We are generally interested in the amount of air that is moved. It could be because regulations call for a particular air change per hour, for example offices typically have four to six changes per hour. Alternatively a machine needs to be cooled and therefore requires a particular volume of air to remove the heat to keep it cool. Volume flow is generally measured and expressed in the following units;
Resistance to Airflow
Air needs to be pushed or pulled to move it from one point to another. When moving any object, force is required to overcome the resistance to movement. Moving air through duct, tubes or equipment requires force to overcome the resistance to flow. The force to push/pull the air is called pressure. The units of measure for pressure are;
Fan performance is represented on a graph called fan curve. Volume flow is on the X axis and pressure is on the Y axis. It represents how much volume of air the fan is able to move against a particular resistance to flow. As the resistance to flow increases, the volume output of the fan decreases. Please refer to fig. 1.
The system be it a duct, room or piece of equipment has a resistance to airflow, known as ‘system resistance’. This resistance to airflow is expressed as pressure. The magnitude of resistance to flow increases as the volume of flow increases. The system resistance increases in a square law function, p is proportional to V2. For example to increase the volume by a factor of 2, means that the resistance to flow increases by a factor of 4. Please refer to Fig. 1 which shows a system resistance curve plotted on the fan curve. Where the system resistance crosses the fan curve is the duty point for the fan in that particular system.
A fan is a good generator of noise! We hear noise by sensing pressure variations on our eardrums. When a fan operates the noise generated from the various components of the fan is transmitted by pressure waves radiated out on the fan.
Sound Power & Sound Pressure
Sound can be expressed in two ways. Sound power - how many watts is converted into sound, or sound pressure where the pressure of sound waves are measured in pascalls. Both sound power and sound pressure measurements are expressed on a logarithmic scale, as the ear is sensitive to noise in a logarithmic fashion.
Comparison of Sound Power v Sound Pressure
An important point to note is that for the same sound level the sound power figure and sound pressure figure will be different. For example a conversation between two people can be measured as 50 dB sound pressure and 70 dB sound power. As a numerical value one looks larger and therefore louder than the other, but in fact it is the same sound level to your ear.
Noise figures are commonly expressed as A weighted. This is a method to provide a figure which provides a sound level which is perceived by the human ear.
Simple Fan Selection
When given a performance requirement, selection can be fairly straightforward. For example if a requirement is given for 600m3/hr at 500 Pa then this duty would fit the graph shown in Fig. 1 perfectly. However, it is rare that a required duty fits a fan curve perfectly. If the required duty was 1000 m3/hr @ 200 Pa. The fan shown in Fig. 1 would actually give 1150 m3/hr @ 275 Pa. The fan would provide more volume flow than required and attempt to move more volume through the system. The system back pressure increases through the square law, resulting in the actual duty point being where the system resistance crosses the fan curve.
The following are some simple laws associated with any type of fan, be it axial, forward curved or backward curved centrifugal.
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