Modern Plastics Handbook 2011 Part 8 pdf

calibrated, they can be connected with displays which continuously display, both graphically and digitally, levels in multiple silos. Both electromechanical and sonar systems are volumetric in that they measure height of material in a silo and require the use of conversion tables to determine actual weight. For a more direct method, another system is used. Load cell systems. By mounting load cells either on the lower side wall section or structural leg supports of a silo, direct measurement of material weight can be taken. This method is accurate and the only choice where “certified” weight requirements exist. Auxiliary Equipment: Material Handling 7.21 Figure 7.7 (Continued) 0267146_Ch07_Harper 2/24/00 4:47 PM Page 21 7.4 Bulk Resin Conveying Systems Pneumatic conveying lends itself almost ideally to the handling of plastic pellets, hence its universal use in the industry. Before we consider different types of pneumatic systems, we will examine the basic principles involved. The term pneumatic conveying itself implies air movement, and it must be clearly understood that in order to move any material, we first must move air. The mechanics of conveying are really quite simple. Air is caused to move through a transfer line by either a pump or blower. The air velocity at the inlet of the system is sufficient to pick up material and keep it in suspension as it is swept along with the airstream. If you simply think of a vacuum cleaner sweeping a rug, you can see the relationship of air movement, particle pickup, and conveying. Numerous factors affect the sizing of conveying systems. The princi- pal ones are 1. Material characteristics. Bulk density and particle size and abra- siveness. 2. System capacity. Throughput, in pounds per hour. 3. Conveying distances. Carefully taking into account elbows, vertical distances, and flex-hose connections. These parameters can vary widely from one application to another, therefore, systems are available ranging from fractional hp units with 1 or 1 1 ր 4 -in lines to 100ϩ hp systems with 6- or 8-in transfer lines. 7.4.1 Vacuum conveying system Figure 7.8 illustrates schematically the basic elements of a simple vacuum conveying system, and shows the following elements: 1. The vacuum power pack with motor/blower to provide air movement in the system. 2. The filter which protects both the pump and environment from con- taminants. 3. The vacuum receiver which accumulates resin during the loading portion of the cycle. The bottom of the receiver is fitted with a “flapper” style of dump throat which closes to provide a vacuum seal during the load portion of the cycle and opens to allow material discharge during the dump portion of the cycle. 4. A level sensing device which signals to the control system. 5. A conveying line which routes the material from the source to the destination. 6. A pickup device, which is either fixed to the discharge of a piece of equipment, such as a silo or surge bin, or a suction lance to allow 7.22 Chapter Seven, Section One 0267146_Ch07_Harper 2/24/00 4:47 PM Page 22 pick up from Gaylord-style containers. The main consideration with pickup devices is that they must have the ability to vary the air inlet and, therefore, the air/material ratio in the system to optimize system performance. A variation of a simple vacuum system is shown in Fig. 7.9. This is referred to as a central vacuum system in that it uses a single vacuum pump to draw material to multiple receivers on different machines by means of a common vacuum line with sequence valves which open when loading is required at a particular station. Systems such as those described in Figs. 7.8 and 7.9 are the most widely used in the plastics industry and are available in a wide range of sizes. Auxiliary Equipment: Material Handling 7.23 Figure 7.8 Simple vacuum system. Figure 7.9 Central vacuum system. 0267146_Ch07_Harper 2/24/00 4:47 PM Page 23 Figure 7.8 illustrates what is often referred to as a batch type of loading system in that material is conveyed in discrete size batches, which fill the receiver and are then discharged into a hopper, with the load-dump cycle repeated until the level switch is satisfied. A variation of this system, which yields higher throughput, albeit at a higher cost, is a continuous vacuum system as shown in Fig. 7.10. With a continuous vacuum system, the flapper assembly is replaced by a rotary airlock. The airlock itself is a cast-iron housing with inlet and discharge and incorporates a cylindrical bore housing a rotor. The bore and rotor are precision machined with tolerances of 0.003 to 0.005 in between surfaces. In operation, the rotor is turning continuously. The tight tolerances provide a vacuum seal while the pockets fill with material from the upper section and discharge continuously from the lower. 7.4.2 Pressure conveying systems The two previous systems both use vacuum or negative pressure to cre- ate air flow. Most vacuum pumps are capable of drawing vacuums up to a maximum of 12 to 14 in Hg, which places an upper limit on their performance. In applications where long conveying distances are encountered, positive pressure systems are often used. The pressure rating of a given blower is typically higher than the vacuum rating, therefore, a blower can deliver more “driving force” in the pressure mode. Material can be routed to several destinations by means of diverter valves with appropriate level switches and controls. A properly designed pressure system can convey material 800 to 1000 ft. Figure 7.11 shows the basic elements of a single positive system, including the following: 1. A pressure power pack with motor/blower to provide air movement in the system. 