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Fan Basics

Session 4 - Airflow and Static Pressure

When you evaluate the performance of fans, you probably look at the maximum airflow and maximum static pressure ratings in catalogs. However, neither of the maximum airflow and maximum static pressure values can ever be obtained in real systems. In this fourth session, let's learn about fan airflow and static pressure.

Definitions of Maximum Airflow and Maximum Static Pressure

Airflow indicates the volume of air that a fan can move per unit of time, and static pressure is the ability of a fan to push air against resistance. Higher static pressure means that the fan can even ventilate inside equipment with high mounting density.
The maximum airflow is defined as the airflow when there are no obstructions on either the inlet or outlet sides of a fan. The maximum static pressure is the static pressure when the fan's outlet is completely blocked. It is impossible, however, to meet either of these conditions in real-world operation, so a fan's maximum airflow and maximum static pressure can never actually be obtained.

Definitions of Maximum Airflow and Maximum Static Pressure

Airflow vs. Static Pressure Characteristics

So, what are the airflow and static pressure under real operating conditions?
In our catalogs, every fan model is provided with a curve titled "Airflow - Static Pressure Characteristics" separate from the specifications table. The airflow and static pressure values under the fan's operating conditions are the points on the curve.
Airflow vs. static pressure characteristics, also called P-Q performance curves, show the performance characteristics of fans, and vary with the fan type and model. This session will explain P-Q performance using a typical axial fan as an example.

Airflow vs. static pressure characteristics (P-Q performance) example

As the above P-Q performance curve shows, airflow is at its maximum when static pressure is 0 Pa, and static pressure is at its maximum when airflow is 0 m3/min. The airflow and static pressure values under the fan's operating conditions lie between these two points.
The shape of P-Q performance changes when fan speed is altered, and also when multiple fans are used.

Performance Change due to Speed Change

In principle, airflow is proportional to rotational speed and static pressure is proportional to the square of the rotational speed. For example, doubling the rotational speed will double the airflow and quadruple the static pressure. Using this rule, you can approximate the P-Q performance curve for your desired rotational speed from the base P-Q performance curve given in our catalogs.

P-Q performance curve example

Performance Change when Combined

When multiple fans are combined, parallel and serial configurations result in different P-Q performance curves. For example, let's think about combining two of the same fan. Theoretically, combining them in series will double the static pressure, and combining them in parallel will double the airflow.

P-Q performance when operating two fans in series P-Q performance when operating two fans in parallel

However, under real-world conditions, the air flow from each fan interferes with each other, so they rarely exactly double. When two fans are placed right next to each other, the interference increases even more, further deviating from the above-mentioned theoretical values.

Moreover, when multiple enclosures with fans are combined, lower-capacity fan's performance might be severely hindered. For example, each of enclosures A and B have a fan mounted, and both fans have sufficient blowing capacities in individual enclosures. But, it should be noted that combining them in one enclosure might render the fan in enclosure A almost non-functional.

Combination of fan-mounted enclosures

As in the above example where there are multiple enclosures in a device, it is common for thermal design to be optimized for individual enclosures. In such cases, it can happen that parts with high mounting density are left with little ventilation. In addition, installing extra components can change the fan's operating environment, so this also needs to be taken into account when designing devices.

Date of publication: March 12, 2018

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