Noise reduction of large heat pumps and large refrigeration systems.

We are receiving more inquiries about the noise reduction of air-cooled large-scale heat and large-scale refrigeration systems with a single output > 200 kW, which require a reduction in noise emissions of 25 dB(A) and more in order to comply with the legal noise limits. In many cases, multiple systems are planned in this performance range on a small area.

The dilemma of installation often arises during the planning phase. Real estate investors do not want to free up space for the investments that can be rented out. As a result, planners are often forced to plan the systems close to the property boundary on roofs, etc., and then quickly encounter the quiet needs of the adjacent residents.

Since the noise emissions of air-cooled systems cover an emission spectrum of 32 to 8K Hz, these noise reduction requests quickly come up against the limits of physics, or rather the sound insulation and sound absorption limits of the materials available today. Further on available areas to be able to build sound enclosures with the envelope necessary for noise reduction.

One difficulty is the high air volumes that these plants require at full capacity, which are usually > 150000 m3/h to call up the full plant capacity. In addition, there are usually multiple compressors that are switched on depending on the power level. Individual scroll compressors in the 400–600 kW range are typically around 80–86 dB(A) Lw in the "Low noise" version, while SuperLowNoise variants achieve a further reduction of around 3–5 dB(A). Noise from the hydraulics and intake noises due to pressure losses on the evaporator side are of course added. In practice, the distance between 1/4 and 4/4 speed can be larger because higher fan speeds are usually driven at full load, this is especially true for refrigeration systems. In heat pump operation, the fan speeds are not significantly reduced even in partial load operation. 

The dilemma now when planning sound enclosures for large heat pumps is to create enough free space for air circulation and to keep the air velocity and pressure losses through the sound enclosure as low as possible. In practice, this means that with an air volume of 150000 m3/h, we need 6.9 m2 of free space to increase the air velocity to 6 meters/sec. and the pressure loss to 24 Pa. and prevent air flow noise.

In limited areas, which is the case in most projects, this means designing the air duct in such a way that noise-critical areas in the vicinity are not directly in the escaping sound of the system. This means that the free air intake areas must now be accommodated on a reduced perimeter without generating direct air vortices.

Since the fans also discharge low-frequency sounds in the range of 32 to 250 Hz, there is still the problem of reducing this sound at the source. Especially in this area, most of the materials available on the market today usually have a very low effect.

Full of hope, I have therefore dealt intensively with acoustic meta-materials in the last few months and have come to the realization that most manufacturers mainly address the frequency spectrum of 250 to 500 Hz and are largely still in the development phase and have hardly any available products.

In the frequency range from 32 to 250 Hz airborne sound, resonant acoustic meta-mmaterials (membrane and Helmholtz type) are particularly effective, which can be specifically tuned to this range. However, a lot of development is still needed here to develop effective products for the masses. However, I am convinced that metamaterials expand the "design space" within existing physics, but they do not create perpetual motion effects, no true hyperlight communication and no violation of conservation or causality principles.
Of course, a few additional dB can be reduced with specifically tuned resonators and/or meta-material panels in the range of 32 to 250 Hz, but the whole thing remains a physical challenge (wavelengths, a lot of leakage potential through the free ventilation openings). Fan reserves, defrosting behavior and recooling temperatures must also be coordinated with the hood geometry and gate resonator arrangement.
In the meantime, the choice of installation locations as far away as possible from noise-critical locations is still the ideal solution and helps in the planning of sound measures, taking into account their performance and the possibility of protecting critical areas from noise emissions through targeted air ducting.

Forcotech Air-Routing Large Heat Pumps und Chillers