Page 2 of 2 FirstFirst 12
Results 14 to 25 of 25
  1. #14
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Don't worry, I'm not going to jump any contactors or anything. : ) I let the journeymen do that sort of thing. But I like to try have things summed up before he gets there. And, I think, they have always appreciated that. And when i'm wrong, they usually teach me why. It's been a great education.

  2. #15
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Turned out it was a problem with the cpc. Somehow some setpoints went awry had to be re entered.

  3. #16
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,064
    Quote Originally Posted by Iceneck View Post
    the display said fan #1 on this rack was "ON". Had a coworker look at the protocol display while I went back up on the roof, I watched the fan (not running) but he said protocol display still said fan #1 on this rack was "ON". I pushed in the contator and the fan cycled on.

    Just curious...did you ask your journyman how it could have been a setpoint error if your above statement is acurate?

    If the controller is already calling for it to be "on" there is no possible setpoint change that will make the output turn "on" more than it already is.

  4. #17
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Phase loss,

    I really have absolutely no clue. He had me up on the roof watching the contactors/fans cycle cycle as he made changes to the cpc and talked with the guy who did the original "programming" over the phone as I yelled down which fan was cycling on. But when he left everything worked as it always has...fan #1 cycled on first, followed by #2, etc. Should note there was also a pc board located up on the roof in the motor control panel for the condenser fan motors. On the board there were leds next to the circuit for each fan (there was a sort of terminal block with a fuse in between it located on the pc board for each contactor/fan). That led was not lighted even though cpc said fan #1 "on".

    It was cool, after he was done I got to pick his brain about other things with the rack I wasn't clear on...I asked about the difference between the head pressure at the discharge line and then at the king valve. So he put his guages on each and let me see (and feel) the difference. I also couldn't figure out the function of this one check valve thing...it went from the discharge line into the liquid line entering the receiver. I'd been trying to figure out what that thing was for for weeks. Turns out it is a valve that opens when pressure in the receiver is low, and it lets some hot gas in to raise it up. I guess it's pretty important when the ambient temp is really low...but we don't see much of that here.

  5. #18
    Join Date
    Sep 2002
    Posts
    12,195

    discharge "check valve" -

    It's not so much for low-ambient - it's primarily for start-up. If the discharge is bypassing for more than a few minutes under low ambient conditions - the system is undercharged or something is not adjusted correctly.

    In a system which contains a lot of refrigerant, on startup sometimes the refrigerant doesn't flow quickly enough at first in order for the machine / system to stabilize.

    So the head pressure is low, the liquid doesn't move fast enough to support the suction pressure, and the compressor(s) unloads or shuts down on low suction pressure. The head pressure is then low again and the cycles repeats itself.

    To avoid that there is often a discharge hold-back valve between the compressor and the condenser. It stays closed until it's inlet pressure is about 180 lbs. on 22 or 404. Tee'd in before the discharge hold-back valve is another valve which remains open any time the pressure After it is about 160 lbs. The outlet of this second valve leads to the top of the receiver. This second valve is a receiver pressurizing valve.

    This puts immediate discharge pressure onto the top of the liquid refrigerant in the receiver - pushing it through the solenoid valves and TXV's - giving the compressors something to pump - and allowing them to build the discharge pressure to the point where everything stabilizes it's operation.

    The fans do not cycle on until the discharge pressure is well above the setpoint of the discharge holdback valve.

    PHM
    -------





    Quote Originally Posted by Iceneck View Post
    . . . . other things with the rack I wasn't clear on...I asked about the difference between the head pressure at the discharge line and then at the king valve. So he put his guages on each and let me see (and feel) the difference. I also couldn't figure out the function of this one check valve thing...it went from the discharge line into the liquid line entering the receiver. I'd been trying to figure out what that thing was for for weeks. Turns out it is a valve that opens when pressure in the receiver is low, and it lets some hot gas in to raise it up. I guess it's pretty important when the ambient temp is really low...but we don't see much of that here.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  6. #19
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Quote Originally Posted by Poodle Head Mikey View Post
    Tee'd in before the discharge hold-back valve is another valve which remains open any time the pressure After it is about 160 lbs. The outlet of this second valve leads to the top of the receiver. This second valve is a receiver pressurizing valve.

    PHM
    -------
    Thanks for that PHM.

    And that is the valve I assume I was looking at. When you say it "remains open any time the pressure after it is 160 lbs" do you mean it's "open" sending the refrigerant only through the condenser, or "open" as in it's sending a portion of the hot gas from the discharge line into the receiver? If the pressure after it is 160 lbs (high enough to allow all the txvs to operate correctly) why would you need any extra pressure in the liquid line?

