When discussing air pressure related to ventilation systems, the basic terms “total”, “dynamic”, “velocity”, “static”, “drop”, and “loss” are often used. How these pressure terms are applied and the means for measuring them can be confusing. In part one of this two part series, I will provide some clarity to understanding pressure terminology.
Static Pressure
The scientific definition of static pressure is “the pressure exerted by a still liquid or gas, especially water or air.” In simple terms, static pressure is the pressure of a fluid or gas that is not moving. Static pressure can be either negative or positive. A good example of static pressure is the force exerted by air on the inside of a balloon. For a ventilation system, static pressure can be measured as the difference between the inside and outside pressure at any point along the system.
Pressure Drop
Pressure drop or pressure loss is the difference in static pressure across an element in a ventilation system. It indicates the amount of resistance that an element exerts against the air flow. A good example of an element that resists air flow in a ventilation system is a filter. The pressure drop caused by the filter can be measured as the difference between the static pressure taken upstream and downstream of the filter. The formula would be written as:
Ps = Pu – Pd
If the upstream static pressure is 0.25” w.c. and the downstream static pressure is 0.15” w.c., then the pressure drop for the filter is 0.10” w.c. (0.25 – 0.15 = 0.10).
Velocity Pressure
Velocity pressure or dynamic pressure is created by the movement of air. It is always positive and increases as the air velocity increases. A good example is sticking your hand out of a car window. When the car is not moving, the static pressure on either side of your hand is equal so it doesn’t move. As the car starts to accelerate, your hand will be pushed back. The acceleration of the car has not changed the static pressure in the front or back of your hand so what you would be feeling is the increased velocity pressure.
Velocity pressure can be calculated at any point along a ventilation system based on the air flow velocity. The formula to determine velocity pressure in inches of w.c. is:
Pv = (FPM/4005)2
Let’s assume that the air flow velocity at the discharge of a ventilation system is 1,500 FPM. The velocity pressure at the discharge would be calculated to be 0.14” w.c. (1500/4005)2.
Total Pressure
Total pressure is the sum of all the static drops across all elements in a ventilation system plus the dynamic pressure at the discharge. The formula for total pressure is:
Pt = ∑Ps + Pv
To illustrate the calculation of total pressure as we have a simple ventilation system that consists of a fan and filter. We will also assume the filter has the same pressure drop as the one we used in the example above and the ventilation system discharge air velocity is 1,500 FPM. The formula for total pressure of our simple ventilation system is:
Pt = 0.10 + 0.14 = 0.24” w.c.
In this first part of this series, we have accomplished understanding pressure terminology. In the second of this two part series, I’ll discuss how these pressure terms apply to selecting the right fan for an application.