One gunuine AlphaGeek answer comming up.

For simplicity, I will use the term freon for any refrigerent.

The orifice tube serves the same purpose as the expansion valve.  But in a slightly different way.  The expansion valve varies it's opening to control freon flow.  The Orifice tube is fixed, and does not vary.  Instead, the compressor is cycled on and off to control pressure and operation.

First thing to understand is temp is a intensity, and heat, measured in BTU, is a volume.  They are NOT the same thing.  Compare a cutting torch to a furnace.  The torch will cut steel, but won't heat the room.  That's a lot of temp, not much heat.   The furnace will heat the room, but not cut the steel.  That's a lot of heat, without much temp.

Second, Refrigerant can absorb and give off heat (volume)  It does this by being warmer or cooler than the surrounding area.  If the refrigerant is warmer, it gives off heat.  If it is cooler, it absorbs heat.

Third, refrigerant has similar properties no matter which modern one is used.  In a circulating system (not static) the pressure will relate to the temperature.  If you create more pressure, the temp goes up.  If you drop the pressure, the temp goes down.  With R-12 this is very close to one psi for one degree F.    100 PSI R-12 is at 100 deg F. but only in a operating system, not a static one.  At higher and lower temps, there is a bit more variation of pressure vs temp.  R-134a is similar but a bit higher temp.

In the A/C system, the compresser creates high pressure gas.  Due to the pressure the temp is high.  Then it goes to the condenser in front of the radiator.   Due to the fact it is hotter than ambient air, the freon gives off heat, which causes it to condense into a liquid.  This is still at the same high pressure and temp, but has less heat content.  It then goes to the restriction which could be an orifice or expansion valve.  At this point the liquid goes through the restriction and drops pressure.  It is now a low pressure cool liquid.  Heat quanity did not change. It then goes to the evaporator inside the car.  Here is is cooler than the interior air, so it absorbs heat.  This causes it to evaporate, and turn into a gas.  It is now a low pressure cool gas, but has absorbed a large quanity of heat.  It then goes to the compressor where it is turned back into high pressure hot gas.  Then to the condenser where it gives off heat to become a high pressure liquid,  and around and around it goes.

This effect called the refrigeration cycle, collects heat from the car interior, and disperses it the the out side air at the condenser.  Cold cannot be created, but heat can be transfered away, only leaving cold.

The orifice will gunk up.  Then it inhibits the flow of hot to cool liquid.  This reduces the ability of the freon to cycle heat, and thus reduces cooling.  It also changes pressures in the system, which further reduced cooling.  If the flow volume is too low, you can't move the heat.  To carry 5 gallons you need at least a 5 gallon container.  You need enough freon to contain the heat.  If you flow too much, you won't get the pressure drop.  Then the freon is too hot to absorb heat from the interior.

The reason for the vacuum pump is to remove the air and everything else from the system.  As stated,  the air will reduce the effectivness of the system.  Air does not have the same heat transfer properties as any style of freon.  Plus, at very low pressures, water and other liquid contamanents will boil.  You can boil water under 50 degrees with enough vacuum.  This causes the moisture to turn to vapor, and the vacuum pump can remove it from the system.  Moisture is a major deterent to proper A/C operation, and a major contributor to the system decay, doing damage from inside out.

I think everyone just got a free lesson on air conditioning from Ken.

Alan

I knew the Alphageek was lurking around out there. Thanks Ken... I obviously couldn't have said it better myself!!!

Dave

 

Thanks Guys...those are the type answers I was looking for.  Most of the time I understand the "hows" but the "whys" no one wants to explain.

 I found a place a few miles from here that'll vacuum the system for me so I'm set.

 I hope to see ya'll in October at C3X or maybe earlier, remind me to buy ya'll a beer or three.

  Jim

I'm always happy to explain the why. ( as long as I know :)  )

Glad to help.

If you don't mind me askin' Ken... what don't you know about cars?  I am amazed sometimes at what I can find out on this site.

 I only wish i could get to a show sometime and meet the faces with the brains..

I just had the local tire shop/garage do some work on my 80 on Friday and they added a couple of cans of Red Tek to my old system and it works just fine. The system was empty I'm sure as the car had been stored for 11 years and didn't give me any cooling effect when I tried it. Went on a cruise Saturday to the nearest city 70 miles away and everything was just fine. The Red Tek is a do-it-yourselfer deal but I had the boyz at the shop do it as I wanted to cruise on Saturday!

Actually, there is a LOT I don't know about cars.

But I am a ASE Master Auto, Master Truck, Master Paint and Body, Advanced engine performance, Engine block machinist, Engine assembly certified tech.  I fixed cars for a living for over 25 years, and started teaching them part time, and now teach them full time at a Technicial College.  I am also a State of Ohio E-Check emission repair trainer, or was when they required the training.  Now it's optional and nobody takes it, so I did not continue my training cert for the state emissions.  I have written over 90 articles in trade magazines, and hosted a call in radio show in Akron called "Car Time" for 5 years.  I worked at Chevrolet from about 77 to 82.  So I worked on C3s when they were new.  Granted at 30 years old they have different problems.  Cars and my family are almost my whole life. I love them both dearly.  Many folks in the area call me the car guy.

All that and 60 cents gets me a coke.  I learn a lot here from everyone. There is so much I don't know.  There is simply so much to know.  When someone tells you they know everything, run away.  They are either lying, or they are fools and actually believe it.  They are the worst ones.

Many folks over the years have wondered what kind of car Ken Styer drives.  They think it's some great insite, and speculate on what it would be.  Everyone has their own idea.  But everyone here knows my favorite of all time, and all cars.  Nothing better than a C3!  Even broken ornery ones. 

Just had my r12 system changed to 134a. I did replace/refurbish ( lotsa  $$ ...) everything myself, reseal the boxes, etc. It seems to work as good as the old stuff, I get 40 degree air out of the vents......so I think I am good to go now.

 

[QUOTE=sfindlan]

Just to add  to the thread - I did mine. Got an interdynamics kit, with all the info, etc to change it over.  Also had a good guage set for 134a.  On mine, it was a matter of drawing it down, charging it with the new oil (which is compatible with the old R-12 oil) and charging with 134a. Took 3 cans and I was good to go.

According to a licensed AC guy the only issue with the 134a swap is that the original oil for 134a was not compatible with the R12 oils and would attack the seals.  This is not supposed to be a problem with the new kits.  Napa, Pep boys, and just about any auto parts place sells the interdynamics kit for about 30-35 bucks.  So it isn't expensive.  Have used this same kit on 3 other cars with great results and have not had any problems after several AC seasons in NC.

Good Luck!

[/QUOTE]

since you have a 79 with a l-48, i assume you have the stock system. did you have a problem with the low side port interfering with the compresser bracket? i got the interdynamics kit and hit a brick wall- the new fitting does not allow the hose to be connected to it to fill the system because the bracket and adj bolt gets in the way. i bought a 90 degree r134 fitting from napa that is a perfect solution, only now i have this problem- the fitting has the valve 90 degrees from the original fitting, but has no way to open the existing valve on the port to put the stuff in. its got the new valve on the end of the elbow, and the orig valve is still underneath, where nothing can open it to fill. do i have to remove the old valve guts (yes system is empty) so that the new one can work? its hard to describe, so i will post a pic when i can.

can anyone answer this question?