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Message |
Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, November 13, 2002 - 1:55 pm: | |
...another subject.. I bought one of those A/C- defroster kits from Ronthebusnut, and am in process of installing it in the front of my bus to keep me happy while driving. Really nice kit!
I was wondering what the preferred piping would be to use between the unit and the compressor (mounted midship). I have a very nice long hunk of that dual/insulated soft copper stuff that's used in household A/C's with remote compressors... is that stuff OK? The lines are 3/4 and 3/8" diameter- I'd think that would be an acceptable sizing.
Here's a photo of the stuff I have:
http://www.heartmagic.com/00ACpipe.JPG
Or should I use solid copper or even hose?
AND another question for those of you who know A/C... The unit has a control panel that turns everything on and off, operates the fans at variable speed, etc. In all the wiring harness, there is evidently no connection to turn the compressor clutch on and off.
There is, however, a 12 volt solenoid that shuts off the high pressure refrigerant line, mounted directly before the expansion valve, and that solenoid is controlled by the A/C on-off switch on the control panel.
My deduction is that the system was perhaps designed to have an "overpressure" switch installed in the compressor's output line, and so when the solenoid on the AC head shuts that line off, the pressure builds and the overpressure switch then shuts off the compressor clutch. Seems like a cheezy design but is that the way it works? Or should I add in a relay to do the job of turning off the clutch?
Thanks
Gary |
Henry R. Bergman, Jr. (Henryofcj) (63.224.197.10)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, November 13, 2002 - 6:12 pm: | |
Hey Gary the only thing that comes to mind immediately is that you may need a little larger diameter line and maybe what they call an accumulator to help keep air pockets from forming in the compressor oil.....
Since the compressor is way in the back of the middle of the Crown and your actual cooling radiator will be in the very front of the Crown up where you will be driving.
Also... what did your eventually decide upon about driving the comressor off the engine belt pully? Reason I am asking is that I am still leaning towards using a PTO off the tranny then trying to.....
Figure out where and how to mount another belt driven do-hicky off the crank where there is simply no room right now no how. CROWNS FOREVER!!! |
Mallie (65.38.24.95)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 16, 2002 - 1:00 pm: | |
Hello Gary,,
I am no expert on MVAC, but I will give you a little basic help.
Depending on the size, it should be OK. The rule of thumb as I recall is, R12 or equivalent requires a line size 50% larger for the same BTU as for R22. The copper is fine. Checking pressure drop can confirm.
The solenoid valve should be completely removed or disabled, unless your system is designed for the refrigerant flow to be completely stopped in that circuit. The results for a normal single system would be sudden high pressure, busted hose, or and a locked compressor in a short time. It was used with multiple evaporators. Would work fine if used an original bus air system. And the solenoid may have been used to cycle, but it will not work on a single system.
You will need some means to cycle your compressor. A simple low pressure switch used on many car AC systems can do this for you. For continues operation, the pressure/temperature must remain above freezing.
For 134a, that is 32F/27.8psi.
For more precise regulation of temp, the thermostat for a window unit can be used in addition to the low pressure in series.
I would guess there may be a thermostat attached in line to the solenoid valve, if this is the case, just use it to control your compressor, in series with the low pressure switch.
A high pressure switches are not usually used, because pressure can not rise above condensing pressure/temperature, but it is a good safety if you are not sure of sizing, air flow etc.
It is important to have adequate sized condenser and proper airflow over same.
The charging of any cap tube system is critical. Watch your super heat at the evaporator.
Mallie |
Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 16, 2002 - 4:45 pm: | |
Thanks Mallie! Now that you mention it, yes that solenoid seems like it would make sense in a dual evaporator system. I'll just pull it out and then I can hook the circuit for it to the clutch instead.
This evap unit has a temperature (and probably pressure) controlled expansion valve built in, so it's not a straight cap system... and it was my understanding that you just let the compressor run full time in this kind of system; the expansion valve either puts the refrigerant thru it's little cap hole into the coils, or doesn't and instead opens up, routing the high side directly back into the return with no restriction, so then there's no pressure in the system... is that correct? Or do I have to cycle the compressor as well...???
Last, the evap says it's made for R134. I still have a big can of R12 laying around, so would it be wise to ignore that and use R134 anyway? I'm Just looking for opinions on this one....
Thanks!
