Handbook of air conditioning and refrigeration / Shan K

fans, two desktop supply outlets, flexible ducts, and a control panel. Inside the mixing box, there is
a variable-speed fan to extract primary air (cold conditioned air from the AHUs) either from the
low-pressure floor plenum or from the supply duct in the ceiling plenum by means of connecting
flexible ducts. Another fan pulls recirculating air from the knee space under the desk. The primary
air and recirculating air are mixed together according to the required fractions by regulating the
opening of their dampers in each of the transporting ducts. The mixture is then drawn through an
electrostatic air filter. The primary air damper is never allowed to close completely, to ensure that
the minimum amount of outdoor ventilation air is supplied all the time. The mixing box is hung on
the back or on the corner of the desk and is connected to the two supply nozzles (outlets) on the top
of the desk by flexible ducts.
Two supply nozzles may be rotated 360° on the horizontal plane. They are installed with horizontal
guide vanes which are adjustable � 30° in the vertical plane.
Control Panel. A desktop DDC unit controller uses adjustable sliders to control the variablespeed
fan and modulate the volume flow of the supply air from the nozzles. The supply air temperature
is adjusted by varying the opening of the primary and recirculating dampers, and thus the ratio
of primary to recirculating air. A 200-W electric radiant heating panel in the knee space is employed
during winter heating. The dimming of the occupant’s task light controls the lighting luminance
and a white noise generator which has a continuous frequency spectrum with equal
energy/Hz over a specified frequency range.
A motion detector–based occupancy sensor sends a signal to the DDC unit which shuts the
desktop system off when the workstation is unoccupied for a specified time.
Operating Characteristic. Each desktop unit can provide 40 to 250 cfm (20 to 70 L/s) of supply
air from the supply nozzles. Even when the two fans in the desktop unit are shut off, 40 cfm (20
L/s) of primary air will still be delivered to the conditioned space for the minimum outdoor ventilation
air requirement.
The maximum outlet velocity measured at the face of the 2.3- by 4-in. (58- by 100-mm) supply
outlet during laboratory tests varied between 6.5 and 24.5 fps (2 and 7.5 m/s). For a typical desktop
system, the primary air temperature supplied from a variable-air-volume (VAV) AHU was 55°F
(12.8°C). After mixing with recirculating air, the supply air temperature from the desktop outlet
was 65°F (18.3°C).
The supply nozzles are 5 ft (1.52 m) wide by 30 in. (0.76 m) deep at the back corners of the
desk. Air is supplied from these two nozzles toward a focus point near the center of the front edge
of the desk. For isothermal air jets of horizontal supply with a volume flow of about 90 cfm
(43 L/s), the average speed at the work location in front of the desk was 3.3 fps (1 m/s).
Performance of Desktop Task Conditioning Systems
AIR SYSTEMS: SPACE AIR DIFFUSION 18.47
Based on the laboratory investigation and field-measured results, the performance of desktop task
conditioning systems was summarized in Bauman et al. (1993) as follows:
● The desktop unit can be controlled to produce a wide range of thermal conditions, allowing occupants
the opportunity to fine-tune the local environment to their individual comfort preferences.
● Under the influence of different load density of the adjacent workstation, or under a warm average
room air temperature, a desktop unit is capable of being fine-tuned to maintain a nearly comfortable
environment, or to maintain a temperature of 1 to 3°F (0.56 to 1.7°C) lower than that of the
workstation without the desktop unit, even at relatively low supply air volume flow rates.
● At the same supply volume flow rate, a larger supply nozzle was shown to deliver lower air
velocity than the smaller nozzle. A larger nozzle reduces the potential to produce draft while
maintaining improved ventilation performance at moderate to high supply volume flow rates.
● Turning off the desktop unit whenever the workstation is unoccupied significantly reduces its energy
use.