Chillers, dry and adiabatic coolers and air condensers are very common sources of environmental noise problems. Fortunately, there are elegant, low-cost retrofit engineering solutions that do not reduce efficiency. Unfortunately, installing ineffective noise control measures that are costly and seriously reduce system efficiency is also common. There is a simple process to determine the best noise attenuation method that is also generally applicable to all air handling unit (AHU) systems. Chiller/cooler, and similar heat pump noise control must be based on understanding the 3 typical noise sources, each with its own characteristics and each with a different optimum silencing solution.
AHU fan noise comes in 2 distinct flavours:
In addition to the AHU noise components, there may also be very characteristic compressor noise
Not using the diagnostic process described below cost one company over £100k. They replaced an air conditioning chiller AHU based on an assessment of the overall dB(A). In fact, the complaints were caused by a different unit generating a low-frequency hum from the fan...
Sound file: dry chiller and AHU noise control at source: 12dB(A) attenuation
Caused by AHU fan-generated air movement that usually dominates the overall dB(A) noise level. By their nature, air-cooled condensing units, air handling units, adiabatic coolers, heat exchangers and air conditioning plant pass high volumes of air through heat exchanger radiators. The airflow 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 AHU or chiller:-
Email us a video from your smartphone for an evaluation of best practice
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 often at a very low frequency at the blade pass frequency of the fans (speed x number of blades) 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 AHU noise tones do not contribute significantly to the overall "A" weighted noise level (the "A" in dB(A)). Not understanding this simple fact is a very common source of very costly mistakes.
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 low-frequency tonal AHU fan noise is either to buy new low-noise units or to fit low-cost aerodynamic fan modifications to reduce the tones. The latter are retrofit inserts and geometric flow control devices we have developed 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 often makes these noise control projects self-financing.
We follow this simple procedure to ensure that the optimum environmental noise control package is applied. You should always do the same. This approach applies to all chiller, AHU, dry cooler, air condenser, HVAC and refrigeration system noise problems.
Unless this process has 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.