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How differential pressure switch works for Exhaust Fans

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The Fan is an apparatus with rotating blades that creates a current of air for cooling or ventilation. it is a powered machine used to create flow within a fluid, typically a gas such as air. A fan consists of a rotating arrangement of vanes or blades which act on the air. The rotating assembly of blades and hub is known as an impeller, rotor, or runner.
Differential Pressure Switches for Exhaust Fans
There are three main types of fans used for moving air, axial, centrifugal (also called radial) and cross flow (also called tangential). The American Society of Mechanical Engineers Performance Testing Code 11 (PTC) provides standard procedures for conducting and reporting tests on fans, including those of the centrifugal, axial, and mixed flows. 

Differential Pressure Switches is installed across fans in order to identify actual fan running condition by the pressure difference so, Differential Pressure Switches positive tube must be installed after the fan or say positive side and negative tube to be installed before the fan or open to the environment or say negative side and pressure range selected as 100 pa for Differential Pressure Switches.
Digital Air Differential Pressure Transmitter

GE-923 Air Differential Pressure Transmitter 

This GE-923 Air Differential Pressure transmitter is produced with mag...


1. Initially when a fan at off condition, before the fan and after fan will be same pressure, so there is no differential pressure across before and after the fan.here pressure differential pressure or PD is zero.

PD=0pa and lesser than 100pa(range)

so here still NC and C remain the same.

2. When the fan starts to run, let us say on the negative side is 40pa and the positive side is 100pa, where the Pressure difference across the fan of the positive and negative side is

PD=100-40pa.

PD=60pa still lesser than 100pa(range).

so here still NC and C remain the same.

3. Immediately when the fan runs at high speed, let say pressure difference before the fan and after fan will be high let say 150pa in positive side and -30pa in the negative side so

PD=150pa-(-30pa)

PD=180pa which is greater than 100pa(range).

so here Differential Pressure Switches triggers NC to NO and C.

This signal will be connected to BMS PLC controllers or any other control circuit to perform the required operation.
Air Differential Pressure Switches

Air Differential Pressure Switches for Exhaust Fans

GE-924 Adjustable Air Differential Pressure Switch for monitoring over...


4.When a room have many fans for ventilation, If one fan is failed, the air flow backwards from other fan, it should have pressure in the duct, then how to install the differential pressure switch to alarm this failure? We have this experience about it. If you want to know the answer, please send a email to us (A.YITE TECH -- ZHOU ENSI LTD), we will answer you separately.

However fan running condition can be monitored from relay contact of the motor control circuit panel, but here Differential Pressure Switches is used to know the actual fan running condition by confirming flow across the fan because from the motor panel will trigger the relay but not know whether fan really running or not, sometimes motor belt damage.

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