Cold
Air Distribution: A new approach for Commercial Buildings
Have you ever operated or maintained a cold air
distribution HVAC system? If not, it looks exactly like a
conventional VAV box distribution system, with supporting air
handlers, chillers and cooling towers. But it operates totally
different! Read more to find out...
Cold Air Distribution
Cold air distribution systems are being installed in more
new buildings in the Washington Metropolitan area. Chances are
that you will one day be assigned to a building that has one.
Several of our engineers currently operate and maintain them.
If you've ever heard of cold deck, then cold air systems
are a variation of that technology. The theory behind this
design is that by using smaller-sized equipment that produces
colder air, you can condition interior space to the same
temperature, thus saving money on the construction of the
building.
A wider temperature difference between the discharge air
temperature and the interior space allows for a lower relative
humidity level at the same temperature. As a comparison, an
interior space at 76 degrees and 30 percent humidity will feel
exactly the same as 74 degrees and 50 percent humidity, such
as a typical VAV distribution system provides.
The equipment used in a cold air system looks the same as
in any other HVAC system. Above the ceiling, the cold air
distribution system looks identical to a fan VAV box
distribution system. However, that's where the similarity
ends. VAV boxes in cold air systems are actually called PIU's,
or Powered Induction Units. By design, they operate completely
opposite than a VAV box. Where a VAV box delivers a variable
volume of air at a constant temperature (typically 55
degrees), the PIU box delivers a constant volume of air at a
variable temperature. The PIU box receives 45-46 degree
primary air from the air handling unit and mixes it with 74-76
degree return plenum air drawn by the fan in the PIU box. As
more primary air is used, the fan in the box slows down. As
less primary air is used, the fan speeds up. PIU boxes serving
exterior zones typically have electric heating elements, also
similar to VAV boxes. However, PIU boxes with heat operate on
a zero-minimum setpoint position, using fan air only when in
heating mode.
Cold air systems typically utilize chilled water. To make
45-46 degree primary air, the chilled water needs to be 38
degrees. For anyone who has operated a chiller, we were taught
that you should never chill the water below 42 degrees, or
otherwise risk freezing the evaporator tubes. Chillers for
cold air systems are designed to operate under lower
temperatures.
As you would imagine, operating, making adjustments, and
maintaining cold air systems involve a different technology
and knowledge than what engineers were typically taught. The
secret to success is to read the design documents and
understand the control sequence of operations. Do not try to
employ economizing strategies to increase efficiency. Cold air
systems are based upon a delicate balance of temperature and
relative humidity, and you will create humidity problems
sufficient to grow mold in the ductwork if you "tweak" the
temperature settings. | 
