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Understanding Static, Dynamic & Total Pressure In Air Movement

Pressure Definitions

Pressure DefinintionsStatic Pressure

Static pressure is signed as Pst or sometimes on fan curves as Pfa.
Static pressure is defined as “the pressure exerted by a still liquid or gas, especially water or air”
This is the difference in pressure either across an element in the system, for example a carbon filter or a heating coil, or between the inside of the system and the outside atmosphere. Static pressure can be either positive or negative. Understanding the static pressure is useful when selecting the correct fan for the application. Fan selection is typically based upon a CFM and static pressure.
In simple terms, Static pressure is the pressure of a fluid or gas if it was not moving. An example would be the pressure inside a balloon. It is used to determine a fans power by showing how much it would increase the static pressure if a fan was blowing into a sealed system. Imagine using a fan to inflate a very weak balloon, a fan with a higher static pressure would inflate the balloon to a much larger size than one with a lower static pressure. The static pressure therefor can indicate how strong a fan is and how good it is at overcoming resistance.

Key Facts:

– Resistance to flow
– Equal in all directions
– Can be positive or negative
– Independent of air velocity

Dynamic Pressure

Dynamic pressure is usually signed as Pd or PDy.
As the name suggests this is pressure created by the movement of air, it increases as velocity increases and is always positive. An example of this can be shown by sticking your hand out of a car window when it is moving. Your hand will be pushed backwards because the pressure in front of your hand is higher than the pressure behind your hand. This is because your hand is bring the air to rest and providing a pressure difference equal to the dynamic pressure of the fluid/moving air. The faster the car is going, the more force pushing your hand back, this is because the dynamic pressure of the air is higher. Note that driving faster does not increase the static pressure at all, as the air would still have the same pressure if you were standing still.

Total Pressure

Total pressure is usually signed as Pt ot PT. This is the sum of all the static elements in a system plus the dynamic pressure at the discharge. So the sum of both the static and dynamic pressure.
Pt = Ps + Pd
In the previous example using the hand out of the window, the measured pressure in front of the hand will be the total pressure while the pressure behind it would be the static pressure. The difference would be the dynamic pressure.
Many people are confused over the total pressure and only include the sum of the static elements, calling this “the total pressure drop” but when using the total pressure curves you must always add the dynamic pressure due to the velocity at the discharge from the system, for example at 10 metres per second, discharge velocity Pd is 60 pascals.

Pressure Definintions
Depending on the application, the difference between total and static pressure may be negligible, but for others, neglecting the difference may result in costly mistakes.
For more information on system pressure calculations or for assistance with fan selection, contact us on 01782 349430 or email us on sales@axair-fans.co.uk

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