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.

• Minimum ventilation for continuous limiting Volatile Organic Content (VOC) and other pollutants from furniture, floor, cleaning agents etc. in e.g. schools, health care or residential premises during non-occupied periods with minimum possible energy loss.
• Cooled or heated spaces with precise temperature control by ventilation. 
• Precise dosage of air specially treated e.g. by ionization, anti-microbial agents etc.
• Ventilation systems emphasizing energy efficient and environmental priorities.

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.

Advantages

• The apparatus can be installed in practically any kind of air duct
• The installation does not reduce the cross section of the duct meaning lowest possible pressure loss and noise
• High measurement precision
• Vast measurement range

Disadvantages

• Non-compact solution, the measurement, controller and control damper actuation device are usually divided
• High cost
• Complex control algorithm

Advantages

• Moderate cost
• Precise measurement, easy calibration
• Compact VAV measurement transmitter-controller-actuator combinations applicable

Disadvantages

• Limitation of velocity for sufficient precision in measurement (>0,5m/s) in compromise with permanently reduced flow cross section
• VAV

Advantages

• Vast measurement range
• High reading precision
• Nearly full flow cross section available
• Favourably low cost
• Compact VAV measurement transmitter-controller-actuator combinations applicable

Disadvantages

• Complex control algorithm

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).

q=k√∆P

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.

• Pressure independent compact variable air flow controller - electronic type.
• Control range of air flow velocity 0,2 - 6 m/s (velocity in equally sized duct)
• Adaptive measurement probe for high efficient dynamic pressure readings on whole velocity range 
• Advanced algorithm for appropriate air flow control even at subliminal duct static pressure (2 Pa)
• Operable at ∆P range 2 - 600 Pa
• Lowest possible cross section restriction for given pressure- / flow parameters resulting low pressure loss, low noise
• Inaccuracy app. 5% on the whole control range
• Leakage class 4C acc. to EN 1751 at pressure up to 1000 Pa
• Complete set of operation and override functions (Open, Close, Vmin, Vmax)
• Sizing: For air duct diameter 100 - 400 mm