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Carrier IDT steam coil

Check to see if there is back pressure in the returns. Break a union AFTER that trap, on the return piping. There should be no air pressure and certainly no steam present there.

If the unit is running at the time that you break the union open, and the trap becomes hot while the piping is broken open, look for bad traps blowing into the returns from a different unit, or from a trap on a main.

The air has to get out if the steam is going to come in.

Noel

Comments

  • john_117
    john_117 Member Posts: 15
    Carrier IDT steam coil

    I have a 39 MVN40005DMK23XGS air handler serving as a 100% fresh air unit with an IDT (inner distributing tube)steam coil. The unit has face and bypass dampers which close to the face of the coil and open 100% the steam valve. The problem I am having is the last 2feet of the coil will not heat up. I have shut the fan off and had the steam valve 100% open and the face damper closed and still the last 2ft will not get warm. The installation is two months old and the coil is clean. The coil is designed for 5lbs of steam and I have 6 lbs supply. Any ideas why the coil won't heat fully?
  • piping?

    Do you have 5 # of steam after the supply valve with the valve wide open and the fan running?

    Is the air flow volume correct?

    Noel
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Is the trap functioning?

    If there is a bypass valve on the trap, can this be temporarily opened to check?

    Also is there a vacuum breaker on the coil downstream of the control valve? Might be hanging up trap operation.

    Is the unit dropping out on freezestat and what is the unit discharge temperature in the duct?
  • john_117
    john_117 Member Posts: 15


    there is 5psig after the steam valve and the air flow is correct 21,000 cfm.
  • john_117
    john_117 Member Posts: 15


    there is not a bypass on the trap, the freeze stat is tripping, there is not a vacuum breaker on the outlet of the coil.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Check what Noel said- good call.

    The vacuum breaker if existing, should be upstream of the coil but downstream of the control valve. Usually this is high in elevation. If it is installed downstream of the coil it is often elevated above the top coil header. You have to let air in on the down-cycle let condensate out. This is as much to protect the coil as anything and it seems that you would need that. In normal operation the steam pressure pushes it out via the trap.

    But Noel is on to this; backpressure is worth checking. An infra-red thermometer is handy, but any contact type thermistor temperature sensor (such as on a multi-meter) works as well. Any temperature above 210 F on the pipe surface is suspicious. Track and map any such temperatures.
  • john_117
    john_117 Member Posts: 15


    followed all suggestions and it turns out that when the coil was made the internal distribution tube must have been out of alignment with the machine that made the tube piercings and did not pierce the last foot or so of the tubes. Which explains why the end of the coil from top to bottom will not heat up.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Interesting

    Is this a one-row coil where the steam and condensate backflows to the same side of the header connection? Or is it a two-row?

    I find it interesting that so many of the tubes (all of them?) had their ends not fed.

    Let me know what the fix is, if it is a field repair or a coil replacement and how well Carrier steps up to the plate.

    Thanks
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