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Designing a pneumatic conveying system
A pneumatic conveying system to move materials with maximum efficiency does not depend only to the best equipment. It also requires experienced designers who know in advance how the loading, transfer, conveying and arrival devices available on the market will affect the final results. Designing an industrial pneumatic conveying system also requires a careful study of the material to be handled and the right chemical and physical parameters.
You should also rely on Apply for this. In our company you will find excellent components as well as profound expertise in dimensioning and configuration. Competent designers and experienced fitters will be able to guarantee you the highest yield, durability and a good return on your investment. Let’s take a look at the main parameters when designing a pneumatic conveying system.
QUANTITY OF AIR AND PRODUCT
These two parameters are expressed as mass flow (kilograms per hour) and not volume. Air is compressible, so its volume depends on the pressure, which varies along the duct. Similarly, the volume of the transported product also changes, depending on the size of the particles.
CONVEYING AIR VELOCITY
For dilute phase pneumatic conveying systems, the air velocity must be maintained at 12 m/s for very fine powders, up to 16 m/s for granular materials, up to higher velocities when larger elements and higher densities materials have to be conveyed. For dense phase systems it is around 3 m/s or even less in some cases. These values are those at the entrance of the transport line.
PARTICLE VELOCITY
This parameter is only evaluated in few cases, because it is not always easy to measure it accurately. In dilute phase systems, where the material is suspended in the air, the transport mechanism can be compared to a dragging force. However, particles velocity will be lower than the air. In a horizontal transport pipe the particle velocity will be about 80% of the air velocity, in a vertical pipe, when the flow is upwards, it will be about 70%. The value will depend on the particles size, shape and density and can therefore vary over quite a wide range.
PHASE DENSITY
This is defined as the ratio between the quantity of transported material and the quantity of transported air. It is a useful parameter for design purposes because it helps to visualize the flow along the entire route. For dilute phase transport a maximum value of 15 is achievable, or even a little more if the distance to be covered is short, or if low air velocities can be used. For dense phase systems, with product moving on the bottom, the minimum phase density is usually 20, while values above 100 are the usual average. This value loses some of its meaning in “cap” transport, as the material has a high air permeability.
CONVEYING CAPACITY
In theory, pneumatic conveying systems are able to handle almost any type of material, but distances impose a practical limit. A hydraulic conveying system is capable of moving material at rates of over 100 tonnes per hour, over distances of even more than 100 km, in a single stage. The limit for a pneumatic system is typically 1.5 km for almost all applications. Water has a density 800 times higher than air; the difference in density between the material being transported and the transport fluid is therefore very different between the two systems. As a result, the velocity of the conveying air is greater than that required by water to move material in suspension.
PRESSURE
Since water is incompressible, in a hydraulic transport system the speed variations along the route are very small, therefore water pressures of up to 150 bar are possible. When designing pneumatic conveying systems based on compressible gases, such as air, 5 bar is rarely exceeded. By definition, high operating pressures are those above one bar. The motion of the air-material mixture is due to difference in pressure between the starting and the finishing points. This difference, known as pressure drop, is directly proportional to phase density, straight length of the pipe and velocity square of the transport air. With this in mind, everything possible should be done to keep the velocity of the transport air low, not only to minimize consumption and degradation, but also to improve the performance of the entire system.
PARTICLE DEGRADATION
Many of the materials to be transported are friable and therefore their particles are subject to breakage when they impact against surfaces, in case of pipe bends, for example. Numerous ways of reducing this problem have been devised, methods which affect transport conditions, pipes geometry and used materials.
HUMIDITY AND CONDENSATION
The amount of vapor that can be contained by a given air mass decreases as the temperature decreases, leading to major condensation. The same situation occurs as pressure increases. In a pneumatic conveying system, changes in temperature and air pressure are frequent and must also be considered from this point of view. It may be necessary to design an additional system to treat the air.
CAPACITY
The system capacity, to obtain a given flow rate of material, depends essentially on the pressure drop on the pipeline and its diameter. Generally the pressure does not exceed 5 bar, so to increase the system capacity, the size of the pipeline must be affected. In many cases the available pressure in the system is defined by using a particular type of compressor or blower: the desired flow rate can be achieved by various combinations of pressure drop and pipe diameter. Where more than one combination is available, an evaluation phase in terms of operating costs and initial investment will be necessary to determine the most cost-effective choice for the customer.
EROSIVE USAGE
Some of the substances to be conveyed have significant abrasion rates and induce hoses and components to deleterious effects due to impacts with the gas and material mixture. To correctly design a pneumatic conveying system, it is essential to know the erosion process caused by handled substances. Then we can determine the steps to be taken to minimize the effects to an acceptable level.
SAFETY AND HYGIENE
Handling many powders introduces safety issues and many materials to be transported are potentially toxic. Pneumatic transport is often chosen to move hazardous materials because the system can be a completely isolated environment. A good design team always keeps these aspects in mind, along with those related to cleanliness and purity of the materials.
SCALE PARAMETERS
In some situations a test phase between design and implementation may be necessary, replicating as closely as possible the final system in a pilot plant. Most of the time, a test phase is suggested, but the assembly of a pilot plant, which is the same as the final one, implies considerable additional costs and time delays. Over the years, thanks to studies, research and growing experience, have been developed parameters that allow to obtain highly reliable projections.
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