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
uair = air velocity in conveying pipe of diameter D (m/s)
Qair = 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
- Qair = 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:
- uair = Qair / (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
- Qair = 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:
- uair = Qair / (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: Qair = uair * π.D2/4.
