Airflow/Volume, Air Pressure and Fan Motor Power

The performance of any fan – centrifugal or otherwise – comes down to three inter-related factors:

  • Airflow – how much air will the fan move in a given period of time?
  • Pressure – how much (or how little) force is needed to move the air from one place to another?
  • Power – how much energy is required to create the pressure needed to move the air?

Let’s examine these factors in more detail…


Airflow refers to the movement or circulation of air caused by a fan or blower. It is typically measured in terms of volume per unit of time, usually cubic metres per hour (m³/h) or cubic feet per minute (CFM). Airflow in fan engineering is influenced by several factors including the:

  • shape and size of the fan
  • rotational speed of the fan’s impeller
  • shape and size of the impeller blades
  • resistance or pressure against which the fan is working.

These factors collectively determine the fan’s ability to move air effectively and efficiently. It’s important to understand the concept of airflow in fan engineering, as it is a critical parameter that impacts the performance, efficiency and specification of fans for all the various applications. Airflow calculations and performance curves enable us to determine the appropriate fan size and type for your given application – taking into consideration factors such as the desired airflow rate, pressure requirements, and efficiency.

Chart showing various MR fan airflow and pressure


Air pressure is defined as the force per unit area exerted by air against a surface perpendicular to the direction of airflow. It represents the resistance or opposition that the air encounters as it moves through a fan, duct or system. Higher air pressure typically indicates higher resistance to airflow, while lower air pressure indicates lower resistance. There are three types of air pressure:

  • Static pressure – this is particularly significant. It is the pressure of the air (in a duct or system) when it is at rest or not in motion.
  • Static pressure can be measured at various points in a duct or system, such as before and after a fan, at bends or elbows in the ducting, or at other points of restriction. The difference between the static pressure at two points is known as the pressure drop or pressure loss, and it is an important parameter used to evaluate the output and efficiency of a fan system.
  • Dynamic pressure (aka velocity pressure) – the pressure of the air due to its motion. This pressure is a measure of the kinetic energy of the moving air. It is used to determine the dynamic effects of air movement, such as the impact of velocity on duct design, fan selection and system performance.
  • Total Pressure – is the sum of the static pressure and the dynamic pressure of the air. It represents the total energy of the air in a duct or system. Total pressure is an important parameter in fan engineering as it indicates the total resistance that the fan needs to overcome to move the air through a system.
Chart demonstrating a fan volumetric flow
Volumetric Flow
Graph showing fan motor current

Motor Current

Chart showing a fan speed

Fan Speed

Measuring Air Pressure

If you’re thinking in terms of PSI or bar – such as the air pressure needed to inflate a car tyre – then you’re using a sledgehammer to crack a nut. The air pressures we need to measure are far lower. So we use:

  • Millimetres of Water Gauge (mmWg)
  • Inches of Water Gauge (in.w.g.)

mmWg and in.w.g both represent the pressure exerted by a column of water of a certain height, measured in millimetres or inches, respectively. These units are used to measure low air pressures where the pressure differences are relatively small. mmWg and in.w.g. are used a lot in fan engineering because they provide a convenient and practical way to measure low air pressures with precision and accuracy. These units are easy to understand. They are used in manometers and gauges designed specifically to measure low pressures.

Electrical test of an industrial fan

How We Test Fans

Fan testing is the responsibility of the engineering/R&D department but its outcomes affect the work of the whole team – notably design engineers, technical engineers, planning engineers, technicians, prototype engineers and electrical engineers. Tests are conducted with reference to:

  • ISO 9001 (quality management)
  • ISO 14001 (environmental management systems)
  • ISO/IEC 80079-34 (application of quality systems for EX product manufacture).

Our engineers use two types of test rigs to determine airflow at either the inlet or discharge points:

  • Inlet Side Test Rig – the flow rate is measured using pressure readings taken at 1 x duct diameter and 0.5 x duct diameter away from an in-duct orifice plate (D and 0.5D), in accordance with ISO 5801 standard for industrial fans. Different diameter orifice plates are used to cover the flow range of the various fan units. The fan static pressure is measured using an inlet side test chamber.
  • Discharge Side Test Rigs – again, the flow rate is measured using an in-duct orifice plate with pressure readings at D and 0.5D, (ISO 5801). And again, different diameter orifice plates are used to cover the flow range of the various fan units. The fan’s static pressure is measured using a duct matched to the size of the fan discharge.
ACI engineer testing an industrial fan
In each case, the tests involve:

  • noting the atmospheric and ambient temperatures
  • taking a series of pressure, temperature and electrical current measurements as the fan pressure is varied between zero – when the fan delivers its maximum volume – up to the point where the fan moves no air and produces its maximum pressure
  • applying the measurements to the BS-approved formula to calculate the performance
  • plotting the results as the static pressure curve where the X-axis is flow and the Y-axis is pressure.

Determining the exact performance of fan designs allows our engineers to recommend the ideal solution for each customer’s specific system requirement as well as helping identify potential areas of improvement to each fan design. By measuring airflow, pressure and power precisely, we can calculate how much money a new air delivery system would save you and how soon you would see a return on your investment. In many cases, it’s an absolute no-brainer in terms of ROI – your new system could pay for itself in a matter of months.

Factors That Affect Fan Output And Efficiency

Every customer and application is unique because there are so many different variables that affect fan performance. We take them all into account when designing the best system for your requirements. And it’s not just the fan specifications such as motor speed, impeller design, volute casing dimensions, and the size of the inlet and discharge point. There are also other factors such as:

  • the resistance to flow of the system, the fan is to operate in
  • the geographical location of the site – air temperature, climate, seasonality, humidity.
  • height above sea level (which affects air density).

Our dedicated fan testing facility also allows us to test customer equipment for any potential resistance to flow that may be caused by ducting or other resistances in the system. Our engineers can help you significantly reduce the amount of money you spend on air delivery applications – especially if you’re currently using supposedly ‘free’ compressed air. The reality is that compressed air is anything but free – it is very energy-hungry and, therefore, extremely expensive.

Fan being tested prior to shipping

Get Expert Technical Advice On Fan Performance And Testing

Contact ACI’s fan engineers for expert guidance on how to solve your fan engineering challenges. With our precision testing, we can help you to specify the best fan for your application. on your investment.


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