This document discusses pneumatic conveying as a method for moving solids in the chemical industry, detailing the differences between dilute and dense phase systems and their respective applications. It outlines considerations for designing pneumatic conveying systems, emphasizes the importance of professional input, and provides pressure loss calculations based on research from notable publications. Additionally, it addresses factors impacting solids movement, such as pipe friction, gas density changes, and the significance of the solids-to-air ratio.

1. Paul Homburg
Process Systems Design
Subject: Bulk Material Handling
June 8, 2017
Dilute Phase Pneumatic Conveying
One of the most popular methods of moving solids in the chemical industry is pneumatic
conveying. Pneumatic conveying refers to the moving of solids suspended in or forced by a
gas stream through horizontal and/or vertical pipes. Pneumatic conveying can be used for
particles ranging from fine powders to pellets and bulk densities of 16 to 3200 kg/m3 (1 to 200
lb/ft3).
Dilute phase vs. Dense phase conveying
The specifying engineer typically has four choice in specifying a pneumatic conveying system.
1. Dilute phase vacuum operation
2. Dilute phase pressure operation
3. Dilute phase pressure-vacuum operation
4. Dense phase pressure operation
1

2. 2

3. 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 for 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 reputable 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.
3

4. * 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:
Vg Gas velocity [ft/s]
ρg Gas density [lbs/ft3]
W Solids mass velocity [lbs/s ft2]
Vp Particle velocity [ft/s]
f Fanning friction factor
L Equivalent length of pipeline [ft]
D Pipe inside diameter [ft]
g Acceleration due to gravity [32.2 ft/s2]
gc Constant [32.174 ft-lb/lb s2]
K Friction multiplier for the solids conveyed
R Solids to gas mass flow ration [lb/lb]
Z Elevation change in pipe line [ft]
4

5. 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 μg is the gas viscosity in lbs/ft s.
Base Equation
!
ΔPT Total pressure loss in the system
ΔPacc Pressure loss due to acceleration of the solids from their "at rest" condition at the pick up point.
ΔPg Frictional pressure loss of the gas
ΔPs Frictional pressure loss of the solids
ΔHg Elevation pressure loss of the gas
ΔHs Elevation pressure loss of the solids
ΔPmisc Pressure losses from miscellaneous equipment
(1)
(2)
(3) modify Zenz equation
(4)
(5)
(6)
!
! = Estimated Misc. losses from the system
!
!
!
!
5

6. 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 ε 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.
For fine powders, the solids velocity can be closer to the gas velocity and a factor of 0.90 may
be more appropriate. Depending on the size of the particles, the slip factor can range from
0.70 to 0.95.
Component Equivalent Length
Bends, 90° bend, long radius
(10 to 1 radius to diameter ratio)
40 x diameter or 20 ft
(whichever is larger)
Diverter Valves
45° divert angle
30° divert angle
20 x diameter
10 x diameter
Flexible Hoses
Stainless steel, with lined interior
Rubber or vinyl hose
3 x pipe length
5 x pipe length
For bends that are less than 90°, use the following equivalent lengths:
L =40 x (Degree of bend / 90)
6

7. 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.
7

8. 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
8