Civil Engineering Reference
In-Depth Information
likely that people feel more comfortable with using this system because of a simpler
access to the diffusers on the fl oor instead of over-head diffusers, and a better air
quality due to its immediate arrival of air to people breathing zone. On the other
hand the main disadvantages and concerns about this system are lack of enough
designer experience with this new system, diffi culty of using these systems in
existing buildings and higher cost for raised fl oor construction.
Bauman in his topic (Bauman
2003
) provides an in depth evaluation and pro-
vides design guidelines for designing under-fl oor air distribution systems.
The sources of energy consumption in this type of systems are the air handling
unit supply fan motor input power, return fan motor input power, electric preheat
strip (if it is used for freeze protection instead of hot water coil) in air handling unit
and baseboard electric units (if it is used). Ductwork (between the air handling unit
and the under-fl oor plenum) air leakage, heat transfer from ducts, and effi ciency
loss in water coils are other sources that can indirectly increase the energy con-
sumption of the system. The small energy consumed by low-voltage power pro-
vided for control valves and dampers is the other source of energy consumption.
8.13.2
Under-Floor Air Distribution Parallel
Fan-Powered VAV
This system (Fig.
8.12
) is similar to a parallel fan-powered variable air volume system
except that (1) instead of using the duct for air delivery, it uses the under-fl oor
plenum space, (2) there is an additional return-air bypass arrangement which is used
to bypass return air to upstream of the fan in order to provide suffi cient dehumidifi -
cation when it is needed, without the need for a reheat coil, and (3) space heating is
delivered through an under-fl oor parallel variable air volume terminal unit that
draws air from the room rather than the ceiling plenum.
Utilizing this system similar to the previous system not only results in saving
energy by delivering air with a higher temperature than what is usually used in
traditional cooling systems, but also creates higher occupant comfort by delivering
air with lower speed than what is used in traditional systems, and where it is most
effective. The sources of energy consumption in this type of systems are the air
handling unit supply fan motor input power, return fan motor input power, terminal
unit fan motor input power, electric preheat strip (if it is used for freeze protection
instead of hot water coil) in air handling unit and electric reheat strip (if it is used
for dehumidifi cation or heating instead of hot water coil) in terminal unit.
Duct air leakage, heat transfer from ducts connecting air handling unit to the
under fl oor plenum, and effi ciency loss in water coils are other sources that can
indirectly increase the energy consumption of the system. The small energy con-
sumed by low voltage power provided for control valves and dampers, and the
power input to the variable frequency drives for supply and return fans that can be
factored in supply and return fan consumption itself are the other sources of energy
consumption.
