Single stage centrifugal fans generally produce airflow at static pressure ranging from 100mmWG to as high as 2000mmWG. These fans propel air mass perpendicular to air incoming axial direction, creating centrifugal effect. It can be envisaged that the leading edge of impeller blades scoop air mass, churning them outwards, before being thrown out at trailing edge of the blades. As such, diameter of blades at leading edge represents main factor in determining airflow volume. The bigger its inlet diameter, the higher its airflow. As static pressure is generated by centrifugal force of the impeller, bigger impeller overall diameter produces higher static pressure of the airflow. For this reason, high pressure range of centrifugal fan has bigger but thinner body.
Centrifugal blower fan impeller is typically reducing in thickness toward outer fringe, due to the fact that air path area increases towards outer edge of the impeller. This prevents over expansion of air density.
Blower fan body
Centrifugal blower body functions as a pressure storage that allows smooth churning out of airflow towards its outlet. A properly designed fan body considers cumulatively increasing airflow volume towards its outlet. Outlet is sized to ensure no constriction of airflow, which causes static pressure loss. It is for this reason that low pressure centrifugal fan, which generally is of higher airflow, has thicker body to provide for enough space for outlet area. Like wise, high pressure blower fan with relatively low air volume to handle does not need thick body for air path, is generally made thinner.
Fan inlet is sized to ensure smooth air flow without big drop in static pressure. Inlet diameter design is intertwined with practical consideration for impeller maintenance removal. For small fan, impeller is removed and installed axially from its inlet. Its inlet shroud is normally built into removable bigger diameter flange. For bigger and heavy impeller, fan body is designed split with flanged connection.
Blower Fan Performance Point
Typical fan curve is an inverse relation between static pressure and airflow. During operation, performance point can sit at any point along the curve, depending on system pressure that the fan is subject to. This is normally combination of ductwork pressure loss and filter resistance. The intersection point between system pressure curve and fan performance curve is fan operating point, as illustrated in curve below.
As the performance point move along the fan curve, power consumption of any centrifugal fan rises with increase in airflow, which is also reduction in static pressure. In other words, power consumption or running ampere is reduced if the fan is shut from airflow by means of damper or high filter resistance. This is opposite of ring blower or roots blower, where pressure or vacuum level determines power consumption. At high pressure end of its performance curve, it consumes more power. A capped pipework to ring blower results in the blower running dry, potentially burn its motor out. That is reason pressure relief valve is installed for ring or roots blower, but not centrifugal fan.
Fan law
At the core of fan sizing are fan laws. You can find mathematical representation of it easily in the internet. For fan size and speed determination, the following 3 of its laws are used.
- Ratio of change in fan volumetric flow rate is proportional to ratio of fan rotational speed, x ratio of fan size to the power of 3.
- Ratio of fan pressure is proportional to ratio of speed to power of 2 x ratio of size to power of 2 x ratio of air density.
- Ratio of fan power is proportional to ratio of fan speed to power of 3 x fan size to power of 5 x ratio of air density.
A base fan with known performance curve is needed before fan law can be applied to establish fan performance of varying size and speed. Fan performance of first base fan can be predicted by series of formulas and tested in wind tunnel.
Air density is a variable in fan law, in many practical application, elevated temperature significantly reduces air density. Higher air temperature has effect to reduce static pressure a fan produces, as well as its power consumption. However, we must bear in mind that static pressure at ductwork is proportionally reduced in similar manner at elevated temperature.
Fan law does also include formula for noise power output from a fan, as a function of size and speed.
In designing and constructing centrifugal fan body and bearing seat, it is extremely important to consider vibration at axial axis, which is often neglected, beside radial and vertical. For a properly designed fan, vertical component of vibration should not exceed that of radial component. It is these 3 components of vibration that are taken at both shaft bearings that fully define fan its vibration condition.