Parallel Boat Compressors
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  1. #1
    Join Date
    Jan 2011
    Location
    Puerto Vallarta, Mexico
    Posts
    7

    Parallel Boat Compressors

    I have visited the sailing vessel Falcon, in Marina Vallarta, Mexico, and found an unusual (to me) configuration, whereby two Danfoss BD50 direct current hermetic compressors have been hooked up in parallel feeding through a water cooled condenser and a receiver to an expansion valve, holdover plate evaporator setup. To solve the obvious problem of oil misdistribution, all of the oil going to one compressor, starving the other, the unknown designer had brazed sump taps and run a 1/4" line between the two compressor sumps. According to the boat owner this system had run satisfactorily for a number of years.

    However, last year, problems developed. One compressor stopped running and the other began cycling on a fault signal (the installation had no LEDs to indicate the nature of the fault. I removed the condensing unit to test on my bench. Both compressors ran independently unloaded, but when R-134a was added to where liquid began to be visible in the sight glass, with a hi side pressure of about 90 psig and a suction of about 20 psig, the individual amperage going to each compressor, climbed to about 9.4 amps and the compressors shut down on overload.

    As I indicated, this configuration is new to me. The compressors are mounted side by side athwart ships, and, I surmise, when sailing healed for long periods on the same tack, all of the oil ended up in the lower compressor, starving the upper compressor, and over time both compressors experienced bearing problems leading to the overload shutdowns.

    Any experience with this type of failure?

  2. #2
    Join Date
    Feb 2004
    Location
    Western New York
    Posts
    1,039
    Apparently Danfoss makes a parallel unit with the BD-250. Maybe the installer tried to copy this. http://www.ra.danfoss.com/TechnicalI...ced100b702.pdf

    I believe these DC compressors were made for "on land" solar/wind applications. I like your idea about parallel imbalance at sea.

    Contact Danfoss tech support... I have found them very helpful.
    Experience is what you have an hour after you need it.

  3. #3
    Join Date
    Jul 2000
    Location
    Guayaquil EC
    Posts
    10,247
    That crossover oil line is a common method for oil equalization, but it depends upon gravity...and levelness...for it to work.

    I'm not familiar with the Danfoss DC compressors, but is it because they don't have a larger capacity model that they resort to twinning them?

  4. #4
    Join Date
    Jul 2012
    Location
    Western KY
    Posts
    1,221
    Could the fault be due to a low charge, lack of motor cooling? The oil would definitely be an issue anytime the unit was tilted very from level in any direction for more then a minute or 2 while running. Sounds cool. Any pics?

  5. #5
    Join Date
    Jan 2011
    Location
    Puerto Vallarta, Mexico
    Posts
    7
    I think, at the time the installation on Falcon was designed, the BD-50 was the largest BD commonly available in the boat industry. So this was an ingenious attempt to increase capacity. Probably, if the compressors had been oriented fore and aft, the system would still be running today, but a sailboat on long crossings can heal to one side for a number of days at a time. I was in the process of replacing the BD-50s with a single BD-80, which has greater capacity, but the boat had to leave Puerto Vallarta before the electronic module arrived. Sorry, I have no pictures, but the configuration was very similar to the dual BD-250 that baub referred to, except using two BD-50s.

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