Grain dryer and air conveyor system noise reduction
Grain dryer silencing can be achieved at a fraction of the cost of conventional techniques such as silencers and acoustic enclosures that involve high capital expenditure and increased running costs. The same applies to pneumatic air conveyor systems. In addition, the problem low-frequency fan hum from these systems remains as silencers and barriers do not attenuate the drone that often causes complaints.
We have developed low-cost environmental noise reduction alternatives that use aerodynamic and other techniques to attenuate the noise whilst improving system efficiency.
Details of how to reduce grain dryer and air conveyor system noise efficiently and cost-effectively are provided below.
Grain dryer noise - fan silencing
Sound file: typical low-frequency grain dryer fan tonal noise
2 grain dryer noise characteristics are generated by the fans:-
- broadband noise from air movement at mid to high frequencies. This usually dominates the overall dB(A) noise level
- fan hum or tonal noise, almost invariably at low frequencies
Broadband grain dryer fan noise attenuation
- conventional silencers: effective at frequencies above about 300Hz. They must be designed to minimise the increased backpressure. It is also possible make use of directional effects and bespoke silencer elements to reduce fan noise by 10dB or more by modifying existing elements.
Grain dryer / air conveyor noise
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- noise barriers: these can also be effective above 300Hz (the higher the frequency, the greater the noise reduction) given the right geometry. The main problem with using barriers to reduce grain dryer noise is that they must be close to the source which can cut airflow and reduce system efficiency, adding running costs.
Important note: acoustic barriers and conventional mufflers do not attenuate low-frequency noise i.e. the typical grain dryer fan hum is almost untouched.
Grain dryer noise - fan hum tonal noise reduction
A very common grain dryer noise problem is a low-frequency hum or drone at the fan blade pass frequency. This is usually at frequencies below 300Hz, in this example the tone is at 221Hz and harmonics. These tones travel a long way, passing through windows with little attenuation - and are at too low a frequency to be attenuated by conventional silencers.
The only effective noise reduction technique is to use source control methods in the form of retrofit aerodynamic fan noise reduction aids to eliminate the tones. This is low-cost, requires minimal downtime and can also increase fan efficiency. This technique has been used to reduce fan hum by around 90%-99% (20dB) whilst simultaneously increasing fan efficiency. The fan modifications for a data center cooling fan noise attenuation project cut the sound by 16dB attenuation and increased fan efficiency by around 23%. Self-financing noise control...
In the case of the 9 centrifugal fan grain dryer shown above, the tonal noise was reduced by around 19dB at source using aerodynamic technology.
What are the sources of pneumatic air conveyor noise and how can they be reduced?
There are usually only 4 potential causes of air conveyor noise, each with specific noise characteristics and each with an optimum attenuation option.
- fans: broadband noise + high levels of tonal noise. Depending on the fan type, the tones and harmonics can be the dominant contributors to the overall dB(A)
- blowers: positive displacement blowers (e.g. Rootes type): multiple mid-high frequency tones
- grain impact noise: broadband noise due to impacts inside the pipework
- silo "honk": intermittent silo noise caused by bridging within a silo that causes resonance in the silo structure
These sources must be ranked in terms of the overall noise level and the sound "character" (tones) that is usually a key cause of complaints. Then select the best noise control technique as described below.
1: Fan noise control
Conventional silencers and/or (depending on relative amplitudes and frequency content) aerodynamic noise control (as above). The aerodynamic option is very effective for this type of problem as it cannot clog (unlike silencers) and it has no hygiene implications.
In this air conveyor noise signature example, the fan tones were reduced by 12dB or more using aerodynamic technology, eliminating the problem.
2: Blower noise (positive displacement) silencing
These blowers are often used in pneumatic air conveyor systems to transport grain or other materials through pipework at higher pressures than fans.
They usually generate very high levels of tonal noise over a very wide frequency range as shown here.
This is typically radiated via the air intake (how much of this travels off-site depends on the location of the intake) and via noise breakout from the pipework.
The former can usually be silenced using conventional (hygienic) silencers on the intake and outlet of the air conveyor blower to cut the pipe noise. The attenuation of the latter can be increased by using local acoustic screening of the pipes.
Air conveyor grain transport noise reduction using Helmholtz resonators
Whilst acoustic lagging is also effective, there can be a problem with corrosion as, in practice, it is virtually impossible to guarantee that there will not be water ingress.
In some cases, the tonal blower noise can be a single discrete tone as in this example. In this case, the 94dB(A) noise level was dominated by a tone at 738Hz/. This was cut by 15dB by designing small tuned Helmholtz resonators for the inlet and outlet ducts (red objects in the picture). Resonators work well - but only at a fixed frequency.
3: Air transport material impact noise reduction
Material impact noise in transport pipework
Grain or other material impacts on the pipe walls cause vibration that is radiated as mid-high frequency noise, a characteristic broadband rattle or roar, particularly at bends. We have developed a very simple, low-cost retrofit modification that cuts this material impact noise component by 10dB - 20dB without introducing the potential water traps (associated with conventional acoustic lagging) that could cause corrosion.
4: Silo "honk" noise control
Example of silo "honk" noise
Silo honk noise is caused by a stick-slip phenomenon inside the silo. The cause is usually the material inside "bridging" until at some point it starts to slip, creating vibration in the silo wall. This then excites a structural resonance in the silo structure, creating a stick-slip feedback mechanism that causes the "honk".
The noise control options include modifications to the geometry and structural damping (including retrofit dynamic vibration absorbers).
