By: Russell M. Keeler, PE

Under floor air distribution, sometimes called displacement ventilation, is a relatively recent innovation in the air conditioning field.  Conventional air distribution is achieved using supply air ducts above or at the ceiling, introducing the air (approximately 57F) at the ceiling, and cooling the entire volume of the space.  Under floor systems introduce 65-68F air at the floor.  The system only attempt s to cool the lowest 7 feet or so (the occupied zone).  As the air rises, it warms, and is taken out of the space.  This approach reduces drafts and noise.  Under floor air distribution is an ideal solution for a performing arts venue, but careful zoning is required so that balconies are not overheated.
Ice based thermal storage is an excellent means to reduce the size of an air conditioning cooling plant. This result is achieved by providing undersized water chillers that are capable of producing chilled water at low temperature (22-24F).  The typical cooling load of a building shows that the peak design cooling is only required for a few hours in the afternoon.  Thermal storage acts as a “battery”, which creates ice during minimal load hours at night, then melting the ice during the peak daytime hours.  Successful installations have reduced the size of the chiller plant by up to 40%, and saved a significant part of the electric demand cost, with no loss of comfort.  The solution is a green one, as nighttime electric production uses the most efficient part of the generating system.  Care must be taken in design; many systems have failed due to overly complex control systems.
Conventional air distribution systems employ conditioning air at 56-60F.  Low temperature air distribution, usually combined with thermal storage, supplies air at 47F.  The major benefit of low temperature air is to reduce the required cooling air quantity by over 40%, shrinking ducts and fans by a significant amount.  The savings are not only in duct and fan size, but also in fan room requirements.  Smaller ducts can increase the ceiling height without increasing the floor-to floor height of the building.
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