Wastewater Treatment Plants - Effects of Machine Chamber Temperature Increase on Ventilation

During the operation of a positive displacement blower, which is also known as a compressor or turbo blower, a portion of the consumed power is released as heat into the room in which the blower is placed. Other heat sources inside this room can include electrical switchgear (such as frequency converters) and pressurized pipes, which have residual heat loss; however, these are not covered in this article.

Radiated Heat Volume

Regardless of the amount, all packaged units lose some portion of energy as heat into the chamber due to the running motor, as shown in Chart 1, which is denoted by

Chart 1: Heat volume as percentage of the motor power for all available Aerzen types

Required Cooling Air Flow QLab for Heat Dissipation

Room Ventilation

Suction of Positive Displacement Blower Outside the Installation Room via Suction Pipe

The above values are guide values for continuous operation, based on practical experience using insulated piping. Additional heat sources are not accounted for. Calculations as per TH1-040/00/DE.

For ventilation, an exhaust fan should be fitted in the installation chamber for the blower.

Suction of the Positive Displacement Blower in the Installation Room

When the differential pressures are sufficiently low, adequate fresh air will be supplied by the volume flow of the total intake of the positive displacement blowers or compressors to maintain the room temperature ∆T < 10 K. However, it is always advisable to use an exhaust fan to prevent the room from being heated up over time. The exhaust air volume flow should be Qvent = 1000 m3/hour.

Where (n⋅Q1) > QLab.

The above values are guide values for continuous operation, based on practical experience using insulated piping. Additional heat sources are not accounted for. Calculations as per TH1-040/00/DE.

Example Arrangement for Room Ventilation with Room Suction

AERZEN’s AERselect is the most recent design tool, and can be downloaded from http://www.aer-zener.de/customernet. This has been used in the examples below to show the impact of a simple change in allowed room temperature from ∆T = 5 °C to ∆T = 8 °C. An important point to be kept in mind is that the volume flow of the exhaust ventilator changes according to the method adopted for suction (whether room suction or piped inlet) and according to the increase in room temperature, within permissible limits. The cross-section of the louver over the air inlet must be sufficiently large.

Calculation of Sound Level: Sound and Silencing Measures

Sources of sound lead to vibrations that make the surrounding liquid, air, or solid bodies also vibrate. The type of noise—whether liquid-borne noise, airborne noise, or structure-borne noise—is determined by the media that convey the vibrations. It should be noted that the human ear has the ability to identify airborne noise of around 16–20,000 Hz, which is the audible threshold. Yet, it would be possible for humans to hear structure-borne and liquid-borne noise only if these are changed into airborne noise.

The Positive Displacement Blower/Screw(-Type) Compressor as Noise Producer

As shown in the following instance of a silenced positive displacement blower, some measures can be taken to silence noise emissions.

Possible sound sources

The sound level for the following is indicated by the emission values:

  • Machine noise emitted from the packaged unit with or without an acoustic hood, which is indicated as a sound pressure level LP < A in dB(A), which in turn is a value computed using values measured from the entire body of the machine (measurement object) at a radius of 1 m. It can even be indicated as a sound power level LW(A) computed as shown in the following section. As specified in DIN 45635, both the values are taken under free field conditions. Consequently, upon installing the packaged unit in a room, it is anticipated that the sound pressure levels would be typically higher.
  • The sound levels from the pipe on the intake side and the discharge pipe (which are supplied on demand). Another technique is to compute the anticipated sound levels outside the pipe when the pipelines are resonance-free and when the pipe-compressor coupling is flexible. Thus, both the frequency level of the sound pressure level within the pipes and the height of the pipes play a vital role in the amount of noise generated and moving outward from the pipes.

At the installation site, noise development arises from:

  • Machine noise: For indoor installations, this is measured as the sound pressure level, which varies according to the conditions that lead to sound reflection within the room. As specified by VDI guideline 2571, the determined level guides the value allotted to the mean sound pressure level, and also incorporates the increase in this level upon placing more than one unit in the same room.
  • Radiation noise generated within the intake-side and discharge pipes: At the time of installation, it is crucial to ensure that the length and dimensions of the pipe are conducive to the interaction of the sound levels radiated into the pipe and the principal exciting frequencies of the compressor to generate a net sound pressure level, which is within the required standard when measured at 1 m over the entire piping system. If the packaged unit is placed indoors, sound reflection must be taken into account as an additional source of sound pressure. VDI 3733 should be consulted to achieve proper pipe dimensions to prevent an increase in sound radiation due to “passing frequencies” and other types of natural frequencies, and also to compute the sound pressure levels outside the pipe based on standard rules. A crucial factor here is the composition of the pipes, whether they are made of thin-walled stainless steel or standard steel.

To summarize, the following points must be taken into account:

  • If the unit is installed indoors, an increase in sound pressure level is anticipated
  • It is also anticipated that there will be an increase if more than one unit is installed
  • Since the thickness of the pipe also has an impact on radiation of noise from the pipe, during the design phase, it is important to choose the correct dimensions of pipe in relation to the diameter and the length, to prevent the same frequency from arising at the natural piping frequencies as well as the compressor’s exciting frequency.

Definition of Terms

1. Sound Pressure, Sound Pressure Level, Frequency

It is evident that airborne noise induces vibration of the air around the source of the sound, resulting in a variation in air pressure above the atmospheric pressure. Although the variation is small, it is significant. The sound pressure p is described as the maximum deviation in pressure within a sound wave, or its amplitude. This is in correlation with the volume of the sound heard.

