With bulk material handling machines they are sizes according to the volumetric capacity and powered according to the gravimetric capacity. So I would generally quote a machine as

maximum volumetric capacity *** m3/hr

maximum gravimetric capacity *** te/hr

then there is no doubt regarding the basis of the machine offer. ■

Does this mean you use lower m3 for volume transport and then apply higher density atop m3 already in hand to obtain te for power ?

It has always confused me, since the tons/hr is the only parameter of power. You must determine which of the densities applies to the transport rate. It cannot be both. Therefore, why not be unambiguous and state the density and ton/hr, then there is the moisture issue. Do we add the moisture to m3 or te?

However, it still is cloudy since if you calculate the power based on volume at a low density with or without moisture, you will consumes a lower power, without factoring moisture, than a power based on high density, without moisture.

I think the inventor was a little confused. There should be a standard rather than be a copy cat of nebulous wisdom. ■

Originally Posted by nordell

I think the inventor was a little confused.

Which inventor was that ■

Hey,

In some instances they both give the same answer as you work it out as tonnes/hour.

However if you have a volumetic feed onto the conveyor you would calcuate the volume based on the lower then times this volume by the higher density.

Hence you are designing for a higher tph. Some call this deisgn tph vs nominal tph.

You should also have a structural density as well.

Best Regards,

Gareth ■

Originally Posted by blakeyg

However if you have a volumetic feed onto the conveyor you would calcuate the volume based on the lower then times this volume by the higher density.

And of course it can make a significant difference if the density range is 350 - 1100 kg/m3 like a job I'm just working on !! ■

Hi All, you certainly picked a contentious point. I am not a conveyor guru as such but have had to work with some variations of products conveyed by Truck and conveyors. It is interesting that the physical volume of, say sandy loam or fine crushed rock. The density of the products can vary by up to 40%, dependent on the source, of from igneous rock to pure silicate granites. Both products have similar strength properties and can be used for the same applications. Another phenomenon is that they are very heavy (Density) in Dry powdered form per m3 volume but bulk by up to 25% of volume when damp which equates to a 20% density reduction per m3 @ 2 to 7% humidity and then start to reduce back to standard densities at around 12% (humidity) moisture(water and product /m3 measured) and then any moisture added thereafter increases the overall mass (Density of product plus water mass). Belt widths need to cater for the increased volume albeit less mass and visa versa. Gearbox/s Drives also have to take in the criteria to be able to operate in all conditions for similar products especially when moving between differing sources of product. This is why most government clients specify delivery as mass or volumetric quantities at below a particular moisture content so they know they have the correct mass of product to complete the job.

This may sound convolute but is a fact of life. ■

The fact is that all particulate solids have variable bulk densities, although some vary considerably more than others. Coarse granules tend to settle quickly to a semi-stable condition that are capable of bearing loads because the contact points of the constituent particles came to rest in a structural array that formed a continuous load path of gravitational orientation. This structure can be disturbed by transverse vibration to form a more dense condition or by high loads that stress the contact points to failure. In a dynamic condition, particle are in a state of dilatation because clearance is needed for the irregular shapes to pass each other, however, the range of density conditions of coarse products is relatively small but the value measured will depend on the means by which the sample is prepared. The value used for a particular calculation should therefore reflect the expected 'worst' condition that will how to be accomodated in the application under consideration

Fine powders are much more variable in density conditions than coarse materials, mainly because the gas content of the voids cannot escape or enter the bulk easily through the small interstices between the particles. The flow potential of the bulk is even more variable, but is also related to the density condition.

In these circumstances the particular value that should be selected should also be based on the 'worst' conditions that are to be accommodated. bbearing in mind that the worst for some duties is not the worst for others. For example, The safe tonnage capacity of a bin should be rated on the lowest density that the material will attain, which is probably as initially delivered to the vessel, although account should be taken of the potential for the initial feedstock to settle before the last is added. The worst condition for flow is that attained after time consolidation from the period of maximum storage time. The torque on a discharge screw feeder should be similarly rated in order to provide the starting load, but the output determined by the dynamic state attained when the screw is running. Once that it appreciated that the material density changes according to the current and historic stresses it is easy to consider what value should be adopted for specific ‘safe’ calculations.

Similar considerations also apply to the flow condition. What is best for flow through a certain size of opening to a feeder is worst for calculating the overpressure under which the material has to be sheared to start the feeder running material, whereas the worst condition for flow reduces the pressure acting on the feeder, so different values should be used for the same material under different conditions of use. The effect is real, as both conditions have to be satisfied; as with dealing with both starting and running conditions, when both power and capacity change with inverse characteristics from the design point of view, high starting load, high density hence high capacity; low running load, low density hence low capacity.

Overall, it is simply 'horses for courses' and use the density value that is appropriate to the particular function being addressed. ■

Hello Shrinivas,

One area where it is essential to specify two densities, one for volumetric design and another for power sizing is when you are handling a material such as biomass that can be compressed or fluffed-up and has different moisture contents; which results in different densities. Unless you are confident that the form and moisture content will not change, I recommend that:

1) You size your conveyor for the required tonnage, using the lowest density, which will give you the greatest volume to be handled.

2) Then you use the higher density on that volume, for structural (load) and power calculations.

If you don't understand that some materials can have various densities, it may seem that you are adding unnecessary safety factors. However, many times in my >30 year career, I have trouble-shooted biomass conveyor installations where the density issue has not been understood by the engineer / designer and they have either installed a conveyor that is too small or have under-designed the drive.

G.F. Paul Janz

Senior Specialist, Biomass and Wood Handling

Ausenco Sandwell

855 Homer Street

Vancouver, BC, V6B 2W2

Canada

E paul.janze@ausencosandwell.com ■

Hello Shrinivas,

Another way to look at it would be to consider that bulk materials have multiple densities throughout their life on the conveyor. We at Martin have observed that bulk density changes as the material is handled. We call it Loose bulk density and consolidated bulk density. The loose bulk density is the density of the material is in a non compacted condition. The consolidated density is the density after the material has been vibrated. As material is "fluffed" as it is transported, the space it takes up will change. I have always used a tons per hour to calculate drive power, but I use loose density to size my belt widths.

I would suggest looking at pages 399 through 400 of our Foundations book for details on this phenomena

http://www.martin-eng.com/apps/f4/?module=login ■