Innovative fibreless HVAC duct silencer based on microperforated elements

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Abstract

A novel environmentally friendly silencer concept for noise control in HVAC applications is presented and its acoustic properties are studied in this paper. As a sustainable alternative to the mass-produced, nowadays unfavourable fibrous material filled silencers, the silencing effect of the new solution is based on multi-layered microperforated elements. A prototype silencer following standard HVAC dimensions has been composed and optimal design and technological aspects of the fibreless solution are described. The paper also provides overview of experimental methods for the determination of primary acoustic characteristics, including low frequency transmission loss spectra in various mean flow conditions and insertion loss results for the silencer. The acoustic effect is hereby validated by comparison of the results with experimentally obtained characteristics of typical fibrous material filled units commercially available in the region. The conclusions of the acoustic study demonstrate the suitability of the silencer type for a variety of HVAC implementations were reliable and eco-friendly noise cancellation is important.

Introduction

Dissipative silencers are widely used in HVAC duct systems [1]. Despite of cost effectiveness and relatively good sound attenuation properties, the fibrous materials used in dissipative silencers are generally unfavoured for the emission of harmful particles when exposed to pulsating media flow. Due to the relocation, contamination and moistening of the fibrous material the acoustic performance deteriorates in use. Additional issues related to the recycling process become apparent at the end of the life cycle, considering the remarkable amounts of disadvantageous fibrous substances contained in this type of silencers. Efforts are made globally to develop environmentally friendly alternatives for the existing porous noise control materials implemented in silencers for a variety of applications. The current study is focusing on the design of a reactive type of silencer aimed for a wide range of HVAC duct applications where eco-friendly solutions are in favour. Four different HVAC silencers commercially available in the European Union (EU) (see Fig. 1) were selected for comparison purposes. The central duct internal diameter of the silencers is 160 mm, the outer diameter of the expansion chambers varies between 270 and 290 mm and the expansion chamber length ranged between 600 and 620 mm. The characteristic properties of the silencers selected for reference are presented in Table 1. All four reference silencers represent the dissipative type units and incorporate mineral wool to provide sound absorption (see Fig. 1).

A proven solution to replace the fibrous materials is based on micro-perforated elements. Since the introduction in 1975 by Maa [2] a variety of micro-perforated elements have been presented and acoustically studied for noise control purposes. The microperforated elements can be regarded as the most significant innovative alternative to effectively substitute nowadays unfavoured fibrous sound absorption materials in most of the applications. The noise attenuation mechanism of this type of element primarily relies on the losses of acoustic energy around the perforated apertures of the panel. As the acoustic performance is directly related to the particle velocity of the elastic media penetrating the aperture, the dimensions of the apertures should be designed smaller than the thickness of the acoustic boundary layer, typically remaining in the sub-millimetre range. Fundamentally, the diminishment of acoustic pressure fluctuation is regarded to originate from the viscous effects, acoustic vortex shedding and grazing flow iterations around the apertures. A number of duct silencer concepts incorporating micro-perforates elements have been presented by acousticians including the authors of this paper, mainly for IC engine applications [3], [4], [5], [6], [7], demonstrating the potentiality of non-fibrous materials and encouraging sustainability. Several manufacturing technologies including CNC laser cutting and additive manufacturing have been tested to produce the microperforated elements. However, currently commonly implemented technique in mass production is the sheet metal stamping technology.

As one of the most important aspects in practical applications the reliability, primarily the contamination of the relatively small apertures has been studied by the authors recently [8], [9]. In these studies, the suitability of the microperforated elements has been concluded, even in the harsh and contaminating IC engine exhaust gas environments. The acoustic performance of the prototype presented is completely characterized and the results are presented and validated in comparison with commercially available silencers.

Section snippets

Microperforated acoustic panels

Based on earlier studies performed on several fibreless acoustic panels [10] a commercially available microperforated sheet metal panel manufactured of aluminium alloy was selected and implemented for the silencer design presented in this paper. Followingly, the main physical characteristics of the microperforated element are presented in Table 2 and in Fig. 2 [11].

Fibreless silencer design

The most common acoustic element implemented in the reactive type of silencer is the expansion chamber [5]. In this study a number

Acoustic experiments

Experimental studies were carried out in TalTech Acoustics Laboratory. The key parameters of the microperforated elements implemented in the silencer as well as the characteristic performance curves of the complete silencer unit were determined by using dedicated test facilities described in the following sections. In general, the most important property of acoustic materials is the sound absorption, provided by the material layer, often coupled with back-cavity (see Fig. 5). The most common

Results

The main results acoustically describing the microperforated materials in various configurations as well as the performance of the designed silencer are presented in Fig. 9, Fig. 10, Fig. 11. The absorption coefficient experimentally determined for the microperforated material layer by using the impedance duct method (see Fig. 5) is presented in Fig. 9. Hereby the single layer configuration located at distance x1 varying from 0 to 50 mm from the rigidly closed termination (see Fig. 5) is

Conclusions

A novel fibreless HVAC silencer concept has been proposed incorporating reactive expansion chambers and dissipative microperforated elements. The developed silencer containing double-layered microperforated material has been composed based on parametric studies and the acoustic performance of the new silencer has been experimentally validated. The paper presents acoustic properties and design features of the silencer in comparison with the existing commercially available HVAC silencers. The

CRediT authorship contribution statement

Margus Villau: Formal analysis, Investigation, Resources, Data curation, Visualization, Writing - review & editing. Hans Rämmal: Conceptualization, Methodology, Validation, Writing - original draft, Writing - review & editing. Jüri Lavrentjev: Supervision, Project administration, Funding acquisition, Validation, Writing - review & editing, Software.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by Estonian Development Grant EAG28 (Smart micro-perforated silencer for heat, ventilation and air conditioning systems).

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