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13	Oktober 2018

anchovy and mackerel used to

The term �sound� is used for air or other gas while �vibration� is used for a similar disturbance of motion in a solid. The sound pressure disturbance impacting on a solid can impart a vibration while the vibration of a solid can result in sound. Some prefer to call the concept of vibration which results in sound �structure borne noise.�

There are two types of noise problems � those that we anticipate from our sound ratings and those emanating from some abnormal condition in the fan. Some of the more common sources of abnormal or unanticipated noise are:

1) Fan wheel unbalance.
2) Resonance of fan or attached components.
3) Rotating components rubbing on stationary parts.
4) Failing Cheap Malik Hooker Jersey , misaligned, or contaminated bearings (on the fan or on the motor).
5) Air leakage. This can allow sound leakage and also generate a whistle-type noise.
6) Belts slipping.
7) Coupling misalignment.
8) Motor noise, especially with improper power supply. Inverter drives may increase motor noise at certain speeds.
9) Air turbulence.
10) Operation in surge.
11) Loose components.
12) High velocity air blowing over fixed components which are not part of the fan.

The other class of noise problems are those we have anticipated because of normal fan sound ratings. Some cures for these problems are:

- Select a different fan. Computerized selection routines allow us to examine many fan types looking for the quietest. Use a custom fan design, if required.
- Relocate the fan to where sound is not a problem.
- Vibration isolation and flexible connectors on the inlet and discharge will reduce structure-borne noise.
- Insulate or acoustically enclose the fan housing if housing radiated noise is a problem.
- Add silencers or duct lining to inlet and or discharge to reduce sound in these directions. However, a silencer on the outlet does not reduce the housing radiated noise or inlet noise; and an inlet silencer does not affect the housing radiated andor outlet noise.
- Look for ways to reduce system resistance since sound output is proportional to fan static pressure.

One final tip which can help to avoid noise problems is to select lower RPM fans. Fans exceeding 3000 RPM are much more likely to tune in to an attached structure resulting in structure borne noise. Structure borne noise easily propagates an entire system and can become a problem at many locations. Also, people tend to become more annoyed with higher frequencies than with lower, increasing the likelihood of a noise problem.

Recently, several changes are developing in the technology of fan noise. Some of these are:

1. The latest codes define testing for fan inlet noise, fan outlet noise, and noise radiating from the housing (or casing). The inlet noise can no longer be assumed to apply to the outlet and vice-versa.
2. A new test code which uses sound intensity is near adaption. In theory, integrating a series of measured sound intensities over an enclosed area yields the sound power directly.
3. Sound criteria is playing an ever more important role in the selection process. Many fans are insulated for sound or use other sound reduction apparatus.
4. Active noise cancellation continues to be difficult to apply to most fan installations. If it ever proves practical, this technique can cancel fan noise by adding a second pressure wave out of phase with the original.
5. Many more fan specifications are requiring AMCA sound certification. Certified sound ratings mean that AMCA has verified that the ratings are generated in accordance with the codes and that at least one sample has been tested in the AMCA Laboratory to verify the ratings.

Additional information can be found at the Canadian Blower company web site http:www.canadianblowerahuindex

Oleg Tchetchel
Air Handling Systems Engineer
oleg.tchetchel@canadablower
Canadian Blower
http:www.canadianblowerinquiryindex
http:www.canadianblowerairhandlerindex
JINAN, Oct. 5 (Xinhua) -- Dwindling numbers of fish are threatening the survival of the once booming fishing industry in east China's Bohai Gulf.

Fishermen in east China's Shandong Province are finding it increasingly difficult to earn a living and face bleak prospects.

Sailing out from north China's biggest fishing port, Zhang Shengli, 46, does not hold high hopes.

"Five years ago, we could catch a boat full of fish after spending four days at sea, but now it takes ten days, and the fish we catch are not worth much," said Zhang, a fisherman from the province.

Yellow croaker, long-tailed anchovy and mackerel used to be abundant in the Bohai Sea. Yet Zhang, who has been fishing for 13 years, hardly sees these types of fish now. His major catch is flatfish.

In Yantai, a coastal city in Shandong, fisherman Li Xiaowei stretched out a 60-meter-long net, only to catch a couple of shrimps.

Although China began imposing a fishing moratorium during the summer 19 years ago, it has failed to reverse the depletion of marine resources. Fishermen in the Bohai Sea mothball their boats a month ahead of the moratorium, which lasts from June to August, due to a shortage of fish.@ Overfishing is blamed for the depletion. Environmental pollution, coastal line changes and a lack of fish bait in the sea have also worsened the problem.

To make matters worse, fish prices have plunged from a year ago as demand for exports remain subdued. Aquatic products are instead sold on the domestic market at a lower price. Flatfish sold on the pier is one third cheaper than a year ago, while the price of cod is nearly 60 percent down from its highest two years ago.

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