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In the social housing market where customers require one fan for the van, an extractor fan needs to be able to achieve a number of criteria and since the change in document F which endorsed the use of dMEV constant running fans all major fan manufacturers have their own version of a dMEV constant running fan. As a result, we are now seeing fan manufacturers switching to Mixed Flow technology, but why?

From various tests carried out, forward curved impellor fans have proven to underperform in two key areas:

  1. Centrifugal is mainly effective against resistance at higher RPM meaning that if constant running/trickle speed is required the fan extract performance is severely affected once a wall sleeve or external grille is added, this is before we add in wind blow back and a longer duct run. Mixed flow and Axial impellor fans perform much better at lower RPM.
  2. Forward curved, centrifugal impellors are renowned for collecting grease and therefore are not used in commercial applications for this very reason. For longevity Mixed Flow and Axial fans are better against collecting grease and grime over time.

With the above in mind we must also need to consider resistance from a longer duct run; it is widely known that Axial fans are not effective once the duct length exceeds 1.5 to 2 metres. As well as against resistance, Axial fans also breakdown upon the criteria for 60 litres per second through a 100mm duct.

One thing which Mixed Flow operation hasn’t overcome (like Centrifugal) is the unit size when 60 litres per second, boost facility is required and we expect further research and development will find a solution here.

For the time being though, it’s worth being aware that for successful dMEV constant running fans where the need for the fan is to perform in all cases (one size fits all) ventilation technology has moved on and Mixed Flow seems to be the way forward for now…