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The formula for the energy effi­ciency

If you want to compare the energy effi­ciency η of various fans, you should consider the entire fan, i.e. the fan system, and not just its indi­vidual parts.

Anyone wanting to purchase a fan system asks them­selves two ques­tions: 1. Which fan will be up to the task? 2. Which fan will be up to the task with the greatest effi­ciency? The second ques­tion is about finding a fan that will provide the best effi­ciency (η).

Uwe Sigloch, Director of Product Manage­ment at ebm-papst in Mulfingen (Photo | ebm-papst)

In essence, a fan system consists of three main compo­nents: the motor, control elec­tronics, and impeller. The effi­ciency of each compo­nent can be described by the respec­tive indi­vidual effi­ciency level. It is usually spec­i­fied in terms of the optimum oper­ating range (i.e. ηmax) by the manu­fac­turers of these indi­vidual compo­nents. If the fan is assem­bled from the indi­vidual compo­nents mentioned, the indi­vidual effi­ciency levels are normally multi­plied and docu­mented. However, this can only be a theo­ret­ical overall effi­ciency that cannot be achieved later in oper­a­tion. Why is that?

The effi­ciency describes the ratio of effort to benefit. In the case of the fan, the benefit is that it moves a volume of air against a resis­tance (pres­sure), i.e. the air perfor­mance in watts. This is calcu­lated by multi­plying the air flow (in cubic meters per second) by the pres­sure (in Pascal). The effort is the elec­trical power consump­tion, also spec­i­fied in watts. The result is always <1, as energy is always lost. Now you have a value for the level of effi­ciency, for example η = 0.8. This would appear to be an objec­tive value that can be used to compare the effi­ciency levels of different fan systems. Unfor­tu­nately, this is not always the case.

The crucial factor is how η came about. The ques­tion must then be: has the effi­ciency of the fan system been calcu­lated or measured? There is a theo­ret­ical optimum effi­ciency level, and the effi­ciency level resulting from actual oper­a­tion. The optimum η is always higher (i.e. better) than the measured value — some­times by 20 percentage points! That is why many manu­fac­turers prefer to specify the optimum η.

Measure the fan system as a complete unit

But this is not useful for the appli­ca­tion, as it cannot be assumed that all three compo­nents will run at their optimum effi­ciency at the desired oper­ating point. The devi­a­tions are often consid­er­able. A fan system has a sepa­rate overall level of effi­ciency in each oper­ating state, which is very diffi­cult to calcu­late if using the indi­vidual effi­ciency levels of the compo­nents. It is better to measure the fan system as a complete unit.

When spec­i­fying the indi­vidual effi­ciency level of the impeller, there is also the addi­tional partic­u­larity that, in the calcu­la­tion, often total pres­sure increase is used, i.e. the sum of static and dynamic pres­sure. However, only the static pres­sure is rele­vant for a venti­la­tion system. In this way, ­ηmax­im­peller is often inad­miss­ably calcu­lated to appear better than it actu­ally is.

At ebm-papst, we follow the Wire2Air approach. We only state actual, measured effi­ciency levels. In any case, we recom­mend attaching less impor­tance to the effi­ciency level. In prac­tice, it is much more infor­ma­tive to compare the expected power consump­tion for a specific air-move­ment task. 

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