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.
As with the backward curved centrifugal fan, the suction side of the impeller blade draws air from the centre of the cross flow fan which results in a directional change of the airflow between the inlet and the exhaust of 90o.
The optimum operating area for a forward curved centrifugal fan is when it is operating at higher pressure. A forward curved centrifugal fan works best when high pressures against lower volume flows are required. The graph below illustrates the optimum working area…
The volume flow is plotted along the X-axis and the system pressure is plotted on the Y-axis. When there is no pressure in the system, (the fan is blowing freely), a forward or backward curved centrifugal fan will produce the greatest volume flow. As a resistance to flow is applied to the suction or exhaust side of the fan, the volume flow rate will drop.
Caution should be exercised when selecting a forward curved blower to operate at low pressures and highest volume flow. At this point, the impeller is operating in an aerodynamic stall in the same manner as an axial flow fan operating in the saddle point of its curve. At this point noise and power consumption will be at its peak due to turbulence.
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.