- by Peter Koelewijn, 13-06-2018
In my last blog we have covered three types of noise which are common in HVAC systems. Our topic for today is to find a solution for each one of them. Although there are different forms of noise, almost every issue can be brought back to three basics. By knowing the solution for these basic problems, you are ready to face every noise problem in the future.
As an example I have sketched a situation with two cabins, both supplied with air by a centrifugal fan in the neighboring room. I am going to use this situation for all the sound issues and their solutions. First the noise problem, then the way to solve it. Step-by-step we handle every type of noise. If you want a deeper knowledge about the three noise types, check out blog three, which handles this topic.
The centrifugal fan is pumping air into room A and B. Although the fan is placed inside a separate room, mechanical noise can be heard through the bulkheads. Especially in room A, because this room is next to the fan room. The source of this noise is the fan. To reduce it we have to isolate the fan itself. In image 2, I have placed a sound dampening box around it. Another option is to insulate the entire room.
Image 1: Mechanical noise
Image 2: Fan placed in sound box
The mechanical fan noise is isolated by the box. Sound waves will not penetrate the bulkhead. But, because the rooms are connected with an air duct, sound waves traveling the supply air can still enter the room. To reduce the noise we place a sound damper in the duct (see image 3). The longer the damper the better, especially with mechanical noise as the frequency is low and the wave lengths are long.
Image 3: Air flow noise
Ideally the air flow is laminar, which means the air molecules are traveling in layers through the duct. Distortions in the ducting system - such as bends, bottle necks or HVAC equipment - will cause the air flow to become turbulent (see image 3). Air molecules are spinning around in the duct, muttering and swooshing, which causes air flow noise. Turbulent air can be caused by bad duct design or high speeds inside the duct.
Image 4: Dampers placed in ducting
To prevent air from becoming turbulent, the ducting system has to be adjusted to minimize resistance. For example a 90° bend in the system. Make sure these bends are as curved as possible, guiding the air into the right direction and avoiding collisions with the sides. You can also use blades inside the bend that guide the air. Another option is to place a sound damper before and after the bend. By dividing the dampers into different sections, the air will be forced into layers, becoming laminar again. See image 4.
The source of vibrations are moving parts in the system, like the fan. It rotates at speeds of 3600 rpm and the pressure is fluctuating between the inlet and outlet of the fan. All these processes are causing vibrations. These vibrations travel through the floor it stands on and through the ducts which causes them to resonate. Also the fan (box) is positioned against the wall, making contact with the bulkhead, transferring vibrations to the room next to it. See image 5.
Image 5: Vibrational noise
Obviously, we cannot stop the fan from moving. What we can do is to stop the vibrations being transferred to the surroundings. First of all we have to make sure the fan (box) is not connected to anything except the floor. No contact with bulkheads, walls or whatsoever. Secondly we put the fan on vibration dampers, these will dampen the movement transferred to the floor. And finally we make sure the connection to the ducting system is flexible. This way the fan can move freely without transferring its vibrations to the floor, bulkhead and ducting system. For more details, see image 6.
Image 6: Flexible arrangement of fan
The solutions discussed above are typical for ventilation systems. But the same noise problems occur in other systems like a fresh water system, with the circulation pump being responsible for the vibrational and mechanical noise and water that goes turbulent through the pipes. Or what about a cooling plant with freon lines and a compressor. Mechanical parts, vibrations and flow.
There are many other options to eliminate noise in your HVAC system. For example, turbulent air can also appear when the ducts and filters are dirty. The solution still stays the same: reduce resistance. There are many ways to realize your goal. Think out of the box. It’s all about the source where the noise is coming from. If you know how it transfers the sound waves, you can find a way to block it. We summarize as follows:
This concludes my series of blogs on noise control. The reason I have written all these pages is to highlight the importance of acoustic comfort in an HVAC system and give a better understanding of what sound exactly is. We have seen that conditions like temperature and relative humidity are not the only parameters for a comfortable environment. By knowing which types of noise there are, you can find a solution for every HVAC noise problem.
I can imagine there are still some questions remaining or you have a specific noise problem with your HVAC system. Feel free to make contact with one of our expert technicians. We are happy to help you achieve acoustic comfort for your system.