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  1. #1
    Join Date
    Aug 2010
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    2,447

    Variable displacement compressor for home A/C?

    I don't mean like two stage, but I mean 15-100% adjustable capacity.

    Some cars use variable displacement compressor.
    The displacement is controlled by moving the swash plate. The angle of swash plate determines the piston stroke and offers capacity variation down to about 15%.

    In older designs, the adjustment target was to set the suction to a target pressure. As the compressor output increases (with the engine RPM increasing) compressor will cut back to reduce capacity.

    If you're at idle, it will also adjust compressor in response to evaporator motor speed as that affects the evaporator load.

    Some newer designs use an electronic valve which also monitors thermostat setting. So, if the cabin temperature is close to adjusted value, it can actually increase the evaporator temperature by throttling enough to cause evaporator pressure to go up.

    The point of that is, rather than making air that is too cold and uncomfortable out the vent and reheating, it makes the air not as cold.

    MVAC variable compressors are usually 5 cylinder axial type with pistons arranged in pentagon shape.

    Why can't the same setup be used for stationary A/C?

  2. #2
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,914
    I'd rather have a mechanically simple compressor, with an inverter drive to vary its capacity.
    If more government is the answer, then it's a really stupid question.

  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    5,520
    Carrier Green speed and now several WSHP use inverters and change the compressor RPM to go from about I think 30-150% capacity. The +50% I think is only unsed in heating. There are alos VRF system, which are multizone minispits with condensers up to 10 tons that cna alos be tied together for up to 30 tons of combined capacity. Those also have vairable speed compressors.

    I'm waiting for cars to use heat pumps so you don't freeze your a** off the first 5 minutes in winter. Plus the extra load would warm-up the engine faster. Heck just a 2000Watt set of heat strips would help... especially with defrost.

  4. #4
    Join Date
    Jul 2000
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    NW IL.
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    3,935
    Quote Originally Posted by mark beiser View Post
    I'd rather have a mechanically simple compressor, with an inverter drive to vary its capacity.
    Any Trane XV1500's still around?
    Aircraft Mechanical Accessories Technician. The Air Force changed the job title to Air Craft Environmental Systems Technician. But I've decided I'll always be a Mech Acc.

  5. #5
    Join Date
    Aug 2004
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    North Richland Hills, Texas
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    Quote Originally Posted by MechAcc View Post
    Any Trane XV1500's still around?
    Beats me.
    As far as I'm aware, there has only been one outfit in the area that had any parts, or anyone trained and experienced at working on them, but that was 7 or 8 years ago.
    If more government is the answer, then it's a really stupid question.

  6. #6
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    I suppose you could also combine 2 small scrolls with an inverter drive and get 20-150%... but it would be cost prohibative.

    At some point, you have to consider what you're trying to accomplish with variable capacity, and you might find that using a chiller for example would be even better. Plus as mentioned in another thread, you could use chilled water for your refrigerator too and generate r at least preheat heat hot water with the condenser.

  7. #7
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,158
    The problem with warmer evap temps during light load conditions is poor humidity control when you need it the most. However if you combine lower blower speeds with lower compressor speeds you are onto something. The thermostat could be a little computer constantly monitoring temperature and humidity and varying blower/compressor speed "on the fly".

    But with these new systems comes high equipment costs, and proprietary systems. When the system goes down, it could be days before a replacement part comes in. Technicians would need significant training on the newer technology. Furnaces last for 20-30 years in our area, A/C units are good for 10-15 years. High tech system are rarely put in by a builder. When a unit suddenly goes down most customers are willing to spend the extra cash for a high tech system.

  8. #8
    Join Date
    Dec 2002
    Location
    Arizona
    Posts
    4,334
    Quote Originally Posted by MechAcc View Post
    Any Trane XV1500's still around?
    There a still a few in my area, and yup we work on them.
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  9. #9
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,916
    Besides some GM cars (not trucks) anybody else use the variable displacement comps? Why not? Too expensive? I like that over constantly cycling every few seconds at times.

  10. #10
    Join Date
    Aug 2010
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    Quote Originally Posted by BaldLoonie View Post
    Besides some GM cars (not trucks) anybody else use the variable displacement comps? Why not? Too expensive? I like that over constantly cycling every few seconds at times.
    Yes, a lot of cars do and have been since the R12 days.

    It's easier to diagnose constant output compressor like residential units, because obstructions and problems are visible through gauges.

    When you have variable displacement, compressor will reduce stroke and hide symptoms.

    You're stuck with either on or off with constant compressor. With variable displacement, you can actually change the load on engine by changing fan speed.

  11. #11
    Join Date
    Jul 2000
    Location
    NW IL.
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    3,935
    Our summer class installed a 24.5 SEER iQ Drive in one of our class rooms. Compressor, condenser fan and air handler motor all modulate to load. We set the controller at 75 degrees and the humidity is constantly below 50%. It's the best humidity control that room has been since it was made in 2010.

    Just have to love engineers that think bigger is better because they get paid by the tonnage. NOT.
    Aircraft Mechanical Accessories Technician. The Air Force changed the job title to Air Craft Environmental Systems Technician. But I've decided I'll always be a Mech Acc.

  12. #12
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,614
    After the severe thunderstorms and lighting we have been replacing v/s modules x13 motors and circuit boards ,think ill stick with my psc motors and single speed compressor

  13. #13
    Join Date
    Sep 2010
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    21
    Quote Originally Posted by BaldLoonie View Post
    Besides some GM cars (not trucks) anybody else use the variable displacement comps? Why not? Too expensive? I like that over constantly cycling every few seconds at times.
    Nissan/Infiniti has been doing it for some time now also... I believe VW even has one that will destroke to 0% and does not even have a clutch...

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