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4.1 OA Measurement Challenges

Measurements of air speeds in AHUs often rely on Pitot-like sensors that determine the velocity pressure of moving air. The maximum speed of airflow through louvers is constrained because fast-moving air enables water drops to penetrate through the louver. The airspeeds downstream of louvers are even smaller than the air speeds within the airflow passages of louvers. Additionally, louver designs vary and the downstream airflow patterns will vary with the design of the louver, posing measurement challenges that vary with the type of louver used. 

Fisk et al. [18] estimated that maximum average air speeds downstream of three commercially-available louvers ranged from 0.8 to 2.9 m/s (150 to 580 fpm, see Table below). If the air handling unit (AHU) has an economizer, these maximum air speeds will occur when the air handler is supplying 100% outdoor air. When minimum outdoor air is being supplied, which is when outdoor airflow measurements are most needed, the air speeds downstream of louvers are lower, often 80% lower. These low air speeds are difficult to measure accurately. 

Table. Air velocities and corresponding pressure inside and up/downstream of 3 example intake louvers.
 Example Louver 1 (free area = 31% of nominal louver area) fpm m/s IWG    Pa
 Maximum recommended free area velocity 1856 9.4  0.21 53 
 Velocity up/downstream of louver at maximum velocity 575 2.9  0.021  5.1
 Minimum recommended free area velocity
 371 1.9 0.009 2.1
 Velocity up/downstream of louver at minimum velocity 115 0.6 0.0008 0.2
 Example Louver 2 (free area = 31% of nominal louver area) 
 Maximum recommended free area velocity 500 2.5  0.015  3.7
 Velocity up/downstream of louver at maximum velocity 155 0.8 0.001 0.4
 Minimum recommended free area velocity 100 0.5 0.0006 0.2
 Velocity up/downstream of louver at minimum velocity 31 0.2 0.00006 0.01
 Example Louver 3 (free area = 44% of nominal louver area)    
 Maximum recommended free area velocity 696 3.5 0.03 7.5
 Velocity up/downstream of louver at maximum velocity 306 1.6 0.006 1.5
 Minimum recommended free area velocity 139 0.7 0.001 0.3
 Velocity up/downstream of louver at minimum velocity 61 0.3 0.0002 0.06
*Minimum velocity assumed equal to 20% of the maximum velocity, i.e., assuming 20% minimum outside air.

Because many pressure transducers marketed for commercial HVAC applications cannot accurately measure these low velocity pressures, some OAMT systems use electronic air velocity sensors to more accurately measure lower air speeds. However, with use of electronic velocity sensors, the complex airflow patterns continue to make accurate measurements of outdoor air flow rates very challenging.

Outdoor air intake hoods are often used in packaged roof-top HVAC systems. The air intake hoods serve the same function as louvers - they limit moisture entry into the air handler. As with louvers, air speeds inside air intake hoods are limited to minimize moisture entry. The ASHRAE minimum ventilations standard [2] specifies that air speeds should not exceed 2.5 m/s (500 fpm). If the air handler has an economizer, air speeds when minimum outside air is supplied will be much lower, often 80% lower, resulting in air speeds of 0.5 m/s (100 fpm). Accurate measurements of these low air speeds is challenging. At 0.5 m/s, the velocity pressure is only 0.15 Pa (0.006 inches of water, IWG) which is too small for accurate measurement with the pressure transducers used with air handling systems. 

Hood geometry can also force changes in airflow direction which, in-turn, lead to complex airflow profiles and uneven air speeds. Of the two hood designs illustrated here, the top hood forces a larger change in airflow direction.