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Pneumatic Conveying Design
One of the most popular methods of moving solids in the
chemical industry is pneumatic conveying. Pneumatic conveying refers to the Dilute phase vs. Dense phase conveying The specifying engineer typically has four choice in specifying a pneumatic conveying system. 1. Dilute phase vacuum operation Vacuum systems allow multiple product inlets through the use of simple diverter valves. However, it becomes costly to have multiple destinations because each must have its own filter receiver with partial vacuum capability. Vacuum systems are also more "distance sensitive" than pressure systems due to the maximum pressure differential of 5.5 to 6.0 psi. Dilute phase pressure systems can easily achieve pressure differentials of 12 psi. Pressure-vacuum operation (utilizing both methods) are sometimes ideal for a given conveying setup. A very common application is the unloading of a standard railcar. Since the cars cannot be pressurized, air is pulled from the outside, through the car (carrying solids with it) to a filter. Then after the filter, a blower can be used to forward the solids to the final receiver. The choice between dilute and dense phase operation is typically dependent on the solids properties. For example, the lower velocity bulk phase operation is popular ofr highly abrasive products or for those that degrade easily. For example, this method is popular in transporting kaolin clay.
Designing a System for Dilute Phase Operation Considering designing a pneumatic conveying system yourself? Probably not a good idea. There's as much art involved as there is science and such a design should be left to professionals. Consider that even different grades of the same material have been known to convey differently. Testing is a must (as you'll see from the method below). Before you can even make any good judgements from the method presented here, you need to know solid friction factor for your solids (which we'll discuss later) and the minimum gas velocity required to move your particles. So, if you're involved in designing a system from the ground up, seek assistance from reputatable people in the field of conveying. If you're already familiar with your solids, the method below can be used to examine the pressure loss expected in your system. The method presented here is very good and has been stood the test of real systems over time. The design method presented here is based on the work of Dr. F.A. Zenz and Dr. D.F. Othmer as published in their book "Fluidized and Fluid Particle Systems" published in 1960 (see References). This method was presented by A. T. Agarwal of Pneumatic Conveying Consulting Services LLC in the January/February 2005 issue of Powder Handling and Processing. This method has been widely used and is generally found to be within 10% of measured pressure losses. Pressure losses experienced in pneumatic conveying systems are the result of the following forces: Friction of the gas on the inside of the pipe, force required to move the solids through the pipe, forces required to support the weight of the solid and the gases in vertical pipe runs, force required to accelerate the solids, and friction between the solids and the inside of the pipe. Friction losses as the result of the solids being in contact with the inside of the pipe are usually very small and can be neglected when considering dilute phase transport. * Please be aware that using air as a carrier gas should be investigated thoroughly. When some powders are mixed with oxygen, they form an explosive mixture! Nomenclature
Early is their work, Zenz and Othmer used the following equations to described the pressure losses in horizontal and vertical pipes:
It was later concluded that the term (fp Vp/f Vg) should be replaced by the constant K (friction multiplier for the solids conveyed) because the term was dependent upon the physical properties of the solids being conveyed. The value for K must be back-calculated from actual pressure drop data from an existing system or it must be determined experimentally in plant tests. This lead to the following equation for the pressure drop for the solids in the system:
Thus, the equation used for the total system pressure loss is:
where:
The Base Equation is the basis for the following equations used in the pressure loss calculations. The pressure loss is expressed in psi or lbs/in2 in the following equations.
The Fanning Friction Factor In order to determine the fanning friction factor, f, the Reynolds Number must first be calculated:
where mg is the gas viscosity in lbs/ft s. Then, the friction factor is calculated from the following equation derived from pages A-23 and A-24 of Crane's Technical Paper No. 410:
Where e is the pipe roughness factor which can be estimated as 0.00015 for smooth pipes or 0.0005 for shot-peened pipes. Pipe Equivalent Length For straight pipe runs (either horizontal or vertical) use the actual length of the pipe. For bends and other devices, use the table below as a guide:
Solids Velocity Solids also move at a velocity lower than the gas velocity due to drag forces. The difference between these velocities is called the slip factor. For most course or hard solids, the slip factor is around 0.80.
Solids Velocity in Long Radius Bends For a 90° radius bend, the solids velocity at the exit of the bend (Vp2) is around 0.80 times the solids velocity at the inlet of the bend (Vp1). This factor can range from 0.60 to 0.90 depending of the properties of the solids. For bends that are less than 90°, the exit velocity may be expressed as:
After leaving a 90° bend, use a nominal value of 20 pipe diameters for the length of pipe needed for the particles to accelerate back to their original velocity at the inlet of the bend. Solids to Air Ratio, R The solids to air ratio is calculated as:
In the above, m is the the solids mass flow in lb/s and A is the pipe cross sectional area in ft2. Remember that the gas density (and therefore velocity) will change throughout the system (we'll discuss this in more detail later). When calculating the gas velocity, reduce the gas mass flow rate by 5% for pressure systems to account for gas leakage through a rotary valve if such a valve is used to feed the solids. Gas Density and Pressure Loss As the carrier gas moves through the system and losses or gains pressure (depending on whether the system is a pressure or vacuum system), it's density will increases or decrease accordingly thereby changing the velocity of the gas. The gas pressure loss equation is shown in Equation 2 above. The ideal gas law may be employed to calculate the density of the gas throughout the system. Setting Up the Calculation The method presented here can easily be set up in a spreadsheet to show the performance of your system. It is recommend the divide the entire system into 5 to 10 ft sections for such a calculation. The exit conditions of from section become the inlet conditions for the next and so on. Also, be sure to account for any gas losses that are known and for any extra gas inlet points in the system. For pressure systems, start from the end of the conveying line and return to the solids inlet point. For vacuum systems, start from the solids inlet point and end at the blower inlet.
***Special Note: This article was requested by a visitor to this site. Do you have a request? Email me below. References: Zenz and Othmer, "Fluidized and Fluid Particle Systems", Chapters 10 and 11, published by Reinhold Publishing Corp, 1960 Agarwal, A.T., "Theory and Design of Dilute Phase Pneumatic Conveying Systems", Powder Handling & Processing, Vol. 17, No. 1, January/February 2005 Kimbel, Kirk W., "Troublefree Pneumatic Conveying", Chemical Engineering Magazine, April 1998, page 78
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= Estimated Misc. losses from the system
