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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.
The name, ‘Centrifugal Fan’ is derived from the direction of flow and how the air exits the fan impeller radially from the outer circumference of the fan. A Backward curved centrifugal fan is characterised by its cylindrical shape, several large curved blades and a conical inlet nozzle. In the example shown below, the fan rotates in a clockwise direction.
As the fan rotates a pressure difference is created on the impeller blades. On the leading, convex side of the impeller blade, a positive pressure is created as the rotational motion of the impeller imparts a force in the air. The impeller blade pushes the air outwards, exiting in a radial direction. On the concave side of the impeller blade, a negative pressure is created as the fan rotates, drawing air into the space between the blades. This air is then picked up by the following blade and forced outwards radially in a continuous process. The suction side of the impeller blade draws air from the centre of the fan which results in a directional change of the airflow between the inlet and the exhaust of 90o.
The optimum operating area for a backward curved centrifugal fan is an area in the middle of its performance characteristic. A backward curved centrifugal fan works best when medium pressures and medium volume flows are required.
A non-overloading performance characteristic, high static efficiency and flexible installation options are reasons why a backward curved fan provides the best solution for matching airflow requirements in systems with significant flow resistance. To ensure these benefits are delivered an inlet ring, (or nozzle), is required to ensure smooth laminar flow as the air enters the impeller.
The inlet ring should be positioned concentrically and there should be a small overlap of the inlet ring into the inlet of the fan.
As mentioned above a backward curved fan can be used in a basic mounting design for use in a plenum chamber or incorporated into a scroll housing to direct the exhaust flow…
When the required duty point falls in the area of medium system pressure versus medium volume flow on the fan characteristic a backward curved centrifugal fan should be considered. The fan should be selected within its optimum range which is in the centre of its flow characteristic. The point of peak efficiency is in the middle of the fan characteristic curve where it is also is operating at its quietest. Operating outside of the optimum range (at the extremes of high volume flow or high operating pressure) should be avoided as the turbulence and the aerodynamic efficiency of the impeller blade at these points will create noise and system inefficiencies.
Air on the inlet side of the impeller should be kept as smooth and laminar as possible to maximise the efficiency of the impeller blade. Using an inlet ring (Inlet nozzle) overlapping the impeller inlet cone 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.
The non-overloading characteristic, the ability to install the fan with or without a scroll and an impeller design that offers the highest static efficiency of all types of fans means that the backward curved fan is a flexible, adaptable fan design can be used across a wide range of installations.
