Machine Chamber for Wastewater Treatment Plants

When a positive displacement blower, also called a compressor or turbo blower, is operating, some of the power consumed is released as heat into the room within which it is contained.

Other sources of heat within this room include pressurized pipes and electrical switchgear such as frequency converters, which have residual heat loss, but these are not covered here.

Radiated Heat Volume

The amount notwithstanding, all packaged units will lose some energy as a result of the running motor, as heat into the chamber, as the following chart shows, which is signified by

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

Required Cooling Air Flow Q_Lab 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. Image Credit: Aerzen

The installation chamber for the blower must be fitted with an exhaust fan for ventilation.

Suction of the Positive Displacement Blower in the Installation Room

If the differential pressures are low enough, the volume flow of the total intake of the positive displacement blowers or compressors will supply enough fresh air that is adequate to keep the room temperature ∆T < 10 K, and yet an exhaust fan is always advisable to keep the room from heating up over time. The exhaust air volume flow should be Q _vent = 1,000 m³/h.

Where (n . Q1 ) > Q_Lab.

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. Image Credit: Aerzen

Example Arrangement for Room Ventilation with Room Suction

AERZEN’s latest design tool is the AERselect, which can be downloaded at https://www.aerzen.com/worldwide.html. This is used in the following examples to show how much effect a simple change in allowed room temperature from ∆T = 5 °C to ∆T = 8 °C can cause. It must be remembered that the exhaust ventilator has a volume flow that changes with the method used for suction (whether piped inlet or room suction) and with the room temperature increase within permissible limits. The louver over the inlet for air must have a large enough cross section.

Calculation of Sound Level: Sound and Silencing Measures

Sources of sound cause vibrations that make the surrounding air, liquid or solid bodies also vibrate. The media carrying the vibrations determine whether these are airborne noise, liquid-borne noise or structure-borne noise. It is noteworthy that the human ear can identify airborne noise from about 16 to 20 000 Hz which is the audible threshold. However, liquid-borne and structure-borne noise can only be heard by humans after they are changed into airborne noise.

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

Some measures can be adopted to silence noise emissions as is seen in the following instance of a silenced positive displacement blower.

Possible sound sources. Image Credit: Aerzen

The emission values indicate the sound level for:

• Machine noise from the packaged unit which comes with/without an acoustic hood, which is shown as a sound pressure level LP<A in dB(A), which in turn is a value calculated from measuring values from all around the machine (measurement object) at a radius of 1m. It can also be shown as a sound power level LW(A) calculated as shown in Section 3.3. Both these values are taken under free field conditions as specified in DIN 45635. As a result, when the packaged unit is installed in a room, the sound pressure levels are typically expected to be higher.
• The sound levels from the discharge pipe and the pipe on the intake-side (which are supplied on request). Another method is to calculate expected sound levels outside the pipe if the pipelines are resonance-free and if the pipe-compressor coupling is flexible. Thus both the height and the frequency level of the sound pressure level within the pipes determine how much noise is created and moves outwards from the pipes.

Noise development at the site of installation comprises:

• Machine noise: This is measured, for indoor installations, as the sound pressure level, which changes in accordance with the conditions that favor sound reflection within the room. As VDI guideline 2571 specifies, the level determined guides the value assigned to the mean sound pressure level, and also includes the increase in this level that occurs if more than one unit is placed in the same room.
• Radiation noise produced within the discharge and intake-side pipes: During installation it is important to make sure that the pipe dimensions and length are such that the sound levels radiating into the pipe, and the chief exciting frequencies of the compressor interact to produce a net sound pressure level which falls within the required standard when measured at 1m all around the piping system. If the packaged unit is indoors, sound reflection as an additional source of sound pressure must be considered. It is advisable to consult VDI 3733 to achieve proper pipe dimensions so as to avoid a rise in sound radiation because of “passing frequencies” and other types of natural frequencies, as well as to calculate the sound pressure levels outside the pipe according to standard rules. An important factor here is the make of the pipes, whether they are of standard steel or thin-walled stainless steel.

In short, the points below must be considered:

• An increased sound pressure level is expected when the unit is installed indoors
• An increase is also expected if more than one unit is installed
• The thickness of the pipe wall also affects noise radiation from the pipe, so during the design phase it is crucial to select the right dimensions of pipe with regard to the length and the diameter, so as to prevent the same frequency from occurring at both the natural piping frequencies and the exciting frequency of the compressor

Definition of Terms

1. Sound pressure, sound pressure level, frequency:

If we consider airborne noise, it is clear that it induces vibration of the air around the source of the sound, causing a change in air pressure above the atmospheric pressure. This change is small but significant. The sound pressure p is defined as the maximum deviation in pressure within a sound wave, or its amplitude. This correlates with the volume of the sound heard.

