What is air velocity in pneumatic conveying?

Air velocity is the speed at which air moves through a pneumatic conveying line. It helps keep material particles suspended as they travel through the system.

What happens if air velocity is too low?

If air velocity is too low, material may fall out of suspension, build up in the line, cause blockages, reduce productivity, and increase downtime.

How Do You Calculate Air Velocity in a Pipe?

Q: How do you calculate air velocity in a pipe?

Air velocity can be calculated by dividing the volumetric air flow rate by the cross-sectional area of the pipe.

Q: What happens if air velocity is too high?

If air velocity is too high, materials may degrade and conveying components such as pipes and elbows may experience increased abrasion and wear.

In pneumatic conveying systems, it’s crucial to correctly calculate the required air velocity in pipelines in order to preserve product quality, minimize abrasion to components, and keep operations running efficiently. If the air velocity is incorrect, it could not only negatively affect both the product and system, it could also lead to costly periods of downtime and a halt in production.

What is Air Velocity in a Pipe?

In pneumatic conveying, the air velocity is the speed of the air circulating in the conveying pipes. This is what keeps the material particles in suspension as they travel through the conveying lines. Also known as the conveying air speed or air flow velocity, this changes along the pipelines in relation to changing pressure and temperature.

The required air velocity in a conveying pipe depends on the specific conveying line and the materials it’s transporting, as well as other factors, like the solids loading ratio. This is the ratio between the mass flow rate of the material being conveyed and the mass flow rate of the air used to convey the material.

Determining the concentration of particles suspended in the air, the solids loading ratio can be used to figure out the minimum air velocity a material needs in order to get from A to B without causing line blockages, product degradation, or abrasion. For materials being conveyed in dense phase, for example, the minimum air velocity decreases as the solids loading ratio increases.

The type of material being conveyed also affects the air velocity in pneumatic conveying. For example, soft material particles, such as plastics, require a lower air speed in order to prevent unnecessary frictional heat and the formation of streamers, which can lead to material degradation, pipe blockages, and pressure drops. Meanwhile, heavy material particles need to be conveyed at higher air velocities in order to stay in suspension.

What is the Difference Between Air Velocity and Air Flow Rate?

The air velocity in pneumatic conveying lines refers to the air speed, while the air flow rate refers to the volume or mass of air output (known as volumetric air flow and air mass flow respectively). Both the air velocity and air flow rate must be proportional to each other in order to maintain optimal conveying conditions in the pipes.

How Do You Calculate Air Velocity in a Pneumatic Conveying System?

To calculate air velocity in pneumatic conveying systems, you can divide the volumetric air flow rate in m³/s by the passing section of the pipe in m². However, since air velocity changes along the pipelines, caused by the changing pressure and temperature, calculations should be based on a specific reference position, which is either at the beginning or end of the line.

It’s crucial to calculate the air speed for a specific pneumatic conveying line properly in order to keep particles suspended throughout the pipelines. The air velocity must always be kept at the minimum acceptable level in order to ensure this, so while approximations can be given for different types of pneumatic conveying systems, it’s always best to calculate the air velocity that your specific system and its material needs.

If the air velocity is too low, for example, particles can fall out of suspension and block the pipelines. This reduces productivity, compromises the integrity of the system, causes periods of downtime, and loses a lot of money. If the air velocity is too high, on the other hand, particles are more likely to abrade the internal surfaces of pneumatic components, like pipes or elbows.

Here’s an example of how you can calculate the air velocity in a dilute phase vacuum system under different conditions:

Condition 1: Beginning of the pipe

u_air = air velocity in conveying pipe of diameter D (m/s)

Q_air = air volumetric flow rate (m³/s)

D = pipe diameter (m)

Using the calculation formula below, calculate the volumetric air flow rate according to the beginning of the pipe’s specific conditions. E.g. atmospheric pressure, a temperature of 20°C, and an internal pipe diameter of 80mm

Q_air = QVN.T/273*1.013/P = 400*293/273*1.013*1.013=429 m³/h=0.119 m³/s

Once you know the volumetric air flow rate, you can then use this following calculation formula to determine the conveying air velocity:

u_air = Q_air / (π.D²/4) = 0.119/(π*0.08²/4) = 23.7 m/s

Condition 2: End of the pipe

Using the calculation formula below, calculate the volumetric air flow rate according to the end of the pipe’s specific conditions. E.g. a pressure of -0.3 bar g, and a temperature of 20°C

