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Deter­mining and regu­lating air flow

Adjusting the air flow of a fan to the actual demand enables signif­i­cant energy savings and noise reduc­tion in prac­tice. It should there­fore be possible to regu­late the air flow of the fans used in venti­lation, refrig­er­a­tion, and air condi­tioning systems as precisely and effi­ciently as possible to a spec­i­fied setpoint. Depending on the fan type and area of appli­ca­tion, different methods can be used.

In many appli­ca­tions, regu­la­tion of the air flow to a spec­i­fied setpoint offers the possi­bility of signif­i­cantly reducing energy consump­tion and noise emis­sions (Fig. 1). First of all, this applies to the fan itself; in many appli­ca­tions such as air-condi­tioning systems, this also has a posi­tive effect on the energy require­ments of other system compo­nents such as heaters, coolers or humid­i­fiers. In addi­tion, appli­ca­tion-specific and regional legal require­ments must often be complied with, e.g. for venti­lation of resi­den­tial build­ings.


Figure 1: Demand-based adjust­ment of the air flow enables signif­i­cant energy savings and noise reduc­tion. (Photo: ebm-papst)

Spec­i­fi­ca­tions for resi­den­tial build­ings

For example, DIN 18017-3 applies to exhaust fans in window­less bath­rooms and toilets. According to this, the exhaust air flow may change by a maximum of 15% at a pres­sure differ­ence of +/-40 Pa or +/-60 Pa. In indi­vidual venti­lation systems with a common exhaust air line, a reduc­tion in the exhaust air flow of the lowest unit of a maximum of 10% is permitted when oper­ating all units. This is intended to ensure that the exhaust air flow changes as little as possible in the event of external wind forces acting on the building enve­lope.

There are similar legal require­ments for decen­tral­ized venti­lation of indi­vidual rooms or for central resi­den­tial venti­lation units with heat recovery. DIN EN 13141-8 applies to decen­tral­ized systems. Here, the units are divided into three quality classes depending on the resulting change in air flow at a pres­sure differ­ence of +/-20 Pa (Fig. 2).

Figure 2: Spec­i­fi­ca­tions in DIN EN 13141-8 for decen­tral­ized venti­lation systems with heat recovery. (Photo: ebm-papst)


In the case of central venti­lation systems, the exhaust air flow must always be higher than the intake air flow according to the DIBT approval (German Insti­tute for Construc­tion Tech­nology), but the excess exhaust air may not exceed 10%. Satis­fac­tory values can only be achieved here if the air flow of the fans used in the appli­ca­tion can be precisely deter­mined and regu­lated as needed by adjusting the speed.

Methods compared

In prin­ciple, there is wide range of phys­ical measuring methods for deter­mining a volume or mass flow rate; however, not all of them are suit­able for gases and there­fore for fans. Magnetic-induc­tive measuring methods or those based on Cori­olis force are ruled out, for example, as they only work with elec­tri­cally conduc­tive media or the forces gener­ated with gases are much too small. Mass flow measuring methods with thermal sensors are, in prin­ciple, suit­able for fans, but only under labo­ra­tory condi­tions, as the thin measuring wires are very sensi­tive.

Not all measuring methods that are possible in prin­ciple do not neces­sarily turn out to be prac­tical.

Other measuring methods are highly accu­rate and robust, but rela­tively expen­sive to imple­ment and there­fore more suit­able for test rigs. On an ultra­sonic flow meter, two offset detec­tors record the transit-time differ­ence of two ultra­sonic waves propor­tional to the average flow speed (Fig. 3).

Alter­na­tively, flow speeds can be recorded using a vane anemometer or using a vortex gener­ator according to the vortex prin­ciple (Fig. 3). On a vortex gener­ator, the shed­ding frequency of the vortices propor­tional to the flow speed is detected. In the case of the anemometer, the speed and air flow are propor­tional.

Figure 3: Left: speed measure­ment using ultra­sound; right: vortex gener­ator. (Photo: ebm-papst)

What is prac­tical?

Things that are possible in prin­ciple do not neces­sarily turn out to be prac­tical. Speed measure­ments can be used in the inflow or outflow of a fan in all fan types and the rela­tively small sensors do not cause any rele­vant pres­sure losses. In many appli­ca­tions, however, the addi­tional costs for the sensors, their instal­la­tion, and the effects of aging or cont­a­m­i­na­tion present substan­tial obsta­cles.

In addi­tion, flow speeds measured “locally” – i.e. in one place – require precise knowl­edge of the oper­ating point or instal­la­tion-depen­dent speed distri­b­u­tion in the cross-sectional area through which the flow passes or corre­sponding unit-specific cali­bra­tion to deter­mine the air flow.

