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Improving Exhaust-to-Intake Dilution Calculations for Rooftop Exhaust Systems

Improving Exhaust-to-Intake Dilution Calculations for Rooftop Exhaust Systems 

From ASHRAE Journal Newsletter, June 9, 2020

Diluting exhaust from laboratories is important for the health of building occupants. The 2019 ASHRAE Handbook – HVAC Applications gives procedures for calculating dilution at a receptor of interest using plume theory and other empirical methods. However, this model may sometimes show overly conservative dilution predictions, according to some researchers.

In a recent Science and Technology for the Built Environment article, researchers identify improvements to the ASHRAE model through ASHRAE Research Project 1823. Researchers Jordan Clark, Ph.D., Member ASHRAE and Saba Zakeri Shahvari, Student Member ASHRAE, discuss the research. 

1. What is the significance of this research?

Proper dilution of exhaust from laboratories and other buildings is important for ensuring health and comfort of building occupants and of those in neighboring buildings. This is achieved through specification of both a fan and an exhaust stack located on the roof to ensure sufficient dilution at nearby points of interest. The ASHRAE Handbook gives simplified models for doing this. However, previous work showed these models sometimes led to overly conservative designs, requiring either unnecessarily high laboratory stacks, or an overly energy-intensive design due to overly conservative exhaust flow rates. In this work, we have identified improvements to the model that lead to less conservative designs while ensuring sufficient dilution, thus saving energy and costs.

2. Explain the steps of this research project. What did the process look like?

We first evaluated the existing ASHRAE Handbook model using published data to identify the situations in which the model leads to overly conservative designs. We then identified model parameters that can be improved and/or which lacked physical or empirical justification.

Finally, we attempted to make modifications to the model that both integrated additional physics and more precisely identified model parameters. We checked new model formulations against several published wind tunnel and full-scale measurement databases to ensure model performance.

3. Why is it important to explore this topic now?

Buildings with large ventilation requirements, such as laboratories, are among the most energy-intensive building types. While ventilation cannot be reduced through better dispersion modeling, significant fan turndown can be achieved with more precise and accurate calculations of dispersion from exhaust stacks. As many laboratories seek to achieve designations as high-performing, better exhaust design is critical.

4. What lessons, facts, and/or guidance can an engineer working in the field take away from this research?

In this research we have suggested several modifications to the ASHRAE model for laboratory stack design and fan sizing. Our improved model leads to less conservative designs compared to the current model, which can reduce the required fan power and/or stack height, saving the client energy and operating expenses.

5. How can this research further the industry's knowledge on this topic?

We identified several aspects of the description of exhaust plumes above buildings that can be improved, including for the height of the building recirculation zone, the initial plume spread and the plume trajectory. These assumptions were questioned in our research and some modifications were suggested. This new version of the model showed better performance in predicting the exhaust plume dilution.

6. Were there any surprises or unforeseen challenges for you when preparing this research?

We identified a few facets of the current model that were “grandfathered” in from work done quite a long time ago, including one assumption based on a single study from the 1930s. There are also several assumptions that were accurate for single tall stacks in isolation (for which the model was originally developed) but did not perform as well for shorter rooftop stacks.