Covid-19 is still not entirely understood, in particular the way it spreads indoors through airborne micro-droplets (aerosol spread). However, in recent months IN2 have considered and adopted ventilation design proposals based on sound fundamental principles for our clients to mitigate the risk of Covid-19 transmission should it be identified to spread in this manner. This can be achieved through the design of ventilation systems to maximise freshness and minimise recirculation of contaminants.
We at IN2 have combined three fundamental principles of design in our approach to promoting safer indoor spaces for all.
1. Natural Ventilation to Maxamise Fresh Air
It is generally established that the risk of Covid-19 transmission is lower in outdoor spaces. Outside the fresh air can dilute viral micro-droplets rapidly, reducing the likelihood of these being breathed in by others. To prevent the stagnation of potential viral aerosol spray, indoor spaces require the extent of Natural Ventilation through opening windows to be maximised where comfort conditions allow, to provide a large turnover of air.
This can be achieved through the implementation of a Mixed Mode ventilation strategy where windows are automated to open during spring, mild summer and autumn conditions, with supplementary mechanical ventilation, only provided during cold winter, warm summer, or inclement windy weather conditions where excessive discomfort would occur.
The graph below indicates how determined CO² levels would be particularly good during periods when windows were predicted to be open, primarily between the months of May and September. From this, we can expect a correlation between Air Quality and contamination build-up.
Fig 1: Predicted Indoor Air Quality (IAQ) in Carbon Dioxide (CO2) levels for an office building operating under Mixed-Mode strategy.
2. Displacement Ventilation to Minimise Air Mixing
Fig 2: Predicted Contaminant Spread – Conventional (Mixing) Ventilation
In contrast to the conventional method of Mixed-Air ventilation, Displacement based systems offer an improved method of mechanical ventilation to minimise the risk of potential spread of Covid-19.
Figure 2 demonstrates the predicted contaminant spread from the person seated on the left-hand side for conventional Mixed-Air ventilation. Here the air is supplied at a high level and relatively high velocity.
However, the Mixed-Air ventilation design creates a circulating effect through the entire space, with contaminants entrained to the supply air jet. Whilst any pollutants or contaminants would become diluted by the fresh air component of the mechanical ventilation system, these would also be mixed throughout the entire space presenting a risk of spread.
In contrast Displacement Ventilation involves the supply of air at low velocity and low level.
Fig 3: Predicted Contaminant Spread – Displacement Ventilation
Here Displacement Ventilation causes supply air to “spill” across the floor, with any heat sources (occupants/computers etc.) providing plumes of hot air upwards, displacing any contaminated air upwards with the cool, fresh air from below.
A simulation of Displacement Ventilation shows the person on the right-hand side facing the source of contamination is predicted to be at a lower risk of any biological agent spread in comparison to the Mixed-Air configuration. With Displacement Ventilation aerosols would predominantly rise without mixing and then extracted at a high level, with the lower occupied zone remaining generally clear.
Displacement ventilation is a well-established design approach. IN2 Engineering has extensive experience of successfully implementing in a wide range of building applications on major projects such as TCD School of Business, Lyric Theatre Belfast, O’Donoghue Centre for Drama, Theatre and Performance, and NUI Galway etc.
3. Exhaust Air to Prevent Cross-Contamination
Finally, the plant components of the mechanical ventilation systems would be designed to ensure no potential risk of cross-contamination of Exhaust Air with Fresh Air, either externally or within Air Handling Units (AHU’s).
Adequate separation of Exhaust and Fresh Air Louvres would be ensured and verified by airflow simulations. The potential of cross-contamination within AHU’s would be eliminated by providing plate heat exchangers for heat recovery or run-around coil configurations, where fresh and exhaust air paths are entirely separated, with mixing boxes and thermal wheels avoided.