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head pressure control - split system

have a 35 ton unit with an especially long freon run.there are head pressure issues and concern about returning oil and unit use on cooler days may be a problem.

to improve reliability of unit, installation of a device to modulate condenser fan has been proposed (ie. head pressure control); also condenser fan blade and motor to be replaced too.

interested in comments about this idea



  • TechmanTechman Member Posts: 2,144
    Head press control

    What brand, how many comp's, how many cond fan motors, any type of unloading, freon, length of piping, vertical separation, line sizes, why replacing cond motor and blade, application, minimum ambient comp is to run in?
  • elfieelfie Member Posts: 264
    edited September 2013
    brand is carrier

    and a carlyse compressor

    one 06E compressor

    r22 freon

    piping run 1 way is about 200 feet

    dont know about vertical separation

    not sure why blade and motor are being replaced

    minimum ambient temp is around 60 degrees (system is for a 1200 person auditorium)

  • TechmanTechman Member Posts: 2,144
    edited September 2013
    Head press

    With multiple cond fan motors ,the 1st motors cycled off are controlled by regular pressure or temp controls and the last 1 or 2 motors are controlled by the RPM type of control (Carrier MotorMaster), The MotorMaster setup uses a motor w/ ball bearings instead of sleeve bearings. Check w/ Carrier about which motors are controlled by which type of control. I would add pressure tap access hi/lo at the evap. Barring any piping problems ,the oil should return properly by maintaining the required minimum hi side press. There has to be an "oil press safety" on this size of comp. Comp Unloaders? Possibly a "double suction riser" for oil return? Low points in the suction line? Suction line pitched properly?
  • elfieelfie Member Posts: 264
    installing head pressure controls

    are the any unintended consequences that might occur by installing head pressure controls

    the reason for doing it is to increase the head pressure alittle to assist with oil return on cooler days

    will increased complexity of system due to head pressure controls lead to more maintenance issues (current system has a very long freon run)

  • TechmanTechman Member Posts: 2,144
    edited September 2013
    The Law of Unintended Consequences

    By which ever way you do it, it usually doesn't matter. Maintaining the lowest/minimum allowable hi side press( on cool/cold days) is what counts, that helps oil return! A low hi side press causes oil to NOT RETURN to the comp. So, the LoUC is you are screwing yourself out of killing a comp and all that work in changing out the  comp. You are not a nice Guy! ,,,,,,,,,,,,,,,,, to the compressor maker industry ,that is!

     Is there a LLSV?
  • EmpireEmpire Member Posts: 2,343
    System Design?

    Can you give an idea of how the system is piped for example; Condenser above/below evap, size of refrigerant lines, any traps?, is there any increase or reduction in pipe size due to length, Count up cond. fans, if Unloader, is it suction bypass or hot gas bypass?  I am looking more towards the design of the system and not necessarily they head pressure control as those can be addressed later.  Also what are you current running stats, pressure, temps on everything please.

    Peace Mike T.
  • Fan Cycling

    You seem to be addressing two issues here. One is the use of some type of fan control and the other is referencing oil return concerns.

    Head pressure controls are always desirable as they have the ability, depending on the number of fans and the strategy employed, to maintain the head pressure of the system within a desired range. This can be done by altering the speed of the fans, by cycling them on and off, by controlling the amount of refrigerant that is able to bypass the condenser coil altogether or by controlling the amount of air available for the fans to move (shutters and dampers).

    Oil return is a completely different animal. Adequate oil return can be achieved by ensuring that best field practices were observed when the system was installed.

    For example, if the air handler is below the condensing unit, there should be a trap in the suction line at the base of the vertical run up to the condensing unit. In addition to this trap, there should be an additional trap for each 15 feet (or so) of vertical rise to the condensing unit. There should also be an inverted trap at the very top of the suction line run and the suction line should slope downward to the condensing unit to ensure proper oil return. Assuming that these guidelines have been followed, oil return should not be a concern.
  • EmpireEmpire Member Posts: 2,343
    Hey Eugene long time..........

    With your condenser above the A/H, don't forget to also install inverted style trap on liquid line so refrigerant migration during cooler days does not fall to A/H posing a possible liquid slug on a start of next cycle.  I have been doing that for years now.  How have you been Eugene?  We're getting ready for the winter months ahead.  A few calls have already rolled in for PM service before the rush starts.


    Mike T.
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