We are often asked how big the performance loss of a heat pump, air conditioning or refrigeration system is when it is equipped with a soundproof hood?
In order to give an answer to this, we first have to determine which fan key figures the system has, or how large the static pressure of the fans is in Pascal. Axial fans with a maximum static pressure of 20 Pascal are often used in these systems. If the systems are also designed for heat recovery, the static pressure is usually much higher because the air has to be discharged via appropriate channels. Axial fans are suitable for large air volumes that have to be transported over short distances, i.e. typically when using heat pumps, air conditioning and refrigeration systems with direct air outlet in horizontal or vertical direction. Due to their design, centrifugal fans usually generate a significantly higher pressure than axial fans and are therefore suitable for extracting air over greater distances where branches or bends cause a significant pressure loss.

What does this mean when designing acoustic hoods?
For the systems, which are mostly designed with axial fans, it is important to dimension the acoustic hood volume in such a way that the air volume required by the system can be transported through the limited atmospheric space of the acoustic hood. Furthermore, the free areas that must allow unhindered circulation of the supply air and exhaust air are decisive. This means that the air openings of the acoustic hoods must be designed for the air volume required by the systems under full load. If the air opening, for example at the air inlet, is too small, there is a pressure loss on the condenser, which in speed-controlled systems leads to a higher fan speed and thus increases the noise of the system and the electrical input power. If the air outlet is dimensioned too small, the stagnant air creates a resistance that can no longer be pushed away by the fans and, in the worst case, can lead to a standstill of the system.

So the correct dimensioning of the free air inlet and outlet surfaces is a decisive factor in the design of noise protection hoods.
But even if the free air surfaces are dimensioned correctly, a lack of separation of the air chambers between the air inlet and air outlet can lead to a massively deteriorated performance of the system because in this case there is an air short circuit between the air inlet and air outlet. This means that with a heat pump, for example, the cold air cannot be completely discharged and mixes with the incoming air, which can lead to massive cooling of the incoming air. Keep in mind that the air outlet from a heat pump is around 7 to 9° below the ambient temperature. In the case of an air conditioning or refrigeration system, the exact opposite is the case, but here too the separation of the air chambers is a decisive factor so that the system performance is not reduced by an air short circuit in the acoustic hood. Even small gaps in the air separation are sufficient here to enable recirculation, so it is important to separate the air chambers hermetically.

Properly designed acoustic hoods with a hermetic separation of the air chambers do not lead to a reduction in the system performance in annual operation.
This is the case when the system pressure drop does not exceed 20 to 25 pascals, since this pressure drop is compensated by the static pressure of the fans. This is also the reason why the dimensions of acoustic hoods in systems with larger air volumes are relatively large in order to prevent baffles and to allow the air to flow in and out unhindered. A correctly dimensioned acoustic hood offers the following advantages in addition to noise reduction:

Protection of the installation from atmospheric agents such as: hail, snow, rain, wind and pollution with leaves that can deposit on the condenser and cause pressure loss.

Selectable air line enables air diversion for systems that are close together in order to prevent air short circuits between the systems.

Due to the selectable air flow, systems can also be installed in front of obstacles, since the air diversion can be designed flexibly.

Protection against access by unauthorized persons and against vandalism.

Protection from direct wind on the condenser and fans.

Further information: www.silent-mode.com to edit.

Initial position
An OCHSNER heat pump from the "AIR-85-C14A" model series was used as part of a renovation project. The outdoor unit is designed as a table evaporator. The system has the following dimensions: 2,225 x 1,940 x 1,340 mm L x W x H. The system has 2 axial fans with a maximum volume flow at full load of 20,000 m3/h.

The heat pump is located outside of a kindergarten in Stuttgart.
 
Acoustic enclosure concept 
The basic structure of the acoustic hood is an aluminum plug-in profile frame, the service doors around the system are integrated into this profile frame. The service doors are based on aluminum panels with a thickness of 1.5 mm with internal 40 mm thick insulation made of Stratocell® Whisper, the material has one of the best sound-absorbing properties available on the market.
 
The acoustic hoods have the following dimensions: 2,800 x 2,500 x 1,700 mm (L x W x H) with a weight of 375 kg.
The air openings and the free air inlet openings are designed with around 15% above the required air volume of the system. The air flow is offset to the sides, so that no air short circuit can occur between the exhaust air and supply air. With the air deflection by 90°, the airborne sound is broken, which leads to a significant reduction in the exit noise. The air outlet speed is around 1.98 m/sec.

Noise reduction
The all-round noise reduction across the frequency spectrum from 63 to 8,000 Hz is between 19 and 20 dB(A), with a pressure loss of around 20 Pa.
 
Adaptation to the environment
The acoustic hood was made entirely of aluminum and powder-coated in the RAL color: 5022 (night blue).

Additional Information
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​Large heat pumps are particularly suitable for use in industry, in municipal infrastructure or in large residential, commercial and commercial buildings. While heat pumps for use in private residential and commercial buildings are now standard heating technology and the use of condensing technology is increasingly being pushed back, there is still great potential in the broad application of large heat pumps and industrial heat pumps in the power range from 200 kW to > 1,000 kW.

But their areas of application already show the way that the mostly with multi-stage compressors and high air volume, meet the quiet needs of the residents in the dense construction in cities.

Our modular sound hoods can help here, by combining several modules, practically all mooring sizes can be equipped with a sound hood.

The picture shows a sound hood for a heat pump with a heating capacity of 120kW and a cooling capacity of 315kW installed in our warehouse before shipping.

The system has 5 axial fans with an air volume at full load operation of 85,000 m3/h. The dimensions of the system are 3,600 x 2,250 x 2,320 mm (L x W x H).

The acoustic hood has the following dimensions: 5,000 x 3,300 x 3,100 mm (L x W x H). The acoustic hood is designed for an air flow rate of 92,000 m3/h and brings a noise reduction of 18 - 20 dB(A) at a distance of 1 meter, measured according to the enveloping surface method.

Outdoor swimming pools in the private sector are booming, which means that more and more pool heat pumps are being used.

These are then usually in the quiet zone around the pool and often also in the vicinity of the neighboring property.

Attention pool heat pumps, also known as quiet inverter heat pumps, cause noise like a heat pump that is used to heat the living space.

We are therefore experiencing a great demand for sound hoods (Acoustic Enclosure) for pool heat pumps and often also for pumps and filter systems.

With the use of a sound hood, the noise emissions of a pool heat pump can usually be reduced by between 16 and 20 dB(A). Since the color of the sound hoods can also be adapted to the environment and the roof can also be greened, this is another advantage when it comes to integration into the environment. ​

We are at your disposal for further information
info@silent-mode.com