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
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.
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
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.
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.
