Fan Types - Why choose a forward curved centrifugal fan Full-time Job

Fan Characteristic

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.

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.

Mounting options

As mentioned previously, the forward curved motorised impeller produces high velocity air at the tips of the blade that needs to be directed and slowed to convert dynamic pressure into static pressure. To facilitate this, we build a scroll around the impeller. The shape is created by a ratio of distances from the centre of the impeller to the fan outlet. As with the backward curved fan it is also recommended to have a small overlap between the inlet ring and the mouth of the impeller. Both mounting considerations are shown in the diagram below…

Summary – Why Choose a forward curved centrifugal fan?

When the required duty point falls in the area of higher system pressures versus lower volume flow on the fan characteristic a single inlet forward curved centrifugal fan should be considered. If the requirement for the application is for a high-volume flow in a restricted space envelope a double inlet forward curved centrifugal fan should be considered.

The fan should be selected within its optimum range which is at what is known as the knee of its characteristic curve. The point of peak efficiency is in the closer to the higher-pressure limit on the fan characteristic curve where it is also being operating at its quietest. Operating outside of the optimum range (at the extremes of high volume flow) should be avoided as the turbulence and the aerodynamic efficiency of the impeller blade at these points will create noise and the impeller will also be operating in an aerodynamic stall. At low pressures and high-volume flows consideration should be given to the operating temperature of the motor under load as there is a potential for a motor overheat to occur.

Air on the inlet side of the impeller should be kept as smooth and laminar as possible. To maximise the efficiency at least a clearance of 1/3rd of the impeller diameter should be allowed on the fan inlet. Using an inlet ring (Inlet nozzle) overlapping the impeller inlet will help to eliminate flow disturbances before the air is drawn through the fan, reduce turbulence induced noise, keep the power consumption at the duty point to a minimum and maximise efficiency.