  7. #20
    Join Date
    Sep 2002
    Posts
    12,195

    Maybe make your self a sketch -

    The compressor discharge line is headed for the condenser.

    A hold-back valve is in-between the compressor and the condenser. The hold-back valve reacts to the pressure on the Inlet side of it.

    The hold-back valve stays closed until the pressure on the compressor side gets to 180 lbs. At 181 lbs. it start to crack open and bleed off pressure to the condenser. But don't forget - if the compressor-side pressure falls to 180 again - the hold-back valve would close again.

    Between the compressor and the hold-back valve is a tee. The side of that tee feeds to a receiver pressurizing valve.

    The receiver pressurizing valve reacts to the pressure on it's Outlet.

    The receiver pressurizing valve is open any time the Receiver pressure is less than 160 lbs. When the receiver pressure get to 161 lbs - the receiver pressuring valve closes.

    So the hold-back valve maintains a minimum of 180 lbs in the discharge line and at the tee.

    The receiver pressuring valve feeds that 180 lb. pressure into the receiver.

    This pushes the liquid refrigerant in the receiver out through TXV's, through the evaporator, and back to the compressor suction.

    As soon as the compressor has enough to pump, the discharge pressure climbs above 180 lbs. refrigerant flows through the condensers, and the receiver is pressurized by the liquid line. As soon as the pressure coming down from the condensers is over 160 lbs. the receiver pressurizing is held closed by that pressure.

    As soon as the system starts, refrigerant starts to move around the system, and the pressures starts to come up to normal - the hold back valve is of course wide open and the receiver pressurizing valve is closed.

    Draw that out, include the compressor, the condenser, the liquid receiver, the TXV, and the evaporator. One of each is enough.

    Then label everything, put arrows showing the flow directions, and write in the pressures.

    BTW: I'm just using R-22 pressures in this example - of course for other refrigerants they might be different.

    PHM
    ------





    Quote Originally Posted by Iceneck View Post
    Thanks for that PHM.

    And that is the valve I assume I was looking at. When you say it "remains open any time the pressure after it is 160 lbs" do you mean it's "open" sending the refrigerant only through the condenser, or "open" as in it's sending a portion of the hot gas from the discharge line into the receiver? If the pressure after it is 160 lbs (high enough to allow all the txvs to operate correctly) why would you need any extra pressure in the liquid line?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  8. #21
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    PHM...awesome, thanks.

    I took a pic of the rack...I can't find the hold back valve though. Can you see it in the pic?

    After taking another look at that pic (you can't really see the discharge line or any valves on it because i'm standing directly behind it) I added another of our other condenser (rack c and d). C and D are larger capacity and you'll notice not one but two check valves going from the discharge lines into their respective receivers. But I still don't see the hold back valve...i am missing something. Thanks again for the help, PHM!
    Attached Images Attached Images   

  9. #22
    Join Date
    Sep 2002
    Posts
    12,195

    Just a little guy, isn't it?

    No; I can't pick it out in those pics.

    Although I have never seen it done that way, I suppose that just taking hot gas off the top of the discharge line (to bypass the receiver) and feeding it onto the top of the receiver may create enough of a pressurizing effect to get the system started.

    I am more used to systems with a few thousand lbs. of liquid in the receiver. It takes some serious 'push' to make it start flowing.

    PHM
    ------




    Quote Originally Posted by Iceneck View Post
    PHM...awesome, thanks.

    I took a pic of the rack...I can't find the hold back valve though. Can you see it in the pic?

    After taking another look at that pic (you can't really see the discharge line or any valves on it because i'm standing directly behind it) I added another of our other condenser (rack c and d). C and D are larger capacity and you'll notice not one but two check valves going from the discharge lines into their respective receivers. But I still don't see the hold back valve...i am missing something. Thanks again for the help, PHM!
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  10. #23
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,064
    Your holdback valves are those gray "L" shape valves.

    your bypass valves are those expanded copper looking thingamajigs.

  11. #24
    Join Date
    Aug 2002
    Location
    Orange County CA
    Posts
    1,084
    This thread is the exact reason why I voted no on the "pro forum"

    people helping people

    This guy wants to learn and some people jump in and help.

    Now the guy's curiosity will get him thinking and he will be learning as he goes.

    I've been on other forums where I was lost and was helped. This is the stuff the internet is great for.

  12. #25
    Join Date
    May 2011
    Location
    CA
    Posts
    123
    Quote Originally Posted by NedFlanders View Post
    I've been on other forums where I was lost and was helped. This is the stuff the internet is great for.
    Exactly...

    And again, thanks to Poodle Head and Phase Loss and everybody else who helped me out. I should pay pal you guys money for a sixer each.

Page 2 of 2 FirstFirst 12

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event