The measuring accu­racy at low air flows is signif­i­cantly reduced by the pres­sure drop or pres­sure differ­ence methods.

On the other hand, volume flow control systems based on measuring the pres­sure drop or pres­sure differ­ence are now found rela­tively frequently in air condi­tioning systems or venti­lation units (Fig. 4). A sensor is also required here for pres­sure measure­ment. However, in many cases the measuring points can be applied in such a way that the speed is not measured purely locally, but instead that enables at least an approx­i­mate “inte­gral” measure­ment of the air flow to be taken via the pres­sure signal.

In addi­tion, there are normally no addi­tional pres­sure losses and the processes are rela­tively inde­pen­dent of the inflow and outflow and the oper­ating point. The greatest disad­van­tage of the pres­sure drop or pres­sure differ­ence methods is that the measuring accu­racy at low air flows is signif­i­cantly reduced by the quadratic rela­tion­ship between air flow and pres­sure. In addi­tion, there are appli­ca­tion-specific prob­lems: for example, if the pres­sure differ­ence across a heat exchanger or filter is used in a resi­den­tial venti­lation unit, the measuring signal is heavily depen­dent on cont­a­m­i­na­tion and bypass flows.


Figure 4: Left: pres­sure differ­ence measure­ment at the inlet nozzle of a centrifugal impeller in an air condi­tioning system; right: pres­sure drop measure­ment in a resi­den­tial venti­lation unit. (Photo: ebm-papst)

Sensor­less control

If there is a clear rela­tion­ship between power consump­tion and air flow at a constant speed, an oper­ating point can be deter­mined by measuring the motor current and the speed. These char­ac­ter­istic curves are only found for forward-curved centrifugal impellers. The term “sensor­less” control is often used in the context of elec­tron­i­cally commu­tated blowers because only internal motor vari­ables are used and no external pres­sure or speed sensors are required.

Various ebm-papst blowers use sensor­less control inte­grated into the elec­tronics for constant volume control (Fig. 5). To deter­mine the oper­ating point, this relies on a blower-specific and, in some cases, also unit-specific cali­bra­tion poly­no­mial. However, the rela­tion­ship between power consump­tion and air perfor­mance which is cubic in the first approx­i­ma­tion leads to signif­i­cantly increasing control inac­cu­ra­cies, even in this approach at low air perfor­mances. In addi­tion, a change in the air density results in an error in air flow deter­mi­na­tion.


Figure 5: Sensor­less constant volume control of a forward-curved ebm-papst centrifugal fan. (Photo: ebm-papst)

Effi­cient solu­tion for back­ward-curved centrifugal fans

In the case of back­ward-curved centrifugal fans, sensor­less air flow deter­mi­na­tion is not possible due to their char­ac­ter­istic curve. For this highly effi­cient fan design, the flow special­ists at ebm-papst have devel­oped a ready-to-install plug & play solu­tion: a vane anemometer posi­tioned in the outlet nozzle of the scroll housing (Fig. 6). It contin­u­ously records the actual air flow without signif­i­cant pres­sure losses or addi­tional noise. The data is trans­ferred to the inte­grated central control elec­tronics of the fan.

This adjusts the speed of the EC motor to the desired setpoint and regu­lates the air volume of the blower to the spec­i­fied setpoint regard­less of air density influ­ences. As the speed of the vane anemometer is no longer quadrat­i­cally, but only linearly depen­dent on the air flow and the speed of the EC motor is used as an addi­tional internal correc­tion vari­able, very high control accu­racy can still be achieved even with low air flows. In addi­tion, deter­mi­na­tion of the oper­ating point is barely nega­tively affected by the effects of instal­la­tion through recording the entire air flow.


Figure 6: Constant volume control of a back­ward-curved ebm-papst centrifugal fan with vane anemometer. (Photo: ebm-papst)


With this very robust and fully inte­grated constant volume control, an extremely precise and effi­cient solu­tion can be achieved over the entire control range. For resi­den­tial venti­lation units, this means, for example, balanced supply and exhaust air flows all year round. On the one hand, this prevents the unwanted supply of cold outside air; on the other hand, this prevents warm room air flowing outwards in winter through leaks through the building enve­lope, cooling down and thus creating conden­sa­tion in the outer walls.

The addi­tional impeller does not result in any air perfor­mance losses or disrup­tive noise, meaning that the overall perfor­mance of the fan remains unchanged. Even cont­a­m­i­na­tion is not a problem, as has been demon­strated in tests under extreme condi­tions with dust and increased air humidity.

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