Dear Horacio,

To calculate your (existing and tested) system, some relevant data are missing.

-Compressor data (volume displacement in m3/min, type, maximum pressure)

-Capacity at 45 psi (3.1638 bar)

-Feeding (tank, rotary lock)

-Particle density (1550 kg/m3)

-Bulk density

Your observation that the pipeline is plugging at lower pressures is a bit strange, because a lower pressure indicates a lower SLR and higher velocities, which should stabilize the flow.

Testing an existing pneumatic conveying installation is normally started with varying the feed rate (in fact the Solid Loading Ratio).

Plugging at higher pressures (higher feed rate) causes the compressor pressure relief valve to blow off and thereby lowering the velocity in the pipe, resulting in plugging.

Or, in the case of a rotary lock feeder, an increase in rotary lock air leakage with the same result.

If you can supply the extra information, I will run this system through the computer.

Compressor and rotary lock name plate data give the required information.

All for now

Teus ■

Dear Horacio,

I derived the following set up of your installation:

Carbon

Particle size average 2 mm

Bulk density 1550 kg/m3

Particle density 2200 kg/m3

Suspension velocity approx. 9.7 m/sec

Installation:

Pipe 3’ (78 mm)

84.3 m horizontal

19.7 m vertical up

17.4 m vertical down

(Total length 121 m)

Conveying air 152 cfm (0.0717 m3/sec (from high pressure system)

Pressure tank conveying system with educator (venture system)

I calculate a carbon capacity of 1 ton/hr at a pressure of 3.2 bar (approx. 45 psi)

Sedimentation along the whole pipeline (except for the 17.4 m down) due to the too low air velocities.

Your experiments with higher and lower pressures probably influenced the functioning of the educator, causing the plugging.

Examine the feeder system and increase the airflow at the same conveying pressure.

In addition, consult your supplier.

Success

Teus ■

Dear Horatio,

Booster(s) in bends attribute to the velocities after the injection points.

Adding the booster air in the beginning of the pipeline (Which is the same as increasing the compressor volume) has the most effect.

I can calculate 1 booster compressor, if required.

You state the pressure of the booster as 30 psi, where the pressure of the compressor is 45 psi.

Although a booster has a lower operating pressure than the compressor, the booster pressure, when injected at the beginning of the pipe line, is almost the compressor pressure.

Lower feeding rates, decrease the conveying pressure and reduce the chance of plugging, but also reduces the capacity.

Can the educator be adjusted to lower feeding rates by re-positioning the nozzle?

Again, consult the supplier of the system and perhaps do field measurements.

Success

Teus ■

Dear Horatio,

It is hard to tell the effect of the 2 blowers, as the volumes are not known.

The boosters are installed at 20 m and 37.5 m from the beginning of the pipeline.

The supplied compressor conveying air takes care of the first 20 m, the compressor plus the first booster for the next 17.5 m and the compressor plus the 2 boosters serve the rest of the pipeline (83.5 m)

The pressures will arrange themselves according to the caused pressure drops.

Depending on the system operating under dense- or dilute phase, the result can be a higher compressor pressure as well as a lower pressure.

The use of an eductor is very sensitive for volumes and settings.

A feeding regulating system, whereby the pressure is maintained at a preset value is necessary under all circumstances.

A system used for another material than designed for easily creates problems, because of the wrong amount of conveying air.

Measure the pressure before and after the eductor, carbon capacity, determine the real airflows and observe pressure behavior of changed carbon feeding or other settings and ask your supplier to comment on the gained data. Especially the vendor of the eductor venturi.

What was the expected carbon conveying rate and how high was the lime conveying rate at which pressures and air volumes?

Relevant unknown parameters, s.a. air volume, pressure drop over eductor, conveying rate, etc. make it almost impossible to judge the system.

Success

Teus ■

Dear Horacio,

Calculating the lime conveying gives the following results:

Lime:

Average particle size = 0.035 mm

Suspension velocity = 1.35 m/sec

9 tons/hr (150kg/min) at 1.12 bar after the venturi.

Air flow 0.0709 m3/sec (152 cfm)

The pressure drop over the venturi is unknown.

Calculating the carbon conveying gives the following results:

Carbon:

Average particle size = 2 mm

Suspension velocity = 9.7 m/sec

1.1 tons/hr (18.3 kg/min) at 3.2 bar after the venturi.

Air flow 0.0709 m3/sec (152 cfm)

Whole pipeline shows severe sedimentation, because of too low air velocity.

