01-27-2022, 06:58 AM
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.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.
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.
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…
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.
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…