A (mechanical) ventilation system is necessary to obtain a good indoor climate in classrooms. The ventilation system ensures a sufficient supply of fresh air, so that, for example, CO2 concentrations remain below an acceptable threshold. High CO2 concentrations have a adverse effect on the learning performance of the students, but also on the health of the students and teaching staff.
However, the presence of a ventilation system that supplies sufficient fresh air does not automatically mean that there is a good indoor climate. For example, drafts, due to a poorly designed system or cold air draft near windows, can be an unpleasant experience. It is also conceivable that large differences in air quality can arise in the classroom because certain parts are not or hardly reached by the fresh air.
CFD simulations are particularly suitable for mapping the local differences in air quality before a system is actually installed. As a demonstration, this page shows the difference between two (almost identical) ventilation systems in a classroom by means of CFD results.
The examined classroom is ventilated by an air distribution sock that is suspended from the ceiling over the entire width of the room. The calculation is based on a full occupancy of 24 students and a teacher. In the simulations, these produce heat and CO2. Other heat sources such as a PC, digital whiteboard and lighting are also included for the sake of completeness.
The aim of this study is to calculate the influence of the position of the air exhaust on the indoor climate in the classroom. The following requirements are taken into account:
• Air speed < 0,20 m/s
• Draught Rate < 20 %
• Floor temperature betwee 19 and 26 °C
• Vertical temperature gradient < 3K/m
• Concentration CO2 < 950 PPM
The CFD model is based on a supplied architectural 3D model (Revit). The study compares two possible positions of the air exhaust. In design 1, the air exhaust is divided over two grilles in the wall. In design 2, the air exhaust is an opening in the suspended ceiling.
There is a relatively high velocity region (greater than 0.5 m/s) close to the sock at the ceiling, but not in the occupied zone for both designs. In both designs, the air speed is generally less than 0.2 m/s. There are regions where the air speed is greater than 0.2 m/s. To determine whether this is experienced as uncomfortable, it is better to look at the Draugth Rate.
The “gray cloud” in the figures shows the area where the Draft Rate is 20%. For both situations the Draugth Rate is greater than 20% close to the distribution sock. It is also greater than 20% for a column because some holes in the air sock blow directly against the column. None of these places are in the living zone.
The vertical temperature gradient is smaller than 3 K/m for both situations.
The average room temperature with the air exhaust in the wall is 24.4° C. In the situation with the air exhaust in the ceiling, the average room temperature is 23.3° C. This difference is due to the fact that the air from the air distribution sock in the situation with the outlet in the wall partly short-circuits directly to the outlet. The exhaust in the ceiling is also close to the window and the heat load by the pupils is centered on the same side. As a result, heat is dissipated more effectively in this situation.
The floor temperature is generally greater than 20° C and for the most part less than 26° C for both situations.
The average concentration of CO2 at eye level is 1000 ppm for the situation with the exhaust in the wall and 900 ppm for the situation with the exhaust in the ceiling. This difference is due to the fact that the air from the air distribution sock in the situation with the outlet in the wall partly short-circuits directly to the outlet. The students are also on the same side as the ceiling drain. As a result, CO2 is removed more effectively in this situation. The requirement is that the concentration in the occupied zone is a maximum of 950 ppm. Only the design with the outlet in the ceiling therefore complies satisfies this criterion.
Additionally, the ventilation effectiveness has been calculated for both designs according to the following equation:
The ventilation effectiveness is 0.89 for exhaust in the wall and 1.05 for exhaust in the ceiling. This is higher than 1 because the average concentration of CO2 at the exhaust is higher than that on the surface at eye level.
The situation with the air exhaust in the ceiling complies with the set criteria. However, the situation with the air exhaust in the wall does not comply as the average concentration of CO2 in the occupied zone is higher than the required 950 ppm.
The analysis shows that a good design of a ventilation system is important and that an apparently small change in the design can have a major impact on the indoor climate. CFD simulations are a useful tool for testing ventilation systems and for comparing different ventilation designs at an early stage.