Written by Angirekula Venu
Wednesday, November 23, 2022
Enhance the IAQ of your space with Displacement ventilation system
By
Angirekula Venu
Proper HVAC system plays a vital role in indoor air quality by providing safe and comfortable conditions in both residential and commercial settings. The idea is to provide all room occupants with the best conditions while complying with the set regulations.
In recent years, the world has seen tremendous growth in construction and infrastructure. Architects have worked tirelessly to give us modern buildings and the newest skyscrapers. But there has always been the question about indoor air quality (IAQ) and Thermal comfort in indoor spaces. How do you get these building designs correct in the pre-construction stage? That’s where Autonomous HVAC CFD - AHC comes in! Today you can get accurate designs from the robust CFD simulations way before the contractor pours the first concrete batch.
So, as we are discussing IAQ & Thermal comfort in building design, let’s first understand how to choose which type of ventilation system is suitable for the layout.
Do You Really Require a Ventilation system?
If the room is ventilated naturally and meets thermal comfort and air quality standards, there will be no need for a ventilation system. However, obtaining this type of conditions is very rare, and most building spaces require mechanical ventilation systems to,
- Deliver the designed flow rate to control the indoor air temperature, humidity, and air quality.
- Keep the CO2 levels safe by providing the required outdoor air mixing.
- Prevent mold growth by maintaining healthy indoor relative humidity levels.
- Keep occupants safe from indoor contaminants like Radon, VOC, PM, etc.
- Filter harmful microorganisms and particulates.
Achieving Proper Ventilation
The quality of a ventilation system is measured by air velocity, humidity, temperature, gas, and pollutant(s) concentration. We can use natural ventilation when outside temperatures aren’t a concern (friendly weather). Whereas mechanical ventilation can be helpful in adverse outdoor temperatures (above/below standard temperatures). However, these two ventilation types can also be used concurrently in a hybrid mode. Different ventilation systems are defined in mechanical ventilation based on the location and properties of the supply air. Let’s discuss two major systems in detail now.
Mixing Ventilation
Mixing ventilation or traditional mixing ventilation uses a basic ventilation principle. This type of ventilation controls indoor contaminants density by mixing adequate outdoor air to dilute indoor emissions uniformly. Here the heavier cold air is supplied near the ceiling with high velocities, which will induce the mixing effect.
- Easy to design
- Need lower supply temperatures (~13 ℃).
- Need higher fan speed.
- Low acoustic performance
- Low ventilation effectiveness
Displacement Ventilation
This ventilation method introduces air through diffusers and floor terminals at very low velocities. Just like water floods a room, the cold air will behave the same when it has low momentum. Once this air contacts the heat sources, it will rise and pass through the occupied area until it is exhausted through return placed near the ceiling.
Since the air displacement units are usually installed at a low level, a vertical temperature gradient will exist between the ceiling and the floor. That means that the properties of air supplied will depend on the room's height due to the ceiling and floor gradient.
One more thing to keep in mind while designing this type of system is to place the diffuser at least 2 ft away from the occupants to prevent draft effects.
- Low fan speed
- Better acoustic performance
- Need moderate supply temperatures [>17 ℃].
- Diffusers can be blended into walls and columns - Aesthetically appealing.
Case study
Let’s dissect the two ventilation methods with a case study, shall we?
better understand the two methods, here is a study conducted in a space with the following conditions:
better understand the two methods, here is a study conducted in a space with the following conditions:
- Room dimensions = 7.0 X 5.8 X 3.05 m3
- Occupancy = 4
- Occupant CO2 flow rate = 0.026 m3/h
- Initial CO2 level = 400 ppm
| Mixing Ventilation | Displacement ventilation | |
|---|---|---|
| Supply flow rate [CFM] | 292 | 292 |
| Supply velocity [m/s] | 8.0 | 0.13 |
| Supply Temperature [℃] | ~13 | 17 |
Supply diffuser details for both Mixing and Displacement ventilation systems.
Geometry Setup
(a) Mixing ventilation
(b) Displacement ventilation
Figure 1: Geometry and airside system design of Mixing and Displacement ventilation systems.
