GOOD PRACTICES FOR THE INSTALLATION OF VIBRATION ISOLATORS FOR HVAC EQUIPMENT

The correct selection of anti vibration mounts is key for achieving good vibration isolation values on HVAC equipment. However, if the installation of the vibration isolators is wrong, the expected isolation results may not be reached. In this article we will describe good practices for installation, with examples of what has to be done and what has to be avoided to ensure we don’t face expensive surprises.

Each work is different and has inherent complexities, in fact there are many variables that can affect the vibration transmissibility. For example the materials used in the structure (wood or concrete), the position of where the machinery has to be installed (ground level or rooftop) and also the type of room that is adjacent to the machine room (sensible or not sensible).

Even if it is hard to define all problems that may happen we will show here some common ones and we will describe how they can be avoided.

1.- SUBFRAME STIFFNESS WHEN USING SPRING MOUNTS

The air handling unit manufacturer must prove that its frame is sufficiently rigid to be supported on specific supports. We recommend in all cases to reinforce it by creating a frame in IPN 120 beams fixed at least over the entire periphery. The Spring mounts will first be placed at regular intervals under the equipment frame, the deflection of the spring mounts must then be checked to ensure the installation is flat and level. If the deflection of the vibration isolators is not even across all mounts then the position of them must be adjusted, this can be done by increasing the density of the anti vibration mounts in the most deflected areas.

Fig. 2: Representation of how an additional IPN beam can be added under the frame of an HVAC equipment.

2.- STIFFNESS OF THE BRACKETS

The stiffness of the brackets must be at least 10 times more rigid than the antivibration mounts. If the brackets are softer, their stiffness may affect the vibration isolation of the System and may damage the anti vibration mount.

Fig. 3: Installation of DRD 130 mounts on an HVAC equipment, where the brackets used are weak than the rubber itself.

3.- WIDTH OF THE METAL BASE ON ROOFTOPS

The metal bases used above and below the anti vibration mounts must be correctly dimensioned to ensure adequate support. The complete contact area of the metal interfaces must be supported, this avoids the possibility for uneven loading and damage to the vibration isolators as a result.

Fig. 4: Installation of DRD 130 mounts underneath HVAC equipment, where the metal bases used are thinner than the base of the baseplate of the anti vibration mount. The fixation deforms and damages the anti vibration mount.

4.- WIDTH OF THE BRACKETS WHEN USING TSR ANTI VIBRATION MOUNTS

It is important to recall that the load must be distributed over the entire surface of the elastomer pads. This is crucial to ensure that the elastic properties are achieved, if the contact area is too small it will be necessary to provide a steel distribution plate of sufficient thickness, so that it does not deform when loaded.

Fig. 5: Representation of an HVAC equipment with correctly dimensioned bases for TSR Anti vibration mounts

5.- INSTALLATION OF PIPING FOR HIGH VIBRATION AMPLITUDES

When experiencing high levels of vibration for piping installations, the amplitude can be reduced by adding damping to the system. A good way to achieve this is by adding an inertia base that is fixed on to anti vibration mounts as shown in the image below. This way the vibration energy is decreased and the suspended element is damped. Using Sylomer® bands on the perimeter of the piping and on the base of the anti vibration mounts will help to diminish the transmissibility of high frequecies.

Fig. 6: Representation of a high amplitude vibration pipe on an inertia base with Vibrabsorber+Sylomer® vibration isolators.

6.- INSTALLATION OF HVAC EQUIPMENT WITH A HIGH POSITION OF CENTRE OF GRAVITY

Machines that have the Centre of gravity located at a high position tend to be unstable. The first reaction by the design engineer may be to change the anti vibration mounts and replace them with ones that have a higher stiffness. Althought this may resolve the problem of the stability, the increased stiffness will in turn increase the natural frequency of the system and lead to a decrease in vibration isolation.

A simple way to resolve this problem is to use stabilisers. Stabilisers can be installed at points where the highest amplitude is achieved, however special care must be taken so this point does not transmit excessive vibration to the adjacent supporting members (Frame). The part should only work only when the machine is exposed to shocks adding the lowest stiffness as possible when the machine is working.

Fig. 7: Shows a HVAC installation with a CB anti vibration mount acting as a multiaxial buffer.

Another possibility is to use spring mounts with integrated buffers. The Vibrabsorber+Sylomer® antiseismic anti mounts have rubber based buffers on the side walls that can provide a silent rubber to metal contact when exposed to lateral movements of the HVAC equipment. If additional stability is required, the VISCO version of these mounts can provide additional stability through hydraulic damping.

Fig. 10: Figure of a tall machine with integrated buffers on the Springmounts. 2 VSH Vibrabsorber+Sylomer® antiseismic mounts are installed on a low position.

These anti seismic mounts are easy to install, the below video shows the simple process.

Fig. 11: Video of installation of Vibrabsorber+Sylomer® anti seismic mounts.

7.- SUSPENDING HVAC MACHINERY WITH ACOUSTIC HANGERS

Fig. 12: Elastically suspended HVAC equipment with SRS+Sylomer ® acoustic hangers

It is important to maintain the concentricity of the studs to avoid any metal to metal contact, and a resulting acoustic bridge. Although the SRS+SYLOMER® acoustic hanger from Mecanocaucho incorporates a rubber cup to avoid a metal to metal contact, it is preferable that the stud has no contact with the rubber rim.

Fig. 13: Correct and incorrect installation representation of SRS+Sylomer ® acoustic hangers

If you are interested in getting further information on this topic, more information can be found in CORRECT INSTALLATION OF SPRING HANGERS TO OPTIMIZE ACOUSTIC INSULATION RESULTS.

8. ELASTIC SUSPENSION OF PUMPS WITH ANTI VIBRATION MOUNTS

For pumps, since the dynamic load is often greater than the static load, during stops and starts, the suspended assembly tends to move on these vibration isolators if it is not attached to an inertia block. Consequently, to avoid significant movements during transient phases and therefore to avoid mechanical failures, it is necessary to implement inertial masses under the pumps, this ensures the dynamic loads are relatively low compared to the static.

For further information on this topic, you can click on the following article. “Vibration isolation of machinery, Controlling motion”.

AMC MECANOCAUCHO manufactures anti vibration mounts and has a team of application engineers to help your installation needs so do not hesitate to contact our technical department if you require help on this topic.