dear Mr Cementawi,

A de-dusting installation of equipment is very much related to the design of the equipment and its application.

De-dusting has two objectives that have to be met.

1)prevent over pressure in the system that can occur when f.i. air is blown in to that equipment. (f.i. closed air slides, silo that is pneumatically filled, flat storage)

2)create a little negative pressure inside the system, to prevent dust coming out of possible leaks in the system.

Bucket elevator

The de-dusting should take care of :

-negative pressure inside the casing (approx. 100-200 mmWC)

-airflow in the spout of approx 4 m/sec (cement) against the material flow, to prevent dust coming out. In case the end of the spout has its own de-dusting connection, you might well skip the dedusting on the top of the bucket elevator casing.

Air slide :

-closed air slide

A closed air slide is de-dusted at the end of the air slide.

The air extraction causes an airflow in the air slide casing in the same direction as the material flow. (additional pneumatic conveying)

The amount of air that has to be withdrawn is equal to the fluidizing air.

If the air slide is supplied from a fluidized bed in a silo, where an over pressure of approx 0,3 bar can exist, the released air from the transported cement has also to be withdrawn.

-open air slide

Withdrawal of fluidizing air plus the possible leakage through openings in the system, while maintaining an under-pressure of approx. 100 – 250 mmWC

The place where the air is withdrawn, should have an air velocity of approx. 1 m/sec, to prevent pneumatic conveying of material.

Further in the system, velocities of approx. 5-8 m/sec should be maintained to prevent sedimentation.

The ducting should also be designed in such a way that sedimentation is prevented.

The amount of withdrawn air should be taken as 2 times the calculated amount of air.

The fan should be capable to withdraw that amount of air at a pressure of

abt. 450-550 mmWC. (if possible even higher).

A filter pressure drop is assumed of approx 250 mmWC.

The filter area in m2 should be taken as :

fan air volume (m3/min) / 1,2 (m/min)

1,2 m/min is called the filter load)

A pressure drop calculation should be made to check whether the fan is chosen correctly and the ducting not to big or to small.

If the fan works later on at a lower volume and a higher pressure still maintaining enough negative pressure in the system is no problem, it works and no dust is released.

The fan must have a falling curve. (at higher volumes a lower pressure)

(Do not forget the velocity pressure drop in your calculations)

Also check with your suppliers.

success ■

Good day, Mr Cementawi

In my previous post, I meant the velocities are calculated, rather than measured.

In case the dedusting system is giving problems, than measurements are usefull to determine the cause of the problems.

1)measuring small pressures can be done in the following way.

-weld a “ pipe socket to the casing.

-drill a 6mm hole through the center of the socket.

-connect a transparent flexible hose of approx 6 mm to the socket.

-bend the transparent hose in an U-shape and fill it halfway the U with water.

-The difference in water level in the U-tube is the pressure difference.

-After the measurement close the socket with a plud.

2)By the spout, I meant a possible long ducting from the top of the elevator to a dropping point.

The upward velocity in the duct should not be too high, but high enough to maintaina certain negative pressure.

By de-aerating the dropping point this problem does not exist.

Velocity can be measured with a (self constructed) pitot tube through a hole in the casing with the same U-tube.

( If you wish I can send a dwg-file or dwf-file of the pitot tube with the related mathematics by e-mail, let me know)

3)If the air velocity in the ducting (pipe) between the bucket elevator (or air slide) is too low, sedimentation will occur

A too high velocity would create pneumatic conveying and too much dust is transported to the filter receiver.

Therefore, a very low velocity at the pick up point will reduce the dust transport.

( to control the velocities in certain sections of the system, sometimes regulating air by-pass valves are used)

4)Not the bulk density, but the suspension velocity is important.

suspension (floating) velocity at atmospheric conditions :

v(susp) = sqrt (4/3 * density(prod)/density(air) * particle size/dragfactor)

v(susp) = sqrt (4/3 * 3100/1.2 * 0.00005/0.06) = 1.7 m/sec

I do not have exact figures of dust loading ratios, but in the field where I am working, cement unloaders, the filter systems work well, just 1 meter above the entrance of the cement into the transfer kettles or silos. Locations where the dust concentration must be very high.

