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I'm currently troubleshooting a gravity return 2 pipe steam job. I've been reviewing the Lost Art of Steam book regarding the B dimension and the explanation is a little different than with the A dimension. I understand that the two have different reference points - the A is based on the end of the steam supply and the B is based on the lowest trap. However the A dimension is also based on pressure DIFFERENTIAL between the boiler and the end of the supply main and the B dimension seems to be based on boiler operating pressure - not pressure differential. If the B dimension is based on boiler operating pressure, than does that assume there is a 100% pressure drop across the steam supply side? (Reference - Lost Art, Chapter 9, "The Gravity-Return "B" Dimension section - page 127)


  • Steamhead
    Steamhead Member Posts: 16,718

    The steam side has the usual pressure drop, i.e. as low as you can get. What kills the leftover steam pressure to create a B dimension is a steam trap, water seal, orifice or whatever stops the steam from reaching the returns.

    It is usual for a Vapor system to have an A dimension on a steam main drip that does not go thru a trap, and a B dimension on a dry return drip that is fed thru traps on radiators.
    All Steamed Up, Inc.
    Towson, MD, USA
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