Fan Types - Why Choose an Mixed Flow / Diagonal Fan

By FanManDan     28 November 2019 

Fan types

There are 4 main types of fan to consider when deciding how to move air over, around or through a piece of equipment…

  • Axial fans, delivering high volume flow rates at low pressures and covering a high surface area will be commonly seen in ventilation (Extract fans) or heat exchange (Condenser / evaporator) applications.
  • Forward curved centrifugal fans used in a scroll housing have a steep pressure characteristic developing higher pressure with a lower flow rate. These will be seen in particulate or odour filtration applications (Single Inlet Blower), or in fan coils where a high-volume flow is required in a restricted space envelope (Double Inlet Blower).
  • Backward curved fans with their high efficiency deliver a non-overloading fan performance characteristic that delivers a medium volume flow at medium pressures. Typical applications include air handling and air conditioning units where low noise and high efficiency are critical.
  • Tangential or Cross flow fans produce a wide laminar flow at high velocity and will usually be experienced rather than seen in shop doorways or behind the outlet of a domestic fan assisted electric fire.

These four main types of fan cover a wide variety of processes that require air flow, however, there are some applications that require a mixture of fan attributes top combine the best features of both in a hybrid fan.

Axial fans deliver high volume flow at low pressure and a backward curved centrifugal fan delivers medium volume flow at higher pressure. In the case of the ATCA platform, the space available to install the fan meant that an axial fan didn’t produce enough pressure whilst the backward curved fan did not produce enough flow rate.

One option to solve this problem would be to choose a DC compact fan with a high-speed motor rotating at several thousand revolutions per minute. Producing the volume and pressure required draws a significant amount of power from the supply, reducing the amount of time an uninterruptable battery back-up power supply system (UPS) can operate. Another consideration that needs consideration is the noise output from a high-speed fan when operating at the required duty point.

An alternative option that could be considered is a Mixed Flow fan which combines some of the high-volume flow aspects of the axial fan with the pressure delivery of the backward curved centrifugal fan. The air intake to a mixed flow fan is in an axial direction where it is picked up by Hybrid design impeller blades with axial and radial elements that deflect the air outwards at a diagonal angle in the region of 45o. This combination of forces on the air passing through the impeller maintains a high flow rate whilst boosting the pressure on the exhaust of the fan. This performance can be delivered at speeds that do not put an unnecessary load on the power supply whilst delivering the performance quietly.

For this reason, a mixed flow fan is an ideal choice to bridge the performance gap between Axial and backward curved centrifugal fans.