This is a vast and complex topic which is better addressed in eg. Perry's Handbook for Chemical Engineers or similar. Petrochemical plants sometimes use standpipe theory in effecting the transfer of cracker catalyst between the reactor and regenerator and vice versa. In those applications the large pressure differences between and within chambers are partly responsible for regulating flowrates and not just the pipe diameters. A simpler application is used to seal the discharge from suction ship unloaders. Cargo, powdered, is drawn into a vessel and after the first sealing is acheived, using a valve, the said valve is left open and the standpipe allows a steady discharge while retaining enough powder to maintain a seal for the vessel above. Some clever Swiss used to do it to prolong the life of the said valve.

If you are just concerned with gravity discharge there are many variations on the Beverloo equation which have been tailored to specific situations. ■

A useful start would be to read about flow regimes and particle packing ratios. The particle size range quoted is above that where cohesion is a factor is the particles surface is smooth, firm and not sticky. Although relatively light, the bulk material should be relatively free flowing and present no arching or rahole tendencies when dischaging from bulk storage. A key concept in chute design is the difference betwen confined and unconfined flow. Higher flow rates are possible in unconfined chutes, where the input is controlled to less than filling the chute cross section, but the entry conditions are then generally the determining factor for maximum output. However, should the outlet is temporally restricted on a chute be passing material with a very high cross sectional loading, the transfer capacity can be seriously diminished and not clear until the infeed is stopper to let the chute run clear. Entry conditions are also important for flooded chutes as the mass has to shear in order to converge, so the geometry of the flow channel is a major feature. A mass flow channel has a lower flow rate that a funnel flow condition if the outlet is unresrticted because convergence occurs in confined mass flow, whereas the more dilated exit conditions of funnel flow allows greater freedom of particle motion. Wall friction is important in deciding the minimum inclination for a chute and considertion should be given to the exit velocity of abrasive or friable materials. Radius corners are also useful for avoiding the build up of fines in corners.

There is a mine of information on the net and this site that should make interesting reading for a newcommer. ■

Dear All

I am currently looking at a sugar silo that feeds directly into the company's sweets/lollies production (small family run business). There is no mesh at the point at which the the sugar enters the food production system.

Mysteriously, small pieces of wood/bark were found in the clear lollies and traced to the sugar in the silo.

Commonsense tells me that there should be a mesh of the appropriate size fitted at the point at which the sugar enters the food production.

Is there a requirement for such a mesh and is it set out in a Standard (Australian or ISO etc) or is it required only as part of good practice? The silo about 7 years old.

Thanks a lot

Macron ■

Originally Posted by marcon

Dear All

I am currently looking at a sugar silo that feeds directly into the company's sweets/lollies production (small family run business). There is no mesh at the point at which the the sugar enters the food production system.

Mysteriously, small pieces of wood/bark were found in the clear lollies and traced to the sugar in the silo.

Commonsense tells me that there should be a mesh of the appropriate size fitted at the point at which the sugar enters the food production.

Is there a requirement for such a mesh and is it set out in a Standard (Australian or ISO etc) or is it required only as part of good practice? The silo about 7 years old.

Thanks a lot

Macron

This really ought to be a new thread: not least because it has some very broad issues.

What does the Ozzie Food Authority think? They might be very helpful: perceived food contamination is not unusual and food production is essential.

There needs to be an investigation of the mechanism that allowed wood (or bagasse) ingress at the supply source.

How is the sugar landed into the silo? Is there any chance of pallet splinters or similar?

There needs to be a mesh at the inlet. What keeps the mice and other vermin out?

How was the impurity discovered? Sometimes we hear about mice being peeled off the underside of pizzas even after massively stringent, they say, quality control.

Your remedy will have to be something like a very small vibrating screen under the silo outlet and a coarser mesh at the silo inlet. ■

Much of the literature fails to recognise the differences in the flow dynamics between hoppers, bins and the like and transfer chutes. We have known this for quite some time and have used it to very good effect in developing better transfer chute designs. There is an excellent summary paper titled "On dense granular flows" written by G.D.R. MIDI, a French research group and which can be accessed on line at http:www:lps.ens.fr/recherche/formes/papers/ArtGDR.pdf that you need to look at. In essence what you will find is that in a well designed transfer chute;

> wall friction is basically irrelevant, what is relevant is the effective friction between particles as this will determine the way the material flows.

> Jenike and Johansson and other scaling techniques used to quantify material characteristics are useless as far as transfer chutes are concerned (something we have also known for quite some time). We find already that many in Australia are now realising that the way the suppliers of DEM software are suggesting their software be calibrated is flawed.

> when we get to cohesive and/or adhesive materials they state that we must rely on empirical methods to assess the flow dynamics. Once again we have known this for some time and have developed methodologies whereby we can reasonably accurately assess these affects

The consequence to this is we have applied this knowledge very successfully now by developing a different type of transfer chute that has very exciting prospects. We have applied for patents and will publish more details in the months to come

Colin Benjamin

Gulf Conveyor Systems P/L

www:conveyorsystemstechnology.com ■