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Rotary Positive Blower Intake and Discharge Silencers

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Information On Rotary Positive

The rotary positive or positive displacement blower is a twin lobe rotating mechanism for delivering large quantity of air or gas at relatively constant volume for either pressure or vacuum applications.

The blower installation produce excessive noise and destructive pulsations which can be harmful for the equipment, personnel and the neighborhood. In order to achieve silencing and pulsation control, the silencers at intake and discharge are used irrespective of the blower size and speed. This catalogue deals with different types of intake and discharge silencers for treating airborne noise which can be used depending on the degree of silencing required.

The blower capacities are generally expressed in CFM and the blower size is expressed as gear diameter by rotor length. The pitch line velocity (PLV) or the transition speed is the circumferential velocity of the timing gear i.e. the product of gear circumference and the rotary speed of the blower and generally expressed in feet per minute (FPM). The noise and pulsation energy generated by blower both at intake and discharge are a function of blower size and blower speed therefore these are important factors to be considered for selecting the correct silencer for intake and discharge.

For the purpose of selection of silencers for blowers, the following considerations must be kept in mind. 1. The silencer must be correctly sized i.e. It should have sufficient volume for the air flow and 2. The silencer must be of proper design. Normally, there are two basic types of silencers used in the industry. The one is reactive type silencer which creates series of cross-sectional discontinuities which results in reflection of sound waves back to the source. The design consists of a series of expansion chambers connected with interconnecting tubes. This type is generally used to treat the low frequency pulsations. The other type is more sophisticated combination chamber absorption type design in which acoustical pack sections are added to reactive design to treat the high frequency pulsations. The acoustic energy propagating through the silencer is converted into heat energy in combination silencers for noise reduction.

The pitch line velocity is important criterion for selecting the silencer type. Generally, larger gear diameter and higher rotary speed will produce higher pitch line velocity which results in a greater noise level generated by the blower. It has been recognized by silencing industry that the critical PLV or transition speed for the blower is 2700 FPM for the discharge and 3300 FPM for intake.

For intake applications, the PLV or transition speed of 3300 FPM or higher is considered critical. Blowers operating below this speed will need only chamber silencer to adequately silence as the low frequency pulsations are to be treated. If the blower speed is higher than 3300 FPM then high frequency is required to be treated therefore a combination chamber absorption type silencer should be selected.

For discharge applications, the PLV or transition speed of 2700 FPM is accepted as critical speed. Blowers operating below 2700 FPM can be adequately silenced using a chamber type silencer for treating low frequency noise. The blowers operating above 2700 FPM requires a combination chamber absorption type silencer for treating high frequency noise.

Table 1 shows the nominal capacities of various silencers. The size refers to the intake size of the silencers in inches. The capacities are expressed in intake CFM. Select the silencer size corresponding to the volume and discharge pressure of the blower. For selecting the type of silencer, refer to Table 2, which shows the critical intake and discharge transition speeds (PLV) based on blower gear size. As mentioned earlier, the critical PLV for intake silencers is 3300 FPM and for discharge silencers is 2700 FPM. For intake silencers, if your blower’s operating speed is above the intake value then select the combination chamber absorption silencer and if it is below the intake value then select chamber silencer. Similarly, for discharge silencers, if your blower’s operating speed is above the discharge value then select combination chamber absorption silencer and if it is below the discharge value then select the chamber silencer.

Noise attenuation curves showing insertion loss at each frequency are shown for various models. These curves represent the insertion loss for airborne noise under average conditions. The resultant silenced noise level will depend on number of other factors therefore these curves should be used with discretion and can be used as a guideline for evaluating the noise levels of a blower after the silencer installation.

As a general rule, the silencer must be installed as close as possible to the blower to avoid radiated noise from silencer using a flexible coupling to eliminate stresses and fatigue which can cause premature failure of silencer or piping. Consult our engineers for any specific silencer orientation required.

The silencers described herein are designed to a maximum operating pressure of 15 psig. For applications where pressure exceeds 15 psig, the silencers can be designed to ASME Code, Section 8, Div. 1 for pressure vessel construction. The dimensions are similar to standard models, but the material types and thicknesses are selected to meet code requirements. Consult our engineers for pricing and design information.

The following formulas may be used to calculate pressure drop through the silencers covered in this catalogue. Also see Table 4.

(1) to calculate velocity through silencer, divide flow in ACFM by cross sectional area of silencer intake diameter in square feet.

The chamber absorption silencers described in this catalogue use polyester wool packing as the standard acoustical absorption material. The temperature limit for polyester wool is 492°F. For applications where the temperature exceeds this range, consult our engineers for alternate materials.

To learn more about EI Williams, call us at (905) 428-0950 or toll-free at 1-877-840-3347 or you can also email us at info@eiwilliams.com for further inquiries.

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