Water Vs. Air: Dehumidification Issues

Water Vs. Air: Dehumidification Issues

By Greg Cunniff, Applications Engineering Manager, Taco, Inc.

Another major issue between water vs. all-air systems is how chilled water and DX-type systems treat     humidity, a critical consideration in providing air conditioned comfort. Humid climates in particular   provide a mismatch between the sensible heat ratio and the load in DX systems. In order to adequately remove moisture from a space or zone, cooling coils need to remain online for as long as possible in order to adequately remove all the moisture. However, the sensible heat ratio of the load is generally lower than the sensible heat ratio of the coil. If the coil is cycled based on sensible heat or outside air temperature, then the moisture remains, to be re-evaporated back into the space. Water also is left sitting in the drain pan.

Chilled water systems will provide much better dehumidification due to the fact that coils can be selected to provide a sensible heat ratio of the load. Plus, reheat systems can also be incorporated in extremely humid climates. Chilled water systems can even reheat at the zoned level, if desired. Taken together, buildings cooled with chilled water systems will have higher comfort levels than typical DX systems.

Have you designed A/C systems for use in humid climates using either hydronic or air systems and, if so, how have they fared in operation with regard to controlling humidity. Join the discussion.

Next time: the emerging world of radiant cooling.

5 Responses

  1. I agree with the author’s views and would like to add a few points to further bolster them.
    Chilled water systems are ideal for dehumidification for a few more reasons;
    they track the buildings sensible load by utilizing mixing valves to regulate coil temperature and the chillers supplying the chilled water generally have more stages of unloading than a direct expansion system.
    Add the ability to reduce a chilled water coil’s air flow far below that of a direct expansion coil without the fear of liquid refrigerant flood back and you practically eliminate system off cycles.
    Remember- off cycles are a real problem for dehumidification because a warm coil is a dry coil, and a dry coil isn’t removing water.

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