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charging charts

Lets say you're charging a new, matched system, a/h on second floor. Now the liquid pressure at the coil is going to be about 10 psi lower at the evap than at the odu.

On the the suction side you're reading total sh at odu unit and the line friction loss as well. Would comparision by the chart be more accurate if you adjust for line pressure loss/gain?

Of course I'd also be checking sh and subcool, but I've been wondering this lately. Any thoughts?

Comments

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380


    Typically, the high side and low side pressures as read at the condensing unit are typically accurate enough for superheat and subcooling calculations.

    You mention that you have a 10 psig drop in the liquid line on a 2-story rise... Sounds excessively high. Typcically, the pressure drop on a residential split air conditioning system with a second floor air handler is no more than 1 to 2 psig.

    Excessive pressure drops in the liquid line can often be traced back to undersized liquid line driers or improperly sized refrigerant lines.
  • billygoat22
    billygoat22 Member Posts: 124


    The question was not specific to any particular job, didn't mean to sound like that. I was thinking about the pressure drop due to the head of liquid in the line- about 1/2 psi/ft for the riser.

    Some manufacturers for their fixed metering device units have an adjustment chart for the pressure losses/gain. You adjust the orifice to allow for the change in pressure from odu to coil. That got me to wondering if the chart best relates to the conditions at the odu or coil.

    I've also read that it is "OK" to have up to 30psi drop in a liquid line, so long as no flashing occurs. For a suction line I believe its only 2 or 3 psi drop, and stays above a critical velocity to assure oil return.

    By checking superheat and subcool in conjunction with charts I've discovered a number of undersized suction lines (+5 degree rise from coil to unit) and a few liquid lines.

    With newer, high eff units charging is going to be more critical than ever, so I'm trying to cover all the bases, so to speak. Thank you
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Sorry Bryan

    > The question was not specific to any particular

    > job, didn't mean to sound like that. I was

    > thinking about the pressure drop due to the head

    > of liquid in the line- about 1/2 psi/ft for the

    > riser.

    >

    > Some manufacturers for their fixed

    > metering device units have an adjustment chart

    > for the pressure losses/gain. You adjust the

    > orifice to allow for the change in pressure from

    > odu to coil. That got me to wondering if the

    > chart best relates to the conditions at the odu

    > or coil.

    >

    > I've also read that it is "OK" to

    > have up to 30psi drop in a liquid line, so long

    > as no flashing occurs. For a suction line I

    > believe its only 2 or 3 psi drop, and stays above

    > a critical velocity to assure oil return.

    >

    > By

    > checking superheat and subcool in conjunction

    > with charts I've discovered a number of

    > undersized suction lines (+5 degree rise from

    > coil to unit) and a few liquid lines.

    >

    > With

    > newer, high eff units charging is going to be

    > more critical than ever, so I'm trying to cover

    > all the bases, so to speak. Thank you



  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Sorry Bryan

    Sorry, my brain was running a mile a minute while my eyes were still reading.

    I have run into a numbe of situations and, not to sound too evasive, the best thing to do is follow the manufacturers recommendations.

    If the rise from the evaporator is long, the velocity of the refrigerant slows down, and the mass flow rate of the system decreases. This results in reduced evaporator, compressor and condenser capacity.

    Some manufacturers recommend using tandem liquid lines, while others ercommend decreasing the liquid line size to maintain velocity. Reducing the liquid line size will increase the pressure drop in the line, but will keep the mass flow rate up.

    The adjustment charts you refer to are for fixed bore metering devices, as the TXV will adjust accordingly to maintain the evaporator coil superheat.

    You are definitely right when you refer to the higher efficiency systems being a little more "critical" and, for that reason, I would recommend that we all follow the installation/startup instructions that each manufacture provides.

    Please forgive my vague-ness.
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