The pressure drop over the venturi is unknown.

Calculating the carbon conveying with a booster to increase the air velocity gives the following results:

Carbon:

Average particle size = 2 mm

Suspension velocity = 9.7 m/sec

7.2 tons/hr (120 kg/min) at 2.11 bar after the venturi.

Air flow 0.0709 m3/sec (152 cfm) + booster after the venturi 0.18 m3/sec

Whole pipeline shows no sedimentation, because of too low air velocity.

The pressure drop over the venturi is unknown.

Figures are based on average parameter values and have therefore to be considered indicative for your situation.

Whether the venturi can handle the carbon of 2 mm particle size is unclear, as it was chosen on the basis of lime.

In addition the size and performance data of the venturi are not known in relation to material, air flow and pressure drop.

A new venturi or modified feeder system might be necessary.

Due to the high suspension velocity of the carbon, extra conveying air is required.

Is the feeder tank a pressure tank?

What is the desired carbon capacity?

Have a nice day

Teus ■

Dear Horacio,

I have attached the indicative calculation that refers to the acceptable capacity of 120 kg/min (# 7.2 tons/hr).

This installation with a booster of 0.18 m3/sec (10.8 m3/min) injecting right after the venturi-eductor. The booster pressure is then 2.03 bar.

The reason that the existing system (without booster) is unstable is caused by the too low air velocity. The system then operates in the unstable conveying region as given by the Zenz diagram.

By adding the booster, the working point in the Zenz diagram is shifted into the stable region.

The required air velocity is related to the suspension velocity.

The suspension velocity of the carbon with a particle size of 2 mm is estimated at 9.69 m/sec.

This requires a horizontal air velocity of approx. 44 m/sec in a 3” horizontal pipeline to prevent sedimentation.

The material velocity at the 1st bend is approx. 12.04 m/sec and is almost fully developed.

The exit material velocity from the bend is approx. 5.09 m/sec, which is high enough to prevent choking of the bend.

A long radius bend does not noticeably improve the situation.

A test with a hired compressor of 10.8 m3/min (mobile plant compressor with pressure reducer, set at 2.8 bar) is feasible.

Be careful with high pressures on the installation.

The venturi eductor behavior is still the unknown factor as it is usually.

Hope to have helped you a bit in the right direction.

Teus

Attachments

carbonconvinstallation (PDF)

■

It seems that the above analysis is based on dilute phase conveying for this application. But in most cases I have seen, carbon black is dense phase conveyed.

Suggest we do this analysis both ways if we are not sure

Regards,

Amrit Agarwal

Consulting Engineer

Pneumatic Conveying ConsultingCharleston, WV, USA

Email: polypcc@aol.com ■

Dear Amrit,

I repeated the calculation for higher and lower air flows and indeed, the calculation represents dilute phase conveying.

(Increasing air volume at constant capacity results in an increasing pressure drop).

However, if the air flow is decreased into the dense phase region of the Zenz diagram, the pressure drop increases steeply as well as the formation of sediment, indicating possible plugging.

Keep in mind that the particle size is 1-3 mm

I attached the calculated Zenz diagram.

It would be very interesting to learn the results of your indicative calculation.

Have a nice day

Teus

Attachments

zenzdiagramcarbon (PDF)

■

Dear Horacio,

Pipe diameter 0.078 m (3”). Area = 0.004037 m2

Airflow 0.0717 m3/sec (152 cfm)

Pressure 3.16 bar (45 psi) = 3.16+1 = 4.16 bar(a)

Airflow at 4.16 bar(a) = 0.0717/4.16 = 0.01723 m3/sec

Air velocity at the beginning of the pipeline =0.01723 / 0.004073 = 4.23 m/sec

Temperature and presence of material neglected

This air velocity can accelerate the carbon, however, is too low to keep the carbon fully in suspension.

Dense phase dune conveying in a 3”pipeline over a length of 121 m results in lower capacities and higher pressures as the working point shifts to the left in the Zenz diagram (see previous attachment)

The manifold should not distribute the pressure to the eductor but the airflow (f.i. with a sonic choke).

The venturi delivers a constant mass flow of air as long as the pressures are constant.

If the feeding changes, these pressures change and thereby the airflow. When this runs out of control, plugging is the result.

Consult the eductor/venturi supplier for more technical information about your application.

A dedicated compressor is a good idea, however, what compressor and for which pressure in combination with the venturi is not easy to say.

Have a good day

Teus ■

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Email: derek@airslide.net http:// www.airslide.net ■