The above figure shows the office room with occupant seating arrangement, work setup, TV, and locker, along with the air side system layout of mixing and displacement ventilation systems. For the mixing ventilation (MV) system, two square plaque diffusers and one return are placed at ceiling level. Whereas, for the displacement ventilation (DV) system, the supply diffuser is located in the middle of the left sidewall near the floor. A grill-type return is placed on the ceiling directly above the occupants to purge the contaminants.
The CFD stages, like geometry creation, generating the proper mesh and solving the momentum, heat, and contaminant transport equations, are taken care of autonomously by the AHC app. So, let’s dive into the results to see how both systems performed in terms of occupant comfort and indoor air quality.
(a) Mixing ventilation
(b) Displacement ventilation
Figure 2: Temperature distribution in MV and DV systems represented by a vertical contour.
Temperature Contours
Low momentum air supply near ground level in the displacement ventilation method creates a stale environment in the occupied zone. This results in layers of air at different temperatures stacking one over another commonly referred to as 'thermal stratification'. The results section shows that this phenomenon is captured in the DV system by AHC. While in MV systems, due to high mixing in the room, the air in the occupied zone is mostly uniform and close to 24 ℃.
Though the DV system ran with a higher supply temperature than the mixing, the occupants in both scenarios were comfortable at 24 ℃. This highlights the potential economic advantage of installing the DV system in your space. It shows that displacement systems can operate with low compressor load and still achieve a comfortable environment for occupants.
Streamlines
A better representation of flow from the supply diffusers can be seen from the streamlines. The following figure shows the streamlines for both Mixing and Displacement systems. In Mixing systems, due to high spread, cold air hits the walls, drops to the floor, gets heated, and rises again. This results in recirculation and mixing in the occupied zone. On the other hand, the low momentum supply air spreads on the floor for DV systems, as seen in Fig. 3b.
(a) Mixing ventilation
(b) Displacement ventilation
Figure 3: Streamlines of MV and DV systems
Carbon Dioxide (CO2) levels in space
(a) Mixing ventilation
(b) Displacement ventilation
Figure 4: CO2 contours for MV and DV systems
Not only do DV systems have potential economic benefits, but they also provide better Indoor air quality. They do this by separating the contaminants like CO2, VOC, and other particulates away from the occupied zone. In the current setup, the four human models are emitting CO2 at a constant rate. Thereafter, CO2 is transported from the source to the exhaust by the surrounding air movement. In the DV system, CO2 is carried away by the thermal plume generated by the manikins. Whereas in the MV system, the turbulence in the occupied zone causes the CO2 to get diluted by incoming air, which results in a mostly uniform CO2 concentration.
The following column chart presents the percentage of occupied zone volume inhabited by different levels of CO2. It clearly shows that the DV system has clean air in comparison with the MV.
(a) Mixing ventilation
(b) Displacement ventilation
Figure 5: Percentage of volume by different levels of CO2 in the occupied zone
Cross-contamination in space
The COVID-19 pandemic highlighted the importance of preventing cross-contamination between occupants in crowded spaces like shopping malls, movie theater community halls, and hospitals. Let’s discuss how proper selection and design of the ventilation system can answer this problem. AHC also generates CO2 clouds based on hazard levels. According to ASHRAE 62.1, it is recommended to keep CO2 in space below 1000 ppm for a healthy environment. The following figure shows the cloud generated for CO2 > 1000 ppm. It shows that in the displacement system, exhaled CO2 is directed toward the ceiling. Both ventilation systems are successful in maintaining the space CO2 within acceptable levels. However, in the MV system, we can observe cross contamination ( CO2 clouds from one occupant affecting another) occurring between two occupants due to the strong air current present in the occupied zone.
(a) Mixing ventilation
(b) Displacement ventilation
Figure 6 : High concentration CO2 clouds for MV and DV systems
simulationHub Will Help You Assess the IAQ and Comfort of the Ventilation System Design
Designing ventilation systems for buildings can be a daunting task, especially when there are different variables involved. simulationHub is here to ensure that you don't sweat it - it's time to interact with the Autonomous HVAC CFD app!
The simulations are performed on the cloud, and the results can be viewed on the web browser. The app provides vital information like Predicted Mean Vote(PMV), Percentage of People Dissatisfied (PPD), Relative Humidity, Air Velocity, Temperature, Mean Radiant Temperature of Air, Mean Age of Air, and CO2 levels for all the conditioned space.
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