Many demand oriented ventilation systems are facing one common problem. They are designed for operation with some nominal air flow volume that correspond to air flow velocities of a few meters per second in the duct. This is the sunny side of the life for the most ventilation systems, since the air flow is easily and precisely controlled by simple and affordable means like common VAV (Variable Air flow Volume) controllers.
Though, there are periods when just a tiny fraction of this nominal air flow volume is required. Let′s point some examples for such demand oriented ventilation systems.
During such operation periods the air flow velocities shall be measured deep below one meter per second. Here the most popular control methods hopelessly struggle against massively increasing imprecision.
Getting out of the imprecision trap at low air flow velocity measurement.
Some of the air flow velocity measurement methods seem to solve the low velocity imprecision problem and perform well at the higher velocity end of range as well.
Analyse of the methods brought a possibility for comparison. The simplified diagram shows the result.
The method of ΔP measurement probe attached to and moving with the VAV control damper blade became our favourite with a prospect of getting VAV control and handling for low and middle velocities with outstanding precision comfort for a very reasonable cost. The product to develop has got the name OPTIMA-LV-R. We have used the DNA of our standard VAV controllers from OPTIMA family, like precision, comfort, reliability, updated the measurement hardware and added a portion of sophisticated control algorithms.
This helped us to overcome the basic problem of this method, the floating k-factor. It is genrally known that the air flow volume (q) in an enclosed system can be calculated from pressure drop in this system (ΔP) and a factor that is representing the flow resistivity of this system, called k-factor (k).
A control damper has a different resistivity for every single opening angle opening angle (∠α). So there is an unlimited number of different k-factors (k1…kn , n=∞) for the damper between the fully open and fully closed position.
The control algorithm must therefore continuously read the actual damper position and pressure loss values. To interpolate the instantaneous k-factor values a higher degree polynomial is implemented in control algorithm.
For extremely low duct pressures under 2Pa, when the air flow velocity drops below 0,2m/s, a special procedure protects the controller from undesired oscillations and mechanical stress on the actuator keeping the damper in static waiting position. When the duct pressure recovers to operable value, the controller returns to the normal operation state - air flow control.
All the basic and advanced functionality is packed in a VAV-box with compact actuator/controller unit hard to distinguish from the standard VAV devices.
Peter Duffek Systemair