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ME 340:  Manufacturing of a Silencer

July 2, 2002




1.0 Introduction

The industrial silencer is an important component in manufacturing facilities that generate exhaust and noise. It reduces noise and helps maintain a suitable working environment.
This report will discuss the manufacturing process of the industrial silencer modelled using the procedure observed at ZAP Inc. ZAP Inc. is a US based company that produces noise reduction devices such as  silencers. This report includes a discussion of material make-up, the principal of how a silencer works, and a discussion on the manufacturing process. The three main processes that will be discussed are cutting, rolling, and welding. The focus will be on rolling and how the basic cylinder is formed. Alternate manufacturing procedures and improvements on the current processes will be suggested. The information is obtained from the internet, actual industrial contact, and literature.


2.0 Identification of a Silencer

2.1 Main Components

There are three main types of exhaust silencers used to reduce engine exhaust noise reactive (reflective), absorptive (dissipative) and a combination of reactive and absorptive [9].  This project will specifically discuss the reactive industrial exhaust silencers.  These silencers are mainly used in applications where low frequency noise must be reduced such as in internal combustion engines.  The reactive silencer typically is made up of a large cylindrical chamber called the outer shell with inlet and outlet flanges located on the end pieces.  Inside the outer shell are two chambers separated by a barrier called the bulkhead.  Pipes known as the resonator tubes run through the bulkhead connecting the two chambers inside the silencer.  These tubes normally contain perforated holes in them for added sound dissipation.  The bulkhead and resonator tubes together are referred to as the resonator.  See Figure 1 below for schematic. 

Figure 1:  Schematic of a silencer.


2.2 Dimensions

The physical size of the silencer various significantly with customer to customer as the product is usually custom built for its application.  The typical dimensions of a reactive silencer can be seen in Figure 1.  Overall lengths (D) range from 30 to 212, while inlet and outlet diameters (A) are 1 to 30 openings.  The overall outer shell diameter (B) has a max value of 64 and a minimum value of 7.    Typical wall thicknesses are 14 gage, 11 gage, and 3/16 depending on the application and customer preference [9].  The manufacturing process for silencers with various dimensions is identical to the process described in this report.


2.3 How it works

Noise is a form of sound that has to be avoided in order to create a safe, healthy working and living environment. Reactive silencers are discussed in the report. The principle on which they work can be explained when the sound is modelled as a sine wave.  The silencer consists of two chambers that are separated by the bulkhead with two resonator tubes in it. The bulkhead forces the sound waves to bounce back. Thus, the incoming sound wave and the reflected sound wave interfere with each other. The distance L (Figure 2) has to be chosen according to the sound wave characteristics that have to be damped. The noise will only be reduced if the length L is not a multiplier of π. The ideal case is when the sound waves interfere in such a way that they erase each other completely.

Figure 2: Sound wave patterns in a reactive silencer.

sdfIn reality the waves are not exactly sinusoidal. Therefore, the reduction is not as easy as it is discussed in theory. That means that the incoming dark sound waves are only reduced to a smaller amplitude instead of cancelling out completely. The damped sound waves will find their way through the second resonator tube and are reduced further. That is why there is a need for more than one chamber. The type of silencer is chosen such that it has the ability to damp the sound waves of the required wavelength [2]. 

3.0 Materials

To ensure a durable silencer that requires minimal maintenance the material make-up of the silencer is critical.  High temperatures and/or corrosive gases must also be considered when choosing the correct material.  Carbon steels are adequate for typical applications such as exhaust from diesel-fuelled generators.  Graded carbon or stainless steel may be used when higher temperatures are unavoidable as in natural gas engines.  Aluminized steel is a popular option as it provides more resistance to heat than carbon steel, is relatively inexpensive, and has increased corrosive resistant properties when the costs of a stainless steel system are considered [9].  Plastics can also be used as they do not corrode and are lightweight and easy to work with.  The drawback of using a plastic silencer is that the strength obtained by using metal is lost, thus plastic silencers are only used in low-pressure settings where strength is not much of a concern.  Moreover, plastic silencers cannot be used where high temperatures are present [7].   This report only examines the manufacturing process of the metal silencer and not the plastic silencer as metal is used in 95% of industrial silencer applications [7].      


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