TM 5-805-4/AFJMAN 32-1090
a constant volume delivered to the space served.
determine the static efficiency of the fan, and to
This type of unit is designated as a "series" FPT.
compare it with the maximum static efficiency of
The noise of any air terminal unit can propagate
the type of fan being utilized. The operating static
to the space served via a number of paths, but the
efficiency of a fan may be obtained from the
two prominent paths are (1) via the units dis-
following:
charge ductwork to the connected outlet(s), or (2)
Static Efficiency = (Q x P)/(6356 x BHP)(eq 7-1)
by direct sound radiated from the casing of the
where;
unit into a ceiling plenum and then through a
Q = air quantity is in cubic feet/minute,
ceiling (usually acoustical) into the space served.
P = static pressure is in inches of water, and
Manufacturers publish data giving the octave
BHP = brake horse power.
band sound power level for the unit discharge
This calculated static efficiency is then compared
sound, and the casing sound. These data are
with the maximum efficiency for the fan, which
usually measured in accordance with
may be taken as 80% for a centrifugal fan with
Air-Conditioning and Refrigeration Institute (ARI)
airfoil blades; 75% for centrifugal fans with back-
Standard 880-89. With a VAV terminal unit the
wardly inclined, single thickness blades; 70% for a
measurements for the casing sound measure only
vane axial fan; and 65% for a centrifugal forward
the casing sound. For a fan powered box (FPT) the
curved fan. The ratio of the calculated static
casing sound data includes both the sound radi-
efficiency to the maximum static efficiency is then
ated by the casing, and the fan sound radiated
used to determine the correction for off-peak effi-
from the air intake opening to the unit casing.
ciency as shown on part C of table C-13. For
(1) Noise level prediction. To predict the sound
example if the calculated static efficiency for a
level in an occupied space produced by a terminal
forward curved fan is 62%, then the ratio of the
unit serving a space, procedures suggested in ARI
calculated static efficiency to the maximum static
Standard 885-90 may be used. For the duct borne
efficiency is 62% divided by 80%, or approximately
sound, radiated by the air outlets, the estimation
82%. In other words the actual static efficiency is
procedure involves two steps: (1) reducing the
approximately 82% of the maximum static effi-
sound power of the discharge, by the insertion loss
ciency and the off-peak correction from part C of
(IL) of the duct system between the unit outlet and
table 10-13 is 6 dB.
the space outlets, to obtain the unit sound power
b. Air terminal units. Air terminal units are
emitted into the room from the air outlets, and (2)
components used in ducted air distribution sys-
applying the octave band "Rel Spls" to obtain the
tems to maintain the desired temperature in a
octave band sound pressure levels in the room. For
space served by varying the volume of air. Basi-
the casing radiated sound again two steps are
cally these units consist of a sheet metal box
required to estimate the sound pressure levels in a
containing a damper, controls and a sensor, and
room with a unit located in the ceiling plenum,
they are usually connected to a supply header via
these are: (1) a plenum/ceiling transfer factor
a flexible circular duct. They usually discharge air
which combines the insertion loss (IL) of the
to one or more diffusers via rectangular sheet
ceiling and the absorption of the plenum is sub-
metal ducts. In their simplest form these units are
tracted from the published power level for each
designated as variable air valves (VAVs). How-
octave band, and (2) the room factor for the space
ever, units are also available with an auxiliary fan
is subtracted from the power levels transmitted
in the box to supplement the air delivered by
through the ceiling. Values for the Plenum/Ceiling
mixing induced air from the ceiling plenum with
Transfer for typical acoustical ceilings are given in
the primary air from the supply header. Units
table 12.1. These values are applicable to typical
with these auxiliary fans are termed fan powered
ceiling construction, with some openings for lights
terminals (FPTs), and they are available in two
and return air. These values do not apply when
forms. In one form the fan only operates when it is
the terminal unit is located directly above a return
necessary to mix warm air from the ceiling ple-
air opening.
num with the primary air, and this type of unit is
(2) Noise control Typical noise control mea-
designated as a "parallel" FPT. The intermittent
sures for air terminals, including VAVs, fan pow-
operation of the fan in this type of unit leads to
ered units, and mixing boxes are:
some increased awareness of the noise generated.
(a) Locating units above spaces such as cor-
In a second form the fan operates continuously and
ridors, work rooms, or open plan office areas. Do
handles both the primary air and the return air
not locate VAV units over spaces where the noise
from the ceiling plenum. Both the primary air and
should not exceed an NC or RC 35. Do not locate
return is mixed in varying quantities to maintain
fan powered terminal units (FPT), which are sized
7-2
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