My experience is that multiple de-aeration points on one filter receiver are difficult to tune, because of the uncertainties in the various flow resistances.

much success ■

Dear Mr. Cementawi

The equation is derived from the general resistance formal:

dragforce = * Cw * rho(air) * v2 * A

in which :

dragforce in N

rho(air) = air density in kg/m3

v = air velocity in m/sec

A = cross sectional area of particle = pi/4 * d2

d = particle size in m

the eqation in dimensions becomes :

N = Cw * kg/m3 * m2/sec2 * m2

N = m * g = kg * m/sec2

kg * m/sec2 = Cw * kg/m3 * m2/sec2 * m2

kg * m/sec2 = Cw * kg*m/sec2

Cw is dimensionless and represents the factor to calculate the real air resistance of a particle in an airflow. (or any other medium)

best regards ■

Dear Mr. Cementawi,

I notice that you spread your inquiries over many chapters.

This question is more or less taken care of inmy last reply (29 december 2005) in the section "Dust Control/Dust Suppression/Dust Collection"

Hope to have helped you,

Until next time ■

Dear Mr. Cementawi

In addition to my previous post, the upward dragforce can be taken equal to the downward particle weight.

We derived already :

dragforce = * Cw * rho(air) * v2 * A

With a particle in an upwards vertical air flow the gravity acts downwards.

Thus : N = m * g

m = pi/6 * d3 * rho(material) * g

substituted :

pi/6 * d3 * rho(material) * g = * Cw * rho(air) * v2 * pi/4 * d2

pi/6 * d3 * rho(material) * g

v2 = ----------------------------------------------

* Cw * rho(air) * v2 * pi/4 * d2

4 * d3 * rho(material) * g

v2 = ------------------------------------

3 * Cw/g * rho(air)

This equation represents the situation whereby the weight downwards equals the lift in the air flow upwards.

The particle is then subjected to a resulting force = 0

The particle is not accelerated and stays where it is (suspension velocity of that particle)

The factor Cw/g is called the dragfactor and the resulting velocity is the suspension velocity.

A higher air velocity causes a higher upward force and the particle will be accelerated and moved upwards.

This is the basic principle of pneumatic conveying.

All additional theory, phenomena, technology, etc. is related to this velocity.

Dust extraction is just a special form of pneumatic conveying.

nice talking to you ■

Cementawi,

All the basic information you need for a dust collection system design is contained in the publication "Industrial Ventilation- A Manual of Recommended Practice" published by The American Conference of Governmental Industrial Hygienists.

There you will find recommended exhaust quantities for many standard equipment items, recommended conveying velocities, procedures for sizing the duct system and system pressure calculations for determining the fan capacity.

You do not need to get into detailed calculations of settling velocities etc., the Manual simplifies the whole matter.

Maybe you can engage a consultant to help you, otherwise, your best approach is to ask a baghouse manufacturer to do a complete design, supply and install package.

Certainly, use the Manual as a check on the manufacturer's methods.

Good luck,

Michael Reid. ■

Dear Mr. Cementawi,

The airslide is easy :

20 m3/min fluidization air is the minimum amount of air to be extracted.

Any leakages are not allowed in an air slide, but being on the safe side and to be able to cope with eventual surges, 1,5 times that amount should be the fan capacity 30 m3/min for the air slide.

For the bucket elevator the amount can be smaller as no air is entering the casing.

The only time, that air is displaced is when the bucket elevator is filled.

That can take place with 200 tons/hr # approx. 200 m3/hr # approx. 3,5 m3/min

Depending on the condition of the casing, leakages can exist..

A safe estimate would be 6 m3/min ( leak of 0,1 m2 at 1 m/sec)

Thus approx. 10 m3/min for the bucket elevator.

If the air extraction is with 1 fan, then the fan capacity is approx. 30 + 3,5 + 6 = 40 m3/min.

With a filter load of 1,5, the filter area = 40 / 1,5 = 28 m2

The pressure drop of the fan is built up from the pressure drop over the air ducting plus the pressure drop over the self cleaning filters.

For instance :

50 meter of pipe 350 mm dia + 10 bends 90° = dp = 46 mmWC

Filter resistance = approx. 250 mmWC

pressure drop caused by dust conveying (estimate) = 100 mmWC

Totaling 400 mmWC

Thus: fan specification = 40 m3/min at 400 mmWC (4000 Pa)

If for instance the bucket elevator is feeding the air slide, the air can be extracted at the end of the air slide

If we had a situation sketch or overview, a more specific solution can be given, but that is also something for your contractors or regular advisors.

The bag house or self cleaning filtersystem should also be designed properly.

salutations. ■

Dear Mr. Cementawi

The supplier of the filter and fan has chosen a rather big installation.

Filter load = 83,4 / 62 = 4,35 is OK-

The fan description is somewhat unclear.

