# Dense phase- or dilute phase pneumatic conveying

In the bulk-online forum are several pneumatic conveying questions posed, using the descriptions dense phase conveying and dilute phase conveying.
It seemed then that there was not a general understanding about the definition of the two conveying regimes.
After the discussion on the Forum it became clear that the definition was related to the so called Zenz-diagram.
The Zenz diagram is widely accepted as a description of pneumatic conveying with explanatory properties.
Since the calculation of a Zenz diagram is now possible by an extensive computer program, it is also possible to investigate how the diagram is formed.

The calculation approach is described in the Bulkblog article “Pneumatic Conveying, Performance and Calculations!. By varying the air flow at constant capacity, the resulting partial pressure drops were calculated and combined into a table.
The summation of the partial pressure drops results in the total pressure drop of the system under the chosen conditions.
Dividing the calculated pressure drops by the total length, the pressure drop per meter is derived.

This procedure could also be differentiated to partial pressure drops over partial lengths.
Then it can be checked whether one part of the conveying pipeline is in f.i. dense phase, while another part of the conveying pipeline is dilute phase. This not executed for this article.

Zenz diagram

The curve in the Zenz – diagram represents pneumatic conveying as the pressure drop per unit of length as a function of the air flow (or air velocity).

For this curve the solids flow rate and the pipeline are kept constant.

For a cement conveying pipe line, this curve is calculated.

The calculation curves are given below:

 cement 200 ton/hr pipeline 12″ meter pressure SLR Pumpvolume pressure / meter kWh/ton mu 0,8 24745 134 0,86 55,68 0,9 20475 111 0,82 49,49 1,0 18577 100 0,83 44,54 1,1 17295 93 0,86 40,49 1,13 17048 92 0,87 39,53 1,2 16428 89 0,90 37,12 1,3 15794 85 0,95 34,26 1,4 15333 83 0,99 31,81 1,5 15040 81 1,05 29,69 1,6 14819 80 1,10 27,84 2,0 14612 79 1,37 22,27 2,1 14680 79 1,44 21,21 2,2 14750 80 1,51 20,25 2,3 14875 80 1,59 19,37 2,4 15013 81 1,67 18,56 2,5 15171 82 1,76 17,82 3,0 16175 87 2,22 14,85 3,5 17460 94 2,76 12,73 4,0 18844 102 3,37 11,14 4,5 20340 110 4,05 9,90 5,0 21900 118 4,81 8,91 5,5 23540 127 5,65 8,10 6,0 25260 137 6,57 7,42

From 0.8 m3/sec to 2.0 m3/sec, the pressure drop decreases.

This can be explained as the stronger influence of the decreasing loading ratio, opposed to the

weaker influence of the increasing velocity, which would increase the pressure drop per meter.

In addition, the residence time of the particles becomes shorter with increasing velocity and the required pressure drop for keeping the particles in suspension decreases.

From 2.0 m3/sec to 6.0 m3/sec, the pressure drop increases.

This can be explained as the weaker influence of the decreasing loading ratio and the decreasing pressure drop for keeping the particles in suspension, opposed to the stronger influence of the increasing velocity, which increases the pressure drop per meter.

The lowest pressure drop per meter occurs at 2.0 m3/sec.

Left of this point of the lowest pressure drop per meter, the pneumatic conveying is considered: dense phase and on the right of this point, the pneumatic conveying is considered: dilute phase.

As can be read from the calculation table, the loading ratio (mu) is higher on the left part of the curve than on the right part of the curve.

Regarding the energy consumption per ton conveyed, the lowest value occurs at 0.9 m3/sec.

This can be explained as follows:

The energy consumption per ton is depending on the required power for the air flow.
(solids flow rate is kept constant)

This required power is determined as a function of (pressure * flow ).

It appears that the minimum in pressure drop does not coincide with the lowest power demand of the air flow.

As soon as the decreasing airflow (causing lower power demand) is compensated by the increasing pressure drop, the lowest energy consumption per conveyed ton is reached.

The calculation for an air flow of  0.8 m3/sec indicated the beginning of sedimentation in the pipeline, due to the velocities becoming too low.

From this calculation, it can be concluded that a pneumatic conveying design for the lowest possible energy demand, is also a design, using the lowest possible air flow (or velocity).

The lowest possible velocities are also favorable for particle degradation and component’s wear.

Contribution of partial pressure drops to the total pressure drop

To investigate the physical background of the shape of the Zenz diagram, a cement pressure conveying installation is assumed and calculated, whereby the partial pressure drops are noticed.

