Handbook of air conditioning and refrigeration / Shan K

18.50 CHAPTER EIGHTEEN
Heat Unneutralized
When the supply air from the floor level is returned at the ceiling level, upward airflow lifts the heat
gains in the upward flow direction and returns some heat unneutralized to the ceiling. Furthermore,
the upward airflow captures most of the lighting heat gain in the return airflow. Research has
demonstrated that for a typical office, 15 percent of space heat gain and most of ceiling-level lighting
heat gain can be considered unneutralized because of the localized thermal buoyancy. In offices
where equipment is concentrated, convective plumes, upward warm expanding airflow above the
heat sources, can capture an even greater percentage of space load by the floor-to-ceiling airflow,
and results in greater capability to exhaust heat through ceiling return than the conventional space
air diffusion system.
Maintaining a Consistent Access Plenum Temperature
Because of the smaller supply air temperature differential T r � T s in underfloor air distribution, if the
underfloor plenum has hot or cold areas, the occupants’ comfort will not be satisfied by pulling air from
these localized areas of temperature inconsistency. The following rules are based on test results:
● Supply air colder than 63°F (17.2°C) is perceived as too cold by occupants. Blend air at the floor
outlets and at other supply units.
● Primary air degradation becomes unacceptable when it travels a distance exceeding 30 ft (9 m)
from the point of air introduction within the access floor plenum. Provide sufficient insulated
ductwork to distribute primary air.
● A variation of more than 3°F (1.7°C) across a master control zone may exceed the air transfer fan
adjustment range to compensate for this variation. Practical experience limits the master zoning (a
zone corresponds to an area served by a VAV box or other terminals) to about 3000 ft 2 (279 m 2 ) if
primary air is supplied from a central riser with VAV control, or a maximum of 10,000 ft 2
(929 m 2 ) if primary air is supplied from an on-floor fan room.
Floor Plenum Master Zone Air Temperature Control
Design Considerations
For a VAV system, as space load varies, the supply volume flow rate supply to each master control
zone through an individually controlled VAV box will be varied accordingly. Because of the thermal
storage effect, the floor plenum air temperature does not respond as quickly as required. It takes approximately
20 min before the plenum temperature falls sufficiently to call for proper action.
As the space load reduces, more recirculating air is pulled to mix with the cold primary air. The
primary air rate becomes proportionally greater in the floor plenum. This effect increases the pressure
on the floor plenum, and its pressure becomes positive. The pressurized cold primary air overpowers
the normal downward return airflow. If a temperature sensor is located in the downward airflow
path, this cold air will be sensed by the sensor, with a resulting rapid reponse to cut the
primary air supply flow rate.
● A structural concrete slab of 4-in. (100-mm) thickness that separates the supply raised floor
plenum and the return ceiling plenum is recommended. An attempt to baffle supply and return
sections and integrate them in the plenum of the same level has inherent difficulties due to air
leakages through plenum baffles, short-circuiting of air in the space, and limiting of the supply air
temperature range.