Take a second look at EC fans

EC fans are considered by some as a more energy-efficient solution for cooling in facilities such as data centres. But to consider efficiency alone is to miss the bigger picture, says Carl Turbitt

Carl Turbitt

HVAC drives UK sales manager at ABB

www.global.abb

nergy costs can account

Efor a large proportion of a building’s upkeep, and in specialist facilities like data centres, this figure can be as high as 40 percent. Much of this is due to cooling requirements and the large number of fans needed to keep temperatures low.

In general, there are two main types of fan technology on the market: motor and drive configurations, and EC (Electronically Commutated) fan assemblies, which in the HVAC industry describes a motor and drive package which is integrated together. Both are capable of precise speed control and optimisation of plant, which can therefore save you money on your energy bills.

EC fans may appear cheaper up front, but when you consider the cost over the lifetime of the asset, the motor-drive package will generally come out on top. Having two separate devices rather than one aids with heat dissipation, as in a motor-drive configuration you can situate them apart from one another, unlike an EC fan assembly where the motor and drive must be installed together. Less heat means less wear on components, and a longer lifetime for motors and bearings.

Failures more expensive to fi x

Failures to EC fans can also be more difficult and expensive to fix. EC fans are often made to measure and so must be replaced like-for-like. Moreover, when it comes to replacing an EC fan within an array, you will often be locked into a particular configuration and a particular manufacturer. This in turn could put you at the mercy of long lead times and supply chain issues. Conversely, if a drive or motor fails, it can typically be replaced off the shelf with a similar product from any major manufacturer, meaning very little downtime and greater flexibility in having the ability to choose the drive and/or manufacturer that fits best with your system.

It may seem counterintuitive, but having two devices rather than one can also increase simplicity, as multiple fan motors can be run from the same drive. An EC fan on the other hand requires each fan to have its own drive, which can get complicated in data centre fan arrays with large numbers of units. It also requires additional overload protection.

EC fans have a limited power rating and cannot generate anything like as much torque as a standalone drive, therefore a fan array will typically require several of them to reach the

Operational costs of running a data centre are high with fans contributing a large proportion of energy use

required airflow levels. This could mean more potential points of failure, whereas a drive and motor can go up to 250kW without breaking a sweat, providing more power with less complexity. With existing ductwork the ability to generate enough static pressure is essential, and so EC fans often have to be over-dimensioned in terms of power to reach the nominal static pressure, and thus run far from their best efficiency point.

Replacing motor and drive

In practice, for an array of eight EC fans you are essentially installing 16 devices, and each motor and drive pair will need replacing entirely in the event of a failure. In many cases they will also require a Modbus gateway connection to link them together. In contrast, the same fan array requires just nine devices in a motor-drive configuration.

Harmonics can be a big issue in data centres. These are distortions in current and voltage which can result in nuisance trips and equipment misbehaving or failing. It is a widespread problem, but one that can be underestimated in the building services industry – that is until something goes wrong. Modern variable speed drives typically have harmonic mitigation built in, and in the case of ABB’s Ultra-Low Harmonic (ULH) drive, have a significantly reduced harmonic content compared to conventional drives. In contrast, EC fans not only produce harmonics, but also have no in-built mitigation, and so if harmonic content is too high additional harmonic filters may be required.

Modern VSDs incorporate an increasing amount of additional functionality compared to EC fans. This includes many HVAC-specific features which can either help to improve longevity, efficiency and versatility, or take on the duties of other cabinet components such as thermostats, functional safety apparatus, and energy management and performance analysis tools.

There’s no denying that EC fans are efficient, particularly at peak loads, and then to cost less up front. However, in practice they may not be able to provide everything you need for an effective, reliable and sustainable cooling solution. When you consider the overall package in terms of reliability, longevity, simplicity and versatility, a drive and motor could well be the more cost-effective and sustainable option in the long run. ■