Q_air = QVN.T/273*1.013/P = 400*293/273*1.013*(1.013-0.3) = 609 m³/h=0.169 m3/s

Just as before, you can then use the volumetric air flow rate and the following formula to calculate the air speed:

u_air = Q_air / (π.D²/4) = 0.169/(π*0.08²/4) = 33.7 m/s

You can also figure out the air velocity at different portions of the pipes by using the pipe’s diameter and known air speed of a different pipe. Here’s a step-by-step guide on how to calculate the air speed at different positions in the pipelines:

Measure the diameter of the first pipe the air travels through (e.g. 5 inches)
Measure the diameter of the second pipe the air travels through (e.g. 8 inches)
To get the radius for each pipe, divide them both by two (e.g. pipe one: 5/2 = 2.5 inch radius; pipe two: 8/2 = 4 inch radius)
To calculate the cross sectional area for each pipe, multiply the square of the radius by the number pi (3.14). E.g.:
- Pipe one: 3.14 x (2.5 inches )^2 = cross sectional area of 19.6 square inches
- Pipe two: 3.14 x (4 inches)^2 = cross sectional area of 50.2 square inches
Assuming you want to determine the air speed in pipe two, and know the air speed of pipe one, you multiply the cross sectional area of pipe one by the air speed of pipe one, and then divide that by the cross sectional area of pipe two
In the case that the air velocity in pipe one is 20 feet per second, this calculation would look like:
- (19.6 square inches x 20 feet per second) / (50.2 square inches) = Air speed in pipe two is 7.8 feet per second

How to Calculate the Air Flow Rate

As we’ve just explained, you may have to figure out what the volumetric air flow rate is if you want to calculate the conveying air velocity. You can do this by multiplying the cross-sectional area (i.e. the area of the circular end of the pipe) by the air flow velocity. As with air speed calculations, you should determine the air flow rate under different conditions (the beginning or end of the conveying line) in order to account for the pressure gradient.

You can also calculate the air flow rate if you already know what the air velocity and pipe diameter are. With this information, you can use the following formula to determine the air flow rate: Q_air = u_air * π.D²/4.

How do you calculate air velocity in a pneumatic conveying system?

Air velocity is calculated by dividing the volumetric air flow rate (m³/s) by the passing section of the pipe (m²). Since velocity along the pipeline shifts as pressure and temperature change, calculations should be run at a specific reference point — either the beginning or the end of the conveying line.

What formula is used to determine air velocity in pipes or ducts?

The standard formula is: u_air = Q_air ÷ (π · D² / 4), where u_air is conveying air velocity, Q_air is volumetric air flow rate, and D is internal pipe diameter. First, calculate the volumetric flow rate under your system’s specific temperature and pressure conditions, then apply it to the formula. The same formula works in reverse when velocity and diameter are already known.

Why is air velocity important in pneumatic conveying design?

Air velocity keeps material particles suspended as they move through conveying lines. If it’s set incorrectly, the results range from pipeline blockages and product degradation to accelerated component wear and costly downtime. The goal is saltation velocity, which is the speed at which particles stay floating in a horizontal line.

What happens if air velocity is too high or too low in a conveying system?

If air velocity is too low, particles fall out of suspension, blocking the pipeline and halting production. If it’s too high, particles strike internal surfaces with greater force, accelerating wear on pipes, elbows, and fittings. For soft materials like plastics, excessive velocity can generate frictional heat and cause streamers, leading to material degradation and pressure drops.

What is the minimum air velocity required to move bulk materials?

There is no universal minimum conveying velocity. The right threshold depends on the material in transit and the type of conveying system in use. Heavier particles require higher velocities to stay in suspension, while lighter materials can move at lower speeds. In dense-phase systems, the minimum air velocity decreases as the solids loading ratio increases.

How does pipe diameter affect air velocity calculations?

Pipe diameter is a direct input in the air velocity formula. A larger diameter means a larger passing section, which lowers velocity for the same volumetric flow rate. Because of this relationship, engineers can calculate air speed at different points in a system by comparing pipe diameters against a known reference velocity.

What tools or measurements are used to monitor air velocity in industrial systems?

Air velocity calculations require the volumetric air flow rate and internal pipe diameter, adjusted for temperature and pressure at the reference point. In active systems, flow meters and pressure sensors are commonly used to track conditions and catch deviations before they cause problems.