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  1. #1
    Join Date
    Feb 2008
    Location
    NJ
    Posts
    7

    Confused 404a pressures in low

    I have a r-404a roof-top condensing serving a walk in freezer. pressures are 235 psi head pressure and a 28psi suction pressure. Outdoor ambient is 30degress and indoor box temperature is about 20degrees. System has a head pressure control on roof for hot gas. Wondering if pressures sound right. Seems like head pressure should be lower but not sure. Was at job last week and found a 260 head pressure. Reclaimed a little bit to bring down head pressure but sight glass was still full

  2. #2
    Join Date
    Oct 2011
    Location
    Chicagoland Area
    Posts
    4,564
    What pressure is stamped on the headmaster?
    Officially, Down for the count

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  3. #3
    Join Date
    Feb 2008
    Location
    NJ
    Posts
    7
    Sporlan head pressure control valve
    LAC 4 215
    Also found a heat exchanger about 10' from rooftop condensing unit heating up suction line with liquid line between indoor and outdoor units

  4. #4
    Join Date
    Oct 2011
    Location
    Chicagoland Area
    Posts
    4,564
    The head is too high but not high enough to keep the suction pressure up. You need to determine why you have high head, but I don't think thats the cause of the problem. Did you get a SH reading @ the coil?
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  5. #5
    Join Date
    Jul 2011
    Location
    Lehigh Valley, PA
    Posts
    427
    I think he is working on a freezer. 28psi may be a tad high for desired box temp at 0F. But I agree, check SH at evap.

  6. #6
    Join Date
    Feb 2008
    Location
    NJ
    Posts
    7
    Checked pressures again after box dropped from 35 degrees to 10 degrees after running for a few... 23 psi suction, 225psi head pressure. 20 degree super heat at evap coil. Total system super-heat is 40 degrees. I also have a 10 degree superheat where the liquid from the accumulator passes across the suction line, not sure what it's called but thought I would mention it. Last, the sporlan head pressure control which I believe might have something to do with it. The readings for the valve are. Hot gas inlet - 145 degrees, condenser coil liquid inlet- 42 degrees, valve outlet going to receiver than system-73 degrees. Thanks in advance.

  7. #7
    Join Date
    Oct 2011
    Location
    Chicagoland Area
    Posts
    4,564
    Your evap SH is too high. Should be around 8 degrees with box close to temp. Is there a pressure limiting powerhead on the txv?
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  8. #8
    Join Date
    Jul 2011
    Location
    Lehigh Valley, PA
    Posts
    427
    SH too high at evaporator. You are starving the evaporator. Most guys set TXV for 5 or 6 degrees at evap for a freezer. The bulb is securely wrapped and making good contact? Check this first if you haven't already.

  9. #9
    Join Date
    Feb 2008
    Location
    NJ
    Posts
    7
    Will return to site and check. I know bulb is making good contact because I removed bulb and heater up to check txv operation. Thanks for the help. Will check back in

  10. #10
    Oil insulating the copper from within, where the TXV sensing bulb is located?

  11. #11
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,548
    Quote Originally Posted by Bergen D View Post
    Will return to site and check. I know bulb is making good contact because I removed bulb and heater up to check txv operation. Thanks for the help. Will check back in
    What charging procedure did you use with this unit?

    Units with low ambient control valves require a specific charging procedure to ensure the proper charge.


    Quote Originally Posted by MechanicallyInclined View Post
    Oil insulating the copper from within, where the TXV sensing bulb is located?
    Explain. This would tend to cause the opposite problem, wouldn't you think?

  12. #12
    Join Date
    Jun 2009
    Location
    DFW, TX
    Posts
    664
    Quote Originally Posted by Bergen D View Post
    Checked pressures again after box dropped from 35 degrees to 10 degrees after running for a few... 23 psi suction, 225psi head pressure. 20 degree super heat at evap coil. Total system super-heat is 40 degrees. I also have a 10 degree superheat where the liquid from the accumulator passes across the suction line, not sure what it's called but thought I would mention it. Last, the sporlan head pressure control which I believe might have something to do with it. The readings for the valve are. Hot gas inlet - 145 degrees, condenser coil liquid inlet- 42 degrees, valve outlet going to receiver than system-73 degrees. Thanks in advance.
    Sounds like the headmaster is working properly. The headmasters are stamped with a pressure, but that doesn't mean the system head pressure will always be exactly what that valve is rated at. I have seen 180# headmasters that bypass and maintain a 170# head and they work just fine. In your case, a 215# valve bypassing at 225# sounds just fine to me.

    Yes evaporator superheat sounds high. Confirm the sightglass is clear and solid column of liquid before adjusting the TXV. Good luck!

  13. #13
    Join Date
    Oct 2008
    Location
    Cleveland, Ohio
    Posts
    27
    Sounds like you are over charged to me or you have non condensibles. As far as SH 10 degrees is fine for most applications any thing lower you start getting liquid flood back

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