The sound pressure can be calculated by finding the logarithmic ratio of the sound pressure level.

Frequency f is described as the number of vibrations per second and is correlative to the tone pitch heard.

2. Sound Power and Sound Power Level

A noise source’s sound power P is a term covering the whole sound radiation, and is hence a machine property. Therefore, in contrast to the sound pressure, the sound power does not vary regardless of the distance from the sound source to the measuring point.

3. Sound Spectrum

The sound spectrum represents the sound level distribution within the analogous range of frequency, or, put differently, it defines the sound pressure level/sound pressure within the connecting bands of frequency, for instance, the octave bands. The typical frequency range of a positive displacement blower is as follows:

4. Sound Pressure Level Evaluation (“A”-Weighting)

Although the human ear has the ability to hear frequencies from 16 to 16,000 Hz, it is highly sensitive to sound between 1,000 and 4,000 Hz. Therefore, sounds with frequencies less than 1,000 Hz or greater than 4,000 Hz are heard as softer sounds, in contrast to sounds between these frequencies, which are considered to have a medium frequency, although both sounds could have identical sound pressure. This difference in sensitivity is taken into account while regulating and measuring sounds. For instance, filter curves that fall within the sound spectrum are plotted and stored on measuring instruments. One such curve is the “A-curve”, which is mainly used across the globe to measure sound pressure levels. These values are “A”-weighted sound pressure and sound power levels and are expressed in the units of dB(A).

Sound Measurement: Measured Variables

Standard to be Applied

The basic rules related to sound measurements for machines, with the help of the enveloping surface method, have been specified in the DIN 46535 standard. This defines the standard procedure for the measurement of the machine-radiated noise, or sound power, computed from the sound pressure level measured over a given surface, during the enveloping surface procedure.

Measurement Surface

As detailed above, determination of the sound pressure level A is carried out using a sound level gauge at various points along the given measuring surface S, extending around the machine in a notional shape, for instance, a cuboid shape. After performing corrections for outside noise and measurement-environment interactions, the outcome is the measuring surface sound pressure level Lp(A) expressed in dB(A). The standards for these measurements for blower units, in accordance with DIN 45635, carried out at a distance of 1 m from the outermost points of the free-field contour of the unit, have been described in the workshop manuals.

The Relationship Between Sound Power Level LW(A) and the Sound Pressure Level at the Measurement Surface Lp(A)

The calculation of “A” sound power level LW(A) is specified in the DIN 45635 as follows:

Where

Where S is the contents of the measurement surface (m2) and SO is the reference surface (1 m2).

Where the measurement surface is sound-permeable and has dimensions of less than 1 m2, LS should be subtracted. In contrast, LS is added if the dimensions of the surface are not more than 1 m2. These circumstances correspond to the use of pipes and positive displacement blowers/screw-type compressors, respectively, where in the latter case, the size of the measurement surfaces is 14–20 dB. The values according to increased size can be provided upon demand.

Calculation of Sound Levels

Energetic Level Addition

According to the VDI 2571 standard, which addresses the challenge of radiation of sound from industrial buildings, noise emissions, or the impact of sound radiation at a specific distance from the noise source, can be found by calculating the total sound pressure level by finding the sound pressure levels of each separate sound source, as shown below.

Whether the sound power or sound pressure levels are used for the summation does not have any impact if the reference values match each other.

The equation below is applicable if sound sources have identical individual levels:

Where i is the number of noise producers (dB) and L+ is the difference to be added to the individual level (dB).

Graphical Representation

Energetic Addition of Equal Sound Levels

In case there are four sound sources each at a sound level of 80 dB each, the total generated sound level would be

The following equation is applicable if the noise sources have different sound levels, L1 and L2:

Graphical Representation

Thus, upon adding two different noise sources with L1 = 80 dB and L2 = 76 dB, the total sound level is

Sound Level Reduces with Increasing Distance from the Sound Source

Sound waves are produced as a result of vibrations, which cause increases and decreases of the ambient air pressure in the form of rapid waves. The sound pressure varies corresponding to the increase in the distance from the noise source; however, this decrease is negligible if the measurement is carried out in the near field of the source. Upon simplifying the situation, the following equation is applicable under optimal conditions over long distances (for example, a free field) if the sound source is a point, or very small when compared to the distance:

More precise specifications for computing the amount of reduction in sound when it moves away from the source, considering the environmental damping, sound reflection, and other factors, have been defined in VDI 2571, which deals with sound radiation from industrial buildings, and in VDI 2714 titled “Outdoor sound propagation.”

Calculating Sound Pressure Level in an Installation Chamber

Inside a room, an installed machine’s sound pressure level is dependent on the radiated sound power level of the machine and also the acoustic properties of the room. VDI 2571 specifies the following equation for calculating the mean sound pressure level inside the room:

The following points should be noted:

  • The total sound power level LW(A) is calculated using the individual sound power levels
  • Already measured sound pressure levels and measurement surface levels defined above should be used for calculating the sound power levels
  • There can be considerable variations between the experimental and calculated sound levels at particular points of the machine room due to factors such as screening or reflection of sound
  • The experiential values for T are around 2 seconds when the machine is installed in a conventional factory hall, around 4 seconds if it is installed inside a large echoing room, and around 1 second within a small room with exceptional sound-absorbing limit surfaces

AERselect Calculation Tool

The AERselect calculation tool is an innovative application for performing acoustic calculations in a simple manner.

This information has been sourced, reviewed and adapted from materials provided by Aerzener Maschinenfabrik GmbH.

For more information on this source, please visit Aerzener Maschinenfabrik GmbH.

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