Outdoor air: calculation and delivery

Calculation of equivalent outdoor air ventilation rates is important when comparing, designing or troubleshooting HVAC systems. Ventilation rates affect indoor air quality, energy use, plant design and cost. Simple calculation methods that do not consider distribution of outdoor air to the occupants, do not distinguish between good and poor distribution systems and are likely to result in unsatisfactory ventilation. In recognition of this ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality requires calculation of equivalent ventilation rates where the supply air rate and occupant density vary between zones. However, the method in the standard assumes that the supply to each zone has a common OA fraction, an assumption that is not valid for dual fan dual duct systems, fan powered terminal systems or systems with imperfect mixing of the supply air. A calculation method extending the existing ASHRAE formula to handle such systems and allowing for imperfect mixing within zones is presented in this article. This method is relatively simple to use and can provide meaningful results where traditional methods do not. This article also includes background on ventilation for acceptable indoor air quality, discussion of appropriate system parameters and suggests on how to achieve in practice, what ismore » conceived as theory.« less