Danfoss VZH052 Manual page 27

Manage sound and vibration
Mechanical vibrations
Gas pulsation
A compressor generates some vibrations
that propagate into the surrounding parts
and structure. The vibration level of a VZH
compressor alone does not exceed 76 µm peak
to peak. However, when system structure natural
frequencies are close to running frequency,
vibrations are amplified due to resonance
phenomenon.
A high vibration level is damageable for piping
reliability and generates high sound levels.
Mitigations methods:
1. Danfoss VZH scroll compressors are designed
to produce minimal vibration during operations.
To ensure minimum vibrations transmission
to the structure, strictly follow mounting
requirements (mounting feet, rails etc..). For
further information on mounting requirements,
please refer to "Design compressor mounting".
2. Ensure that there is no direct contact (without
insulation) between vibrating components and
structure.
3. Resonance phenomenon
To avoid resonance phenomenon, pipings and
frame must have natural frequencies as far as
possible from running frequencies.
The Danfoss VZH scroll compressor has been
designed and tested to ensure that gas pulsation
is optimized for the most commonly encountered
air conditioning pressure ratio. Manifolded
compressors are equivalents to lagged sources
of gas pulsation. Therefore, pulse level can vary
during time.
This could be challenging on a variable system as
all resonant frequencies between min speed to
maximum speed will be exited.
It is mandatory to check that piping vibrations
are acceptable across speed range. This test
can be done by increasing slowly speed and
monitoring piping behavior through, strain gage,
acceleration, or displacement measurement.
As alternative visual check with strobe light can
also emphasis high piping displacement.
If some resonant frequencies generate high
piping vibration, problem can be solved by
increasing piping stiffness with brackets or
changing layout. Dampers can also be installed
to mitigate vibration.
If some frequencies continue to produce
unacceptable vibration levels, speed by-pass is
adjustable in the frequency converter, in order
to avoid some frequency ranges. Four by-pass
ranges are adjustable, and settings can be made
in parameter group 4-6.
Mitigations methods:
If an unacceptable level is identified, a discharge
muffler with the appropriate resonant volume
and mass can be installed.
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