Acoustic bonnet for a dry cooler

Initial situation
As part of the refurbishment of the first floor of the Bubenberghaus on Schanzenstrasse in Bern, the cooling system is also being replaced and extended. The capacity of the cooling system was increased compared to the previous system, which meant that the precautionary noise values were not met. The recooler is used to cool the entire building, but in particular the MRI systems of the radiology centre located in the building. Noise emissions must be reduced by at least 14 dB(A). This is achieved by using the installed acoustic bonnet.

The recooler has the following dimensions: 7,646 x 2,420 x 2918 mm (L x W x H). The system has 12 EC fans. The total air volume of the system at full load is 220,000 m3/h.

Sound enclosure concept
The basic structure of the sound enclosure consists of 6 modules of an aluminium plug-in profile frame that are connected to each other in place. The service doors around the system were integrated into the profile frames. The inside of the doors is lined with 40 mm ‘StratocellWhisper’ insulation. The acoustic bonnet was built from 6 modules that were simply mounted on top of and next to each other and statically reinforced. The air chambers between the supply and exhaust air are hermetically separated by the use of separating panels that run in a rail system of aluminium U-profiles. The panels can be opened at some points so that access to the fans is possible at any time.

The assembled modules have the following dimensions:
9,446 x 4,100 x 3,618 mm (L x W x H) with a weight of 2,040 kg.

The air outlet speed at full load is around 10.2 metres/sec.

In terms of energy, it is interesting to compare the ambient air temperature with the temperature of the air entering the condenser. Despite the relatively high air inlet velocity, this is around 5.4° Celsius lower than the air temperature outside the bonnet.

The reason for this is obviously the shading of the condenser on all sides in combination with the movement of the air, which leads to this relatively high cooling of the air as it enters the condenser. This will also have a positive effect on the cooling capacity and power consumption of the dry cooler.