Page 37 - Climate Control News Magazine July 2021
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                Product Focus
    Figure 7
FAR LEFT: Providing the compressor assembly with elastic bearings reduces the direct excitation of the unit’s frame.
LEFT: The compressor assembly is mounted on an intermediate frame to decouple it elastically from the rest of the system.
Figure 8
products made from natural or synthetic rub- ber and classical rubber products such as EPDM or SBR. This is why the annex to ISO 18437-1 explicitly draws attention to the pro- nounced amplitude dependence of the dynamic properties of rubber products on account of their soot content.
Small excitation amplitudes, which typically occur in HVAC systems, lead to significant stiff- ening under dynamic loading conditions. Polyu- rethane does not exhibit this behaviour to the same extent (Figure 6).
Rubber and polyurethane layers exhibit a dif- ferent dynamic response, even though they dis- play the same static deflection at the same static stiffness. Rubber products exhibit a significantly higher level of dynamic stiffness compared with polyurethane products.
This stiffness results in higher natural fre- quencies and a reduction in the effectiveness of the vibration isolation.
Utilising elastic elements inside the HVAC sys- tem, directly at the source of the vibrations, re- duces the direct excitation of the unit's frame, which in turn can have a positive impact on pri- mary airborne noise (Figure 7).
Note that the feed and return lines of the re- frigerant are soldered rigidly in place on the com-
Figure 9
pressor which will be damaged if the elastic at- tachment is not made correctly.
Fractures leading to the loss of refrigerant must be avoided. Thus, it is important that the static stiffness of the elastic bearing on the compressor itself closely matches that of the rubber bearings supplied by the compressor manufacturer.
The result will be a significantly improved vi- bration-isolation effect. Damage to the sensitive solder connections is avoided by mounting the entire compressor assembly on an intermediate frame that is elastically decoupled from the rest of the system (Figure 8).
CASE STUDY: AIR HEAT PUMP IN A DETACHED HOUSE
An air/water heat pump in a detached house in Austria, installed on a concrete pedestal with rub- ber bearings, seriously affected the well-being of the family.
The vibrations generated by the compressor could be felt inside the house and caused a considerable amount of noise in the bed- rooms, especially in winter. The entire heat pump was fixed on a spring-polyurethane ele- ment (Figure 9). At a main excitation of 50 Hz, this solution brought about an improvement
of 14.7 dB over the conventional rubber bear- ing (Figure 10).
BEARING CHOICE
The practical choice of the bearing elements with an app like EquipCalc is based on the model of the single mass oscillator. The main parame- ters are the mass of the machine and the number of support points.
The elastic bearings should not be more than 1.5 m apart. The location of the overall centre of gravity of the HVAC system must also be considered.
“WHEN A HVAC UNIT IS
OPERATING NORMALLY VIBRATION DAMPING PLAYS A SUBSIDIARY ROLE.”
There are hardly any restrictions regarding the installation of polyurethane. Elastomer cuttings can be fashioned as point, strip or full-surface bearings and polyurethane-rubber-elements.
If trimmed pieces of an elastic material are to be bolted in place, additional elastic decoupling between the bolt head and the oscillatory system will be required in form of a pre-loaded bearing. Ignoring this requirement will cause vibrations to be transmitted directly into the foundation.
The bolted connection cannot be made using the usual tightening torques. If this is unavoida- ble for stability reasons, we recommend the use of a steel sleeve, as this will dissipate the forces required to pretension the bolt.
If polyurethane is used for the elastic decou- pling, note that the elements should be com- pressed. Tensioning is to be avoided, as high-ten- sion peaks are generated at the edges of the fastening surfaces which result in damage even when only small cracks are present. Similarly, per- manent shear deformation of the elastic bearings must be avoided. ✺
        Figure 10
ABOVE: Installing the heat pump on a suitable elastic element significantly reduces noise levels. ABOVE RIGHT: Improvement as shown in measurement report
CLIMATE CONTROL NEWS JULY 2021
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