Belt Feeder Preferential Reclaiming From Back Vs. Front
Dear Designer,
Yes, the preference is to reclaim from the back of the hopper for the reason stated: belt wear and also power consumption.
We have studied back and front reclaiming. The Mexican Hat reclaim system was invented to control the nature of how material is reclaimed. There are too many factors that can affect the reclaim mechanism. However, the Mexican Hat is a type of reclaim where the vertical flow onto the belt is partially controlled by internal vertical partions within the hopper or bin.
Typically, the partions are organized to either provide equal mass flow of all partions or perferentially from the back. The belt resistance (friction) with material must be more than the wall friction over the slot length between vertical partions. In this way, the material will reclaim back to front in this zone. There are special geometry configuration that influence this condition. The Mexican Hat can partion many zones within a long slot hopper and systematically feed the prescribed amount per each zone so that all partions flow with equal mass.
Hoping words meet expectations. ■
Re: Belt Feeders Under Slot Hoppers
Can you elaborate on the "Mexican Hat", it's not a term I'm familiar with. ■
Feeders Reclaiming From Front Or Back
As per my opinion the material is dragged from the front in case of belt feeders. I think, the factor which decides whether material will be dragged from front or back is dependent on which force is greater, force to shear the material at slot height or the friction force between material and carrying member (belt or pan). If the friction is less which is true for belt and smooth pan, material will be reclaimed from front. In case of apron feeders, having beaded pans, friction force is higher than shear force and hence material is sheared at slot height and carried by the pan from the back. I have observed these physically also.
With regards,
C B Ray
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Hopper Extraction By Belt.
Apart from the fact that it hardly seems to matter whether a dead region of stored material is at the back or front region of a hopper, the question appears to accept that extraction cannot take place along the length of a long, hopper outlet slot, which is perhaps a reflection of the state of many approaches made to hopper design. An analysis of the relative shear force against belt and wall friction would indicate where material will be drawn by a belt feeder from on a long, parallel outlet slot, (This usually turns out to be from the back, but not always), but anyone capable of securing these values and undertaking this calculation, which involves the awkward assessment of outlet overpressures, should be able to design a belt feeder interface with progressive extraction that gives superior flow and bulk storage characteristics, with savings on power and belt wear. For those who can’t, I suggest they follow the style shown in my paper at the 2006 MHEA Seminar. Details are available from lyn@ajax.co.uk for those who are unable to lay their hands on a copy. It may be worth mentioning that the use of a mass flow section immediately prior to the outlet permits the use of a narrower slot to counter ‘arching’; and hence smaller belt width and cut-off devices than with a non-mass flow design, and that ‘ratholing’ cannot take place in either a mass flow or ‘V’ shaped hopper. There is more advice on hopper design procedure and bulk solids behaviour from the same source. ■
Re: Belt Feeders Under Slot Hoppers
Originally Posted by Lyn Bates
When trouble shooting the performance of a machine an understanding of how it generally works always helps.
the question appears to accept that extraction cannot take place along the length of a long, hopper outlet slot, which is perhaps a reflection of the state of many approaches made to hopper design.
That extraction along the length of a parallel slot hopper results in preferential feed from one area unless special design features are included has been a staple of papers and articles on bunker flow for at least 25+ years to my knowledge. In the case of chain conveyors used as feeders I've seen this for 40+ years.
An analysis of the relative shear force against belt and wall friction would indicate where material will be drawn by a belt feeder from on a long, parallel outlet slot, (This usually turns out to be from the back, but not always), but anyone capable of securing these values and undertaking this calculation, which involves the awkward assessment of outlet overpressures, should be able to design a belt feeder interface with progressive extraction that gives superior flow and bulk storage characteristics, with savings on power and belt wear. For those who can’t, I suggest they follow the style shown in my paper at the 2006 MHEA Seminar. Details are available from lyn@ajax.co.uk for those who are unable to lay their hands on a copy. It may be worth mentioning that the use of a mass flow section immediately prior to the outlet permits the use of a narrower slot to counter ‘arching’; and hence smaller belt width and cut-off devices than with a non-mass flow design, and that ‘ratholing’ cannot take place in either a mass flow or ‘V’ shaped hopper. There is more advice on hopper design procedure and bulk solids behaviour from the same source.
I'd appreciate a copy of the paper referred to and have sent you a PM with my email address.
General methods of designing the interface between feeders and mass flow bunkers are well known from many papers, but differ in detail according to the style of feeder (belt, chain, screw etc). I would suggest that these methods are also worthwhile with non-mass flow bunkers. ■
Belt Feeders Under Slot Hoppers
This subject came up with some colleagues today, in particular "in a slot hopper does the material feed preferentially from the back or front of the hopper".
I expressed the view that while I had seen published papers where some said "back" and some "front", I was of the opinion that the preference was from the back as with a chain feeder.
However, two colleagues said that they had witnessed a belt feeder under a coal hopper where at the back of the feeder inlet coal appeared to sit stationary, the belt slipping under the coal rather than carrying it from the back. Hence they were of the opinion that the coal was discharged from the front of the hopper. They further said they had seen belts removed from feeders which appeared to have longitudinal score marks in the top cover and that this could only have been as a result of belt slippage with the coal causing the scores.
Anyone have any practical observations/experience on this subject they can share? ■