommend to hard-plumb A/C lines?

#3) Since they're hidden, would you insulate the lines to improve efficiency?

06-04-2007, 09:12 PM
Ok so to clear something up , While if its 100 degrees out side you will get 60 degrees ,,,at first, if you have a recirculate mode.
Also looking at the system you are running figure the high side usually is ran in a 1/2 in to 5/ 8 but the low side (remember this is a heat laden gas going to the compressor).
Now as for hooking up a system I would use shrter runs of aluminum piping(I find you can scanvenge out at junkyards and get some great deals on long striaghts and such and on later model cars you may even find R134A fittings.lines that work for your application but you will have to cut off the crimp and find someone with the replacement crimp sleaves.Also looking in NAPA or Carquest AC pic catalogs and see if anything can be made to work.
Most AC lines are in aluminum so if careful you can bend it to fit certain ways.
Also a lot of the AC companies do have compnenets to sell.
NOW always use barrier hose for AC regardless of refrigerant type.
Good year makes the Galaxy hose in bulk and its good and several other companies make barrier hose.
Barrier hose is a b--ch to work with but the neoprene inner liner keeps the refrigerant inside the system.
Also remember that to be safe you should put an information tag (the conversion tags work good or get one engraved)that states what type of oil installed(POE,PAG,ROC,etc and the viscosity/quantity, which is dependant on the compressor brandsome want 46, 100 or 150 so choose wisely) and the refrigerant amount installed.
And make sure it has the fittings for the type of refrigerant used (this is EPA law and could keep you from getting your system serviced) and if you shoot R134A int oan R12 unit and someone tells you its contaminated and wont work on it well thats thier right,,if they suck the mixed stuff out it can contaminate my AC or someone elses,,,and its ilegal.
Basicly too long a run may cause system problems but basicly just add to system volume.
common sence is the rule here but remember some systems run 15 or 20 ft of line to the rear of vehicles so it can be done.
Now as for compressor size the A6 or Sanden units work fine and will work for jsut about any system. Heck I used an A^ AC compressor for an air compressor for my air ride for 2 1/2 yrs.
Now I have a York (old Ford AC compressor)906 now and after installing an el;ectric air pump I will make it my AC unit probably,, just for fun.
Oh yeah fan SPEED has nothing to do with it you want proper air flow or CFM.
IF it goes 500 MPH but only flow 5 CFM it isnt gonna work but if it goe 100 MPH but moves 1200 CFM then its gonna work better.
Remember your MOVING HEAT no creating cold air.
Air conditioning isnt condidtioning except its is removing heat back to the outside and dehumidifying.