​The summer has barely begun when we start rolling out various projects for our acoustic enclosures for large heat pumps. In June 2025 alone, the following acoustic enclosures for large heat pumps will be launched:

1 acoustic enclosure 7,800 x 4,430 x 3,440 mm L x W x H (single enclosure)
1 acoustic enclosure 6,900 x 3,800 x 3,800 mm L x W x H (single enclosure)
1 acoustic enclosure 8,900 x 6,100 x 4,100 mm L x W x H (double enclosure)
1 acoustic enclosure 10,800 x 4,800 x 3,810 mm L x W x H (double bonnet)
1 acoustic bonnet 5,480 x 2,590 x 2,788 mm L x W x H (single bonnet)
1 acoustic bonnet 18,500 x 4,600 x 4,150 mm (L x W x H (triple bonnet)
1 acoustic bonnet 3,100 x 2,200 x 2,472 mm (L x W x H) single hood

In total, the 11 large heat pumps are planned to reduce noise emissions by 209 dB(A). All of the systems are located close to sound-sensitive neighbourhoods and cannot be operated without sound measures in compliance with noise regulations. The projects are spread across Germany (Stuttgart, Berlin, Esslingen, Munich), Austria (Innsbruck) and Switzerland (Lucerne).

Around 1,350 m2 of 2 mm thick aluminium sheets and around 1,600 linear metres of aluminium profiles plus around 1,300 m2 of insulation were used to produce these acoustic enclosures. In addition, locking systems, sensors and a large number of fastening materials were used.

At a small hospital in France, we installed a sound bonnet for a recooler used to cool the operating theatre and computer tomographs.

The acoustic bonnet was required to reduce the noise emissions from the recooler to the bed wing. The recooler moves 28,000 m3/h at full load. The recooler was installed directly adjacent to the plant room on the ground floor. Among other things, 6 heat pump boilers, each with a capacity of 500 litres, are installed in the plant room. The plant room is not heated.
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The idea quickly arose to use some of the warm exhaust air from the recooler to raise the temperature in the plant room and thus supply the 3,000 litres of service water.

As the heat pump boilers use the ambient air, a thermally insulated duct with adjustable louvres was installed directly on the exhaust air side of the acoustic bonnet in order to channel some of the warm exhaust air into the plant room.

The free cross-section, which is 1.94 m2 when the louvres are fully open, is designed for full-load operation of the refrigeration system of 28,000 m3/h at an air speed of 4 metres/sec. The position of the louvres is controlled by a pressure and temperature sensor. The louvre position can thus be positioned between 0° and 90°.

This reduces the Delta-T for the domestic hot water treatment and achieves a temperature lift of the domestic hot water. Around 1,000 m3/h of air volume is required per heat pump boiler at full load. At full load, the 6 heat pump boilers that provide the service water for the overhead laundry room, sterilisation and kitchen have a total air volume of 6,000 m3/h.

To remove the cold exhaust air from the room, a thermally insulated duct with a cross-section of 0.417 m2 was installed, which was connected directly to the exhaust air outlets of the heat pump boilers. This means that all the exhaust air is discharged to the outside. At full load of the 6 heat pump boilers, we have a maximum air velocity of 4 metres/sec in the duct.

A load profile measurement, which we will now carry out over the next 12 months, shows how much electricity can be saved in the domestic hot water treatment with this concept.

In addition to the noise reduction of the recooler, it has already been achieved that negative pressure and room cooling no longer occur in the plant room when the heat pump boilers are in operation.

The following advantages were utilised in this project. Installation of the recooler directly at the smallest possible distance from the plant room and the fact that the heat pump boilers were already in place. 

Image Legend
1) Heat pump boiler 2) Air inlet duct 3) Water pipe 4) Exhaust air duct 5) Consumer (washing machine etc...) 6) Recooler acoustic bonnet 7) Cold air inlet 8) Hot air recooler 9) Service access in the acoustic bonnet to the recooler

For us, the use of StratocellWhisper with a thickness of 50 mm or more in our sound insulation hoods and sound insulation walls for HVAC systems is undisputed. The material properties and the sound effect speak for themselves. For projects with very high sound insulation requirements, we also install a cavity between the insulation and the external panels.This results in several acoustic and technical advantages:

1)
Significantly improved sound absorption: A cavity of 50 mm significantly increases sound absorption, as the distance between the wall and the insulating material acts like a resonator. This means that low-frequency sound waves are better absorbed, and the reflected sound energy is reduced. This is often used specifically in acoustics to increase the effectiveness of absorbers.

