Performance losses in HVAC systems due to the use of acoustic hoods

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.