Chillers, dry coolers and air condensers are a very common source of noise problems. Unfortunately, installing ineffective noise control measures that are costly and can reduce system efficiency is also common. There is a simple process to determine the best noise attenuation method. Chiller noise control must be based on understanding the 3 typical noise sources, each with their own characteristics and each with a different optimum silencing solution.
Not using the following process cost one company over £100k. They replaced an air conditioning chiller unit unnecessarily based on the overall dB(A) whereas the complaints were due to a different unit generating a low-frequency fan hum...
Sound file: dry chiller and AHU noise control at source: 12dB(A) attenuation
Caused by air movement and usually dominates the overall dB(A) noise level. By their nature, air-cooled condensing units, air handling units, heat exchangers and air conditioning plant pass high volumes of air through heat exchanger radiators. This airflow velocity through less than ideal paths generates turbulence and therefore sound. This type of noise is reasonably directional and can be attenuated by conventional silencers and acoustic barriers. However, great care must be taken to avoid reducing the efficiency of the chiller:-
Compressor hum is caused by pulsation vibration transmitted into the structure that is then radiated as sound or via direct vibration from badly mounted units. You typically get tonal noise at the rotation speed (usually c 49Hz) and many harmonics over a wide frequency range. Particular high-level tones occur at the pumping frequency from pipework pulsations as shown here. Anyone can easily diagnose this issue using one of the many free spectrum analyser apps on a smartphone. The most effective chiller compressor noise reduction options involve the following:-
Once these have been addressed, then enclosure and/or barriers can be added if necessary. Note that enclosing without considering isolation and damping is often completely ineffective.
Vibration isolation: you cannot use sound as the diagnostic tool due to complex reflections of tones. You must use vibration measurements to determine the transmission paths. However, it is often possible to evaluate common issues by inspection. Is the compressor vibration isolated effectively (not hard mounted, are there any vibration breaks in the pipework, no short-circuits, not mounted off thin panels)? How is the high vibration pipework attached to the frame?
Vibration damping: compressor and pipework vibration are fed into the chiller frame and thin sheet metal panels that then behave as efficient loudspeakers. The simplest (and low cost) solution is to introduce high-efficiency damping to the radiating surfaces to dissipate vibration energy to reduce the noise. This can be applied either by replacing thin panels with laminated versions or by retro-fitting high loss damping in situ e.g. https://sounddampedsteel.com/laminated-metal-how-does-it-work/.
If the compressor is the dominant source, these engineering source control techniques can reduce chiller noise by 5dB - 15dB.
The hum often associated with chiller condenser fans (a very common cause of complaints) is at the blade pass frequency of the fans (speed x number of blades) and is caused by poor aerodynamic flow through the impellers. One of the most common and costly mistakes involves wasting resources on silencers and acoustic barriers in an attempt to deal with the hum. Neither is effective at these low frequencies, typically in the range 40Hz - 200Hz as shown here. Also note that these low-frequency tones do not contribute to the overall "A" weighted noise level (the "A" in dB(A)).
Once again, this source is easily diagnosed using one of the many free smartphone spectrum analyser apps - no sophisticated instrumentation is needed to avoid the costly mistake of assuming silencers or barriers will work.
The only effective solution to reduce chiller fan noise is either to buy new low noise units or to fit aerodynamic fan modifications to reduce the tones. The latter are retro-fit inserts and geometric flow control devices that eliminate the pressure fluctuations causing the hum at source by up to 99% (20dB) or more. This is a very low-cost solution that can also improve fan efficiency - by as much as 23% on one data centre relief fan application. This can often make these noise control projects self-financing.
We follow this simple procedure to ensure that the optimum noise control package is applied. You should do the same. This approach applies to all chiller, AHU, dry cooler, air condenser, HVAC and refrigeration system noise problems.
If this process has not been followed, then the selection of, and expenditure on noise control measures has been based on guesswork - which can be very costly.
We provide the whole process as a turnkey service, usually without a site visit via remote control of noise. The initial analysis and diagnosis are completely free - we provide you with the results and a detailed evaluation of the costed noise control options using current best practice. This can be followed by detailed recommendations for implementation by local contractors.
Email us a video from your smartphone for an evaluation of best practice