Handbook of air conditioning and refrigeration / Shan K

18.34 CHAPTER EIGHTEEN
● A terminal velocity of 150 fpm (0.75 m/s) is recommended at the 5-ft (1.5-m) level near the window,
to offset cold drafts in winter.
● An evenly distributed air volume flow rate between the vertically and horizontally discharged
slots is recommended.
Select the Specific Supply Outlet from Manufacturer’s Catalog
After an optimum T 50/L (T 0.25/L) or T 100/L (T 0.5/L) and characteristic length L are determined from
the preliminary layout, the supply outlet can be selected from the manufacturer’s catalog from
known T 50 (T 0.25) or T 100 (T 0.5) and supply volume flow rates, after the following parameters have
been checked:
Sound Level. The combined sound level of terminal and outlet should be at least 3 dB lower than
the recommended NC criteria in the conditioned space. For optimum noise control, the recommended
air velocities at the supply outlet are as follows:
Residences, apartments, churches, 500 to 750 fpm (2.5 to 3.75 m/s)
hotel guest rooms, theaters, private offices
General offices 500 to 1250 fpm (2.5 to 6.25 m/s)
The outlet velocity for the ceiling diffuser can be calculated by dividing the volume flow by area
factor A k, given in the manufacturer’s catalog.
Drop of Cold Air Jet. Drop of a cold air jet should be checked if the cold jet enters the occupied
zone directly. Figure 18.21a shows the drop and other data of typical sidewall outlets mounted
within 1 ft (0.3 m) of the ceiling with an adjustable vane deflection of vertical 5° up and horizontal
0°. Figure 18.21b shows the drop for sidewall outlets without surface effects and a vane deflection
of vertical 15° up and horizontal 0°. Both are cold air jets based on a supply temperature differential
of 20°F (11.1°C).
Total Pressure Loss of the Supply Outlet. The total pressure loss of supply air when it flows
through a slot diffuser with a slot width of 0.75 in. (19 mm) is usually between 0.05 and 0.20 in.
WC (12 and 50 Pa). For a ceiling diffuser, it is between 0.02 and 0.2 in. WC (5 and 50 Pa). A total
pressure loss higher than 0.20 in. WC (50 Pa) is unsatisfactory.
Determination of Final Layout of Supply Outlets and Return Inlets
The determination of the volume flow rate per outlet, the number of outlets, T 50/L (T 0.25/L), and the
selection of specific outlets from the manufacturer’s catalog is often an iteration procedure. After
that, final layout of supply outlets and return inlets can be determined.
In the perimeter zone, the high side wall outlet in recently constructed hotel guest rooms are often
located on the partition wall adjacent to the window. In the interior zone, ceiling and slot diffusers
are usually located directly above the center of the conditioned space.
As mentioned in Sec. 18.4, if a ceiling plenum is used as return plenum, the return inlets should
be located outside the supply air jet, above the return airstream, or near a concentrated heat source
for a better effectiveness factor �.
The recommended face velocities for return inlets are as follows:
Above the occupied zone 800 to 1000 fpm (4 to 5 m/s)
Within the occupied zone 400 to 600 fpm (2 to 3 m/s)
Door louvers 300 to 500 fpm (1.5 to 2.5 m/s)