The installation is described by:

Horizontal conveying length              =         71        m

Vertical conveying length                  =         28        m

Number of bends                               =         2

Pipe diameter                                     =         243      mm (10”)

Capacity basis for Zenz diagram        =         200      tons/hr

The compressor airflow is varied from 0.5 m3/sec to 3.0 m3/sec

The calculation results are presented in the following table.

 Compressor flow in m3/sec 0,50 0,55 0,60 0,65 0,70 Pressure drop mbar/meter intake 0,10 0,10 0,10 0,10 0,10 acceleration 0,62 1,03 1,27 1,42 1,61 product 14,33 12,26 10,70 9,58 9,63 elevation 5,73 5,00 4,45 4,01 3,65 suspension 21,46 16,93 14,10 12,06 10,35 gas 0,12 0,13 0,13 0,14 0,17 filter 0,02 0,03 0,03 0,04 0,04 total dp 42,39 35,48 30,78 27,35 25,55 kWh/ton 0,90 0,86 0,84 0,83 0,85 SLR 97,90 87,60 79,30 72,60 67,00 Sedimentation Sub turbulent flow Turbulent flow

 Compressor flow in m3/sec 0,75 0,80 0,85 0,90 0,95 Pressure drop mbar/meter intake 0,10 0,10 0,10 0,10 0,10 acceleration 2,65 2,76 2,86 2,95 3,04 product 8,72 8,98 9,16 9,27 9,34 elevation 3,35 3,09 2,86 2,67 2,49 suspension 8,95 7,80 6,89 6,14 5,52 gas 0,19 0,22 0,25 0,29 0,33 filter 0,02 0,06 0,06 0,07 0,08 total dp 23,98 23,01 22,18 21,49 20,90 kWh/ton 0,87 0,89 0,92 0,96 0,99 SLR 62,20 58,20 54,30 51,40 48,60 No sedimentation Turbulent flow

 Compressor flow in m3/sec 1,00 1,25 1,50 2,00 2,10 Pressure drop mbar/meter intake 0,10 0,10 0,10 0,10 0,10 acceleration 3,12 3,55 4,01 4,96 5,16 product 9,37 9,11 8,53 7,22 6,96 elevation 2,34 1,79 1,45 1,06 1,01 suspension 4,90 3,33 2,45 1,59 1,49 gas 0,37 0,61 0,91 1,66 1,84 filter 0,09 0,14 0,20 0,35 0,39 total dp 20,29 18,64 17,65 16,95 16,95 kWh/ton 1,02 1,20 1,39 1,80 1,89 SLR 46,10 36,60 30,30 22,60 21,50 No sedimentation Turbulent flow

 Compressor flow in m3/sec 2,20 2,30 2,40 2,50 2,60 Pressure drop mbar/meter intake 0,10 0,10 0,10 0,10 0,10 acceleration 5,35 5,55 5,75 5,94 6,14 product 6,72 6,48 6,25 6,03 5,82 elevation 0,96 0,92 0,88 0,85 0,82 suspension 1,40 1,33 1,26 1,20 1,14 gas 2,02 2,20 2,39 2,59 2,79 filter 0,43 0,46 0,50 0,55 0,59 total dp 16,98 17,05 17,14 17,26 17,40 kWh/ton 1,99 2,08 2,18 2,29 2,39 SLR 20,60 19,70 18,80 18,10 17,40 No sedimentation Turbulent flow

Compressor flow in m3/sec

2,70

2,80

2,90

3,00

Pressure drop mbar/meter

intake

0,10

0,10

## 3 thoughts on “Dense phase- or dilute phase pneumatic conveying”

1. goutam kundu says:

GOOD

2. dineshpanta says:

Hello sir, I wasn’t able to post my question on the forum. So I contacted u..We are designing a pneumatic conveying system to transport brick dust and particles(size upto 20mm at max), particle density(2500kg/m3), conveying distance of : horizontal(26m), vertical(4m), material flow rate:2 tons per hour. Please help me with the cfm required and pressure requirements.

3. Dear dineshpanta,

Your information is far from sufficient.
It is not clear, whether you are referring to a vacuum system or a pressure system.
Particles up to 20 mm (20.000 micron) at a particle density of 2500 kg/m3 have a suspension velocity of approx. 32 m/sec, leading to an air velocity of approx. 80 m/sec.
No information is given about the dust size, nor the particle size distribution.

Then, regarding the short distance and the very low capacity, a mechanical system is likely more feasible.

best regards
Teus

## A weblog for the worldwide powder and bulk solids handling and processing community.

Single Sign On provided by vBSSO