Gary |
Henry R. Bergman, Jr. (Henryofcj) (63.224.197.10)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 16, 2002 - 5:33 pm: | |
If given the choice I would stay away from R12 if you have the newer stuff. Who knows what law may lay in wait years from now to nail you? CROWNS FOREVER!!! |
Mallie (65.38.24.92)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 16, 2002 - 11:22 pm: | |
An expansion valve maintains specific super heat. Which means, it measures the temperature at the outlet of the evaporator with a sensing bulb that controls one side of the diaphragm of the expansion valve. Since you do not have to worry about flood back, charging is a lot easier. Just measure Sub cool, or get a clear sight glass. You still need a low pressure cut out switch to keep it from freezing during low pressure.
Most compressors will work well on the 134a. Just be sure to use Polly or Pag oil. When you add the oil, distribute it equally in the condenser/receiver, and the vaporator/accumulator as well as enough in the compressor to lubricate the compressor at start up. And a good vacuum is a must. I would add a dryer accumulator on the low side, and a receiver with a sight glass on the high. |
Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 16, 2002 - 11:54 pm: | |
Woah!! Thanks for the cool answer Mallie (pun intended), but you just left me in the dust regarding superheat,sub cool,etc, where to stick a sight glass and what to do with it when it's there...and how to properly do the charge.. Could I implore you to come down closer to "dummy" level a bit with your explanation, so I can hopefully grokk what you're telling me?
thanks!!
Gary |
Mallie (65.38.24.92)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 17, 2002 - 2:49 pm: | |
The sight glass is installed between the condenser and the Expansion valve, to determine when you have a clear stream of liquid to the device. Works pretty well on expansion valve system, not so good on cap tube. But it may never be completely clear, so comes the sub cool. What you want is, the condenser condensing, but not holding liquid, at a time when high condensing capacity is required, or it will drive up the head pressure unnecessarily because the liquid it occupying valuable heat exchanger space. Thus the liquid receiver is placed in the system just past the condenser. And BTW,, the liquid receiver usually has a sight glass in the top. Available at any parts store.
The sub cool is a way to determine how much liquid is in the condenser, kind of a checking process. It is how much the liquid has cooled below the condensation point. Pressure/temp are only accurate if measured at the same point, and on an auto, the nearest tap is probably going to be at the compressor. Take that reading, and subtract 8psi for pressure drop through the condenser. It is pretty simple if you have 134a scale on your gages, but most do not have. So, get a Pressure/Temperature chart for 134a, take the reading at the compressor, say 200psi for a 95 degree day, less 8 would be 192psi. On the chart, 192psi corresponds to 128Deg F. The liquid line leaving the condenser should have a temperature less than this, but not more than about 10deg F on a 95 degree day. Head Pressure may be considerably more if condenser is smaller. With the sight glass, sub cool, and head pressure, you can determine if the condenser/fan speed is properly sized, and properly charged.
BWT,, additional oil will have to be added for the length of line beyond a normal system.
What you have is a piece of a system, and you will have to match the rest,, may take a little adjusting. |
Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 17, 2002 - 3:06 pm: | |
Ok, I'm starting to get it. Thanks soooo much- I need to fully understand this system before I start throwing more $, parts and refrigerant at it!
Another question... in reading up on the internet, for domestic systems I keep seeing the use of traps and similar things to pay attention to when the compressor is higher/lower/same level as the rest of the system. Is that an issue in a bus? Since I'll have some 20' lines going here and there, do I have to pay any particular attention to their levelness/slope/etc?
Thanks again!!
Gary |
Mallie (65.38.24.92)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 17, 2002 - 4:25 pm: | |
In a domestic system AC system, A simple rise in the lines from the evaporator will form a trap that will keep the oil/condensed liquid from draining back during the off cycle. The same with the compressor if it is mounded high. Accumulators and receivers are not always used on a residential AC, tho are usually on heat pumps. The straight AC is where the problem can arise if one end is raised substantially above the other, with out use of traps. If the compressor height is great enough, oil can get stranded.
The use of an Accumulator/Dryer on the low side will pretty well take care your system, tho I would still make a rise from the evaporator anyway. |
Gary Stadler (Boogiethecat) (68.7.217.217)
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, November 17, 2002 - 5:17 pm: | |
My compressor will be just about centered with the condenser height, and the both of those about 2 foot below the evaporator level. So am I getting it right that you "want" the oil to run down to the compressor during the off cycle, but don't necessarily want liquid refrigerant to go there hence the accumulator?
How's this: do I have it right? Any traps needed anywhere, and do I have all the parts in about the right places?
http://www.heartmagic.com/00ac.diagram.jpg
Last, I see the big suction return lines always have insulation around them on domestic units.... Necessary?
Thanks!
Gary |
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