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6. KNOW YOUR UPSTREAM PROCESS To properly design a vacuum conveying system, you must define the upstream process that is supplying the material. Find out if the material is coming from a loss-in-weight feeder, volumetric feeder, mixer, reactor, extruder hopper, or any other equipment used to move material. These all influence the conveying pro- cess. Additionally, the frequency of material coming out of these ves- sels, whether batch or contin- uous, affects the conveying process and how the material will behave when it comes out of the pro- cess. Upstream equipment affects downstream equipment. It is important to know everything about the source. 7. KNOW YOUR HEADROOM REQUIREMENTS This is an important consideration when installing equipment in an existing plant. What may have been designed for manual opera- tion may not offer enough room for an automated process. Even the smallest conveying system for powder handling will require at least 30 in. of head- room over your pro- cess, taking into con- sideration maintenance requirements for filter access, discharge-valve inspection, and access to equipment below the conveyor. Applications requiring high throughput rates, which require large headroom, can use a filterless vacuum receiver. This approach allows some carryover dust to pass through the receiver, which is collected in another ground-level filtered vessel. Scaling valves or positive-pressure systems may also be a consideration for headroom require- ments. 8. IS YOUR PROCESS BATCH OR CONTINUOUS? It is important to define the type of operation that you are feeding: batch or continuous. For example, small conveyors discharging into a surge bin is a batch process. Know whether a batch of material will be received in the process via a feeder or an intermediate hopper, and whether your conveying process can handle the surge of material from a batch. Alternatively, vacuum receivers may use feeders or rotary valves to meter the material directly into the process—that's continuous conveying. Or, material can be conveyed into the receiver and discharged at the end of the conveying cycle in a metered way. Extrusion applications typically utilize both batch and continuous operation, feeding materials directly into the mouth of the extruder. 9. KNOW YOUR GEOGRAPHIC OR ATMOSPHERIC CONDITIONS Geographic and atmospheric considerations are important design considerations, particularly where altitude plays an important part in sizing the system. The higher the altitude, the more air is required to convey the material. Additionally, consider plant environmental conditions and temperature/humidity control. Certain powders that are hygroscopic can have discharge prob- lems on humid days. Understanding particle size and size distribution is very important when sizing the receiver and filter assembly. To maximize dust control, bulk-bag unloaders incorporate enclosed vacuum conveying lines, and bag-dump stations integrate dust collectors. Your conveying process may be batch or continuous. Shown here is a continuous process whereby the vacuum conveying system transfers bulk materials to a weigh hopper on load cells. 50 JULY 2017 Plastics Technology PTonline.com T ips & Technique s