2. Arotary airlock on the outlet of the material source (silo or in-plant bin). 3. A blow-through style material pick up. 4. A conveying line. 5. A cyclone separator. 6. A level switch. 7.4.3 Combination systems Most often encountered as railcar unloading units, combination systems are hybrid vacuum-pressure systems. Figure 7.12 illustrates a combination unit in its simplest form with a single blower. The vacuum side of the blower provides a continuous vacuum to draw material 24 Chapter Seven 0267146_Ch07_Harper 2/24/00 4:47 PM Page 24 Figure 7.10 Continuous vacuum system. 7.25 0267146_Ch07_Harper 2/24/00 4:47 PM Page 25 Figure 7.11 Pressure blower system. 7.26 0267146_Ch07_Harper 2/24/00 4:47 PM Page 26 from a railcar into a cyclone separator. The rotary airlock seals the vacuum from the pressure side of the system and also dumps material into the positive pressure airstream. Combination units are typically high-volume systems and in many cases are provided with two power packs, one for the vacuum side and one for the pressure side, as shown in Fig. 7.13. 7.4.4 Points to consider All of the systems described utilize filters at some point in their pneumatic circuit. The type of filter selected depends on the nature of the material being conveyed and the dust loading of the air being filtered. Inadequate filtration is one of the most common reasons for system underperformance. Abrasive material may require special materials in the construction of receivers, cyclones, airlocks, and conveying lines. When sizing systems, future growth must be taken into account. Auxiliary Equipment: Material Handling 7.27 Figure 7.12 Combination vacuum-pressure system (one pump). Figure 7.13 Combination vacuum-pressure system (two pump). 2727 27 0267146_Ch07_Harper 2/24/00 4:47 PM Page 27 7.5 Bulk Delivery Systems 7.5.1 Truck delivery Bulk shipment of resin is accomplished either by special bulk trucks or railcars dedicated to that service. As explained earlier, all that is nec- essary to receive material by bulk truck is a storage vessel large enough to hold the quantity delivered and a fill line for the truck to connect its delivery hose. Since trucks are equipped with their own blower systems, no additional conveying equipment is provided at the plant level (Fig 7.14). Several considerations are, however, worth noting. It is important that silos be equipped with high-level switches and an alarm—either audible or visual—to alert personnel that the silo is full and avoid a situation where material backs up and clogs the fill line. It is, of course, always advisable to assure that the empty volume in a silo is sufficient to hold a truck load of resin prior to calling for delivery. If multiple silos are on site, each holding a different resin, great care must be taken to avoid cross-contamination. The simplest method is to put locks on the connection fittings of each silo. Each lock should have a different key, and the keys should be in the possession of plant personnel who must select the right one after verifying which resin is being delivered. 7.5.2 Railcar delivery With ever-rising resin consumption patterns, delivery of plastic resin by bulk railcar has become increasingly popular. The main reason, of course, is the significant savings which can be enjoyed. On low-density PE, this can be as much as 5 to 6¢/lb over the Gaylord price, which amounts to $10,800 on a typical 180,000-lb railcar shipment. With such savings, the cost of the storage and unloading facility can be recovered quite rapidly. Of almost equal importance is the fact that during a period of tight resin supply, the customer can be assured of a large raw material inventory in storage. 7.5.3 Types of systems Two basic types of pneumatic conveying systems have been adopted for railcar unloading service: the vacuum system and the combination negative-positive pressure system. Each possesses certain advantages which make it useful for different types of applications. Figure 7.15 shows schematically a straight vacuum type of loader that utilizes a silo-mounted vacuum chamber. In this type system, the vacuum pump 7.28 Chapter Seven, Section One 0267146_Ch07_Harper 2/24/00 4:47 PM Page 28 is generally located in the skirt of the silo. Air-return lines extend up to the vacuum hopper. On multiple silo installations, each vacuum hopper has a vacuum line extending to a central area near the pump. A manual flex hose switching station is utilized to selectively draw the vacuum on any silo loader. Fill lines extend from each vacuum chamber to a central area. The silo fill lines are always equipped with male disconnect fittings, while the air-return lines are fitted with female disconnects. By utilizing stainless-steel flex-hose connections, unloading manifolds, and accessories, the hookup to the railcar discharge can be accomplished. When the pump is started, the unit functions identi- cally to smaller vacuum loaders in that it runs for a period of time until the chamber is full. It then allows the material to dump into the silo. This process is repeated until either the silo is full or the railcar compartment is empty. The vacuum hopper and extension are of weld- ed aluminum construction. The hopper has approximately a 200-lb capacity of 38-lb/ft 3 material. The extension is equipped with a clean out door which allows inspection and maintenance of the flapper assembly. Note: The chamber shown is used only for clean pellet applications. Where powder is to be handled, special chamber and filter designs are required. The chamber is fitted with a pellet screen to prevent material being sucked back to the vacuum pump. In addition, the pellet screen Auxiliary Equipment: Material Handling 7.29 Figure 7.14 Silo fill line by truck (blower). 