A: Doan Pendleton, Vac-U-Max, says:
This is a common question when a manufacturer is seeking a vacuum conveyor to fill a process role, solve a problem, or move powder or granular bulk material within a plant, as it’s less expensive to purchase equipment than it is to move material manually.
A basic vacuum conveying system comprises five equipment components: vacuum producer, pickup point, conveying tubing (conveying line), vacuum receiver, and control panel. Each component must complement the others for the system to operate efficiently. If one component is too large or too small, it will keep the system from working properly. A vacuum conveying system’s budget depends on knowing the application’s parameters because they dictate conveyor equipment choice and design.
The most important factors in vacuum conveyor design and expense are the application description, the specific industry, the material’s bulk density and characteristics, the conveying distance and rate, and the material pickup point. To relate how these important factors can influence equipment selection and expenses, here’s a brief description of the five basic vacuum conveying components and the particular factors that impact each component’s selection and operation.
Vacuum producer. A vacuum producer or air mover moves material through the conveying line and is the heart of any vacuum conveying system. Bulk density, conveying distance, and conveying rate will directly influence the necessary size of your vacuum producer.
Bulk density helps determine how many cubic feet per minute (cfm) of air is necessary to move the material through the conveying line. Generally, materials with bulk densities below 55 lb/ft3 and above 25 lb/ft3 are fairly easy to convey, while heavier materials require up to 50 percent higher conveying speed and vacuum level with a corresponding increase in motor horsepower for the vacuum producer. Longer conveying distances also require larger vacuum pumps because of the additional airflow needed to pull air through the conveying line.
Most vacuum conveyors operate on a cycling basis that comprises a conveying function and discharge function. The operating cycle can have a considerable effect on the vacuum producer’s size. An example of this would be when a processor or manufacturer wants to move 5,000 lb/h into a mixer but needs to move the batch into the mixer within 15 minutes. Although the 5,000-pound cycle is only needed once per hour, the actual transfer rate is higher during that 15 minutes and is calculated as moving 20,000 lb/h; thus, a more powerful vacuum producer is required.
Venturi power units, available in single to quadruple venturi versions, are a budget-friendly way to power vacuum conveying systems. The venturi units function using compressed air, have no moving parts, and are easy to install and operate. Because these units use no electricity, they’re excellent for use in explosion-hazard areas.
Positive-displacement (PD) vacuum pumps, primarily used for dilute-phase and semi-dense-phase conveying systems, use a rotary lobe pump that’s belt-driven by a motor to create the vacuum. PD vacuum pumps are useful in dense-phase conveying where the material moves in slugs through the conveying line. A regenerative blower is lower in vacuum and higher in airflow than a PD vacuum pump and is excellent to use when conveying granular or pelleted materials.
Pickup point. A pickup point is where the material enters the conveying system, and a material’s flow characteristics are important factors in material movement into the conveying line. Once material is in the conveying line, whether it’s free-flowing or not, it usually conveys without issue, but sometimes getting material into the conveying line can be problematic.
The pickup point is perhaps the most customized component in a pneumatic conveying system because it’s crucial for feeding material into the system. A pickup point in a vacuum conveying system can be a wand to convey material from a drum, a bag-dump station, a bulk bag unloader, or even a truck or railcar. In more sophisticated systems, a pickup point could be the discharge point from upstream processing equipment. The volume of material to transfer or conveying rate also helps determine how much automation is necessary for a system’s pickup point.
Conveying material from a container with a wand to a vacuum receiver is a low-cost system, where an operator can safely convey up to 5,000 lb/h. To achieve rates higher than 5,000 lb/h, the material must be fed by gravity into the conveying line from a bag-dump station or bulk bag unloader.
Bag-dump stations convey material from smaller bags such as 25-pound bags. The operator lifts the bag onto the bag-dump station grate, cuts a slit in the bag and turns it over, and the material falls by gravity into the conveying line. If the material isn’t free-flowing or is sticky, then a screw discharger may be used to meter the material into the conveying line. An operator dumping 50-pound bags into a bag-dump station can achieve approximately 100 lb/min (two bags per minute). Bulk bag unloaders can achieve much higher rates.
A bulk bag unloader, also known as a bulk bag discharger, handles large bulk bags (2,000 pounds). Bulk bags come fitted with a loading spout and a discharge spout. The filled bulk bag is placed on the unloader’s frame, and then an operator unties the discharge spout to allow the material to flow by gravity into the conveying system.
Conveying tubing and hose. The conveying tubing and hoses that comprise the conveying line are vital to a vacuum conveying system, as they enclose and transport materials from one point to the next. A material’s bulk density, conveying rate, and tubing route determines the necessary size of the conveying tubing.
One pneumatic conveying advantage is that moving material vertically is calculated the same way as moving it horizontally — in linear feet. However, each 90-degree sweep (elbow) in the system equals 20 linear feet; thus, if you’re moving material 120 feet horizontally and 120 feet vertically with four 90-degree sweeps, then the vacuum producer should be sized for a conveying distance of 320 feet.
Conveying tubing is always constructed from metal, which is commonly stainless steel. Twenty-foot lengths of conveying tubing connect using compression-type couplings. In sanitary systems, the tubing is polished and connected using ferrules. Flexible conveying hose, like that used with wands, is always static conductive; and in sanitary design, smooth-bore hoses manufactured with FDA- and USDA-accepted materials are available.
Vacuum receiver. Conveyed material enters the vacuum receiver, also known as a filter-receiver, from the conveying line and discharges from the vacuum receiver into other process equipment, packaging equipment, or intermediary vessels. Vacuum conveyors move material from the pickup point into the vacuum receiver until it reaches a predetermined fill weight or volume, then and the receiver’s discharge valve opens, and the material drops into its destination point.
Vacuum receivers come in all different shapes and sizes — from 0.25 cubic feet to 300 cubic feet in capacity — and proper sizing depends on the material’s bulk density and the system’s conveying rate (lb/h). The vacuum receiver must also be able to handle the airflow provided by the vacuum source, and filters located in the receiver must have the proper air-to-cloth ratio so the filter media doesn’t shred or blind.
A fine powder with low bulk density, such as fumed silica, presents its own set of challenges and may require more filtration and, therefore, needs a larger vacuum receiver than a medium bulk density powder would.
If the vacuum producer is the heart of a conveying system, then the filters are the lungs. Like vacuum receivers, the filter selection depends on the application. Static conductive filters and pulse filter cleaning systems keep the filters clean and the system running efficiently.
Control panel. The control panel is the brain of a vacuum conveying system and is configured according to the application. Even the simplest system uses a micro-PLC to control the system’s conveying, discharging, and pulsing functions. For more involved batch weighing systems and multi-ingredient handling systems, an advanced PLC control panel can be furnished and can include one or more touch screen panels that allow an operator to control every aspect of the conveying system.
Consulting with a vacuum conveyor expert or company with testing capabilities and vast material expertise early in the buying process ensures that manufacturers and processors reap the many benefits of a reliable vacuum conveying system, including optimized throughput, a safer and cleaner environment, minimal maintenance, and a long equipment lifespan.
Vac-U-Max, Belleville, NJ, designs and manufactures pneumatic systems and support equipment for dry material conveying, weighing, and batching.