Humidification Basics Part 12: Humidification Specification and Steam Absorption Distance

By Chad Edmondson (JMP) and Norman Hall (RLD)

The absolute best way to ascertain that you have adequate absorption distance is to use sizing and selection software. We think a good choice is the DriCalc program from DriSteem.  By entering in a few values, including the entering and leaving air conditions, you not only get equipment options, but also the corresponding absorption requirements, which should be noted within the plans as a performance specification. The manufacturer can and should decide from there exactly how many rows of dispersion tubes and what orifice sizes will be required to meet the specification.

Finally, it’s important to place the dispersion assembly in a location that will best facilitate absorption. That is going to be the point where the air is the warmest and there is sufficient duct length. 

The drawing below shows a duct layout with potential options for humidifier location (A, B, C, and D).

In this case, point A is the best choice for the dispersion assembly location.  This because it is downstream of the heating and cooling coils which will provide laminar flow with heated air.   If not enough space for A then we could consider B but this could be an issue during change-over periods if the cooling coil is turned on.   As a third choice, we could look at location C if there is adequate space to install the dispersion assembly, but we would have to be careful of turbulent flow which could impact the absorption distance.  All three of these locations occur after the heating coil, so the air is at its warmest.  Point D would be a poor location do it the cold air stream.

Notice that there is a duct high limit humidity sensor positioned after the dispersion tube to make sure that humidification does not exceed set point.  We recommend using a modulating high limit transmitter for this safety.  This allows the high limit to operate in conjunction with the room humidstat and if we start approaching the high limit setting in the duct we would start reducing the humidifiers output.  Another option to consider is temperature compensation control.  With this option the humidifier is provided with a temperature transmitter that continually monitors the interior window glass temperature to calculate the dew point.   If the window temperature falls below the dew point we would automatically decrease the relative humidity set point so moisture does not form on the windows. That’s a clever way to make sure you never exceed suitable humidity levels within the space.

Lastly let’s consider where to put the steam dispersion assembly if we are near an elbow.  The drawing below shows two possible locations (A or B).

In this scenario location A is the best choice.  This is because better absorption occurs downstream of the elbow.  If there is not enough straight duct at this location, then B can be considered but we must be careful to not wet the turning vanes.  If B is chosen we highly recommend installing a multiple tube unit to make sure you achieve complete absorption.  In either case, it is best to discharge the steam against or perpendicular to the air stream to provide the best possible absorption.