Minimum ventilation is necessary to ensure sufficient oxygen is available inside the barn and to remove harmful substances. Because of this, it is very important that the minimum speed settings are correct. In this blog we will explain how to make sure the minimum ventilation is sufficient in the two most common systems.
We will use the ’Fan Laws’ as basis for this explanation. Depending on the ventilation system, you can save a lot of energy by following these rules and using a frequency drive.
We will now explain the two most common systems:
Ventilation system without controlled inlets:
This is a tunnel ventilation system, without controlled inlets and thus a changing static pressure depending on the airflow.
This means that the highest static pressure (∆Pstat) is at the same level as the maximum airflow. The pressure will go down quadratic with the airflow.
An example: the airflow on maximum ventilation is 100% and the static pressure (∆Pstat) = 30 Pascal (sum of resistance from inlet, friction of walls and surface, pad cooling, light traps etc.).
If this ventilation amount goes down to 20 % (in the night because the temperature drops) the total ∆Pstat will drop to (20/100) ^2 *30 Pa = 1.2 Pa.
That means the new air curve at the lower speed should reach at least 1.2 Pa. We recommend to use, for safety, the speed where the fan curve reaches at least 20 Pa. In our graph the 30Hz line.
Controlled ventilation system:
Buildings with controlled air inlets. In these buildings, the ∆Pstat is constant on a certain level. When the demand of air is going down, the inlet opening will get smaller to keep the inlet speed and so ∆Pstat the same.
An example: The maximum airflow is 100% and the inlets make sure that the static pressure in the building is 20Pa (adjustable in the computer setup).
During the night, the temperature goes down and the animals only need 30% of the air amount.
The inlet opening will adjust to a smaller surface, belonging to that air amount, and the fan will slow down until the ∆Pstat is 20Pa.
That means you can use the curves, which reaches 20 Pa. We recommend using an extra 10 Pa to be certain it is sufficient. In this case, you would only use the curve that reaches 30Pa. In our graph, this is the 35Hz line.
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