Fan Types – Why Choose a Backward Curved Centrifugal fan?
Backward 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 Backward Curved Centrifugal Fan work?
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. 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 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.
The peak efficiency is at a point in the middle 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 backward curved motorised impeller is that it does not have a stall point on its characteristic. This means that there is no point on the fan characteristic curve that it should not be operated. Backward curved motorised impellers also have the highest static efficiency of any fan type and the mechanisms used in creating the airflow through the fan mean that it can be equally used on its own in a basic plenum or it can be enclosed within a scroll to direct its airflow.
Other fans such as axial, diagonal and forward curved centrifugal fans have areas in which operation is inadvisable. In these areas, turbulence can be formed which can have detrimental effects on the long-term reliable operation of the fan.
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 used in a plenum chamber, as often employed in air handling units, the designer has many air delivery options...
Air entering from the duct on the suction side of the fan may have passed through some elements that have resistance to flow, (particulate filters, heat recovery cell, heat exchangers etc…). As the air passes through the fan it turns through an angle of 90o and is exhausted radially. This airflow pattern provides the option of positioning the exhaust duct in a radial or in-line position.
When used in a scroll housing the air exhaust becomes directional and the performance of the impeller changes…
Using a basic scroll reduces the velocity of the air and converts some of the dynamic pressure energy into static pressure which produces a steeper pressure characteristic. The curved nature of the scroll also smooths the exhaust air providing quieter operation.
Mounting considerations – Clearances
It is important to ensure sufficient clearance on both the suction and exhaust sides of the fan.
Insufficient clearance on the suction side of the fan will increase the inlet velocity which will lead to turbulence. This turbulence will be increased as the air passes through the impeller which makes the transfer of energy from the fan blade to the air less efficient, cause the creation of more noise and reduce the fan efficiency. Similarly, placing an obstruction to flow close to the exhaust side of the fan will create turbulence and buffeting which will increase the resistance to flow which the fan will have to overcome.
General recommendations for inlet and exhaust conditions are:
- No obstruction or change in flow direction within ½ a fan diameter distance from the inlet of the fan
- The hydraulic diameter should be greater than 2.2 x fan diameter
- For inline flow direction - no obstruction within a fan diameter distance from the rear of the fan
(See layout drawing and graph above for details)
Summary – Why Choose a backward curved centrifugal fan?
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.