Handbook of air conditioning and refrigeration / Shan K

18.24 CHAPTER EIGHTEEN
FIGURE 18.17 Mixing flow using slot diffusers: (a) perimeter and interior zone, cooling; (b) perimeter zone,
heating.
induction ratio and to produce a very good surface effect. These ceiling diffusers are often used in
variable-air-volume systems.
Mixing Flow Using Slot Diffusers
Slot diffusers are narrower and longer than ceiling diffusers and grilles. When supply air is discharged
horizontally from a ceiling slot diffuser, it has a thinner vertical spread and an excellent
surface effect. Figure 18.17 shows mixing flow using slot diffusers discharging vertically downward
into the perimeter zone and also discharging horizontally along the ceiling in both the perimeter and
interior zones of a large office.
Because of its superior surface effect, the horizontally discharged cold air jet remains in contact
with the suspended ceiling even when the supply air volume flow rate is reduced in a variable-airvolume
system. The occupied zone is then filled with the reverse airstream at a more uniform temperature
and air velocity.
In order to counteract natural convection along the inner surface of a window in the perimeter
zone, the air jet is often projected downward toward the window. A sufficient throw is important to
produce a downward air jet that offsets the cold draft at the inner surface of the window during winter
heating. The occupied zone fills with induced airflow at an air velocity lower than 50 fpm (0.25
m/s).
For a mixing flow using slot diffusers, the location of return slot is preferably aligned with the
supply slots.
In addition to their excellent surface effect in mixing flow, slot diffusers have a linear appearance
that can be coordinated easily with electric lights and ceiling modular arrangements. Therefore,
they are widely used in buildings using VAV systems with moderate loads and normal ceiling
height.
Mixing Flow Using Sill or Floor Outlets
The purpose of using a sill or floor outlet is to counteract the cold draft flowing downward along the
inner surface of the window when the outdoor temperature is below 30°F (� 1.1°C). Figure 18.18
shows a mixing flow using a sill outlet.
During cooling, a stagnant zone may form above the eddies beyond the terminal zone of the air
jet if the supply air velocity is not high enough. Meanwhile the space under the cold air jet is filled