2)
More efficient use of material properties: Stratocell Whisper is already designed as a sound-absorbing polyethylene foam that offers high sound insulation values thanks to its closed-cell structure. In combination with cavities, the effect of the material is further enhanced, as layers of air act as an additional damping layer.

3)
Improved mechanical decoupling: The distance reduces the direct coupling between the wall and the insulating material, which further reduces the transmission of structure-borne sound and vibrations. On the side of the insulating material, we naturally do not cover it with perforated sheets or similar, so that the entire absorption surface of the insulating material is used.
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Conclusion:
A cavity of 50 mm when installing Stratocell Whisper ensures significantly higher sound absorption, improves acoustic effectiveness and offers practical advantages in terms of installation and technical equipment.
ity to improve sound absorption

The design of acoustic enclosures for large heat pumps and large refrigeration systems is based on the following parameters. 

1) Physical dimensions of the systems 
2) Required air volume at full load 
3) Static pressure of the fans 
4) Sound insulation to be achieved
5) Access required for service and maintenance work.

The following physical parameters must be set in the correct ratio during planning:

Air volume / free areas (open Areas) / air velocity / pressure loss.

Free air inlet and air outlet areas (net open areas in which the air can enter and exit the bonnet). These open areas must be dimensioned so that an air velocity of 7 metres/sec is not exceeded. This is due to the pressure loss of 28 Pa. at 7 metres/sec. This pressure loss is partially compensated for by the static pressure of the fans, which have to overcome a resistance on the evaporator on the air inlet side and have to expel the air above the fans on the pressure side. 

Up to an air velocity of 7 metres/sec, the ratio between heat transfer and pressure loss is ideal. At higher air speeds, the heat transfer is negatively affected and the pressure loss becomes too high. In addition, a higher air velocity leads to flow noise which results in an increased noise level.

What does this mean for example for the design of a sound bonnet for a system with an air volume of 170,000 m3/h? 

To calculate the required free area at an air volume of 170,000 m³/h and an air velocity of 7 m/s, the air volume flow must first be converted into the appropriate units in order to then use the formula for the air volume flow.

The free area for an air volume of 170,000 m³/h at an air velocity of 7 m/s is around 6.93 m².

As the exhaust air and supply air areas are separated to prevent recirculation of the air, the free area of at least 6.93 m2 must be ensured both on the intake side and at the air outlet.

Heat pumps and refrigeration systems are caught between energy-efficient performance and acoustic compatibility with the neighbourhood. A central phenomenon in this context is sound masking effects, which significantly influence both the technical design and the acoustic perception of these systems. These effects result from the complex interaction of different frequency ranges and operating conditions, which often lead to unexpected noise emissions, even if individual components have been optimised.
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Sound masking effects describe the phenomenon in which the reduction of certain frequency ranges leads to other frequency components being perceived subjectively louder. This effect is based on psychoacoustic interactions: The human ear is less able to localise low frequencies and perceives them as more dominant when the background noise is reduced.

With heat pumps, such effects are typically caused by:

1
Frequency overlaps between fan noise (usually medium to high frequencies) and compressor noise (low frequencies),

2
Operating state-dependent modulation as typically occurs during de-icing cycles, in L/W heat pumps where fan speed changes shift the frequency spectrum.  

3
Reflection from building structures that amplify certain frequency bands through constructive interference.

Air-to-water heat pumps emit sound in the range of 30-70 dB(A), with the critical frequency bands lying between 63 Hz (low humming) and 4 kHz (high humming). Masking effects occur in particular when high frequencies are attenuated by sound insulation measures, making low frequency components more prominent in relative terms. For example, attenuating a 2 kHz signal by 10 dB(A) can result in a 100 Hz hum being perceived as 6-8 dB(A) louder

One of the main causes of masking effects are thermodynamically induced changes in operating conditions. At air temperatures around 0°C with high humidity, ice forms on the evaporator fins, which leads to the following effects:

4
Pressure losses in the air flow force higher fan speeds (frequency increase of 15-30%).

5
Compressor load changes when switching to defrosting modes generate pulse-like low-frequency oscillations

6
Material expansion on iced components cause additional resonances in the 80-200 Hz range.

These dynamic changes are superimposed on the basic noise spectrum and lead to non-linear masking effects that can hardly be detected by stationary sound measurements.

The interaction of these spectra leads to complex superpositions. For example, the 100 Hz component of the compressor can suppress the perception of 800 Hz fan sounds, while at the same time harmonics at 1600 Hz are amplified by resonances in the housing.

The use of AI tools is already saving us a lot of time in the planning, design and production of our acoustic enclosures for heat pumps and refrigeration systems.

