MIL-HDBK-1191
quality concerns associated with the latter, internal duct lining should be
avoided when possible and may not be used in inpatient areas.
8.11.4.3.
Air Fixtures. Air distribution supply, return, and exhaust
fixtures (diffusers, grills, etc.,) shall be sized to provide air
inlet/outlet velocities consistent with room NC level requirements as
provided at Appendix A. Designers must be aware that diffuser
manufacturer's published noise characteristics are based upon idealized
inlet conditions: crinkled flex duct, abrupt branch duct connections,
elbows located immediately at the diffuser collar, and similar poor
connections may result in unacceptable noise levels. Spin-in or other 90
degree duct drop connections to diffusers shall be equipped with equalizing
grids as necessary to assure uniform air distribution at the diffuser
inlets.
8.11.4.4
Air Velocity. Designers shall limit air velocities in ductwork
(see additional guidance below), air transfer grills, or door undercuts to
values consistent with ASHRAE recommendations to control noise generation.
8.11.4.5
VAV/CAV Terminal Units. Variable Air Volume Terminal units and
constant velocity controllers are a frequent source of noise generation in
air distribution systems. Designers shall specify or schedule units with
minimum inlet sizes for incremental ranges of flow, and shall indicate
maximum sound power output for each unit, at the maximum inlet static
pressure which the designer anticipates that the unit will be exposed to.
If integral sound attenuating devices are required, these shall be
indicated for the respective terminal unit(s) is specs or drawing
schedules.
8.11.4.6
Exterior Noise Sources. Designers shall evaluate the sound
characteristics of exterior equipment provided as part of the project
design (such as cooling towers, emergency generators, etc.) to assure that
such sources do not result in interior noise levels exceeding limitations
provided in Appendix A.
8.11.5
Duct Design. Duct systems shall be designed in accordance with
references 8a, 8b, 8e, and 8l. Maximum velocity in ductwork mains shall not
exceed 760 M/m (2500 fps), and velocities in branch ducts and takeoffs
shall not exceed recommended levels in these standards. Ductwork plans
shall indicate the static pressure class required for sealing and
reinforcement for all types of duct. Access panels shall be provided as
necessary for access to fire dampers, smoke dampers, and control equipment,
and to facilitate periodic cleaning or disinfecting of ductwork. All
supply air, with the exception of air transferred between spaces for the
purpose of pressurization, shall be provided in sheet metal ductwork.
8.11.5.1
Non Corrosive Ductwork Material. Ductwork installed downstream
of high efficiency final filters (90% or greater, see Appendix A) or trim
humidifiers, serving critical spaces, shall be of stainless steel, or
aluminum, including all accessories such as dampers, fasteners, and turning
vanes. This provision does not apply for ductwork downstream of high
efficiency filters when these are located at the air handling units
(filters noted in the "intermediate" column in Appendix A). Exhaust ducts
for glass washers, dishwashers, and cart washers shall be non-corrosive and
shall have soldered or welded joints and shall be pitched to drain.
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