Every good HVAC (Heating, Ventilation and Air Conditioning) system design must be able to ensure a proper air renovation inside the building it is installed at the lowest possible cost. This objetive is reached through the correct positioning and orientation of the supply and extraction air systems, and an adequate value of air changes per hour, also called air change rate. The use of CFD to assess and optimize the installation’s design is paramount in order to reach the best results.
There are different measurement parameters that allow the evaluation of the ventilation system quality:
First of all, the above-mentioned number of air changes per hour (ACPH) in the building. It is defined as the rate of the air volume added to a finite space in one hour divided by that spaces volume. If a perfectly mixed model is considered, the air renovations per hour parameter is a measure of how many times the air in this space is replaced each hour.
This parameter, despite being useful, may not represent completely how renewed is the air inside the space, since the disposal of the ventilation systems, as well as the elements inside the space, can generate flow patterns that make some areas be under ventilated. For this reason, it is also useful using the concept of age of air.
The local mean age of air is defined as the time that particles contained in a differential volume around a point (as is in the case in a cell of a CFD simulation) have been inside the space.
If it is assumed that the age at the inlets is 0 (which equals to say that pumped air is completely new), this parameter evaluates the residence time that particles spend in the building from they enter until leaving.
This value does help assessing whether areas are renewing the air more frequently, since the residence time in them is lower. From this value, it is possible to obtain the mean age of air (MAA) in the building, which is calculated as the average of the local mean age for each point in the space.
The efficiency (ε) of the ventilation system is defined as the ratio between the minimum time that a particle spends in the space from the input to the output, and twice the mean age of air. This concept allows to seek a balance between a good air quality and a reduced air flow.
In addition, the evaluation of the minimum residence time enables the possibility of tracking for short-circuits of new air in the system. Along with this parameter and assuming a complete mixing model it can be stablished that:
- The optimal value is obtained at ε=50%
- For ε<50%, it can be considered that the space is lack of ventilation, or that the minimum residence time is too low, indicating a possible air short-circuit
- For ε>50%, it can be considered that the air Flow ratio is over dimensioned and it is advisable to decrease it
CFD techniques allow not only to determine the age of air in a room. It is also possible to introduce pollutant emission sources, chemical reactions and thermal effects in the models. This way, the pollutant concentration in the area of study can be analyzed. This measure can provide an even more valuable information than the age of air in terms of predicting more accurately which areas of the room are not being properly renovated, and optimize the HVAC installation from the pre-design stage.
The concentration of a chemical compound in a large volume of air is often measured in milligrams of compound per cubic meter of air (in some cases per kilograms) giving a unit of particles per million (ppm).
To put this in perspective, let’s consider a Waste Water Treatment Plant (WWTP). These facilities are attached to a series of emissions of contaminants, perhaps the more characteristic being the hydrogen sulfide (H2S), produced during the decomposition of some amino acids, as well as the reduction of sulfates to sulphites by certain microorganisms.
Hydrogen sulfide is a colorless, flammable gas with an atomic weight of 34 g/mol and a density of about 1.5 kg/m3 at ambient conditions, being slightly heavier than air, tending to accumulate near the ground. Its odor is very unpleasant and it is associated to “rotten eggs”.
The limit of perceptibility for human beings is 0.02 ppm, with some people being able to detect it at 0.0005 ppm. At higher concentrations can result toxic, produce metabolic changes and even cause death. In addition to this, it is one of the main causes of corrosion in this type of facilities, attacking in moist ambiences to iron and concrete with ease.
The dispersion effect that the air impulsion from the HVAC system can cause over these particles can be properly analyzed with a CFD model, along with the study of alternatives in the flow inputs, system location and effect of the different sources. The SDEA_Engineering team presents a broad experience in the HVAC and Computational Fluid Dynamics field and can help in the design assessment and study of the HVAC system.
