Importance of Velocity Pressure

This past weekend I watched an excellent video on balancing airflow from CaptiveAire. If you haven’t seen their content before, it is worth taking a look!

This video demonstrated the challenge of achieving balanced air distribution through diverging branches like tee or cross fittings. A common solution to achieving balanced air distribution is the use of dampers. Their video explores alternatives to this, using methods such as Equal Friction and Static Regain, highlighting the unique challenges of each method with practical examples.

Their video also explores how static and velocity pressures are affected by system design like duct length, duct diameter, and fitting configurations. Somewhat counterintuitively, static pressure can increase downstream of fittings. This phenomenon is the basis behind the Static Regain method, where the goal is to optimize the design so that the static inlet pressure at the next downstream fitting is nearly equal to the prior fitting’s inlet pressure.

Why does static pressure increase downstream of a fitting? Minus the losses through the fitting, total pressure must be conserved. When flow splits at a tee, the velocity must change if the inlet and outlet duct diameters are identical. This means static pressure increases to compensate for the decrease in velocity pressure.

What is velocity pressure? Velocity pressure or dynamic pressure is the pressure exerted on an object in the direction of flow. Imagine the pressure (force sensation) on your hand by air when you hold it out of the window of a car in motion. Velocity pressure is proportional to fluid density and fluid velocity.

The typical gauge which mounts at a tee perpendicular to the direction of flow only measures static pressure. It can be inconvenient to install a gauge which captures dynamic pressure because it introduces additional losses to the system by obstructing flow (the gauge must enter the flow stream to capture dynamic pressure) and is not as easy to install in piping.

Sometimes the static gauge pressure measurement is mistakenly taken as the total pressure of the flowing system, whereas the true total pressure of the system is the addition of the static (gauge) pressure and velocity pressure.

In a lot of cases, neglecting to consider dynamic pressure will not change the results or is well within the margin for error. However, neglecting this aspect in engineering calculations could result in significant error in cases where the fluid undergoes a significant change in density or velocity.

Let’s look at an example. Let’s say a gas undergoes a phase change through a length of a heat exchanger. Velocity pressure depends on density and velocity squared. When a fluid’s density drops during the phase change from liquid to gas, velocity increases to maintain the same mass flow rate, causing an increase in velocity pressure.

In the equation above, differential velocity pressure is also expressed in terms of mass flow rate, cross sectional flow area, and fluid density at both points in the flow stream.

Mass flow rate is conserved and area is assumed to be constant, so we find that the driver of change in dynamic pressure is change in density. A large mass flow rate, small flow area, or substantial difference between liquid and vapor phase densities will increase the dynamic pressure differential.

If this factor is large enough, using the difference in what is measured between the two standard gauges and calling this the total differential pressure between the liquid and gas phase points in the example above would not only be incorrect but may result in significant error.

If the engineer pairs their test results with a predictive calculation, the results would not be in alignment with the calculated results, which could lead the engineer to make adjustments to their predictive calculation based on misrepresentative data. Now imagine they try extrapolating their model which was “validated” by test data to other operating conditions. What a mess that could make!

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