The calculation of the sound pressure is by finding the logarithmic ratio of the sound pressure level

Frequency f is defined as the number of vibrations per second, and corresponds to the tone pitch which is heard.

2. Sound power and sound power level:

The sound power P of a noise source is a term which covers the whole sound radiation, and is thus a machine property. Thus, unlike the sound pressure, the sound power remains the same whatever the distance from the sound source to the measuring point.

3. Sound spectrum

The sound spectrum is a term which represents the distribution of the sound level within the corresponding range of frequency, or in other words, defines the sound pressure/sound pressure level within connecting bands of frequency such as the octave bands. A positive displacement blower typically has the following frequency range:

4. Sound pressure level evaluation (“A”-weighting)

The human ear can hear frequencies between 16 to 16 000 Hz but is most sensitive to sound between 1 000 and 4 000 Hz. Thus sounds with frequencies lower than 1000 Hz or higher than 4000 Hz are heard as softer sounds compared to sounds between these frequencies which are said to have a medium frequency, even though both sounds may have the same sound pressure. This sensitivity difference is considered when regulating and measuring sounds. For example, filter curves falling within the sound spectrum are plotted and stored on measuring instruments. One of these is the “A-curve”, which is primarily used across the world to measure sound pressure levels. Such values are “A”-weighted sound pressure and sound power levels and are expressed in dB(A).

Sound Measurement: Measured Variables

Standard to be applied

The DIN 46535 standard specifies the basic rules regarding sound measurements for machines, using the enveloping surface technique. This sets down the standard procedure for measuring the machine-radiated noise, or sound power, which is calculated from the sound pressure level measured over a specified surface, as part of the enveloping surface procedure.

Measurement surface

As mentioned above, the sound pressure level A is determined with the help of a sound level gauge at several points along the specified measuring surface S which extends around the machine in a notional shape such as that of a cuboid. Corrections are applied for outside noise and measurement-environment interactions, and the result is the measuring surface sound pressure level Lp(A) expressed in dB(A). The standards for such measurements for blower units, according to DIN 45635, taken at a 1m distance from the outermost points of the free-field contour of the unit, are specified in the workshop manuals.

The relationship between sound power level LW(A) and the sound pressure level at the measurement surface LP(A)

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

Where

S: measurement surface contents [m²],
S_O: reference surface [1 m²]

If the surface of measurement is sound-permeable, and is less than 1 m², LS must be subtracted, while it is added if the surface measures more than 1 m². These situations correspond to the use of pipes and positive displacement blowers/screw-type compressors, respectively, the latter having measuring surfaces of 14-20 dB in size. The values per size increment may be requested if necessary.

Calculation of Sound Levels

Energetic level addition

The VDI 2571 which addresses the issue of sound radiation from industrial buildings states that to find noise emissions, or the effect of sound radiation at a particular distance from the noise source, the total sound pressure level may be calculated by finding the sound pressure levels of each separate sound source, as shown below.

It does not matter whether the sound pressure or sound power levels are used for the summation, provided the reference values match each other.

If sound sources have identical individual levels, the following equation applies:

i: number of noise producers [dB]
L⁺ : difference to be added to the individual level [dB]

Graphical representation:

Image Credit: Aerzen

Energetic addition of equal sound levels

If a situation arises in which there are four sound sources each at a sound level of 80 dB each, the total sound level generated is

If the noise sources have different sound levels, L₁ and L₂, the following equation applies:

Graphical representation:

Image Credit: Aerzen

Thus if two different noise sources with L₁ = 80 dB and L₂ = 76 dB are added, the total sound level is

Sound level reduces with increasing distance from the sound source

Sound waves result from vibrations, which cause the ambient air pressure to increase and decrease in rapid waves. The sound pressure is different as the distance from the noise source increases, but this decrease is insignificant as long as the measurement is performed in the near field of the source. If the situation is simplified, the following equation applies under ideal conditions over long distances such as a free field, if the sound source is a point, or very small in comparison with the distance:

More accurate specifications for calculating the degree of reduction in sound as it moves away from the source, taking into account the sound reflection, environmental damping and other factors, are given in VDI 2571 dealing with sound radiation from industrial buildings and VDI 2714 titled “Outdoor sound propagation”.

Calculating sound pressure level in an installation chamber

Within a room, the sound pressure level of an installed machine depends upon the radiated sound power level of the machine as well as the room’s acoustic properties. VDI 2571 gives the following equation for finding the mean sound pressure level within the room:

Some points to note include:

• The sound power levels must be calculated from already measured sound pressure levels and measurement surface levels set out above
• The total sound power level LW(A) is determined from the individual sound power levels
• The experiential values for T are about 2 seconds if the machine is installed in a traditional factory hall, about 4 seconds if within a large echoing room and about 1 second in a small room that has excellent sound-absorbing limit surfaces
• The calculated and experimental sound levels may differ significantly at specific points of the machine room because of factors such as reflection or screening of sound.

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.