Static Pressure Basics

Understanding static pressure in air handling equipment is essential to a good design.  Static pressure considerations are also one of the fundamentals that are the most expensive to fix in the field if a unit was designed under poor assumptions or if an error was made during the design process.  Today we are going to cover some of the fundamentals of static pressure in HVAC equipment.

Static pressure, usually expressed in inches water column(iwc), is the pressure exerted on a surface at rest with respect to the air moving in duct, but not the pressure due to the motion or velocity of the air, and is also known as resistance, friction, friction loss, or pressure loss. Read that last sentence again.  Static pressure is the force exerted on the duct not due to the pressure from the moving air.  Another type of pressure in a duct is known as velocity pressure.  Velocity pressure is the force that the moving air exerts on a surface in the direction of the moving air.  The final type of pressure in a HVAC system is total pressure.  Total pressure is the algebraic sum of velocity pressure and static pressure.  Expressed mathematically

$P_{total}=P_{velocity} + P_{static}$

What is important to understand is that static pressure is exerted equally in all directions and that velocity pressure is exerted only in the direction of airflow.  This makes it difficult to directly measure velocity pressure in a duct.  Simply put, because static pressure is also pushing in the direction of airflow, you can never measure just velocity pressure. Practically, velocity pressure is calculated by measuring pressure perpendicular to the airflow(Static Pressure) and also measuring pressure parallel to the airflow(Total Pressure).  Once you have these two values you can just subtract static pressure from the total pressure and derive velocity pressure.

$P_{velocity}=P_{total} + P_{static}$

Static pressure is always measured relative to another pressure.  Typically it is measured with respect to atmospheric pressure, but it can be measured between any two points in the system.  Furthermore, because of the additive relationship between pressure measurements, you could take a pressure reading at two points in a system with respect to atmospheric and then subtract those two values and you will find that the derived value is equal to the pressure if it were measures strictly between the two points.  Mathematically

$P_{1 to 2}=(P_{1}-P_{atm})-(P_{2}-P_{atm})$

$P_{1 to 2}=P_{1}-P_{atm}-P_{2}+P_{atm}$

$P_{1 to 2}=P_{1}-P_{2}$

The final topic I want to touch on in this introduction to static pressure is Total Static Pressure(TSP), External Static Pressure(ESP), and Unit Static Pressure(USP).  The first thing to note is that total static pressure is different than total pressure.  Total Static Pressure is defined as the sum of External Static Pressure and Unit Static Pressure.

$TSP=ESP + USP$

External static pressure is the static pressure in the supply and return duct work that a fan would typically need to work against.  Unit static pressure, also known as Internal Static Pressure, is the pressure drop across filters, coils, and twists and turns inside the air handler.  As a custom unit designer it is your responsibility to calculate and measure the static pressure drop through your unit.  The external static pressure is usually a given to you.  The ESP is set by the building design engineer as they layout their ductwork, diffusers, and terminal devices that your custom unit would need to serve.

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