Installing an acoustic enclosure for an R290 refrigeration unit on a flat roof
Installation of an acoustic enclosure for a refrigeration unit using R290 on a flat roof. At full load the unit requires an air volume of 50,000 m³/h. During installation, ambient temperatures on the roof reached around 39 °C.
During a site visit, the client asked whether the planned free areas for air inlet and air outlet are large enough for the required air volume even at these high temperatures.
The answer
High air temperatures reduce the effectively usable free ventilation area. The reason is the lower air density: pressure differences become smaller and the volume flow per opening area decreases.
The change in air density between 20 °C and 35 °C results in a reduction of the volume flow per opening area of around 5 %. To cover this effect as well as additional convective influences in warm, humid summer air, we size the free ventilation area of refrigeration units with a 7 % allowance over the design at 20 °C.
Sizing the free ventilation area
- Required volume flow at full load: 50,000 m³/h
- Assumed mean air velocity: 5 m/s
- Reference conditions: 20 °C and 1013 hPa
- Required free opening area at reference conditions: approx. 2.8 m²
- Allowance for higher outdoor air temperatures: 7 %
- Resulting design area: approx. 3.0 m²
This allowance accounts for the reduced air density at high outdoor temperatures and helps prevent high-pressure faults in the refrigerant circuit.
Planning note
Ventilation must be routed directly to the outside. Additional requirements from EN 378 and the respective national guidelines for flammable refrigerants must be considered separately – for example requirements for emergency ventilation in the event of a leak and for the minimum air change rate in the machinery room.
Originally published in German on silent-mode.com