Handbook of air conditioning and refrigeration / Shan K

Selection of Silencers
not exceed 1500 fpm (7.5 m/s). When the duct velocity exceeds 2500 fpm (12.5 m/s), the turbulent
noise may completely mask fan noise, and the effect of an active silencer is reduced to nil.
● Pressure drop of a passive silencer is often 0.2 to 0.35 in. WC (50 to 63 Pa). The electric energy
required to produce the canceling sound is only 40 W, a substantial saving compared to a passive
silencer.
● Due to the development of the microprocessor-based technologies, in 1997, the price of the controller
used in an active silencer dropped substantially compared to that in 1987.
Performance. Gelin (1997) compared the insertion loss, in dB, of an active silencer, a passive silencer,
and a duct liner for a large fan of 5000 cfm (2360 L/s) volume flow rate as follows:
Octave bands, Hz 31 63 125 250 500 1000 2000 4000
Active silencer 5–8 10–15 10–15 2–4 — — — —
1-in. Lined, 84-in.- (2.1-m-) long duct 2 2 4 10 20 17 15
84-in. (2.1-m) Prefabricated silencer 8 17 26 43 43 26 19
The pressure drop of the 84-in. (2.1-m) prefabricated silencer is 0.24 in. WC (60 Pa).
The active silencer has a good sound attenuation in frequencies between 31 and 125 Hz, the duct
liner provides effective sound attenuation in frequencies of 500 Hz and more, and the 84-in. (2.1-m)
prefabricated silencer is effective in frequencies between 63 and 4000 Hz.
During the selection of silencers, the following points should be considered:
● Dominating frequencies of noise. For a low-frequency fan “rumble” noise, an active silencer or a
reflective dissipative silencer should be selected. For a broadband noise, a combination of active
and dissipative silencers or a dissipative silencer with thick splitters, of 2- to 4-in. (50- to 100mm)
thickness, is preferable.
● Design requirements. For clean rooms, fiber-free all-metal silencers are often selected. For health
care facilities, fiber-free packless silencers should be selected.
● Wet surfaces. If there are wet surfaces in the air system, fiber-free all-metal silencers should be selected.
For a specific type of silencer, variations in its length and free area ratios often meet most of the requirements
for IL and pressure drops. Silencers are usually made in lengths of 3, 5, 7, and 10 ft
(0.9, 1.5, 2.1, and 3 m). The IL and �p sil of a silencer are proportional to its length.
A lower face velocity and greater free air ratio for a specific silencer at a given length always result
in a lower pressure drop and therefore lower energy consumption. If space is allowed, a lower
v sil is beneficial for both IL and �p sil.
19.5 FIBERGLASS IN HVAC&R SYSTEMS
Problems
SOUND CONTROL 19.17
Fiberglass-made duct liners, duct boards, silencers, and insulation layers have been widely used in
HVAC&R systems for decades. On June 24, 1994, the U.S. Department of Health and Human Services
(DHHS) announced that fiberglass will be listed as a material “reasonably anticipated to be a