The main challenge in moving these powders is getting them into the conveying line from the pick-up point, feeding them at a constant rate, and getting the powder to discharge from the material receiver. Once the non-free flowing materials are entrained, they are fluidized and convey easily and do not build up on the conveying tubes.
When non-free flowing material is introduced at the pick-up point, some bulk material may go in but it could bridge or rat-hole at that point. If the bulk material does not flow down into the pick-up point, only air will be conveyed, not product.
There are several options that make it possible to get sticky or non-free flowing materials into a conveying line at a constant rate. It is critical to have the proper solids-to-air ratio. If it is too high, the line will plug. If it is too low, the pneumatic system sucks-in mainly air and does not transport much material.
One way to enter bulk materials at a constant rate is to introduce the material into the convey line where the air flow is mechanical, typically with a rotary valve, or physical, typically done with with a screw feeder. Other ways to help mechanically move the powder or bulk material, and ensure the correct solids-to-air ratio, is with the aid of specially devised flow promotion devices, specifically vacuum receivers and suitable filters placed in the correct locations. Flow promotion devices can be vibration on the outside of a bin or hopper that knocks the material loose, or a small air cannon that injects air into the material to prevents it from sticking to the hopper and feeder walls.
When repurposing a conveying system, a common problem is that it will not work with new or other bulk material. It is important to evaluate every single powder that is difficult to convey, even small differences, like processing plants that have two locations, each using powders from different suppliers or different grades and material characteristics, all may have an impact on processing, production and end-product quality.
There are different grades of calcium carbonate, which when mined from the earth, is rocky and free-flowing. The more it is milled down to a superfine powder, the more cakey and sticky it gets.
Titanium dioxide – a very popular additive ingredient in white paint or anything that requires a whitening additive – is similar. The finer it gets, the worse the pneumatic conveying becomes.
Unless the material is something like granulated sugar that is easy to convey and similar no matter who makes it, each difficult powder may need to be handled differently. Typically, pneumatic conveying system vendors duplicate a factory’s conditions in a test lab to determine if a quoted system will work and, will make adjustments if it does not.
Before Contacting a Supplier
Before approaching a vendor to source a fully automated pneumatic conveying system for batch or continuous processes, have a good understanding of what the equipment will be expected to do. It may be as simple as moving a bucket of bulk materials from one place to another.
Know what type of bulk material will be conveyed and its bulk density, or weight in lb/cu ft. This helps determine the size of the tubing, filter, receiver, and vacuum pump in the conveying system. It is much different to move powders at 5,000 lbs/hr (2,268 kg/hr) that are 100 lbs/cu ft than those that are 500 lbs/cu ft.
Know the characteristics of the powder to move. Free-flowing products that are unproblematic and commonly conveyed may be plastic pellets, granular sugar, rice, salt, any granular powders. It is easy to test whether a material is free-flowing by taking a scoop of the material and pouring it out. If a dry powder has a slight moisture or fat content to them, or can absorb moisture, like flour, or if they are oily, they aren’t free-flowing.
Know whether the material is subject to breakage and if the integrity of the material should be maintained when it is introduced into a packaging machine or to the final consumer. This is important to know in order to specify the correct handling system. Handling a fragile mix, such as cereal, so it doesn’t come out as dust, is different from handling a coated tablet so it comes out intact without breakage.
Know the horizontal and vertical distances the bulk material will be conveyed. The further away the pickup point is from the filter receiver or destination, the larger the system becomes. Conveying 10’ vertical and 20’ horizontal at 5,000 lb/hr is different from 50’ vertical and 200’ horizontal at the same speed.
Know the layout of the equipment in the plant and if there are any 90 degree bends in the conveying path. Each one is equivalent to adding an extra 20’ of conveying distance into the system. For conveying, the closer together the equipment, the better, but sometimes close is not possible. Equipment may need to be kept separate for cross contamination reasons, be subject to cleanroom constraints, for safety reasons to contain materials if there is a spill, or if different operators work in different locations.
Know how many pick-up points and what type of sources bulk materials will be delivered in. Material may be located in different places from different sources; a 30-gallon drum from one supplier and a 50-lb bag, supersack, or bulk bag from another. A vacuum conveying system can’t pick up from multiple points at the same time since air takes the path of least resistance.
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