Bernoulli’s Principle

Have you ever thought about the relationship between velocity and pressure? If you think about a fluid being forced through a small orifice you can fairly easily picture the fluid speeding up as it goes through the orifice but did you also know that the surrounding static pressure DECREASES at the point of high velocity?

Energy Vanguard has a great video demonstrating this phenomenon

This lower pressure in a high-velocity fluid (in this case air), also explains entrainment of air into a high-velocity supply air stream that helps mix the air in a room. When the velocity of an air stream is higher, the static pressure around that air stream is lower which draws in air from around that stream (entrains it) and increases the total amount of air being moved in that direction with additional air being drawn in from the sides.

This is also the principle that causes both primary air to be drawn in around the stream of fuel leaving a nozzle as well as secondary air to be drawn into a gas appliance around the flame as the hot air pulls up and through the appliance.

Remember that this effect relies on a directional velocity flow of a stream of fluid (air, water, gas etc..) that is free to flow in the direction of its velocity. As soon as you add in turbulence and containment it isn't all as clear and we do see that directional motion turns into “static” pressurization like we see inside a duct system or in a refrigerant circuit.  We will most often see this principle at work in it's pure form in gas burners, flues, and chimneys and at supply duct registers.

Either way, Bernoulli's Principal is a good thing to be aware of and now that you know about it you may just start to see it at work everywhere.

— Bryan

Related Tech Tips

Pressure / Enthalpy Diagram Example
This article is written by my buddy and Canadian Supertech Tim Tanguay. Thanks Tim! This P/E chart shows R410a at 100°F Saturated Condensing Temp, 10°F SC 40°F Saturated Suction Temp, 20°F SH at the compressor. The green highlighted thumb shape is the saturation zone. Everything that occurs in the saturation zone is a latent (change […]
Read more
Glide at Static Pressure
We've been pretty spoiled in residential and light commercial in the USA because we haven't needed to deal with glide much. R22 has no glide and R410a is a near-azeotropic blend which means it has almost no glide. The days of being able to ignore glide are coming to an end. Carrier has announced their […]
Read more
Static Pressure, Manometers and Magnehelics
When you start talking airflow, it can get pretty in-depth pretty quick. There is a big gap between what is useful for the average tech to apply every day and the whole story so let's start with the simplest part to understand, Static Pressure. Static pressure is simply the force exerted in all directions within […]
Read more

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

en English
X