will be properly aligned. The base assembly should
duct mufflers can usually furnish self-noise data for
be stiff enough to permit mounting of the entire
their products.
equipment load on individual point supports, such
e. Muffler pressure drop. In any installation
as "soft" steel springs. Equipment installations
where exhaust or inlet pressures are of concern,
that involve close-by vibration-sensitive equip-
the designer should request the muffler manufac-
ment, instruments, or processes are beyond the
turer to provide pressure-drop data for the pro-
generalized recommendations given here. The ba-
posed mufflers, and these values should rechecked
sics of vibration isolation (criteria, materials, and
and approved by the engine manufacturer.
approaches) are given in chapters 4 and 9 of the
N&V manual. -The term "engine assembly" is used
35. Ventilation duct lining.
here to include the engine, all driven equipment
Duct lining is used to absorb duct-transmitted
(such as gear, generator, compressor, etc.), and
the engine base. The term "engine base" is used
noise. Typically, duct lining is 1 in. thick. Long
here to include a stiff steel base or platform that
lengths of duct lining can be very effective in ab-
supports the engine assembly and a concrete iner-
sorbing high-frequency sound, but the thin thick-
tia block to which the steel base is rigidly attached.
nesses not very effective for low-frequency absorp-
t i o n . The ASHRAE Handbook and Product
a. Concrete inertia block. A concrete inertia
Directory-Fundamentals (app. B) can be used to
block is required under each engine assembly un-
estimate the attenuation of duct lining. Lined 90
less stated otherwise. The concrete inertia block
square turns are very effective in reducing high-
adds stability to the installation and reduces
frequency noise. Turning vanes or rounded 90
vibration. For reciprocating engine speeds under
turns, however, provides neglible amounts of high-
about 360 rpm, the weight of the concrete inertia
frequency loss.
block should be at least 5 times the total weight of
the supported load; for engine speeds between 360
36. Vibration isolation of reciprocating
and 720 rpm, the inertia block should weigh at
least 3 times the total weight of the supported load;
engines.
and for engine speeds above about 720 rpm, the in-
Vibration isolation of reciprocating engine
ertia block should weigh at least 2 times the total
assemblies is discussed for two general locations:
weight of the supported load. Even small inertia
on an on-grade slab, such as in a basement or
blocks should be thick enough to provide a stiff
ground level location, and on an upper floor of a
base for maintaining alignment of equipment when
multifloor building. Suggestions given here are
the inertia block is mounted on springs around the
based on acoustical considerations only; these are
perimeter of the block. Additional vibration isola-
not intended to represent structural design re-
tion details are given below as a function of location
quirements. These suggestions apply to both the
and engine speed and power.
engine and all attached equipment driven by the
On-grade
location. The chart in figure 32
engine. It is assumed that the mechanical engineer,
b.
shows the paragraphs below that give recom-
structural engineer, or equipment manufacturer
mended vibration isolation treatments for various
will specify a stiff, integral base assembly for the
combinations of engine speed and power rating.
mounting of the equipment and that all equipment
312
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