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  1. #1
    Join Date
    Sep 2002
    Posts
    12,277

    Let's talk about automatic expansion valves

    Any reason not to replace a cap tube with an automatic expansion valve?

    The cap tube in question is clogging (I think - it was OK until the POE oil compressor went in) with crap because they let the condenser clog. It certainly seems like the easiest way to fix it.

    No equalizer line to add and no finding a place for the TXV bulb.

    Why aren''t AEV's more common?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  2. #2
    Join Date
    Jun 2003
    Location
    Chicago, IL
    Posts
    4,396
    I have never seen an automatic expansion valve, but from what I understand, it's a horrible metering device for most applications. It's either starving or flooding the evaporator in most cases unless you continuously adjust it. believe it or not, the cap tube, although a fixed metering device, adjusts to the load placed on the system because of how that affect the pressures in the system.

    Heres two examples of where an AEV is bad.

    imagine a system thats running with an extremely low load, say the evaporator fan broke. This will cause the suction pressure to drop. The AEV sees this low pressure an trys to fix it by dumping more refrigerant into the evaporator, which could cause floodback to the compressor.

    Now, think of a system with a high load. The evaporator pressure will be high, so the AEV will reduce the refrigerant flow, slowing the cooling process. However, one benefit is that in this sense it works as a crankcase pressure regulator to prevent overloading of the compressor.

  3. #3
    Join Date
    Dec 2006
    Location
    Middle Tennessee
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    11,347
    Quote Originally Posted by Poodle Head Mikey View Post
    Any reason not to replace a cap tube with an automatic expansion valve?
    what board due you plan on driving the AEV with?



    .

  4. #4
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,559
    Not an EEV, and AXV. Solely regulated by pressure.

    Probably not the best choice for reasons already stated. If the load was constant, then you would be OK, but if it varies a lot, then you will have problems.

  5. #5
    Join Date
    Dec 2008
    Location
    Dixiana, AL
    Posts
    2,609
    Put a fine mesh strainer with service valves ahead of your cap tubes and go with that. AXV's are nothing more than a refrigerant pressure regulator. They're great for applications with extremely stable loads, or that don't require very close control of the evap superheat (and that have relatively small evaps). Most apps you see with AXV's are things like air dryers and refrigerated purges on low pressure centrifugals. Not saying it won't work in your case, just that there's a better than even chance it won't work well enough. Can you get a TXV on this one? Even at that, most TXV's that small have a strainer on the inlet that you'll have to deal with.

  6. #6
    Join Date
    Jun 2009
    Location
    Charlotte, NC
    Posts
    32
    I think its a horrible idea. Kinda like the new eepr's for racks. Nothing but trouble.

  7. #7
    Join Date
    Sep 2002
    Posts
    12,277

    This is what I did -

    I used an externally equalized TXV. Outlet into the run of a tee. Side of the tee to the TXV's equalizing port. Other side of the tee feeding the evap. Suction sensing bulb on the suction line after it leaves the box.

    I got the evap inlet piped but there was no way to get to the rest of the evaporator without major surgery. And nowhere to put the TXV in the evap housing anyway.

    Set the valve up, insulated everything, and let it fly. Seems to run better than ever. But maybe that's the new condensing unit. <g>

    I was leery of flooding back on start-up with an internally equalized valve on a freezer.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  8. #8
    Join Date
    Jun 2003
    Location
    Chicago, IL
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    4,396
    Quote Originally Posted by Poodle Head Mikey View Post
    I was leery of flooding back on start-up with an internally equalized valve on a freezer.
    Glad to here everything worked out.


    Teeing the external equalizer into the inlet of the evaporator effectively make the valve internally equalized. An external equalizer is recommended in coils that have high pressure drop. Im assuming the unit you're working on is a small unit since it had a cap tube, so an external equalizer shouldn't be necessary anyways.


    an external equalizer allows the valve to take it's pressure reading at the same place as it takes the temperature reading, allowing it to maintain a more precise superheat setting than an internally equalized valve. It has nothing to do with off-cycle equalization, and will not affect off cycle floodback.

  9. #9
    Join Date
    Sep 2005
    Location
    florida
    Posts
    5,504
    Quote Originally Posted by tpang1985 View Post
    I think its a horrible idea. Kinda like the new eepr's for racks. Nothing but trouble.
    Well that's news to me we have lots of racks with eepr's on them their great as long as you keep up with your temp sensors.
    I love the smell of phosgene first thing in the morning:

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  10. #10
    Join Date
    Sep 2008
    Location
    Western PA
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    25,559
    Quote Originally Posted by crackertech View Post
    Well that's news to me we have lots of racks with eepr's on them their great as long as you keep up with your temp sensors.
    X2

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