CHAPTER 10 | Noise and Vibration

As with any mechanical equipment, cooling towers can generate objectionable noise and vibration. This chapter will not provide an exhaustive examination of either but merely point out potential pitfalls and provide some practical observations.

Large installations are typically laid out by experienced people who have a 'feel' for the amount of noise or vibration generated by mechanical equipment. They gather equipment away from noise sensitive areas, add concrete walls as barriers and employ acoustical consultants when necessary.

Sound problems are infrequent but among the most thorny. Minimally designed condominiums or industrial plants that border residential areas are typical. Situations where one piece of equipment has replaced another predominate.

Sound complaints don't always involve rotating machinery. My first boss liked to joke about the funeral home that had a cooling tower float that would 'moan' at the most inappropriate times. It seems a suprise how quiet it is when a radio is turned off and the fluorescent lights are extinguished at the end of the day. We regularly accept a high degree of background noise without notice but a new, unfamiliar noise can be most objectionable.

Replacing a centrifugal fan cooling tower with an energy saving prop fan design is almost sure to be noticed particularly if neighbors can see the fan rotating. The solution may be as simple as turning the tower so that the critical neighbor does not face the loudest side of the tower and see the fan. Comparing the sound data sheets for new and old towers can also help in the decision as to suitability. One easy trick is to oversize the tower and slow the fan down for reduced noise levels.

Another is to utilize centrifugal fan towers which are inherently quieter.

The designer must strive to reduce changes in noise level in noise sensitive situations. Motors starting or changing speed, belts squealing and the like all draw attention to the tower. Variable speed drives or 'soft starters' can help although drives themselves will sometimes exhibit annoying highly pitched sounds related to motor speed.

Once a cooling tower is deemed too noisy, it is difficult to fix the problem. Invariably, the question to the complainant of "OK, how is it now?" is interpreted as "Can you still hear it?". The answer is obvious and the resolution slow in coming.

Every effort must, therefore, be taken to make a quiet installation before the tower is started and to correct any malfunctions immediately.

Two identical towers side by side can exhibit the phenomenon where sound pressure levels are slightly in and out of phase causing an annoying addition of sound pulses- like that of a twin engine turbo prop commuter plane. Intentionally making the fans operate at different speeds through the programming of VFD's- or simply changing one of the sheave diameters slightly on a belt drive tower- will help.

Factory designed sound attenuators that attach to the tower can be helpful in difficult situations; However, they tend to be expensive and generally require more fan hp. They can also make routine tower maintenance difficult. It is better to first consider oversizing the tower and slowing the fan before employing factory supplied attenuators. Also, new, wide blade axial fan designs are becoming more prevalent to address noise at its source.

Vibration can also be annoying and difficult to solve. When towers are on floors or rooftops with people below, it is wise to employ spring type vibration isolation. The amount of static deflection is an indicator of isolation efficiency- generally 1" minimum and 2" maximum.

Another consideration when using vibration isolators is to first mount the tower on a sturdy frame. Then, place the isolation underneath. This is especially critical for multi-cell towers... All cells must be mounted on a single frame before being isolated. Otherwise, the reduced water weight that occurs when valving off and draining one cell allows its springs to extend causing tremendous stresses to flume boxes and interconnecting piping.

Rubber pads offer a small measure of vibration isolation and work ok when placed under a uniformly distributed load; However, some tower designs have highly concentrated load points that will cut through the pads. Worse yet, the underside of some towers can become distorted when they exhibit variable loading characteristics- much like a person attempting to sleep on an overly soft mattress. Rubber pads should be avoided unless the designer is sure they are appropriate.

After taking the effort to isolate the tower, it is mandatory that the piping have flex connectors. One short connector can't always do the job; Two flex connectors separated by several feet of pipe may be required. Make sure the floating components aren't grounded out anywhere. Then, make sure there are adequate constraints for wind loading and seismic events.

Cooling tower manufacturers can provide sound rating data sheets upon request. They can be very helpful in assessing various towers.

Some manufacturers also print "sound manuals" that help the designer assess sound data sheets and estimate installed noise levels. Ask your cooling tower sales representative.