Handbook of air conditioning and refrigeration / Shan K

18.40 CHAPTER EIGHTEEN
Ventilating Ceiling
flows through the raised floor grating and is returned to the recirculating air unit at the top of the
clean room.
Figure 18.23b shows a horizontal unidirectional flow for a clean room. Instead of flowing downward,
the clean airstream discharges horizontally from the HEPA filters on one side of the room and
flows through the working area. The contamination level near the return inlets of a horizontal unidirectional
flow may be higher than those of a downward unidirectional flow.
Figure 18.23c shows a clean workstation that provides a small noncontaminated working area
for a single worker by means of horizontal or downward unidirectional flow and HEPA filters.
Clean workstations can achieve a high degree of contamination control over a limited area for many
practical applications.
To provide parallel stream lines, air velocity of 60 to 90 fpm (0.3 to 0.45 m/s) is required. Unidirectional
flow provides a direct and predictable path of submicrometer-size dust particles and
minimizes the opportunities for these particles to contaminate working parts. It also captures internally
generated dust particles and carries them away. Most dust particles in unidirectional flow
reestablish their parallel stream lines after the downstream eddies of an obstruction.
The supply temperture differential T r-T s for unidirectional flow for clean rooms depends mainly
on the required space velocity and the cooling loads to be removed within the working area. A case
study of a class 10 clean room is discussed in Chap. 30.
A ventilating ceiling is sometimes called a perforated ceiling. It creates a downward uniform flow
similar to the downward unidirectional flow for clean rooms. In most cases, ventilating ceilings discharge
conditioned air through the entire ceiling to form a downward uniform flow, except in the
area occupied by light troffers. The primary differences between unidirectional flow for clean
rooms and downward uniform flow from ventilating ceilings are as follows:
● Unidirectional flow requires a 60 to 90 fpm (0.3 to 0.45 m/s) air velocity and ultraclean air in the
working area, and ventilating ceilings usually have a mean air velocity of less than 15 fpm (0.075
m/s) of conditioned air.
● There is no mixing of supply and space air in unidirectional flow; whereas just below the perforated
ceiling, supply air is mixed with the ambient air at a vertical distance of less than 1 ft (0.3
m) in downward uniform flow from the ventilating ceiling.
FIGURE 18.24 Ventilating ceiling.