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Forward Curved Motorised Impeller

When we have defined the volume flow rate that we require, whether this is to provide fresh air or process cooling, we need to combine this with the resistance to flow that the fan will encounter in the application. The volume flow rate, (in m3/hr) and the pressure (in Pascals – Pa), are combined to become the duty point against which the fan must operate. It is important that we select a fan whose performance characteristic meets the required duty point on or near the point of peak efficiency. Using the fan at its peak efficiency minimises the power consumption and noise emitted from the fan whilst delivering the required performance.

How does Forward Curved Centrifugal Fan work?

The name, ‘Centrifugal Fan’ is derived from the direction of flow and how the air enters the impeller in an axial direction and then propelled outwards from the outer circumference of the fan. The difference in flow direction between a forward and backward curved centrifugal fan is the direction that the air exits the impeller circumference. With a backward curved impeller, the air exits in a radial direction whereas with a forward curved the air exits tangentially from the circumference of the fan. 

A forward curved centrifugal fan is characterised by its cylindrical shape and lots of small blades on the circumference of the impeller. In the example shown below, the fan rotates in a clockwise direction.

Unlike the backward curved impeller, the forward curved impeller requires a housing that converts high velocity air leaving the tips of the impeller blade into a lower velocity static force. The shape of the housing also directs the air flow to the outlet. This type of fan housing is commonly known as a scroll; however, it can also be referred to as a volute or a sirocco housing. By installing the forward curved impeller in a scroll housing, we usually refer to it as a forward curved blower.

There are two types of blowers that employ a forward curved motorised impeller as shown below…

The single inlet blower on the left, draws in air from one side of the housing through the round inlet and directs it to the square outlet, (seen here with a mounting flange). The double inlet blower has a wider scroll housing drawing air in from both sides of the scroll delivering it to the wider square outlet.

The peak efficiency is at a point called the knee of the characteristic curve. At this point the ratio of the output power of the fan (Volume flow (m3/s) x Static Pressure development (Pa) and the electrical power input (W) is at its greatest and the sound pressure being produced by the fan will be at its quietest. Above and below the optimum range of operation the flow across the fan becomes noisier and the efficiency of the fan system decreases.

The benefit of using a single inlet forward curved motorised impeller is that it has a steep fan characteristic. This is particularly useful in systems that require consistent levels of filtration. As air passes through a particulate filter the filter arrests airborne dust and pollen, the finer the grade of filtration the smaller the particles arrested by the filter. Over time the filter will become increasingly clogged with dirt and debris which has the effect that more pressure is required to deliver the same air volume. Using an impeller with a steep characteristic curve in this case means that as the filter becomes increasingly clogged, the volume flow remains constant while the pressure across the filter is increasing.

The benefit of using a double inlet forward curved impeller is that from a relatively small size blower it can deliver a high-volume flow. The compromise with using a double inlet blower is that it has a lower pressure development meaning that it can only work with lower pressure systems.