Understanding the Difference Between Flow, Velocity, and Pressure

Flow, Velocity, and Pressure Defined

One issue that seems to cause universal confusion when designing a blower-based system is understanding the differences between flow, velocity, and pressure and knowing when each metric is important. This article investigates this topic with a focus on how they relate to each other in applications with industrial blowers.

First, let’s define each term:

• Flow is a measure of air output in terms of volume per unit of time. The common units are litres per minute, cubic feet per minute (CFM), etc.
• Velocity refers to how fast the air is moving in distance per unit of time. The common units are feet per second, metres per second, etc.
• ​Pressure is the measure of force applied on an area. The common units for pressure are pounds per square inch (PSI), Pascals (Newtons per square metre), etc. There are also some traditional measures such as inches of water or inches of mercury which are defined as the pressure exerted by a column of water (or mercury) of 1 inch height.

How are Flow, Velocity, and Pressure related?

The relationship between flow, velocity, and pressure can be explained using Bernoulli’s principle. Bernoulli’s equation states mathematically that if a fluid is flowing through a tube and the tube diameter decreases, then the velocity of the fluid increases, the pressure decreases, and the mass flow (and therefore volumetric flow) remains constant so long as the air density is constant. This principle holds true for gases so long as the gas is moving far below the speed of sound and does not vary in temperature (as this would cause an increase in volume).
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Bernoulli’s equation is derived from law of conservation of energy in that if the kinetic energy of the fluid increases due to increased velocity, the corresponding energy associated with pressure (energy per unit volume) must go down.

A Blower-based Explanation

Firstly, as background it is important to understand there are two different pressure measurements that are used when designing a blower-based system: local pressure and cumulative pressure. Local pressure is the pressure at a specific point in the system, while cumulative pressure (sometimes called back pressure) is the total pressure exerted against the blower by the entire system. The length of tube or pipe used, any bends, nozzles, or inline heaters attached to the outlet of the blower will all contribute to the amount of cumulative pressure the blower must overcome to output a volume of air per unit time.
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Cumulative pressure is usually the value referenced by blower suppliers, as the amount of cumulative pressure affects the volumetric flow of the blower. As cumulative pressure increases, the volumetric flow rate of a blower decreases in accordance with Bernoulli’s principle. At the extreme end, if the outlet of the blower is completely blocked (extremely high cumulative pressure) there is zero air flow. If a blower is venting directly to atmosphere (zero cumulative pressure) it will be capable of its maximum rated air flow. A blower manufacturer can supply a blower curve which describes the relationship between pressure and flow for a particular model to aid in system design and equipment specification.

​Recommended Reading:

• Understanding Blowers as part of a System
• How to Interpret a Blower/Fan Curve

Determining which Variable is Important

​Understanding the interdependent nature of pressure, flow and velocity is important when designing a system using an industrial blower; but more important is understanding when each variable is the critical design parameter.

Velocity

​Velocity is critical when the application involves using the impingement force of the air to do work. A good example is a water blow-off system using air knives. In a blow-off system the air is required to make physical contact with the surface water with enough force to knock the water free. The force comes from the air impacting at high velocity. The velocity is increased by forcing a volume of air through a constricted outlet. Air knives are designed such that the air exits from a long thin gap creating a sheet of high velocity air. While the air is moving very quickly, it is not necessarily a high volumetric flow rate; this is a common misconception.

Flow

​Flow is critical when the air needs to fill a space. For example: supplying hot air to an oven/furnace, HVAC systems, heat tunnels, and preheating molds. In all of these cases you need to supply a given volumetric flow rate that continuously fills and replenishes the application space.

Pressure

If an application is expected to have a high back pressure, the blower must be rated to operate at that pressure. This will determine the type of blower you should use in your application. For example, you may decide to choose a high-pressure regenerative blower over a low-pressure centrifugal blower. Characteristics of a system with a high level of back pressure include: multiple lengths or long lengths of hose or pipe, inline heaters, applications where air is forced through small openings, and water/fluid aeration.

Recommended Reading:

• Regenerative versus Centrifugal Blowers
• Whitepaper: All About Blowers

This article is meant to give a brief overview of the concepts of pressure, flow and velocity but is by no means exhaustive. Every application has its own complexities and the solution may not be obvious. Take advantage of our Engineering Services for help determining the right solution for your application problem.
 
Originally published: 4/14/2014