Fan efficiency isn"t intuitive

Few would deny the importance of energy efficiency in ventilation fan systems – after all, by far the largest component of the lifetime cost of any fan system is the cost of the energy it uses, and every business is today mindful of the impact its operations have on the environment. But in the highly competitive field of ventilation fan efficiency, it pays to be cautious; it’s all too easy to be misled by claims that at best could be described as over enthusiastic, and at worst as downright misleading.

Furthermore, when specifying fans, it’s crucial to bear in mind that intuition is a fickle guide – a fan system that looks at first sight to be inherently efficient, may on closer investigation, turn out not to be the best option.

Before looking at these claims in more detail, let’s make it clear that we’re talking about the extended product approach to efficiency, which takes into account not only the efficiency and losses of the motor system but also the efficiency and losses of the driven system – in our case, the fan. This is the most meaningful way of considering efficiency and is widely used in international standards such as IEC61800-9, which covers Eco-design and Minimum Energy Performance Standards for Power Drive Systems.

Let’s move on to consider fans that are driven by integrated electronically commutated DC motors. Commonly called EC fans, these would appear to have many attractions. For a start, under some operating conditions, DC motors are more energy efficient than conventional AC induction motors.

Of course, DC motors have the disadvantage of needing a commutator, and in older motors, this is an electro-mechanical device that needs regular maintenance. Electronically commutated motors don’t have this problem and the electronic systems used in modern types also allow them to be fed directly from an AC supply. This same electronic system also makes provision for controlling the speed of the fan.

What’s not to like? It’s an efficient motor, matched to a dedicated control system and presumably, also matched to the fan. Surely this must be an efficient arrangement? It probably is, but it may well not be the MOST efficient arrangement. Let’s dig a little deeper, taking into account our earlier statement about the high efficiency of DC motors. It is true, but only when the comparison is made with what might be called “traditional” squirrel cage AC induction motors. If the AC motor is a modern permanent magnet (PM) type, the inherent superior energy efficiency of the DC motor is much more questionable.

Control electronics

Now let’s turn to the control electronics. The system supplied with an EC fan is undoubtedly dedicated and purpose designed. But does that make it more energy efficient than a VSD from a long-established manufacturer with vast experience in drives and a strong commitment to maximising the energy efficiency of its products? The answer is almost certainly not.

The reality is that dedicated variable speed drive suppliers are invariably the first to introduce new technologies that improve efficiencies. An excellent example is Automatic Energy Optimisation (AEO), which reduces the voltage and current supplied to the motor when it is operating under steady state conditions. This relatively small adjustment, which has no effect whatsoever on the overall performance of the fan, typically cuts operating costs by between 3% and 5%.

Next to consider is the match between the fan and the motor. The match is no doubt excellent in the EC fan, but that’s not the whole story, as selecting a suitable motor to drive a fan is not a particularly difficult task. More important is the design of the fan itself, and this is an area where there have recently been interesting and significant developments.

To maximise energy efficiency, fans must make use of both static and dynamic pressure. Many of the fans currently in use are, however, centrifugal types which are unable to use dynamic pressure. This limits their efficiency: they are incapable of achieving efficiencies greater than 90%, and in reality many designs fall well short of this maximum. Axial fans have no such limitations and, the latest designs achieve efficiencies of up to 92%, which cannot be matched by any other type of fan.

At this point, it’s time to return to the consideration of overall system efficiency. We’ve just seen that the fan itself can have an efficiency of 92%. Good PM motors are 95% efficient and Danfoss VLT HVAC drives achieve efficiencies of 98%. The overall system efficiency is the product of these individual efficiencies, which is 92% x 95% x 98% = 85.6%. As far as the author is aware, no integrated (EC fan) solution can get anywhere near to this figure. In fact, many EC fan solutions have overall efficiencies that are as much as 20% lower.

We believe that we’ve made a good case that specifiers and users of ventilations fans should be wary of assuming that integrated solutions offer the best efficiency, and should instead dig a little deeper and consider what can be achieved by choosing separate components.

While they are doing so, they might also want to bear in mind that separate component solutions also offer other benefits, such as potentially easier availability of replacement parts later in the life of the systems and, if the components are chosen carefully, easy access to local support. These are important virtues that, along with others, will be considered in future articles.

Of course, there will be those that say this is all very well, but I still have concerns about choosing the right combination of fan, motor and drive. For them, there is fortunately a simple answer: get advice from an expert with proven experience in the field of fan applications.

Danfoss VLT Drives

Oxford Road

0330 808 6888

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