Noise Control Spotlight - Acoustical Nozzles

PRODUCT SPOTLIGHT: CONTROLLING COMPRESSED AIR NOISE

John C. Dolehanty

Winter 2006

Compressed air is a common and efficient source of energy for air motors, vacuums, pneumatic tools, painting etc. It plays such a large role that it is often referred to as industries ‘Fourth Utility’. One consequence of such prevalent use can be the introduction of excess noise when compressed air is used for cleaning, drying, cooling and part movement applications.

The noise generated by these processes alone is often loud enough to require employers to place affected employees into an on-going Hearing Conservation Program (HCP). This noise needs to be reduced as much as possible to minimize the risk of noise induced hearing loss.

There are several products readily available that are proven to reduce the noise associated with compressed air. These products are either nozzles that are applied to the end of open pipe for blowing, or silencers that are used to quiet pneumatic exhaust.

Air Nozzles

When the fast moving air stream from an open pipe comes into contact with the surrounding static air, turbulent air flow is generated. One of the resultant by-products is excess noise. Often a nozzle will be placed on the end of a open-pipe. In industry, the main purpose for adding an air nozzle is to modify the blowing pattern of the open-ended compressed air pipe. When using a silencer nozzle, it is desired that the nozzle also maintain the blowing force (thrust) while reducing the sound level. Modifying the air pattern can be accomplished through the use of a variety of nozzles. Keith Timmons, the Managing Partner of Silvent North America states, “most products like this in the market place do nothing more than dispense the air, there is no engineering into the nozzle; it’s just a machined piece of metal that the air passes through”. However, it takes a special type of nozzle to maintain the thrust and produce less sound and to use less energy (compressed air) at the same time.

Silvent is the leading supplier of engineered compressed air nozzles in the world. According to Mr. Timmons, Silvent products are developed “from the beginning to first and foremost reduce compressed air noise at the outlet of the nozzle. As a result, there is a lot of engineering that goes into the outlet orifices on the nozzle tip, as well as the size of the opening, positioning. Everything is designed to process the air through the nozzle tip so that noise is reduced. We break up the air into multiple high-velocity steams of air [via the multiple orifices] that reduce the turbulence, therefore reducing the noise”. Silvent makes nozzles that area able to provide an efficient blowing force from a fraction of an ounce to over 20 lbs. of blowing force.

WHY SHOULD YOU USE NOZZLES? There really are no good reasons NOT to use air nozzles. The proper nozzle application will:

Reduce noise due to the use of laminar air flow.
Lower the use of compressed air due to the more efficient laminar air flow.
Produce a safer work space due to the elimination of a 30 p.s.i. skin contact source. (See Air Wands, below).
Efficiently use compressed air – which reduces energy costs.
Provides an overall better working environment.

The pictures below show an example of three different Silvent nozzle types. Each nozzle is engineered to reduce the sound level, maintain the thrust (compared to open pipe), consume less compressed air, and meet the OSHA 30 p.s.i. requirement.


Model 208	Model 511	Model 710
The SILVENT 208 is SILVENT’s most popular nozzle, installed in thousands of different applications, where the noise level has been cut in half and energy consumption drastically reduced. The SILVENT 511 slot nozzle generates a directed air jet. This nozzle is well suited to all-purpose blowing and blowing in confined spaces. Due to its compact size, this nozzle is frequently used in machines and tools where clearance is limited. The 700 series of blowing nozzles is specially manufactured entirely of stainless steel. This nozzle series features aerodynamic slots to achieve optimal utilization of the compressed air while, at the same time, keeping the noise level to a minimum. All Silvent nozzles fulfill OSHA safety regulations, which stipulate that air pressure may not exceed 210 kPa (30 psi) in direct contact with skin. These nozzles combine the advantages of low noise level and low air consumption with high blowing force.

Air Wands (Air Guns)

The first priority in eliminating an air wand with an open-ended compressed air pipe is the danger associated with compressed air coming in contact with skin. U.S. Federal O.S.H.A. does not allow sources of compressed air where the dead-end pressure (compressed air pressure that comes in contact against your skin) to exceed 30 p.s.i. All Silvent nozzles meet this requirement – even the nozzles that can produce up to 20 pounds of blowing force. According to Mr. Timmons, this is because “with each nozzle design, with its multiple orifices, it is impossible to dead end enough of the outlet ports with enough surface area to develop 30 p.s.i of pressure against the skin – there is always a relief somewhere”.

