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airconditioning metering device

I replaced a compressor in a bryant rooftop. it also had a badly plugged liquid line filter drier. I replaced the filter drier, added the freon but the pressures weren't correct. only about 45# on low side with about 160# head pressure. it was a cool day under 70 degrees. the compressor right next to it had 60# back pressure and 160 head, I added more freon to get the head pressure up to 200# but could only get the back pressure up to 55#. I called up carrier and tech support told me I probably have a blockage at the entrance to the evaporator.
there are no capilliary tubes and it doesn't have a tx valve. the liquid line connects to a manifold, then from the manifold 4 smaller lines lines enter the evaporator at different points. the tech assistance guy said there is an orifice in each of those lines and that's where my blockage is.he said I should change the manifold and the orifices.
question is; can I get rid of the manifold with the orifices and replace it with a TXV? I could connect the liquid line to the entrance of the TX valve. then at the outlet of the TX valve I could make my own little manifold without the orifices and connect a 1/8 inch line from my new home made manifold into each of the entrances to the evaporator where the orifices were. I think doing it that way would save money and work better. think it will work?
thank you dennis


  • don_207
    don_207 Member Posts: 12
    the boss

    The boss being the evaporator are you sure the coil is clean and you have the correct airflow across it?

    Try this.disconnect the fan speed while the unit is running and let the coil freeze up.If the distributor tube are clogged you will not see any ice on those tube.

    Rooftop unit are know for having clogged evaporator coils and blower wheel.
    Checking superheat and subcool will tell you where your refrigerant is in the system.

    Remember you need some load on the indoor coil to raise the suction pressure.On a mild day with poor airflow it not hard to see those type of pressure with poor airflow and a orfice for the metering device.

  • Empire_2
    Empire_2 Member Posts: 2,343
    Better Yet

    cycle the RTU in heat and check the air flow that way. I do not recall the formula, but this can give you an Idea if you are near the proper air flow for that unit. "Don, do you remember the equation? I think it's net BTU/s X 1.08 or something like that.

    Mike T.
  • ddenny
    ddenny Member Posts: 75
    changing metering device

    hello again. ok I'll try what you suggested. one thing I didn't make clear is that condensing unit with what I thought had the better pressures (60# and 160#) is using the bottom half of the same evaporator. It's a 2 stage system. you know how that works. It's really two separate systems with the evaporators and condensers stacked on top of each other. I also forgot to mention it's an r-22 system but I know you figured out that by yourself. I gave the evaporator a visual check and it looked ok. I also figured that if the bottom half of the condenser was clean enough to get decent pressures, the top should be about the same. filters are fresh.
    I was still wondering, if I do determine the problem is a partial plug in the evaporator orifices, can I replace it with a TX valve? I've heard that expression "the evaporator is boss". what does that mean. thanks
  • don_207
    don_207 Member Posts: 12
    The boss

    The evaporator heat load and the liquid requirement of the evaporator are factors influencing the density of the suction vapor.And the suction vapor will influence the work of the compressor,highside pressure,and level of liquid storage in the condenser.

    The evaporator is the boss and all other componants does what it said by the way of vapor density.

    You have to get the book..pressure enthalpy without tears.
    It relly helps give you a better understanding of whats going on when it comes to vapor density.

    Pay closer attention to your saturated temps.Also start checking the temp out of the condenser and evaporator to help with trouble shooting.

    Hi Mike ...that equation is only helpful if he has some electric heat strips in the system.
    I think its volt x amps to find the wattage.Then wattage x
    3.413 = btus.
    Then its btu devided by 1.08 x temp rise=cfm.
    Keep in mind that is only for sensible heat.

    Sure a txv will keep your super heat in check under all loads.The problem is if you have low airflow issue it will give you problems on the other side of the system.Like stacking refirgerant.
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