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Fundamental diagnostics...

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  • Fundamental diagnostics...

    My A/C problem is in my Airplane.
    It used to have an electrically driven R-12 system
    This was so marginal so as to be practically useless...
    So about two years ago I upgraded to an engine driven system.
    Also changed to R134 at the same time.

    Immediately after the upgrade it worked really well.
    Its now been two years and its back to behaving marginally.

    Today I went out and checked the pressures...
    With the A/C operating I lee low side 48 to 50PSI and High side some where around 185 PSI.
    This is at 72F external ambient and 80F inside ambient.

    The Dryer and expansion valve were changed when the system was upgraded.
    The condenser was evacuated and reused.
    The the evaporator was evacuated and reused.

    So since basic system pressures seem sane and normal I need to do some additional debugging.
    Since this is in an airplane the replacement components need to be FAA/PMA approved and are breathtakingly expensive,
    I'd really like to narrow it down to the exact faulty component before I start replacing things.

    I'll start with the measured temps at the condenser inlet/outlet and evaporator inlet/outlet.
    What other data should I gather?


  • #2
    Ok, your condensing temp is 125 deg F at the high side pressure you list. With the condenser seeing 72 deg so you are 53 deg above ambient, this is bad and points to a condenser problems. Look for anything restricting air flow, bent fins, etc.
    As a "non-current" A&P I was going to ask about the approval, but you seam on top of that.
    Hail, rain etc can damage fins and air flow.
    The closer you can get the condensing temp to ambient the better it will do. In reality air cooled condensers are usually in the 30-40 above ambient range.
    You can find pressure/temp charts here:
    R-134a vs R12 Temp Pressure Chart (http://www.acsource.net/acforum/viewtopic.php?f=1&t=8490#p30284) Postby ACProf (http://www.acsource.net/acforum/memberli


    • #3
      If your condenser is the old tube and fin style look close for corrosion where the fins meet the tube. Any corrosion reduces heat transfer.
      You can test the system if you can spray water on the condenser while the system is running, (watch out for the prop!) this will temporarily increase the heat removed, if the high and low side drop and the outlets blow cool, that would be a good confirmation that the condenser is the issue.
      I have not been involved with aircraft in decades, so don't know what is available, but if upgraded condensers, like a parallel tube construction types that also carry the needed approval, they would be a better choice for R134a


      • #4
        Its an ancient 1980 era fin and tube condenser. The water spray test is a good test.
        The FAA has recently publish a note that for older aircraft they are going to be more lenient about things like this....
        so they might say yes....

        I have my A+P license, I'm not an A/C tech.
        Looking at the aircraft docs it does not have a charge weight, just says to charge til the bubbles go away....

        I ran it some more today, it was good for about 15 minutes then stopped making cold....(20F drop to 0 F drop in ~30min)
        So by basic thermodynamics this makes me suspect the condenser.


        • #5
          R134a is a smaller molecule than R12, it needs more surface area in the condenser to move the same heat load. It was what started the change from tube and fin to serpentine and then to parallel flow type condensers. I my personal opinion it has a great deal to do with the loss of cooling when changing from R 12 to R134a.
          In my own stuff, I oversized the condenser a fair bit and went to parallel flow and saw every bit as good cooling as with R12.
          I am no engineer, and my observations are just that, one persons observations, but it is where I would look also.
          When changing refrigerants, the common rule of thumb is you charge the system with R134a to about 80% of the R12 charge by weight. Overcharging essentially is the same as reducing condenser area, so you want to be careful when the exact design charge weight is not known. Once liquid refrigerant starts to back up in the condenser, that area is lost for condensing hot gas to liquid.
          It has also been my observation on land based vehicles that tube and fin condensers have a thermal bond loss between the fin and tube as they age. It may be to the dis similar metals most are made of (copper tube/aluminum fin). A white powder forms between the two and little heat make it across the jct.
          It is a pity they didn't spec an amount in weight for the R12 or R134a (in the conversion).
          If you can feel the condenser in operation, you want even heat loss from the hot side to the cool outlet, if the top is hot and then it gets cooler with little temp change after 1/2 to 3/4's of the way, it may be over charged, It is not a 100% confirmation, but just a possibility.


          • #6
            So the condenser fan was dead. I fixed it.
            Today I gathered some data with it running on the ground.
            Temps are Condenser In, Out Fin and air temp on the condenser air intake..

            Had about 15 deg drop 80F to 65F inside.
            Did not measure pressures this run as it requires removing the cowl.

            I do have a New Old Stock condenser that looks to be in perfect condition.
            I've not yet installed it.
            Remind everyone this is a R134A system, engine driven compressor and old tired looking R12 Fin and Tube condenser.

            The maintaince manual for the A/C does not have a pounds of refrigerant.
            (Old R12 manual said 4 lbs)
            New Manual says to fill till bubbles in sight glass just go away...
            Alas the Filter Dryer provided with the engine driven STC has no sight glass...

            So what additional info do I need to gather now the condenser is working to know if it needs more refrigerant?
            Or an idea how to fill it if I swap out the condensor?

            The whole A/C thing in the Aerostar is half assed...‚Äč

            You can see where I shut off the A/C at the end as inlet/outlet temps dropped and fin temp climbed as I had no condenser fan running...
            Last edited by pbreed; 2 weeks ago.


            • #7
              In ground based stuff, we shoot for 75-80% by weight, The obvious choice of installing an inline sight glass would be modifying the system, not likely permitted.
              Best I can suggest is go with the above and tweek it if what you choose shows signs of a problem.
              when I built my own, I searched in vain for a simple way to determine how much I needed to put in, but never found any formula other than the 75-80%, and since my stuff was not stock, even that didn't work.
              Sorry I don't have more.