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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.
How does an axial fan work?
An axial fan blade works on the same principle as how an aircraft wing generates lift. The wing of an aircraft has an aerofoil shape which as it is propelled forwards the air is diverted above and below the wing. The air above the wing has farther to travel than the air under the wing which results in a difference in air velocity above and below the wing. The higher velocity air stream above the wing has a higher dynamic force which results in a lower static pressure. Below the wing, the air is moving slower which means it has a lower dynamic force and higher static pressure. The difference in higher pressure below the wing and lower pressure above it generates lift.
In the case of a fan we use an impeller blade like an aerofoil however instead of generating lift we keep the fan blades rotating in a stable axis and this creates airflow. This follows Newton’s third law of motion that for each action there is an equal and opposite reaction. The air above the aerofoil is drawn into the fan by the reduction in static pressure and pushed out through the fan creating a positive pressure area.
To direct the airflow and obtain optimum performance, we use a mounting plate with a bell-mouth shaped inlet cone to create a smooth laminar flow into the suction side of the fan. The fan is held centrally in the mounting plate by the mounting guard which maintains a small tip clearance between the fan and bell-mouth.
There are other methods of mounting an axial fan including placing it in a plain diaphragm hole or in a short radius, however, these mounting methods compromise performance. Using a full bell-mouth mounting plate will ensure that optimum performance from the fan will be achieved.
Reduction in the effective diameter using alternative mounting methods
The optimum operating area on an axial fan characteristic is below what is called the stall dip. An axial fan works best when a relatively high-volume flow is required against a relatively low system pressure. 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), an axial 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 called the knee of the fan characteristic curve. At this point, the ratio of the output power of the fan (Volume flow x Pressure development) and the electrical power input is at its greatest and the sound pressure being produced by the fan will be at its quietest.
Above the optimum range of operation, the flow across the upper and lower sides of the impeller blades becomes turbulent and the impeller starts to go into an aerodynamic stall.
At this point, the fan will start to vibrate and create lots of noise. Operating at this point can potentially cause the motor driving the fan to overheat as the turbulent forces on the impeller blade disrupt the motor operating efficiency.
Mounting considerations – clearances
When installing an axial fan, it is important to consider several factors relating to how it is mounted to the application. Ideally, when we choose an axial fan we could allow for lots of room on both the suction side and exhaust side of the fan to promote an even airflow into the fan and a controlled dispersal of the air on the exhaust side of the fan. Unfortunately, this may not always be possible to achieve and a compromise must be reached.
As a rule of thumb, the clearance (x) on the suction side and the exhaust side of the fan before an obstruction is reached should be at least 1/3rd of the diameter of the fan.
X >= diameter D x 0.33
e.g. For an axial fan of 450mm diameter, there should be at least 150mm of clearance between the fan and an obstruction to flow (Coil, Filter, wall etc…)
Mounting considerations – position of the fan in the system
The position of an axial fan in the application will affect its operating conditions. In the example below the choice for cooling an enclosure containing electronics dissipating a heat load is shown…
As mentioned above, the components of an axial fan are designed to promote a smooth laminar flow of air on to the impeller blade. A laminar flow will maximise the efficiency of imparting a force on the air to move it from the suction to the exhaust side of the impeller blade. On the exhaust side of the impeller blade, the flow will become random and turbulent if flow straightening devices are not used.
Placing the fan at the inlet and blowing into the enclosure is one option to consider. We can use the turbulent exhaust of the fan to spread the air around the enclosure. This will ensure that all surfaces of the components within the enclosure will encounter some of the cool supply air. Being turbulent the random air path will create more pressure in the system as the air makes its way towards the exhaust. More pressure in the system will create more noise from the fan and require more power to deliver the required airflow.
On the other hand, we could place the fan at the exhaust which will change the system characteristic. The air will follow the path of least resistance which means that some of the components will experience less cooling. The pressure in the system will be lower resulting in quieter operation and lower power consumption. The fan will see a higher operation temperature as the air passing over the motor will have an increased temperature caused by the dissipation of heat from the components inside the enclosure.
Summary – why choose an axial fan?
When a relatively high-volume flow is required in a low-pressure system and axial fan should be considered. The fan should be selected within its optimum range either at the point of peak efficiency or below. The point of peak efficiency is an area known as the knee of the fan characteristic curve where it is also is operating at its quietest. Operating in what is known as the stall dip and at system pressures above this point must be avoided. The turbulence and the aerodynamic efficiency of the impeller blade in the stall dip and above creates noise and may cause the motor to overload.
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 a full bell-mouth mounting plate and eliminating flow disturbances before the air is drawn through the fan will reduce noise and keep the power consumption at the duty point to a minimum.