Pneumatic conveying flow rate refers to the rate at which air or materials in pneumatic conveying pipelines travel. The airflow rate directly correlates with air velocity, air volume, and air mass flow, and can impact the solids loading ratio, the chances of pressure loss, the quality of the product being conveyed, and the integrity of internal components.
What is the Flow Rate of Air in Pneumatic Conveying?
In pneumatic conveying, flow rate of air can refer to either volumetric air flow or air mass flow. The volumetric flow rate is the volume of air that passes through a specific cross-sectional area per unit of time. Meanwhile, the mass flow rate measures the mass of air that passes through a specific cross-sectional area per unit of time.
For pneumatic conveying systems to be able to keep material particles suspended as they travel through the pipelines, there needs to be an adequate volume of air. Tied to the air volume is the air flow rate, which must be adjusted according to the volume of air in order to prevent the particles from abrading the pipework.
The flow rate of air in pneumatic conveying systems must also be proportional to the air velocity in order to maintain quality of the product. For example, if the air velocity is too high, it increases the chances of frictional heat as the speed at which the particles move through the pipelines causes them to wear against the walls, forming angel hairs as a result.
While air velocity refers to the speed at which the air is moving in distance per time unit, airflow rate refers to the air output in volume per time unit. The ideal air flow rate differs depending on the product, and any changes made to it must have corresponding changes made to the volume of air, air velocity, as well as the air mass flow rate.
Why Getting the Flow Rate Right Matters
The pneumatic conveying flow rate of air directly impacts the material being conveyed and, in turn, the components that come into contact with it. To prevent both product and equipment degradation, it’s crucial to get the air flow rate right.
As we mentioned earlier, if the airflow rate and velocity is too high, it can cause softer materials, such as plastics, to abrade against the internal walls of the pipelines. This is due to them traveling too quickly, creating frictional heat and causing the material to melt. This leads to the formation of angel hairs or streamers, resulting in product degradation and color contamination, as well clogged filters.
On the other hand, if the air flow rate and velocity is too low, heavier material particles will fall out of suspension as they travel through the pipelines. The side effects of this happening are blocked pipelines and pressure losses.
Maintaining the optimum pneumatic conveying flow rate is also important for improving productivity. By implementing flow control systems that automatically adjust the airflow according to certain parameters, such as internal volume, it ensures that the system operates at maximum efficiency while minimizing risks of blockages and pressure drops in the pneumatic conveying system.
How to Calculate Pneumatic Conveying Flow Rate
Both the volumetric air flow rate and the mass air flow rate can be used to provide data on the air solids ratio, pressure drops, and the air velocity at any point in the pipeline.
The volumetric airflow rate can be calculated according to the following formula, with A representing the cross-sectional area, and v representing the flow velocity:
Volumetric flow rate = A x V
While the volumetric air flow rate in pneumatic conveying systems is usually given in normal conditions (Nm3/h), it should be calculated at different conditions, specifically at the very beginning or the very end of the line due to the pressure gradient.
This is an example of the volumetric air flow rate calculation under different conditions:
Q1 = air volumetric flow rate in (known) conditions (m3/h)
Q2 = air volumetric flow rate to be determined in conditions 2 (m3/h)
P1 = pressure in conditions 1 (Pa)
P2 = pressure in conditions 2 (Pa)
T1 = temperature in conditions 1 (K)
T2 = temperature in conditions 1 (K)
In order to calculate the air mass flow rate, you must first calculate the air volumetric mass at the conditions we’ve just described in the above equation (P & T). You can also use the perfect gas law to figure this out, as shown in the equation below:
ρair = volumetric mass of the air at the conditions considered (kg/m3)
P = pressure at the conditions considered (Pa)
T = temperature at the conditions considered (K)
Mair = the molecular weight of air 0.029 kg/mol
The air mass flow rate can then be calculated by multiplying the volumetric mass by the volumetric flow rate, as shown in this equation:
mair = air mass flow rate (kg/h)
Qair = air volumetric flow rate at the conditions for which the volumetric mass has been calculated m3/h
ρair = volumetric mass of the air at the conditions considered (kg/m3)
