Airflow

People are normally interested in the amount of air that is moved. This is most commonly expressed as a Volumetric Flow Rate. Typical units of measurement are: -

• m3/h metres cubed per hour
• cfm cubic feet per minute
• l/s litres per second

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 requires a force to overcome the resistance through a duct or tube. The resistance to flow is due to the viscosity of air and the friction of each molecule passing over one another. Typical Units of measurement: -

• mBar millibar
  
  
• Pa Pascals
  
  
• InWg Inches Water Gauge

Fan performance

Fan performance is presented on a graph called a fan curve. Volume flow is on the X axis and pressure is on the Y axis .It shows how much volume of air is moved against resistance to flow, or pressure development of the fan. As you increase the resistance to flow, the volume output of the fan decreases.

System resistance

The system be it a tube, black box or a room has a resistance to airflow, along it or through it. This resistance to airflow is expressed as a pressure. The value of this resistance to flow increases with increase in volume. The system resistance curve is a square law. Pressure changes in relation to air volume to the power of two.

P is proportional to V²

For example twice the volume is 2² which = four time the pressure.

Half the volume 0.5² = 0.25 of the pressure.

This can be plotted on a fan curve and where the system resistance crosses the fan curve will be the duty point of the system with that fan fitted.

Noise

A fan is a good generator of noise! We hear noise by sensing pressure variation on our ear drums. When a fan operates the noise generated from various parts of the fan is transmitted by air molecules bouncing against each other and spreading out as small pressure waves.

Sound power and sound pressure - Sound can be expressed in two ways. Sound power, how many watts of energy is converted into sound, or sound pressure where the pressure of the sound waves are measured in Pascal's.

Decibels dB - Both sound power and sound pressure are expressed in units of measurement called decibels. The measurements are on a logarithmic scale as the ear is sensitive to noise in a logarithmic fashion.

‘A’ Weighting - Noise figures are commonly expressed as A weighted. This is a method to provide a figure which gives a sound level which is perceived by the human ear.

Next topic - Fan Laws >>>