the principal of fan engineering

Gain a grounding in the basics of fan engineering by learning to read fan curve graphs, acquainting yourself with fan laws and understanding the impacts of temperature and altitude on fan performance.

#1. How to understand fan curves

Fan curve graphs might look complicated, but once you understand the principles behind them, they are relatively easy to decipher. Essentially, they are just graphs showing fan performance data that is generated by connecting a fan to a laboratory test chamber.



The ability to read fan curve graphs is helpful to anyone who is involved in specifying industrial fans – it allows them to verify fan selection, trouble-shoot if the fan is not operating to design conditions and future-proof for flexibility.

ACI Fan Curve Graph

#3. Fan terminology explained

A good place to start is to look at some of the terminologies around fan engineering.

– Static pressure (Pa or Ps): the resistance pressure the fan has to blow against in order to move air in the desired direction. If the fan is blowing against high pressure, it requires more power and delivers less air.

– Dynamic pressure (Pd): this is the pressure created by the movement of air. It increases as velocity increases and is always positive.

– Total pressure (Pt or pT): this is the sum of all static elements plus the dynamic pressure at the discharge of the fan. Static pressure + dynamic pressure = total pressure.

– Zero flow, block off, shut off, or static: all of these terms have the same meaning – the point of zero airflow on a static pressure curve. This is when the fan moves no air and develops maximum pressure.

– Free air, free delivery, or wide-open performance: the point on a static pressure curve when a fan delivers its maximum volume. This is the point at which static pressure is zero.

– Duty point, operating point, or design point: the point on the fan performance curve where the fan is operating when connected to the system. This is also the point where the system resistance curve intersects the fan performance curve

#4. Reading fan performance graphs

The first thing to note about the fan curve graph is that it shows the relationship between three performance variables at a constant speed (RPM):

– Horizontal X-axis: volumetric flow rate, typically expressed in cubic feet per minute (cfm), cubic metres per second (m3/s), cubic metres per hour (m3/h), or litres per second (l/s).

– Vertical Y-axis: static pressure: typically expressed in inches of water gauge (in. w.g., or in. H2O), Pascals (Pa), or millimetres of water gauge (mm H2O).

– Secondary Y-axis: power, expressed as HP or kW.

The static pressure curve shows the relationship between the static pressure capabilities of the fan and the fan’s airflow. This is the basis for all airflow and pressure calculations. For any given static pressure there is a corresponding airflow rate at a given RPM. For example, on the curve shown, a static air pressure of 4InWg results in an airflow of 6.5CFM.

#5. How to select the right fan for your application

Generally, a centrifugal fan will exhibit erratic airflow that generates excessive noise and vibration when operated to the left of peak pressure. The recommended selection range is the stable ‘right side of the fan curve’.

The power curve shows the relationship between the fan’s air volume flow rate and brake horsepower. Once you have selected your desired CFM, if you trace a line vertically up to the point where it intersects the BHP curve, this will give you the operating brake horsepower BHP rating you need for your fan motor to achieve this airflow.

Generally, the BHP required by a fan increases as flow through the fan increases. With many airfoil and backward-curved or backward-inclined fans, however, power reaches a peak value and then decreases as flow increases. This type of power curve is known as ‘non-overloading’. If the driving motor power is greater than the peak power on the curve with allowances for temperature and drives, etc., the fan should not overload in operation. The power curves of many axial propeller fans and ventilators will show power decreasing as flow increases to the point of free delivery.

In the case of most industrial exhausters, pressure and combustion blowers, and forward-curved fans, the power curve increases as flow increases to the point of free delivery.

Some graphs will also include a total pressure curve. As well as static pressure, this includes the dynamic pressure due to the velocity at the discharge of the system (fan total pressure is equal to the sum of fan static pressure and fan velocity pressure).

#6. What are the fan laws?

You might question whether, given the technological progress of the last few decades, fan laws are still applicable. The answer is ‘yes’ – these laws are regularly used by engineers to help them predict the implications of redesigning or upgrading industrial extraction and ventilation systems. Fan laws are the foundation that determines the relationship between fan airflow rate, static pressure, speed, and power.

ACI Fan Law

Law 1
This law tells us that if the speed is changed, the air volume will change proportionally. E.g., if the propeller speed is increased by 10%, the airflow rate will also increase by 10%.

Law 2
This tells us that the change in total static pressure will increase by the square of the change in speed. E.g., if the propeller speed is increased by 10%, total static pressure will increase 21%.

Law 3
This tells us that the power requirement will increase by the cube of the change in speed of the fan. E.g., if the propeller speed is increased by 10%, the power required to run the fan will increase 33.1%.

#7. How can we use fan laws?

Here is an example of where the laws might come into play: a company extracts waste heat from its processing plant to maintain a constant factory air temperature. Following an investment in additional machinery, it has to increase the number of daily air changes from three to five. Using fan laws, the company can calculate what RPM the fan would need to operate at to deliver this increase in airflow and how much more power the upscaled system would consume. This is assuming the original fan has the capacity to run at higher speeds, otherwise, a new fan would need to be installed.

As the relationship between these variables is non-linear, fan laws would show that to achieve an incremental increase in airflow would necessitate a disproportionately large increase in power consumption.

Another scenario might be where a company requires more airflow but doesn’t want to increase the size of its exhaust fan because of the prohibitively high cost of changing the ductwork. Calculations using the fan laws can establish whether this is feasible by changing motor speed.

This type of calculation can help companies de-risk decisions around upgrading and redesigning air moving systems to avoid costly mistakes and inefficiencies.

In addition, the laws can be used to adjust operating parameters. If you do not need to use the maximum recommended pressure of a fan, in an air knife system for example, but adjust the speed to meet the desired pressure in the system, you can save a considerable amount of energy. As can be seen in the laws above, a reduction in fan speed of only 10% results in an energy saving of more than 25%!

#8. The effects of temperature, altitude, and humidity

Fans are tested in laboratory conditions and will not perform according to their manufacturing specification outside normal temperature and pressure (NTP) conditions. These assume a standard air density of 1.2kg/m3, an air temperature of 20°C and barometric pressure of 101.32 kN/m3. Whenever a fan is operated in an environment or system where any of these conditions vary, adjustments must be made.

Increasing temperature causes most materials to expand. Air acts the same way. As the air temperature rises, the molecules in the air move further apart, making the air thinner, lighter, less dense. The same is true is true of altitude – the air density at the top of Everest is much less than at sea level.

Fan air flow is not affected by air density (the same volume of air passes through the fan – just thinner air). However, system pressure and fan motor power will change in proportion to the change in air density ratio. At higher temperatures both the pressure and the power demand will be less – it requires less horsepower to move the same volume of lower density air. Conversely, at lower temperatures, both the pressure and the power demand will be more. This might mean that a different motor needs to be specified, for example.