Air Conditioning/Inside the Pipes

[Just Plain Jeff]

The air conditioning system life and the most efficient use of air conditioning depends upon the chemical stability of the refrigeration system itself, to a large extent. The most widely used gas used in modern systems is R-134a. It is very important for all materials in the refrigerant system be chemically compatible. The only suitable compound for use with 134a systems is PAG, Polyakylene Glcol, a synthetic lubricant.

Generally speaking, coaches need about 1 ounce of PAG for each 7 feet of hose after the first 15 feet of hose. Roughly, a 40-foot coach will use about 92 feet of refrigerant (depending upon conversion, OTR A/C and a variety of other factors).

Take 15 feet off the measurement and you have 77 feet of line. Divide the 77 by 7 and you will find that you should have about 11 ounces of PAG from a fully evacuated system.

Taking the matter one step further, the 11 ounces will comprise about 18% of the entire capacity of the refrigerant system. This will create a total volume of about 61 ounces, or 3.8 pounds of R-134a.

R134a is a non-explosive, non-flammable and non-corrosive gas. There is hardly any odor to it and much heavier than ambient air. Yet, proper care and handling of 134a is required. At sea level, with mean atmospheric pressures and temperatures it will evaporate so quickly that it will freeze just about anything with which it comes into contact. The open container boiling point for 134a is -21.7F. This low boiling point makes it an excellent refrigerant. The tremendous heat transfer which occurs when a liquid boils forms the basic principle of an air conditioning system. The amount of heat required to raise or lower the temperature of one pound of water by 1F equals one BTU.

On a fully charged, enclosed system, the high side of the compressor can be as much as 200PSGI.

Yet, this is not a universal measurement, as ambient temperatures significant affect the temperature.

Here is just a sampling:

At 20F PSGI=18.43
30F=26.10
50F=45.48
70F=71.19
90F=104.40

Most experienced air conditioning mechanics do not measure the amount of 134a introduced into an evacuated system. Instead, they input air conditioning into the low side (Always the larger hose) at the compressor. They then slowly run the compressor, with an eye to the sight glass. Once the sight glass gets foamy and then stabilizes, they then consider the air conditioning system to be fully charged. This does away with the considerations of ambient air temperature and pressure regulation.

This is important for coach owners.

A quick measure to determine if an air conditioning system requires charging is to look at the sight glass while the air conditioning system is on (compressor running). If the sight glass appears to be 'foamy,' it is time to charge up the system with the requisite 134a and PAG, often sold in the same container.

Do not buy higher priced 134a refrigerant that has a 'leak stop' component, especially if you have OTR A/C. The Carrier compressor has very fragile reeds and seals on the inside and they can easily become clogged if PAG is introduced above the indicated levels.

[Joe Cannarozzi]

Jeff, we are a couple of steps closer to otr a/c. While nosing around inside the dash repairing the wiper squirters I noticed alot of oily resedue around the ducts below the evaporator so I pulled it and had it tested, its o/k. I then ordered the expansion valve that the evaporator threads into Im pretty sure this was the culpret because the hoses are good, either that or it was a loose fitting. Were going to vaccum it down again and if it holds vac this time. I am undecided if I will install the freon, have a local shop do it, or take it to a prevost charter co with a service garage that Im sure has done it a thousand times. Having spent almost 600 bucks for the R-12 I dont want any mistakes. Also I remember seeing somewhere you posted correct high and low pressure levels what were they again. One more thing, Jon also suggested I replace the compressor seal since the system is currently empty and could be a potential leak, it has also has arrived and needs to be installed. Lets get real specific like we know you can! Thanks.

[Just Plain Jeff]

That's a tough call to decide to change out the seal. It isn't a shade tree mechanic deal; there are some very specific procedures that must be performed when changing out a seal; for example: You can't handle a seal with your bare hands, it must be handled with air conditioning oil on your hands and the guts of a Carrier compressor are very finnicky.

Jon has been editing a story (since 2005) about the seal replacement, which was performed by an experienced air conditioning expert, which pretty much provides the entire procedure.

I guess it comes down to this: If you are at least a pretty good mechanic, or have access to an experienced AC person, I would go ahead and replace the main seal while you are at the stage you are.

One of the ways to check how your system is working is to put nitrogen to it at about 125lbs at least overnight and check it in the morning. If you have a leak, you'll know it for sure.

It's likely that Jon may run across this thread and we can move him to finish off the seal and recharging stories and Jim can get them posted.

Hope that this helps.

[Joe Cannarozzi]

Does anaybody know what the high side pressure should be for a cruise air?

[MangoMike]

Joe, When AAP installed my Cruise Air - this was the gauge readings.





Mike

[Jon Wehrenberg]

Those pressures are temperature dependent. Mike, do you remember what the outside temperatures were?

Joe, Lester taught the beer cool method at POG I. Add vapor at the suction side and as soon as you feel a distinct drop in temperature on the suction line with your hand it should be charged properly. (I'll bet the guys that make the Cruise Airs went by the gauges however.)

[MangoMike]

See Joe,

Sometimes you do have good Karma.

MM

[Joe Cannarozzi]

Thank you oh wise one The bus thermostat has to be turned down or it gets too cold. This is good! The drivers air doesnt seem to do as well. Is this normal? Plenty of blower just doesnt seem to get real cold?

[Ray Davis]

Having just had my drivers air worked on, mine blows very cold now. One thing I learned a week ago, which confused me a bit. I had a little fogging going on driving home a week or two ago, so I turned the dash control to defrost, and turned the thermostat up to heat the air.

Later when I turned it back, I noticed that my dash A/C wasn't blowing very cold. Given that I'd just had it worked on, I'm still sensitive to whether or not it's functional.

Well, I guess it turns out that turning up the thermostat lets engine heated water into the core, to warm the air. I guess that makes sense. What I didn't realize, that when I turned it back to cold, it doesn't happen instantly, as it just stops putting hot water in the core, but that the hot water remains until the 190 degree water itself is cooled.

So, make sure your thermostat is all the way down. I thought my dash was broken again when this first happened!

ray

[Joe Cannarozzi]

These A/C compressors do not cycle on and off like you see in cars and trucks. When you turn the A/C on the compressor clutch kicks on and STAYS ON. The only way to regulate temp. when it starts to get cold enough is to introduce a little heat. If I had to guess I would say due to its massive size it would be too hard on the clutch for it to be kicking in and out all the time.