MONTO Y ACTUALIZACIÓN DE LA GARANTIA:
C. P. Luis Manuel Avila Marinez SOLARES JIMENEZ SA DE CV
The ventilation system is a series of duct components that provide pressure resistance losses and sound gen-eration and mitigation. These components contribute to the overall pressure loss and sound at the receptor.
The methodology integrates the calculation of pressure loss and sound generation and attenuation of each component in the system using the computer software Mathcad for documentation and validation of engineer-ing calculations. This software is particularly helpful be-cause it provides a fully documented calculation that can be easily reviewed and altered for different projects. The program is structured by common variables, common geometry, component pressure, component sound, total pressure, and total sound.
Common variables and geometry are defined at the front end of the software program to allow for simplifying input and reducing errors. The same reasoning applies to de-fining common geometric parameters for plenums, damp-ers, and silencers at the front end of the program.
Pressure
Each air pathway in the fan forward and reverse direction is considered when calculating pressure loss as shown in Figure 2. Often only one pathway needs to be deter-mined if it can be shown to be the highest resistance path. If there is no clear distinction of the highest resis-tance pathway, all air pathways should be evaluated.
I.E. Idlechik’s Handbook of Hydraulic Resistance provides resistance coefficients for various components within an airstream such as elbows, tees, structural interferences, and dampers, as well as sudden expansions, sudden con-tractions, and diverging and converging transition losses.
Figure 3 is a sample calculation from a Mathcad file de-scribing the pressure loss for a seven foot silencer. In this example, the face velocity (Vs) is calculated to obtain the pressure loss in the forward direction.
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Figure 2 - South Roosevelt Station elevation layout
Figure 3 - Silencer pressure loss from Mathcad®
EXHAUST / INTAKE
PUSH / PULL EFAN
EFAN
Length of Silencer
Face Velocity Pressure Loss (SMACNA pg 9.10) Lz : = 7. ft.
Counter z := z + 1 z = 10
Pressure Loss/Silencer
Pressure loss through the silencer may be given by manufacturers’ data (IAC Type L Sound Attenuator), however, the pressure loss is taken by SMACNA pg 9.10. The fl ow rate for this type of system is considered a medium attenuator.
= 1875
: =Qf .
As
ft.
min Vs
–0.3 in Pfz :=
Fire and Life Safety Ventilation Systems
Sheet Metal and Air Conditioning Contractors' National As-sociation (SMACNA) has published that pressure loss is a function of velocity. Similarly, a pressure loss is to be calculated for every ductwork component along the highest resistance pathway.
Sound
In conjunction with the pressure term, a sound power component is considered for all segments in which sound is either regenerated or attenuated. The sound reduction or absorption is defined as the insertion loss (IL). When sound travels through a duct component such as a silencer an insertion loss occurs.
Equation 1 describes the sound power after a compo-nent loss where Lw1 is the sound power level before the component loss and Lw2 is sound power level after the component loss (Reference 2). The insertion loss reduc-tion is calculated over each of eight frequency bands which range from 63Hz to 8000Hz. The human ear is only sensitive to this range of frequencies.
Equation 1
The regeneration gain by the silencer is used to calculate the total sound power (Lw3) as shown in Equation 2. The total sound power after the silencer insertion loss (Lw2) is then added logarithmically to the regeneration (LwR) val-ues (Reference 2). The decibel (dB) scale is logarithmic and as such a doubling or halving of energy changes the sound level by 3dB; it does not double or half the sound level as might be expected.
Equation 2
A silencer insertion loss and regeneration values for a si-lencer are available from manufacturers' data for each of the eight frequency band levels in both the forward and reverse fan direction. Insertion loss can be determined for any component in the system. Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) provides methods to calculate insertion and regeneration values for various types of ductwork components.
Total Pressure/Sound
The total pressure loss of the ventilation system is deter-mined by a summation of losses for components along
the flow path. After each component has been evalu-ated for pressure loss, the total summation is used to evaluate the fan brake horsepower. The fan sound power is evaluated from the total pressure loss as shown by Equation 3 (Reference 2).
Equation 3
When calculating the total sound power at the recep-tor, each component is evaluated from the fan to each receptor location. In other words, the fan sound power calculated from Equation 3 is taken to be reduced from the insertion component by Equation 1. For ponents with regeneration, the total sound is com-bined using Equation 2. Sound power, which is a mea-sure of the sound intensity, must then be converted to sound pressure or the power component that directly affects the ear drum. The sound pressure levels are evaluated for the receptors, typically in a tunnel, sta-tion, or ambient locations.
Example
During the design of the ventilation system at U-District station, the use of tunnel fans to exhaust the atrium pre-sented a challenging sound control problem. The fans were in close proximity to the atrium receptor. This al-lowed very little attenuation to occur. Early in the design, it was determined that the sound levels did not meet project criteria with the configuration of the atrium damp-er in-line with the fan.
Three options were investigated to attenuate the sound.
The first option was to add a matrix of silencers at the atrium wall opening. This was a viable solution but not considered to be the best choice due to the cost and aesthetics. The second and third options were to offset the damper to provide additional elbow attenuation. The second option utilized an unlined elbow, whereas the third option used an acoustically lined elbow.
Figure 4 shows the final configuration of the ventilation system. The acoustically lined elbow option provided the necessary attenuation to meet project criteria with minimal impacts to the fan horsepower. The program allowed exploration of different options to reach a fea-sible, cost effective, and architecturally appealing solu-tion by understanding the parameters that controlled the sound.
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Fire and Life Safety Ventilation Systems
Conclusion
Traditionally, pressure loss analysis has received more at-tention in the design of tunnel ventilation systems than sound analysis. In recent years, environmental policy, reg-ulatory requirements, and space constraints are making it more important to understand sound mitigation. This methodology of combining sound and pressure simulta-neously in the design of ventilation systems provides an efficient tool for understanding how both sound and pres-sure are influenced by each system component. In addition to demonstrating compliance with the regulatory require-ments, the method provides a means to explore other op-tions more efficiently than before. It allows for an optimum design that minimizes motor power requirements, meets sound requirements, and minimizes space requirements.
References
1. Idelchick, I.E., Handbook of Hydraulic Resistance. 4th Edition.
2. HVAC Sound and Vibration Manual. s.l. : Sheet Metal and Air Conditioning Contractors' National Associa-tion, First Edition Dec. 2004.
3. HVAC Systems Duct Design. s.l. : Sheet Metal and Air Conditioning Contractors' National Association, Third Edition 1990.
Michael MacNiven is a Senior Mechanical Engineer with a tech-nical background in HVAC, piping, and fire life/safety systems
along with large scale tunnel emergency ventilation systems. DECEMBER 2014 http://www.pbworld.com/news/publications.aspx
ADD ACOUSITCAL MATERIAL TO
WALLS
SOUND PATHWAY
STAIR-6
SOUND POWER FOR SINGLE ATRIUM DAMPER
SOUND POWER PROVIDED AT AMBIENT AT TOP
OF SHAFT
SOUND POWER PROVIDED AT TUNNEL DAMPER
AT CEILING OF PLATFORM
LEVEL (TYP)
Figure 4 - Sound attenuation material at U-District Station