- Model Update
According to the feedback from the pervious check-in session, some updates are made to my model. The width of timbers in my beam system is too large, so I narrow them to 12×24 timber beams. My section leader also suggest that I can remove the columns next to the structural walls, and apply wall foundations under my structural wall. That is reasonbale, but I think it’s not a big problem, and the columns are symmetry for aesthetic purpose.
- HVAC System Conceptual Design
At the very early stage of this module, I need to confirm my fundamental Heating and Cooling strategies, so that I can have a better understanding and control of my mechanical model. Call back to the previous module4 check in session, we have some building site exploration:
My center is located at Beijing, which has an unique climate mode. Beijing is cold in winter and hot in summer, so I need both positive and negative heating and cooling design. In addition, there are enough winds in Beijing, especially in Spring, autumn and winter. Thus the normal ventilation is not needed in my center, all air flow should be for cooling and heating purpose. It is necessary to mention that water resource is lack here in Beijing (since it’s super dry), so I would only apply air cooling and heating strategies instead of the water ones.
The Climate Consultant 6.0 is applied to do a thorough research on the local limate. In the temperature range figure, we can find that Beijing is pretty cold in winter(15°F -9.4°C mean in Jan) and hot in summer(80°F 26.7°C mean in July). Thus we need to design a powerful and efficient HVAC system to make the temperature more comfortable. In the psychromatric chart, we find that the place is 6.7% naturally comfortable. Plenty strategies,(993 hours of sun shading of windows, 1357 hours of cooling and dehumidification, 3756 hours of heating and humidification…) are applied. Since Beijing is cold in winter and hot in summer, we need 15.5% of active cooling and 42.9% of active heating. Heating is a bigger problem needs to be solved.
- Overall HVAC strategies
- What type of heating and cooling loads? How much?
So at this point, let’s jump into some details for the overall HVAC strategies. For internal loads, most of the rooms are for regular function and purpose(ex: exhibition, restroom, office, meeting/confernce room, cafe…), so the internal load is low(or normal) and we do not need much for this part. For external loads, as mentioned before, since the temperature here in Beijing is relatively extreme, the external load is high and need further strategies to reduce it.
For our climate, my center has more of a heating problem (42.9% need to warm the spaces to keep them comfortable) compared with a cooling problem (15.5%need to cool the spaces).
Regardless of what strategy you use for heating and cooling, I still need to provide adequate ventilation to meet the minimum fresh air levels required. As mentoned before, I can use natural ventilation throughout much of the year, but there will likely be at least a few times where the ventilation will need to be supplied mechanically by my HVAC system.
- What type of heating systems can I use?
My center has a huge amount of curtain walls all around the main building. This passive design feature could possibly capture pretty much natural heat and meet part of my heating requirements during the warming months. My center also has shading systems on my curtain wall to prevent excess unwanted heat gain during the cooling months.
As mentioned before, the water resource here in Beijing is limited, so I choose air-based ducted heating system for my center.
- What type of cooling systems can I use?
We have the same shading system taht can shield my building from the solar heat using passive design features to reduce the cooling load during the cooling months. It is not applicable to use natural ventilation, air movement, or water features (evaporation) to provide some of the cooling naturally, according to the local climate situation.
As mentioned before, the water resource here in Beijing is limited, so I choose air-based ducted cooling system for my center.
- Create the Mechanical Model
The mechanical model is created, and the architectural model is linked to the mechanical model. The rooms are copied, and the space types and condition types are set properly. Since I apply both cooling and heating strategies in my center, I set most rooms to cooling and heating, except for the stairways, which are set as unconditioned.
Then I check the number of people expected in each space, and find if they are all reasonable. The number of the people in the exhibition is pretty high, but I would take it as a conservative design instead of a problem to be solved. The only one that looks weird is the cafe, which has an estimate of 160.975 people. I change it manually to 70.12.
After that I assign two zones(public space and functional space) base on their functionalities. I did not choose the service type here since I did not find any one fit properly.
- Calculate the HVAC loads
After the mechanical model is created, I need to figure out how much HVAC loads(cooling and heating) are needed base on the analysis. Firstly, I set up the location as usual:
After that, I define the chematic properties of my building in the energy settings, and select options that are as close to my design as possible:
Then I run the energy analysis and get my analytical spaces model and analytical surfaces model:
I also add an Air Handling System, add a piece of Zone Equipment, and use the System-Zones tool to specify which Spaces to assign to each piece of Zone Equipment.
At this time, I am supposed to ran the energy model and get the detailed report for further HVAC cooling and heating loads data. However, I ran my model for 10+hours and there was no response. I asked Professor Glenn for help, and he told me that revit might be “confused “ about the mdoel, and that’s why it took that long time. Professor Glenn then mentioned that:
“But in the meantime, if you'd like to get started on placing air terminals and routing the ducts to them. you can approximate the required CFM by using a simple rule-of-thumb.
Try this... take the number of people calculated for each space and multiply this by 40 to estimate the CFM required to cool the heat created by each of the people. You can enter this as the Specified Supply Air Flow in your Space Schedule and start adding air terminal to meet this need.
So, for example, your Level 1 Cafe is estimated to hold 70 people. Multiply this by 40 to compute 2800 CFM required. Then, provide air terminals to meet this need.”
Thus I calculated the specified supply air flow for each space, and assigned them to each room.
- Model the HVAC system elements
First we need to figure out the upper bound of the mechanical system, which is the lower bound of the structural system:
As shown in the figure above, the lower bound of the beams is 13’ above the floor level. Thus the ducts should not be higher than this. I would leave 24” for the maximum height of the ducts, so the height of the air diffusers are also set as 11’ above the floor. I would consider to use the ceiling, and this would be designed in the final part according to the situation.
The size of the ducts are all set to be 22”×22”. I tried my best to apply the duct sizing tool to determine the exact size, but there always exist some errors. Thus I use this approximate size in my model. The following as some figures that show my mechanical system：