Recently a graduate student asked me for some help on using the NF-FF model when local exhaust ventilation is present. Since this was for a student, I decided to point out considerations but not give an answer. Perhaps some of this will be helpful to others.
- In general, the local exhaust ventilation Q(local) is a minor contributor to the total room ventilation Q(Total). However, this depends on the absolute quantities of each. For example, If Q(local) is >10% of Q(Total) then Q(Local) will have some effect on the Far Field concentration.
- In NF/FF modeling, it is usually Beta, the near field, far field air exchange rate that most affects the NF concentration. In general, Q(Total) does not do a lot until the generation ceases, and room purge begins. However, this again depends n the value of Beta compared to Q(Total)
- What is the “capture efficiency” of the local exhaust ventilation system? Review the ACGIH Industrial Ventilation book, https://www.acgih.org/docs/default-source/industrial-ventilation-manual/errata_listing_29th-edition-(081318).pdf?sfvrsn=e005bc0d_4
If you can estimate the local exhaust ventilation efficiency, you can reduce your generation rate value accordingly. - The size of the near field space assumption, the random air velocity in the Beta calculation and the quantity of air exhausted by the local exhaust system need to be considered. The Q(Local) should be quite a bit smaller than the total air exchange rate across the NF-FF boundary. for the model results to have any relevance.
- Dr. Paul Hewett and Dr. Gary Ganser developed NF-FF model equations that explicitly consider local exhaust ventilation. “Models for nearly every occasion: Part II – Two box models”, January 2017, Journal of Occupational and Environmental Hygiene 14(1):58-71, DOI: 10.1080/15459624.2016.1213393. Note that Paul offers software that implements this work in a free “student” version. https://www.easinc.co/
Please feel free to add discussion about this.