Non-Condensables? Are you sure?

  Non-condensables in a residential split air-conditioner can be, for most technicians, a pandora's box to diagnose.  Fortunately, there is a simple process that can identify this condition without spending hours and hundreds of dollars in virgin refrigerant.  I think we all are aware that R-22 has hit an all time high, so getting this right is more important than ever - if you want to retain your customer.
  There are several ways air and/or water vapor can enter a sealed system.  First, improper evacuation practices appear to be the most common these days.  If you are not currently evacuating a system using the Triple Evacuation Method, I highly recommend it - along with the manufacturer of the equipment you are installing if you want to keep that warranty! So do a quick search on the web, read it, it's simple.  Second, careless service by the technicians. When connecting your gauges to a closed system, be sure to purge the air (non-condensables) from your refrigerant lines.  This seems like such an easy thing, but hurried technicians working in extreme conditions tend to make mistakes, particularly after a long week of such.  The final way that I am aware of air entering a system is one with a leak, so bad that the suction line pulls into a vacuum, pulling air into the system.  When a technician finds this situation, the first instinct is to add refrigerant and then diagnose the issue.  There is a leak, do not knowingly vent refrigerant into our environment by adding!  Use dry nitrogen to pressurize an empty system, then use the Triple Evacuation Method once the leak is fixed!
  Non-condensables can cause many problems in a working system.  The problems arise when that air settles in the condenser coil, taking up room and board without pulling it's weight!  Since air cannot be condensed, it remains stuck there in the condenser or accumulator, increasing your head pressure.  Since the area of your condenser becomes reduced for the refrigerant to reject heat, the head pressure elevates to compensate, now relying on temperature difference instead of surface area of the coil.  When this happens, you will get higher discharge/condensing temperatures and reduced capacities.  Based on the only value I could find over the years,"a 10 psi increase in condensing pressure will increase power consumption of compressors by 6%." ("Air Tech Notes". T. Quello, 2004)  If there is enough air in there, you could cause the amperage of the compressor to raise to a point of internal overload.
  If you think you have non-condensables in a working system, because some joker is keeping you gainfully employed,  be sure to verify it is not any of the following conditions causing your high head pressure:
  • Clean Condenser Coil
  • Proper condenser airflow
  • Recirculation of condenser air
  • Overcharged; Check Subcooling
  Then use this process to verify your unfortunate situation:
  1. Turn off the condenser using the disconnect switch (you still need a call for cooling).  
  2. Remove the wires to the compressor, so that only the condenser fan is operable.
  3. Apply voltage to condenser by turning disconnect back on.  Only the condenser fan should run.
  4. Measure the temperature of the air entering the condenser.
  5. After a few minutes, use your Pressure/Temperature (PT) Chart for your selected refrigerant to identify the saturation pressure - based on the Condenser Entering Ambient.
  6. Your Liquid Line Pressure should equal the Saturation Pressure, based on the PT Chart.  This is within a couple of degrees of the outdoor ambient, to allow for tool accuracy.
  This process works because of the Pressure/Temperature relationship and Dalton's Law of Partial Pressures: Total pressure of a mixture of gases is equal to the sum of their individual pressures.  For instance, if there was non-condensables in a system, your actual pressure will be higher than your supposed saturation temperature.  If your gauges were supposed to read 155# for R-22 because it is 85F outside, but instead they read 175#, then you have 20# of said non-consensable air.  This could increase compressor power consumption by as much as 12%!

1 comment:

  1. Fully agree with you Chris....except for one thing.....tell the 'technician' that it's a really good idea to use a set of 'properly designed' digital gauges since your gauge on your high side might be at 2% FS accuracy yet with a 500 psig boudon tube you are subject to a 25 psi error band!!
    Also if you use a cheap digital, they are even worse for error band. We are CONSTANTLY using 1930s technology in these bourdon tube gauge manifolds to diagnose 2012 equipment.....a very, very bad mistake....

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