0267146_Ch07_Harper 2/24/00 4:47 PM Page 29 [...]... generated directly from the production operation, such as runners and sprues or extrusion edge trim, or from reject finished parts Whatever the source, economics dictate that the scrap be reintroduced into the material stream The first step in the process is to grind the parts into a particle size small enough to be mixed with virgin pellets and flow through the various blenders, dryers, and loaders upstream... Once parts have been ground, there is often a need to remove dust or oversize slivers of material from the regrind prior to its reintroduction into the process Removal of fines can be accomplished by means of elutriation or air scalper systems which are shown in Fig 7.24 Mechanical screen separators, such as Fig 7.25, have the ability to remove both fines and oversize particles in one step 7 .8 Material... drying hopper, and a process air filter The dehumidifier itself includes the desiccant regeneration system A single desiccant-coated honeycomb wheel rotates slowly, exposing part of the wheel to process air, part to regeneration, and part to cooling prior to returning to the process Single rotating bed 0267146_Ch07_Harper 2/24/00 4:47 PM Page 49 Auxiliary Equipment: Material Handling 7.49 This is usually... the chamber and provide a cooling action to prevent melting 7.7.6 Combination shredder/granulators For very large parts or extremely high volumes, a two-stage shredder/granulator process is often used The first stage consists of multiple low revolutions-per-minute rotors which tear the parts into smaller pieces which then are fed into the grinder section 7.7.7 Noise and safety considerations By their... achieved over all ingredient feed rates As with any press-mounted system, careful consideration must be given to allow access for maintenance, calibration, and cleanout 0267146_Ch07_Harper 7. 38 2/24/00 4:47 PM Page 38 Chapter Seven, Section One 4 Floor-mounted blenders Certain situations arise that preclude the use of a blender mounted on a machine The most common of these is the situation where a dryer... STYLE DEHUMIDIFIER DRYER Figure 7.26 High-temperature material drying system 0267146_Ch07_Harper 2/24/00 4:47 PM Page 47 Figure 7.27 Multimachine central dryer 7.47 0267146_Ch07_Harper 7. 48 2/24/00 4:47 PM Page 48 Chapter Seven, Section One applications such as molding PET bottle preforms may necessitate dewpoints as low as Ϫ50 or Ϫ60°F 4 Residence time Given proper airflow, temperature, and dewpoint,... recommended; therefore, a 200-ft3/min dryer is needed 2 Process air temperature A temperature of 160 to 180 °F is recommended 3 Dewpoint A dewpoint of Ϫ30 to Ϫ40°F is recommended 4 Residence time A time of 3 h is recommended; therefore at 200 lb/h, a vessel holding 600 lb of material is required 7 .8. 2 Drying equipment Desiccant-type dryers are widely used where low dewpoints are required A desiccant... inventory control purposes If equipped with conveying systems using load cell–weigh chamber technology, as shown in Fig 7. 18, all material brought into the plant Figure 7.17 Surge bins 0267146_Ch07_Harper 2/24/00 4:47 PM Page 35 Auxiliary Equipment: Material Handling 7.35 Figure 7. 18 Weigh chamber is weighed and totalized This greatly simplifies the tracking of resin usage patterns over a given period... number of drying hoppers, size of hoppers, and cubic feet per minute of central dryers Future growth in throughput and additional materials must be taken into account at this stage 7 .8. 3 Dryer system controls Almost all modern dryers utilize either microprocessor or programmable logic controllers which monitor process parameters such as temperature, airflow, dewpoint at numerous points in the system,... evaluating multistation systems, it is best to use the longest material run in the system 3 Figure 7. 28 applies to pellet conveying systems If powder is being conveyed, the expected throughputs are approximately one-third lower than those indicated 0267146_Ch07_Harper 2/24/00 4:47 PM Page 51 Figure 7. 28 Loading system throughput curves 7.51 0267146_Ch07_Harper 7.52 2/24/00 4:47 PM Page 52 Chapter Seven, . valves which open when loading is required at a particular station. Systems such as those described in Figs. 7 .8 and 7.9 are the most widely used in the plastics industry and are available in a wide. reject finished parts. Whatever the source, economics dictate that the scrap be reintroduced into the material stream. The first step in the process is to grind the parts into a particle size small. of air movement, particle pickup, and conveying. Numerous factors affect the sizing of conveying systems. The princi- pal ones are 1. Material characteristics. Bulk density and particle size and

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