Application reports

N° 21 : Noise level of oil-free pumps

Noise level

Question: Recently I have replaced an oil-sealed rotary vane pump by an oil-free model. The oil-free pump is very loud, especially during pump down. Which actions can I take to reduce the noise level?

Answer: Oil-free pumps - as the name already suggests - do not have any lubricant as noise-damping fluid. For that reason oil-free pumps normally are louder than rotary vane pumps with comparable pumping speed and base pressure. Lower noise level can be achieved by an exhaust silencer, noise reduction cover, or regulation of rotational speed.

Additional information: Noise level can either be measured or can be a personal perception. Noise can either annoy a user or can disturb a measurement or an experiment. Both is harmful. However, only in measurable cases a pump manufacturer can take effective actions.

Noise level of oil-free pumps is higher during pump down than during operation at base pressure. If noise during pump down is the main concern, exhaust silencers can effectively suppress the noise. adixen's "ES 25S" for the ACP series completely suppresses pump down-related noise.

If noise level at base pressure is the main concern, noise reduction covers can be used. Depending on cover type noise level reductions up to 10 dB(A) can be achieved. This is the limit for adixen's passive covers. With fan-equipped active covers noise reduction can even be better. Please observe manufacturers' specifications for maximum ambient temperature when using a noise reduction cover. Maximum temperature may be limited to 30°C.

Of course, exhaust silencer and noise reduction cover can be used at the same time.

An additional possibility to reduce noise level is variation of rotational speed. If a certain frequency disturbs an experiment rpm regulation with the standard frequency converter on adixen's ACP pumps can eliminate this frequency.

A last option is using a buffer volume. After evacuation of the buffer volume the mechanical pump can be isolated and switched off. Any gas coming from the vacuum vessel is now stored in the buffer volume. This works best in applications without process gas flow. If the pressure in the buffer volume is monitored the mechanical pump can be switched on in case that the pressure gets too high.

Whatever your noise problem may be - manufacturers' catalogues will probably not be a great help. There are hardly any data regarding noise level or even frequency distribution of vacuum pumps. This can be related to noise level measurements according to various existing regulations. In this case we can only advise you: discuss your application with us.

Glossary:
Frequency converter: A frequency converter is a system for controlling the rotational speed of an alternating current electrical motor. In adixen's ACP series it is used for setting a constant rotational speed no matter which power supply voltage or frequency is applied. In addition various rotational speeds can be set.

Buffer volume: A buffer volume is a container in the vacuum line between mechanical pump and vacuum vessel. After evacuation to a certain pressure level the primary pump can be isolated with a suitable valve (observe leak rate!). The buffer volume now maintains the vacuum; e.g. as backing vacuum for a turbo molecular pump. Duration of this operation mode depends on size of vessel and buffer volume and the needed vacuum level.

Noise level measurements according to various regulations: There are various regulations for noise level measurements. E.g. noise may be measured at ear level at a distance of 1.5 m to a pump standing on the floor. Another regulation demands for measurements on every side of the pump plus the top. Distance is one meter each. The five measurements are assessed. Due to measurements according to different regulations pumps which are emanating the same noise level can be specified completely different. In our lab we have measured deviations of up to 9 dB(A).