Why Choose an Mixed Flow Fan

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.

Advanced Telecommunications Computing Architecture (AdvancedTCA)

Back in the early noughties, the ATCA specification was agreed by more than 100 companies, setting out the future of telecommunication equipment. Inevitably the demand was to provide greater reliability and performance in a smaller space envelope with less susceptibility to electromagnetic interference.

To achieve this a Blade type PCB containing more components with a greater power requirement was chosen to be mounted into standardised telecom racks. More components with greater power density and less space available introduce an overheating risk and loss of service which in today’s environment can be costly.

What was needed in this case was a compact fan that could deliver a high-volume flow whilst overcoming the resistance to flow created by the closely packed environment.

Mixed Flow Compact Fans – Bridging the gap between Axial and Centrifugal

As mentioned previously, 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.