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Rawal Variable Expansion Valve

Although Mike refers to this device as an exoansion valve, it is a hot gas bypass, not an expansion device. It simply monitors and maintains the suction line temperature at the inlet of the compressor.


  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Rawal APR

    Does it do what it claims?

    Such as:

    1) Continously variable (modulating) compressor unloading with a corresponding change in energy consumed?

    2) Effectively produce a constant temperature dehumidifier in outdoor conditions like 75° @ 90%+ relative humidity?
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Let's examine

    Not having any experience with this particular control, let's examine the laws of physics a little.

    Anytime you cause discharge gas to recirculate from the discharge port to the suction port you are lowering the discharge presusre and raising the suction pressure. This will cause a decrease in compression ratio and, as a result, the compressor will operate more efficiently (Less work to do). This component definitely allows for hot discharge gas to flow back to the compressor inlet.

    GOing only by the piping diagram that was provided on the website, it appears that hot gas flow from the compressor is modulated into the device where it is desuperheated by liquid from the condenser. Remember that the temperature of the discharge gas from the compressor can be as high as 220 degrees F and the subcooled liquid from the condenser can be in the range of 90 degrees. This will cause the hot gas to cool, while still maintaining the high pressure.

    Also note that the top of the device feeds refrigerant back to the suction line. The low pressure at the top of the device will help any liquid from the condenser to vaporize in the chamber. (Note that the refrigerant returns to the suction line from the top)

    The device also has a thermal bulb setup to take a suction line reading. I am assuming that the design of the system allows for the opening of the device as the suction line temperature drops. A drop in suction line is one (but not the only) indication that the load on the system is low.

    In a nutshell, it appears that the device is acting to keep the suction line temperture above a predetermined set point.

    I have dropped an e-mail to RAWAL and hope to have threir literaure in hand in a few days. I will keep you posted.

    In the mean time, if any of you air conditioning guys out there have any experience with this device, please provide some information about your experiences with it.

    Keep smiling!
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Spoke With the Designer

    I just got off the phone with the gentleman who invented the device and it seems to be a very useful device. The APR, which stands for adiabatic proportional refrigerant control is, as mentioned a means to maintain the desired suction line temperature and is a hot-gas bypass of sorts.

    I invited Davis, the patent holder, to join in the conversation here so you can direct questions directly to the man who made it all happen.

    Welcome and have fun!
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    Thanks So Much Eugene

    You seem rather intrigued--just like I was when I first read about this device a number of years ago.

    Keep hoping to hear from someone, somewhere who has actually used the device.

    Some say, "Oh, it's just a hot-gas bypass like has been used for a long, long time." While my understanding of condensing/evaporating cooling systems needs a LOT of work, my understanding was also, "a hot-gas bypass of sorts".

    It would seem wonderful for areas like mine where the latent heat load is extreme and A/C equipment is typically oversized to rapidly deal with unusually high sensible loads.

    Right now I'm showing 99.9° outside with about 45% RH. Oversized A/C unit for my office area is running about 80% of the time with very short "off" cycles in which I can literally feel the humidity build. We don't call this "Swampeast Missouri" for nothing!
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    No problem, Mike

    It's my pleasure and privilege to assist you guys in any way possible.
  • jerry scharf_3
    jerry scharf_3 Member Posts: 419
    Reading my mind

    I was about to post this question today as well. I understand the problem that Mike is talking about and it sounds great for that.

    My question is as follows. If I run the compressor in a 50% reduced output mode for twice as long, will it use more electricity, the same or less. over the period.

    I decided the APR was too much to add to the system for startup. If it looks really promising, I may add one after a year or two on the system to compare it to.

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    All other things equal

    All other things remaining the same, if you run a compressor at 50% capacity for twice as long, the power condumption will be lower, given that the high amperage draw on startup (LRA)will occur less frequently.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Am really close to getting one for my master suite system. A bit pricey, but love the idea of a modulating A/C system that acts as a nearly constant dehumidifier. If anything like modulating heat should be able to be more comfortable at warmer temps. Hope it will offset the extra electricity consumed from the nearly constantly running fan.
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Call Tech Support

    Mike, you can call tech support on the website and ask them. Who knows, you might even get to talk to Davis!
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Talked to Davis years ago when thinking about using it for a single condensing unit operating three evaps. Unfortunately evaps were of such wildly varying size that there would have been problems if the smalled called alone--also (in this application) it made the refrigerant piping system extremely complicated and, I was told, trouble-prone.

    Guess I'll play Guinea pig again like I did with the Vitodens without low-loss header on a TRVd gravity conversion.
  • Dave Yates (PAH)
    Dave Yates (PAH) Member Posts: 2,162
    Is this

    like the Inverter system linear expansion valve that meters refrigerant to match up with the variable speed motors (all motors - including the compressor) throughout the system for true variable-speed Btu production?

    We've installed a number of the Mitsubishi Inverter units and just received a Daiken Inverter unit for an install next week.

    From reading their web site, it looks like the motors, including the compressor, continue at full speed with this device absorbing the excess refrigerrant and metering its flow?

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