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  1. #1
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    Subzero FF Evaporator failure with an odd twist - Refrigeration Engineers sought



    Broken fridge; need refrigeration engineer help for insurance and maintenance purposes. Many technical geeky issues involved. Long post follows

    My house came with an expensive Subzero 550 refrigerator/freezer.
    During our recent Vermont floods, power was off for two days. The refrigeration compartment got to room temp (about 65F), with a medium payload. We were out of town during the outage.

    The freezer compartment was set to –12F, and its contents stayed fairly frozen, if I use the ice tray as an indicator.

    After the power returned, which was just before we did, the fridge failed to attain temperature, and when I measured it with an Omega HH303 thermometer, it showed about 60F. After a full night of operation, it was still in the high 40’s.

    Calling a repairman, he blamed the problem on a clogged evaporator, which replacement I am currently awaiting. (BTW, Subzero/Wolff Company is providing this at no charge, out of warranty! Nice touch, folks. I am impressed!)

    The tech showed me a pressure reading of negative 5 PSI where a positive 10 should have been visible. Not sure what the measure point is, but it’s where he connected his combo gauge. It uses R134A refrigerant.

    I’ve been monitoring the empty unit for the last two days. A graph of the time/temp curve is here.

    http://dl.dropbox.com/u/176503/Subzero%20Test%201.jpg

    I warmed the interior to room temp the night before, and I’ve been running it since, with only two door openings, clearly visible in the graph. This is with the temp setting at 5 on a 1-10 scale (i.e., mid scale).

    Here’s the question:

    It appears to be regulating, but the hysteresis seems excessive, and its cooling rate is too low. Specs on the sealed system indicate 280 BTUs, which seems small if it means 280 BTUs/hr.

    A long time ago, I wrote software for a cryo freezer (-190C!) using combo *****/ammonia refrigerant and recall something called “de-logging”, which was a 10 minute break in compressor operation per 8 hours of constant operation during pull down. I am sure this Subzero doesn’t have a de-logging feature. It was to prevent compressor oil from gelling. The tech on my unit blamed oil as the culprit in the blocked evaporator.

    I need to build the case that the unit is broken because it ran excessively without a delog feature, traceable to the power outage and resulting cool down demand. I also would love to know if there is any way to clear an oil clog in the evaporator without replacement. I have access to the entire evaporator surface (from the front), and the unit is obviously sufficiently charged.)

    Any/all advice info greatly appreciated. I am an electrical engineer and know enough about refrigeration to get frostbite. Thanks in advance!



  2. #2
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    Re: Subzero FF Evaporator failure with an odd twist - Refrigeration Engineers sought

    Quote Originally Posted by fauxscot View Post
    Any/all advice info greatly appreciated. I am an electrical engineer and know enough about refrigeration to get frostbite. Thanks in advance!
    Hi fauxscot

    1st mate, we would need alot more info to assist you with an accurate response, both high/low pressure readings & a number of tubework temperature readings, without which we can only confirm there 'appears' to be a blockage

    Next (again can only take guess here) it may not be the case that your unit is still fully charged, due to the gauges being installed on a system with a critical charge...how much if any was lost during this procedure or was it the case as low side had negative pressure... was non-condensibles added to the system

    Personally I would sit back...allow evap to be replaced as the diagnosed fault & if not happy with performance of your unit after that....try to get a new replacement

    R's CM
    If the World did not Suck, We would all fall off !

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    Re: Subzero FF Evaporator failure with an odd twist - Refrigeration Engineers sought

    And what is the freezer temp and compressor duty cycle?
    If the freezer and compressor is fine, it may just be a frozen damper flap or motor that supplies cold air from the freezer to the fridge. Or could be the power surge that happened when power was lost or regained, stuffed up the PCB which controls the damper motor that controls the airflow to the fridge.
    edit: A vacuum running on a domestic fridge is not surprising, r134a is at a vacuum running at -30C.
    10PSI is 6F or -14C, how is a freezer going to freeze stuff at -20C if it is running at 10PSI?
    Last edited by paul_h; 08-09-2011 at 03:37 PM.

  4. #4
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    Re: Subzero FF Evaporator failure with an odd twist - Refrigeration Engineers sought

    Hey Paul,

    This FF has separate compressors for the refrigerator and freezer. The freezer section is working fine. There is no interaction between the two compartments. (That's probably one reason the unit costs $10,000 US.)

    Also, I'm just relating what the repair tech related; as I mentioned, I am an EE, not an RE. I'll go see if I can determine where the monitor point is on the compressor.

    Any thoughts on the delogging? Does that even apply to domestic units operating in these relatively mild thermal regions? (It seemed awfully important on the Revco cryo freezers I programmed.)

    Thanks for the inputs, folks.

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