The calculation of fan key figures, air volumes, pressure losses and, for example, optimum air speeds, can be greatly accelerated through the use of AI. Furthermore, the targeted use of AI tools simplifies the prediction of noise emissions and noise immissions, taking into account various atmospheric and topographical conditions.

The use of AI in production serves to optimise the four-cutting of raw materials such as insulation, aluminium sheets and frame profiles, thereby shortening production planning.

The use of AI does not solve any questions that cannot be answered without AI, but it speeds up the process of finding answers enormously.

Perplexity's announcement to integrate ‘DeepSeek R1’ into its AI platform offers the possibility to access DeepSeek's logical capabilities within the Perplexity framework, which promises to further optimise the performance of AI tools for mathematical and physical problems for the benefit of planning technical building services engineering.

We spent the last two days of January in Hamburg discussing future projects, changes and optimisations in our company.

We focussed in particular on current and upcoming projects. We discussed specific optimisation measures to make our processes even more sustainable and efficient, with a focus on the use of AI. The digital integration of production and installers was particularly valuable in order to develop the best possible solutions for our customers.

Finally, we visited a property where our acoustic bonnet for two large heat pumps will be installed in the coming weeks - a measure that both reduces noise emissions and optimises energy efficiency.

With lots of new impetus and a clear vision, we are motivated to take the next steps. 

More and more often, we are forced to plan several refrigeration systems or heat pumps in a relatively small area. 

In most cases, the arrangement of the system is therefore planned on the basis of the available space. 

This one-sided focus can have drastic effects on the free air exchange of the systems, both for the supply air and the exhaust air of the systems. 

The architectural and atmospheric conditions around the systems can have a massive impact on their performance. Recessed roofs and walls, for example, can have a negative impact on the air flow due to turbulence. On the air outlet side, recirculation of the exhaust air can lead to a massive loss of system performance due to the higher air inlet temperatures during heat transfer. 

Another factor that is not often taken into account is the distance between the units, which not only influences the air flow, but can also lead to a massive recirculation of the exhaust air depending on the wind conditions. If the systems are installed one behind the other, the advantage is that the air flow remains free on all sides (Fig. 1). If the systems are arranged next to each other, it must be taken into account that, for example, with 3 systems, both sides of the air flow with the centre system are in the intake area of the two opposite systems (air shadow). Depending on the air volume of the systems, the distance between the systems must therefore be planned to be considerably greater than if they are installed one behind the other. (Fig. 2). 

The design of the systems is also a decisive factor in the arrangement of the systems. A particular example are systems in table-top design, often drycoolers whose condensers are usually installed a short distance from the floor (Fig. 3). If such units are installed too close to each other, the air flow of the supply air is negatively affected during simultaneous full-load operation, i.e. the inflow area is too small, which can lead to a reduction in performance of the units of up to 30%. One way of installing the units closer together is, for example, to elevate them on a grating to increase the air inflow area from below (Fig. 4). An additional advantage of elevation is the greater ground clearance which, depending on the nature of the substrate, e.g. soil or grass under the grille, also has a positive effect on the delta T between the supply air temperature and the cooling temperature, as soil heats up significantly less in direct sunlight than a concrete substrate, for example. In terms of sound, soil also has the advantage of absorbing sound and not reflecting it, as is the case with a sound-reflecting surface such as concrete.

Ejectors in heat pumps
Ejectors are innovative components that are used in heat pumps to significantly increase their efficiency. They enable a significant increase in efficiency by optimising and partially recovering the compression work required in the compressor.

How ejectors work
Ejectors work according to the Venturi principle. A high-pressure flow is accelerated through a nozzle, which leads to a drop in pressure. This pressure drop creates a vacuum, which makes it possible to attract a low-pressure flow (suction flow) that mixes with the high-pressure flow. The kinetic energy in the diffuser is then converted back into pressure energy, which leads to an increase in the pressure of the total mass flow.

Advantages
By recovering throttling losses, part of the compression work can be saved, which increases the overall energy efficiency of the heat pump

Ejectors can significantly reduce the required power of the compressor or even replace a compressor stage in multi-stage systems.

Ejectors allow adaptation to different operating conditions and can be used in different configurations to optimise performance.

Challenges
Despite their advantages, ejectors are not yet widely used in practice. Ejectors need to be precisely matched to their operating conditions, which places high demands on the design. Furthermore, uncertainty about the cost-benefit ratio is one reason why many manufacturers do not implement this technology.

Overall, ejectors offer promising opportunities for increasing efficiency in heat pump systems, but require further research and development in order to optimise their application in industry and make them economically attractive.