Handbook of air conditioning and refrigeration / Shan K

18.14 CHAPTER EIGHTEEN
Slot Diffusers
types of inner cone and vanes. The air discharge pattern of many ceiling diffusers may be changed
from horizontal to vertical by means of adjustable inner cones or special deflecting vanes. Ceiling diffusers
are often mounted at the center of the conditioned space and discharge air in all directions.
An important characteristic of a ceiling diffuser is its induction effect. Induction is the volume
flow rate of space air induced into the outlet by the primary airstream. It is also called aspiration.
The ratio of the volume flow rate of induced air to the volume flow rate of supply air V˙ ind/ V˙ s, both
in cfm (L/s), is called the induction ratio. A ceiling diffuser with a higher induction ratio (see Fig.
18.8b) is beneficial for a higher supply temperature differential �Ts for cold air distribution in an
ice storage system, which is discussed later in Sec. 18.5 and in Chap. 31.
Figure 18.8c shows a perforated ceiling diffuser. A perforated surface increases the induction effect.
It is also used to match the appearance of the ceiling tile. The diameter of the holes on the per-
forated panel is typically in. (4.8 mm). Some ceiling diffusers are designed to discharge air
evenly in all directions so that the surrounding ceiling surfaces are less likely to be smudged.
The size of a ceiling diffuser is determined by its neck size and is therefore closely related to the
neck velocity of the diffuser. Ceiling diffusers are made of an outer shell and a removable inner
core. The opposed-blade damper with an equalizing device of parallel blades mounted within a
frame is often used to adjust the volume flow and to provide a more uniform airflow.
Ceiling diffusers are usually made of aluminum-coated heavy-gauge steel, extruded aluminum,
or teak wood. In addition to high induction ratios, ceiling diffusers have a good surface effect and a
shorter throw and can supply air in all directions. Horizontally projected ceiling diffusers are suitable
for conditioned spaces with low headroom.
The performance data of a ceiling diffuser are defined by the following parameters:
● Neck velocity vneck, that is, mean air velocity at the neck of the ceiling diffuser, fpm (m/s)
● Volume flow rate s � Ak vs, in cfm (L/s); area factor Ak, in ft2 (m2 V˙
), is determined at the testing
laboratory, and the average supply velocity vs is usually measured at a specified position at the
outlet by a deflecting vane anemometer, often called a velometer
● Total pressure loss �pt, in in. WC (Pa)
● Throw, ft (m)
● Noise criteria curve
3
16
A slot diffuser consists of a plenum box with single or multiple slots and air deflecting vanes, as
shown in Fig. 18.9. The slots are available in 0.5-in. (13-mm), 0.75-in. (19-mm), 1-in. (25-mm),
and 1.5-in. (38-mm) widths. The 0.75- and 1-in. slot widths are most commonly used. Slots are typically
2, 3, 4, 5, or 6 ft (600, 900, 1200, 1500, or 1800 mm) long. Air discharged from a slot diffuser
can be projected horizontally or vertically. With a single-slot diffuser, air is always discharged
in one direction. With multiple slots, air can be horizontally discharged either left or right, or a
combination of both, or one slot can discharge vertically while another discharges horizontally. Slot
diffusers are often called T-bar diffusers, as they are compatible with standard ceiling T bars.
Air enters the plenum box from the round inlet at one side. The function of the plenum box is to
distribute the air more evenly at the slot. The plenum box is sometimes insulated internally. After
passing the deflecting vanes, the air is discharged from the slot horizontally (left or right) or vertically
according to the required airflow pattern. Figure 18.9 shows the airstreams discharged from
slot diffusers in various directions. Different manufacturers use their own patented air deflecting
vanes. Deflecting vanes can also be used to block the airflow entirely if necessary.
Extruded aluminum is widely used as the material for the structural member of the slot, and a
galvanized sheet is used for the plenum box. The performance of a slot diffuser is defined by the
following parameters:
● Width of slot, number of slots, and length of slots