Pressure Drop Basics

Whether it is a furnace, air makeup unit, paint booth, or some other type of system, every application that requires filters, requires a fan pushing air through it, creating air pressure.  Air pressure is good, since it is the reason air circulates through the system.

There are factors in the system that will reduce the amount of air pressure.  This reduction is called “Pressure Drop.”

For filtration purposes, Pressure Drop is the reduction in air pressure caused using a filter.  It is a trade-off inherent in filtration: less efficient filters tend to have a lower Pressure Drop; more efficient filters tend to have a higher Pressure Drop.

In North America, Pressure Drop is measured in Inches of Water Column, or "WC.  In other parts of the world, it is measured in pascals, or pa.  We will only use "WC.

Initial Pressure Drop (sometimes called “Initial Resistance") is the Pressure Drop caused by a brand new filter, a filter which has never been used. The Initial Pressure Drop of a filter will vary, based on the type, thickness and efficiency of the filter.

As a filter is used, it collects particles and gradually it becomes more difficult for air to pass through it – the Pressure Drop caused by the filter increases.  Eventually, a filter will reach its Final Pressure Drop

Final Pressure Drop (sometimes called “Final Resistance.”) is the pressure drop at which a filter is meant to be replaced. Using a filter beyond its Final Pressure Drop will deter the performance of hardware and force the fan to work harder to push the same amount of air through the system.  This can increase the risk of expensive hardware failure.  Usually, the Final Pressure Drop of a filter will be either 1”WC or 1.5”WC, but this can vary based on the filter and application.

The Initial and Final Pressure Drops of a filter can be found in its specifications. Usually, the airflow at which the Initial Pressure Drop was measured will be specified. If not, it is usually 500 Feet Per Minute (FPM). If you look closely at the specifications for filters with the Initial Pressure Drop specified for different speeds, you’ll notice that the higher the FPM, the higher the pressure drop.

An Example of Pressure Drop:

I have a paint booth in my cabinet factory.  I install an Andreae AF813 filter, which I purchased from Taff-Guard Filtration, in my paint booth’s exhaust.  I check my booth’s Pressure Drop indicator and see it has an Initial Pressure Drop of 0.8”WC.

I use my paint booth for a few months and check the Pressure Drop indicator again.  It reads 0.4”WC.  The specification sheet for my AF813 indicates it has a Final Pressure Drop of 0.5”WC.  I can still use my filter.

A month later, I check my Pressure Drop Indicator again.  It reads 0.5”WC.  My filter has reached the end of its service life.  I remove the filter, dispose of it safely and install a new AF813.

Written by: Michael Lopez