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Is it time to go with the mixed flow?

Selecting the most appropriate fan for a project requires a number of factors to be taken into account. Mixed flow in-line fans are a case in point, says Alasdair Howie
When selecting fans for a project, there are now many more things to consider than would have been the case a few years ago. Delivering the required performance now has to be achieved with minimum energy consumption and, ideally, low noise. Aesthetics are also important in some projects so the ability to conceal the fan may enter into the equation.

In some ways this may be viewed as having made things more complex but in fact, in narrowing the field of fans that meet all requirements, it's actually simpler.

In recent years in-line fans have become more popular - not least because many in-line fans are able to provide the required performance while offering a compact and low profile design that makes it easier to conceal them.

Where in-line fans are selected, mixed flow in-line fans are generally the best solution for medium pressure ducted systems such as general supply and extract. Unlike axial fans where the air flow is normally axial, with mixed flow fans the initial air flow is axial but is then deflected through 45 degrees by the impeller.

The result is an increase in pressure due to the centrifugal force, making them ideal for overcoming the inherent resistance of ducted systems. At the same time, the design of mixed flow fans retains the benefits of compact dimensions and low profile.

Using mixed flow in-line fans also contributes to energy efficiency as the mixed flow configuration allows for a very efficient impeller and, when combined with a high efficiency EC (electronically commutated) motor, provides very low specific fan power (SFP). This is an important metric as Part L2 of the Building Regulations requires fans to meet an SFP of 0.6W/l/s. Some of the mixed flow fans recently introduced to the market are able to meet this requirement at many different working points and system pressures, helping specifiers and installers to comply in a wide range of projects.

As mentioned earlier, noise levels must also be considered and a useful guide to selecting low noise fans is the Quiet Mark.

This mark of approval is operated by the Noise Abatement Society and is designed to reduce noise-related stress in living and working environments. It covers a wide range of product types, including fans, which are subjected to rigorous testing by the Association of Noise Consultants before the Quiet Mark is awarded.

Until quite recently, low noise in-line mixed flow fans were only available in relatively small fans but advances in design mean that low noise can now be achieved for higher flow rates. For example, it is now possible to deliver flow rates of 1,660 cu m/h with sound pressure levels as low as 52dB(A) at the inlet and 57dB(A) at the outlet.

In addition, future changes in European regulations are set to influence fan selection even further. Following on from the first wave of fan minimum efficiency grades (FMEG) that came into effect from 1st January this year, there will be a second stage at the start of 2015. The European regulation pipeline also has new minimum efficiency requirements scheduled to cover residential ventilation products and commercial ventilation products such as box fans, roof fans and AHUs.

As this legislation stands at the moment it will probably signal the end of many product types and configurations that cannot deliver the required efficiencies.

Among the 'endangered species' are axial roof fans and just about any product
fitted with a forward curved centrifugal fan. It appears that backward curved centrifugal fans are the energy-efficient fan of choice at the EU.

The consultants to the legislature in Brussels are, it seems, more interested in imposing theoretical energy-savings on products across the 27 member states, rather than looking at other design parameters (such as noise or space restrictions), industry norms and cost considerations.

Fortunately, the mixed flow fan may provide the perfect compromise between energy-efficiency and 'usability' in many low to medium pressure ducted systems.

SFP overview
Specific fan power (SFP) is a measure of the efficiency of fans in air movement systems and may be expressed in W/l/s for W/cu m/s. Thus the SFP figure takes account of the resistance in the system to provide an indication of overall efficiency when the system is in operation.

The SFP may be used to refer to the total power demand for supply and extract fans in a building, or it may refer to individual fans. For individual fans, the definitions are different for air handling units with supply and extract air, compared to separate supply air or extract air handling units and individual fans.

The SFP for supply and extract units is the total amount of electrical power supplied to the fans, divided by the largest supply or extract air flow rates under design load conditions.

// The author is marketing manager with S&P UK Ventilation Systems //
17 April 2013

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