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All you need to know about precision-controlled airflow
Systemair experts outline the advantages and disadvantages of different airflow velocity measurement methods
Efficiency
Many demand-oriented ventilation systems face one common problem: They are designed for operation with some nominal airflow volume corresponding to airflow velocities of a few meters per second in the duct.
This is ideal for most ventilation systems because the airflow is easily and precisely controlled by simple and affordable means, like common VAV (Variable Airflow Volume) controllers.
Minimum ventilation
There are instances when only a tiny fraction of this nominal airflow volume is required. Examples include projects with:
Minimum ventilation for continuous limiting Volatile Organic Content (VOC) and other pollutants from furniture, floor, cleaning agents in 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 by ionisation, anti-microbial agents and other methods
Ventilation systems that emphasise energy efficiency and environmental priorities
Airflow velocity measurement methods
The airflow velocities should be measured deep below one meter per second during the operation of such projects. As such, the most popular control methods hopelessly struggle to manage imprecision.
The best approach is to use the most suitable air flow velocity measurement method. Some of the airflow velocity measurement methods help solve the low-velocity imprecision problem and perform well at the higher velocities as well.
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Acoustic (ultrasonic) flow measurement principle
Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect.
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 the 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
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Restrictions flow measurement principle – Venturi tube, nozzle, orifice
The velocity of the air must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressure must decrease in accord with the principle of conservation of mechanical energy (Bernoulli's principle).
By measuring the pressure, the flow rate can be determined
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
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ΔP measurement probe attached to and moving with the VAV control damper blade
The differential pressure sensors, are characterized by extremely high flow-through impedance. When compared to other sensors with lower flow impedance, differential pressure sensors with high flow impedance require less parasitic flow to make a measurement and hence, cause fewer disturbances to the main flow.
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
What is the best choice for you?
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In analysing the methods, we can arrive at a comparison table, which is exhibited in the simplified drawing.
The method of ΔP measurement probe attached to and moving with the VAV control damper blade was determined as the best method allowing VAV control and handling for low and middle velocities with outstanding precision comfort for a very reasonable cost.
This serves as the basis for the development of 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 sophisticated control algorithms.
The updates to the product helped us overcome this method's fundamental problem, the floating k-factor. Generally, the air flow volume (q) in an enclosed system can be calculated from pressure drop in this system (ΔP) and a factor that represents the flow resistivity of this system, called k-factor (k).
q=k√∆P
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A control damper has a different resistivity for every single 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.
Therefore, the control algorithm must continuously read the actual damper position and pressure loss values. To interpolate the instantaneous k-factor values, a higher degree polynomial was implemented in the control algorithm.
For extremely low duct pressures under 2Pa, when the airflow 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 a static waiting position. When the duct pressure recovers to an operable value, the controller returns to the normal operation state - airflow control.
All the basic and advanced functionality is packed in a VAV box with a compact actuator/controller unit that is hard to distinguish from the standard VAV devices.
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OPTIMA-LV-R
Below are a few additional features that make OPTIMA-LV-R best suited for the project:
Pressure independent compact variable airflow controller - electronic type.
Control range of airflow velocity 0,2 - 6 m/s (velocity in an equally sized duct)
Adaptive measurement probe for high efficient dynamic pressure readings on whole velocity range
Advanced algorithm for appropriate airflow 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 in 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
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