Page 36 - Climate Control News Magazine July 2021

Product Focus
Figure 3
When a system operates at a constant rotation frequency of 50 Hz, an elastic bearing with a nat- ural frequency of 15 Hz will provide an accepta- ble isolating effect (> 90%).
In units using frequency-controlled refriger- ant compressors, which are typically found in VRF units, the lowest expected working point must be considered. To achieve effective vibra- tion isolation even at the lowest rotational speeds, the frequency ratio should be . If the lowest rotation frequency of the compressor is 15 Hz, the natural frequency of the elastic bearing should be less
MATERIAL DAMPING
All elastic materials, such as polyurethane or rubber, have a damping as well as an elastic re- sponse. Damping can be described through the loss factor as a material characteristic.
A one-off external stimulation of an oscilla- tory system will cause the latter to vibrate at its natural frequency. In this case the damp- ing causes a defined decay in the amplitude of the vibration, a phenomenon known as vibra- tion damping.
This can be significant in situations where an HVAC system has been installed on a roof to quickly dampen the unwanted excitations
Figure 4
caused i.e. by strong gusts of wind. In these situ- ations, a combination of a steel spring with a core of damping polyurethane is recommended (Figure 3). If lifting forces are also anticipated during strong wind events, special elastic instal- lation elements must be used (Figure 4). Such compression-tension elements demonstrate a high damping response, particularly in the ten- sion direction.
When a HVAC unit is operating normally, vi- bration damping plays a subsidiary role. The stimulation comes not from one-off events, but from continuous excitation. Damping of the elas- tic material has a direct effect on vibration isola- tion, as it also reduces resonance superreleva- tion. This is important if the HVAC unit is subject to frequent start-stop cycles like combined heat and power plants (Figure 5).
Whenever the system is switched on or off, the resonance point is briefly but repeatedly traversed. To prevent a build up to excessive vi- bration amplitudes, it is not advisable to use so- lutions with low levels of damping, such as pure steel springs but spring-damper combinations instead. Note as well that a high level of damp- ing also reduces the effectiveness of the vibra- tion isolation for excitation frequencies
. The exclusive use of highly damp-
FAR LEFT: Steel springs are used
for roof installations. These springs possess a damping core made of polyurethane.
LEFT: Compression- tension elements demonstrate a high damping response in the tension direction.
ing materials for vibration isolation is therefore not recommended.
VIBRATION ISOLATION
Decoupling elements based on rubber materials are normally used for vibration isolation. Their elastic effect is enhanced through plasticisers, which are layered between the polymer chains of the rubber material, but do not bond to it.
They can therefore leach out of the elastomer due to outgassing or when the plasticisers dis- solve on contact with fluids. The elastic proper- ties of rubber materials weaken significantly af- ter a while and the material becomes brittle.
In the long term, rubber materials – as op- posed to polyurethane – cannot provide a con- sistent level of vibration isolation. Due to their basic design, polyurethane materials are much more suitable for use in elastic vibration-isola- tion components.
They contain no plasticisers, so their elastic properties remain constant for decades. Regard- ing the use at temperatures below 0°C, polyure- thane materials display no tendency to tempera- ture-related stiffening.
They can be used from -30°C to 70°C, with short-team peaks of 90°C permitted. Another important aspect is the dynamic stiffening of
ABOVE: Combined heat and power plants can be effectively decoupled using spring-damper elements.
RIGHT: Rubber, unlike polyurethane, stiffens under dynamic loading.
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Figure 5
Figure 6
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