DILUTE: AIR FLOW MEASURING STATION (AFMS)
A dilution strategy centers on the practice of increasing the volume of outside air, as a portion of the air delivered to the space and do so at an increased rate of air changes over time. By doing so, the density of airborne virus is greatly lessened as fresh air is introduced and recycled air is exhausted at an increased cycle.
Challenge #1: Seasonal Outdoor Air Volumes: Typically, minimum ventilation requirements occur during warmer and cooler outdoor air conditions. Doing so, lessens the load on mechanical heating and cooling support systems. There are limitations on these mechanical systems impacting their ability to fully perform outside their design conditions. (See below graph)
Challenge #2: Air Distribution Restrictions & Challenges: The ability to increase air volumes through an air distribution system is limited by:
Fan motor capacity
Air blower capacity
Distribution devices (registers and diffusers) capacity
Ignoring these items will create havoc within the system. Variable Air Volume (VAV) systems further complicate the situation. VAV systems vary the volume of conditioned (55 degree) air based on load conditions. Individual spaces are served by VAV (air regulating) boxes. Different sized boxes serve different sized spaces. Design maximum airflow is delivered under peak load conditions (summer full occupancy) and regulates down to minimum airflow as load conditions decrease. During winter months, the VAV boxes regulate to minimum airflow, a reheat coil activates heating conditioned air to warm the space. It is under this winter condition, where VAV boxes are at their minimum design flow, that the overall air ventilation rate is at its lowest.
Challenge #3: Daily Operation: During normal, occupied conditions, outside air is utilized to meet minimum ventilation requirements and minimize its impact on mechanical systems. System operation is typically determined by a weekly occupied/unoccupied schedule where systems are enabled and disabled, based on time of day, without regard to actual percentage of occupancy. Many buildings operate under an access control system, ensuring credentialed occupants enter and exit, at times limiting access to certain restricted areas. A building automation system (BAS) operates HVAC systems maintaining environmental design condition. Why are these systems independent?
A Practical Approach: Total Building Technology, bringing all building systems together under a Single Pane of Glass(link) bringing user visibility and system interoperability to deliver a dilution strategy based on occupancy data:
Determine the full capacity of the installed mechanical systems.
Develop an integration strategy of all building systems
Utilize an access system to determine % occupancy.
Direct the dilution strategy where the occupants are located
Measure and archive airflow data
WHY USE AN AIR FLOW MEASURING STATION?
Thermal Dispersion Air Flow Measuring devices offer the highest degree of airflow measuring accuracy from 5000 feet per minute down to zero feet per minute! Thermal Dispersion is the most accurate and stable method of measuring air flow at low flow (Winter) conditions when air measuring is most critical.
The Ruskin (a Johnson Controls brand) thermal dispersion air measuring devices not only are the most accurate, they also:
Provide the least flow resistance with their aerodynamic design
Easily integrate into building systems providing industry standard (BACNet) communications
Provide both air temperature and flow data