In the past, a common method to measure ventilation was to determine the difference between return air and supply air measuring stations. This difference was the approximate amount of outside air introduced into the building.
However, if each airflow-measuring station in the supply and return air was 3-percent accurate, an error rate of 6 percent for the total system air could result.
For example, if there is 100,000-cfm supply air system with a 6-percent error rate (or 6,000 cfm) compared to the requirement of 15,000 cfm outside air, the total error of the outside air system is 40 percent. This demonstrates the driving trend toward today’s more preferred system: direct measurement of outside air. By directly measuring outside air, the system designer minimizes the error and increases the accuracy.
In the last few years, several products have come into the market that are designed to directly control the outside air introduced into a building.
One method uses two outside air-measuring stations to control the outside airflow through the unit. One station is designed for 25-percent airflow and the other sized for 75-percent flow.
The control scheme is designed to keep the pressure signal viable for control. These stations stage their operation to keep the signal pressure useable, thus maintaining ventilation rates.
Another method integrates an air-measuring station into an outdoor air intake louver. Stability and control signals are amplified in this scheme, since air is measured at the highest velocity point in the outside air system. It is important that the manufacturer test this as a package. The louver can impact the performance of the air-measuring station if it is not designed and tested as a combination.
The fan-injection method consists of a separate fan and air-measuring station and is used to control the minimum outside air introduced into the HVAC system. These types of systems require extra space to install. Also, the extra energy required to operate the injection fan must be taken into consideration. It is possible that the injection fan could overpower the system and relieve air through the outside air damper.
Measuring and controlling outside air with an integrated air measuring station/damper assembly is becoming more popular. For constant-volume applications, these are usually set up to maintain minimum ventilation; a separate, standard damper for economizer operation obtains free cooling.
Accuracy is increased in measuring and controlling the minimum outside air versus the total air because the signal (or velocity pressure) from Pitot arrays is limited to 300 to 400 fpm on the low end. (The velocity pressure at that point is only 0.01 inches wg.)
Typical commercially available transducers have limited accuracy at these low pressures. By directly measuring and maintaining minimum ventilation rates, again the error rate is minimized.
Ruskin has the most comprehensive line of air measuring and control solutions in the industry. Products include differential pressure probes for high velocity applications, combination units that measure and maintain flow, and highly sophisticated, intelligent solutions that incorporate thermal dispersion technology with microprocessor based controls that communicate with any building automation system.
For more information about Ruskin’s complete product line, application and design support, and our state-of-the-art manufacturing capabilities, contact your local Ruskin representative nearest you or Contact Ruskin directly at (816) 761-7476.