I calculated a fan that can deliver 40 m3/min at a fan pressure of approx. 4000 Pa

The supplier could mean :

1)A fan that can deliver 83,5 m3/min maximum (pressure = 0)

and a pressure of 1500 Pa maximum (Air flow = 0)

2)A fan that can deliver 83,5 m3/min at a fan pressure of 1500 Pa

Even the smallest fan (1) will create an air flow above the minimum required airflow (20 + 3,5 m3/min), but in case the filters gets dirty after a while and the filter resistance increases, the air flow will become too low and over pressure in the system will be generated.

Fan specification 2) is a little bit better.

A bigger filter than I proposed is no problem (apart from investment may be)

The fan pressure however should be higher to secure the proper working of the system over a longer period of time (maintaining negative pressure in the system)

success ■

Dear Mr. Cementawi

I do not have a full description nor a calculation program regarding dust filters or de-dusting installations.

In cases like yours, I follow the general rule that the air volume that enters the system has to extracted from the system plus some extra volume in order to generate a small under-pressure in the system.

This under pressure will prevent dust coming out of any possible leaks.

The arrangement of a de-dusting installation varies with the arrangement of the installation and is very difficult to catch in a calculation algorithm that suits all installations.

Common sense is here a much better guideline.

Experience is also very useful in these projects.

Rules of thumb are :

-The extracted air volume = 1,5 to 2 times the air volume that is blown in

-The fan pressure is between 3500 to 5500 Pa, but has to be checked against pressure drop of the ducting plus the pressure drop of the filter.

-The filter-load should be between 1,25 to 1,6 m/min

-On flat-storages the fan capacity and the filter area should be 3 to 4 times the air volume that is blown in.

-On buildings, like flat storages, the location of the fan outlet must be considered in relation to wind pressure, because that can increase the required fan pressure substantially.

-De-dusting should only convey dust (the smaller particles, i.e. < 10 microns) and the applied velocities must match these particle sizes. The air velocities should therefore be considered at any location in the de-dusting system.

Velocity not too high, to prevent conveying of bigger particles.

Velocity not to low, to prevent sedimentation of the smaller particles.

The ducting layout can also contribute to a better perfomance in avoiding sedimentation spots.

This forum is a good way to exchange experience and that is what we are doing

Greetings ■

Cementawi,

Please post a sketch (or send by private email), showing the layout of your system, identifying the equipment items to be exhausted, the relative location of the baghouse with distances and I will try to give you more guidance.

Regards,

Michael Reid. ■

Dear teus,

I am newly employed as a Process engineer in a cement industry.

I'm having a problem with a baghouse, Fuller kinyon pump frequent tripping and cement temperature in the mill. What exactly can i look into.

Thanks. ■

dear Okoji

As your question does not give very much information and is just general,

try the following procedure.

-check the design specifications

-check the equipment specifications (name plates)

-check the operating conditions and data (pressures, flows, temperatures, energy demands, etc.)

-Check, whether the operating data correspond with the design data. The difference should indicate the problem source

succes ■

Dear Teus,

Thanks for your response, its well appreciated.

The design parameter for the baghouse filter are

Filter type DP 20 x 12/3.6

Gas volume: 50000m^3/hr

DP = -550daPa

Cloth area: 814m^2

We make use of water spray in the mill (111 oC to 117oC)

Fan maximum rpm 1600 and we operate our rpm between 350 and 450 @ 100% damper opening. Why? Both cement transport pump and compressor trip at high load. Kindly advise me also on the rule of thumb concerning the delivery pipe in terms of the number of allowable bends and distance of the pipe.

Thanks. ■

Dear Teus,

At the outlet duct of the mill (Inlet of the Bag house filter) we are experiencing some steam coming out from the sampling point. While at the oulet of the baghouse filter, we have water dripping.

Please advice me on what area to further look into.

Some of the measurement are:

Static pressure @ Inlet of baghouse filter= -2.1mmWg @ 112 oC

Static pressure @ Outlet of baghouse filter= - 41mmWg @ 88 oC

Fan rpm 431

damper opening @ 100%

Thanks. ■

dear Okoji,

Baghouse 50000 m3/hr # 840 m3/min

Filterarea = 814 m2

Filterload 840/814 = 1.02 m/sec

This seems OK

If the 840 m3/min is at 1600 rpm, then the filterload at 431 rpm is approx 0.28 m/min, which should be considered low

It seems that you have water condensation in the baghouse, which is not so good for the lifetime and the proper function of the filterbags.

If the FK pump and the compressor are tripping on overload, then the cause should be found in overloading the installation.

The material feed should be lowered until the power demand is within the limits.

Pipeline diameter, pipeline length and number of bends are free to choose.

The consequence of a configuration design is that the capacity is limited or the diameter and airflow has to be adapted until the desired capacity is reached

have a nice day ■