In addition, an air nozzle equipped blow-off gun generates less noise, and uses less compressed air. The picture to the right shows a Silvent model 007 air wand. This gun features a unique valve design with a two-step system that reduces both noise levels and energy consumption considerably. The first step, variable position, allows variable adjustment of the blowing force and is generally more than adequate for most types of work. This generates a noise level of only 75 dB(A) and permits energy savings of up to 50%. The gun’s second step, the so called “booster position”, delivers twice the blowing force for the most demanding operations. From the maximum variable position, only a slight squeeze of the trigger is needed and the booster position kicks in, instantly doubling the effect. Despite this dramatic increase in blowing force, the sound level is only half that of conventional air guns.

Model 007 Blow-off Wand

Pneumatic Exhaust Control (Mufflers)

The noise from pneumatic valves can greatly increase the risk of noise induced hearing loss. Un-muffled exhaust can easily generate sounds in excess of 100 dBA. All pneumatic exhaust ports can be fit with a standard industrial silencer (muffler). The typical silencer works by dispersing the exhaust air steam over a larger are to reduce the noise. These mufflers typically provide about 20 dBA of noise reduction. The picture to the below shows a typical pneumatic exhaust muffler. Note the threaded end that can be inserted into the pneumatic exhaust port. The multiple holes on the left end of the muffler picture allow the air to be dispersed evenly, thus reducing the overall noise level.
The typical silencer works by dispersing the exhaust air steam over a larger are to reduce the noise. These mufflers typically provide about 20 dBA of noise reduction. The picture to the right shows a typical pneumatic exhaust muffler. Note the threaded end that can be inserted into the pneumatic exhaust port. The multiple holes on the left end of the muffler picture allow the air to be dispersed evenly, thus reducing the overall noise level.
Typical Exhaust Silencer
The major drawback to mufflers of this type is that they have the potential to clog and reduce equipment operating efficiency. In addition, the dispersed air can atomized any oil in the compressed air line and create an unnecessary oil mist in the air. A unique alternative to the standard pneumatic exhaust muffler is the Silvent Hose silencer and the Silvent Central silencer. The hose silencer, shown in the picture to the right, provides approximately 25 dBA of noise reduction while preventing the atomization of any oil in the air line. In addition, the hose silencer, due to its design, it can never clog.
When noise is generated by many valves the best solution can be to connect multiple exhaust ports to a Central Silencer. This control allows for reduced maintenance since many ports are being serviced from a single point. A Silvent OPUS Central silencer provides approximately 30 to 34 dBA of noise reduction. The larger silencers on the market are often designed to lead air to them idle, to a central tube sapped filet. The disadvantage of the construction is that, sooner or later, the filter clogs, resulting in interruption of service.
Silvent Opus Central Silencer System

Why Are Open Pipes Still In Use Today?

In a single word: Education. The use of compressed air to clean, dry, cool and move parts has been accepted for many, many years. However, the application of silencer nozzles and exhaust mufflers is relatively new. It is very common for employers or employees to assume that a quieter product is “less efficient” or “has less power”. With regards to compressed air, the opposite is true. The more noise generated means the less efficient the air flow. In the case of compressed air, less noise equals more power.

Silvent provides a small booklet entitled The Ten Truths of Blowing with Compressed Air that lays out in layman’s terms may of the daily compressed air truths we take for granted, and communicates and educates how controlling compressed air noise can be an advantageous endeavor.

For questions or information on compressed air nozzles and silencers you can contact Keith Timmons at Silvent (800) 263-5638 or visit their website at www.Silvent.com. Silvent products are available directly through the McMaster Carr supply catalog as well as the MSC Industrial Supply Company catalog.

Mr. Dolehanty is the President of Phase To, Inc., and has over 18 years of experience in hearing conservation and noise control. Mr. Dolehanty can be reached at (517) 886-9379 and at johndolehanty@phaseto.com.

Phase To, Inc. (PTI) does not represent, sell, or market any noise control materials or hearing conservation products. References to any specific products in this or any other PTI article does not express explicit endorsement by Phase To, Inc. PTI focuses specific attention on innovative items or products that we find to be superior in the noise control or hearing conservation marketplace.

Copyright © 2006 Phase To, Inc.