Modern Plastics Handbook 2011 Part 8 pdf

calibrated, they can be connected with displays which continuouslydisplay, both graphically and digitally, levels in multiple silos.Both electromechanical and sonar systems are volumetri

calibrated, they can be connected with displays which continuouslydisplay, both graphically and digitally, levels in multiple silos.Both electromechanical and sonar systems are volumetric in that theymeasure height of material in a silo and require the use of conversiontables to determine actual weight For a more direct method, anothersystem 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 theonly choice where “certified” weight requirements exist

Auxiliary Equipment: Material Handling 7.21

Figure 7.7 (Continued)

7.4 Bulk Resin Conveying Systems

Pneumatic conveying lends itself almost ideally to the handling of tic pellets, hence its universal use in the industry Before we consider dif-ferent types of pneumatic systems, we will examine the basic principles

plas-involved The term pneumatic conveying itself implies air movement,

and it must be clearly understood that in order to move any material, wefirst 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 blow-

er The air velocity at the inlet of the system is sufficient to pick up rial and keep it in suspension as it is swept along with the airstream Ifyou simply think of a vacuum cleaner sweeping a rug, you can see therelationship of air movement, particle pickup, and conveying

mate-Numerous factors affect the sizing of conveying systems The pal ones are

princi-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 114-in lines to 100 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

vac-uum conveying system, and shows the following elements:

1 The vacuum power pack with motor/blower to provide air ment in the system

move-2 The filter which protects both the pump and environment from taminants

con-3 The vacuum receiver which accumulates resin during the loadingportion of the cycle The bottom of the receiver is fitted with a “flap-per” style of dump throat which closes to provide a vacuum sealduring the load portion of the cycle and opens to allow material dis-charge 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 thedestination

6 A pickup device, which is either fixed to the discharge of a piece ofequipment, such as a silo or surge bin, or a suction lance to allow

pick up from Gaylord-style containers The main consideration withpickup devices is that they must have the ability to vary the airinlet and, therefore, the air/material ratio in the system to optimizesystem 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 bymeans of a common vacuum line with sequence valves which openwhen loading is required at a particular station

Systems such as those described in Figs 7.8 and 7.9 are the most

wide-ly 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.

Central vacuum system.

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 theload-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 ous vacuum system, the flapper assembly is replaced by a rotary air-lock The airlock itself is a cast-iron housing with inlet and dischargeand incorporates a cylindrical bore housing a rotor The bore and rotorare precision machined with tolerances of 0.003 to 0.005 in betweensurfaces In operation, the rotor is turning continuously The tight tol-erances provide a vacuum seal while the pockets fill with materialfrom the upper section and discharge continuously from the lower

continu-7.4.2 Pressure conveying systems

The two previous systems both use vacuum or negative pressure to ate air flow Most vacuum pumps are capable of drawing vacuums up to

cre-a mcre-aximum of 12 to 14 in Hg, which plcre-aces cre-an upper limit on their formance In applications where long conveying distances are encoun-tered, positive pressure systems are often used The pressure rating of

per-a given blower is typicper-ally higher thper-an the vper-acuum rper-ating, therefore, per-ablower can deliver more “driving force” in the pressure mode

Material can be routed to several destinations by means of divertervalves with appropriate level switches and controls A properlydesigned 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 A rotary 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

sys-tems are hybrid vacuum-pressure syssys-tems Figure 7.12 illustrates a

combination unit in its simplest form with a single blower The

vacu-um side of the blower provides a continuous vacuvacu-um to draw material

Figure 7.10 Continuous vacuum system.

Figure 7.11 Pressure blower system.

from a railcar into a cyclone separator The rotary airlock seals thevacuum from the pressure side of the system and also dumps materi-

al into the positive pressure airstream Combination units are

typical-ly high-volume systems and in many cases are provided with twopower packs, one for the vacuum side and one for the pressure side, asshown in Fig 7.13

7.4.4 Points to consider

All of the systems described utilize filters at some point in their matic circuit The type of filter selected depends on the nature of thematerial being conveyed and the dust loading of the air being filtered.Inadequate filtration is one of the most common reasons for systemunderperformance Abrasive material may require special materials inthe construction of receivers, cyclones, airlocks, and conveying lines.When sizing systems, future growth must be taken into account

pneu-Auxiliary Equipment: Material Handling 7.27

Figure 7.12 Combination vacuum-pressure system (one pump).

Combination vacuum-pressure system (two pump).

27 27

7.5 Bulk Delivery Systems

7.5.1 Truck delivery

Bulk shipment of resin is accomplished either by special bulk trucks orrailcars 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 itsdelivery hose Since trucks are equipped with their own blower sys-tems, 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 isfull and avoid a situation where material backs up and clogs the fillline 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 fordelivery

If multiple silos are on site, each holding a different resin, great caremust be taken to avoid cross-contamination The simplest method is toput 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 sonnel who must select the right one after verifying which resin isbeing delivered

per-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, ofcourse, is the significant savings which can be enjoyed On low-densi-

ty PE, this can be as much as 5 to 6¢/lb over the Gaylord price, whichamounts to $10,800 on a typical 180,000-lb railcar shipment Withsuch savings, the cost of the storage and unloading facility can berecovered quite rapidly Of almost equal importance is the fact thatduring a period of tight resin supply, the customer can be assured of alarge raw material inventory in storage

7.5.3 Types of systems

Two basic types of pneumatic conveying systems have been adopted forrailcar unloading service: the vacuum system and the combinationnegative-positive pressure system Each possesses certain advantageswhich make it useful for different types of applications Figure 7.15shows schematically a straight vacuum type of loader that utilizes asilo-mounted vacuum chamber In this type system, the vacuum pump

is generally located in the skirt of the silo Air-return lines extend up

to the vacuum hopper On multiple silo installations, each vacuumhopper has a vacuum line extending to a central area near the pump

A manual flex hose switching station is utilized to selectively draw thevacuum on any silo loader Fill lines extend from each vacuum cham-ber to a central area The silo fill lines are always equipped with maledisconnect fittings, while the air-return lines are fitted with femaledisconnects By utilizing stainless-steel flex-hose connections, unload-ing manifolds, and accessories, the hookup to the railcar discharge can

be accomplished When the pump is started, the unit functions cally to smaller vacuum loaders in that it runs for a period of timeuntil the chamber is full It then allows the material to dump into thesilo This process is repeated until either the silo is full or the railcarcompartment is empty The vacuum hopper and extension are of weld-

identi-ed aluminum construction The hopper has approximately a 200-lbcapacity of 38-lb/ft3 material The extension is equipped with a cleanout door which allows inspection and maintenance of the flapperassembly

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 materialbeing 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).

is surrounded by a shroud which eliminates direct impingement ofmaterial on the screen The entire hopper-extension assembly is bolt-

ed in place on the center dome of the silo

A high-level switch must be utilized with this unloader Most often,the rotating paddle type is used The switches themselves are mount-

ed in the silo deck with the paddles mounted on extension shafts.Extensions must be used because of the angle of repose the materialadopts when loaded into the silo When material contacts the paddle,the unit is shut off The pumping system itself utilizes a positive dis-placement motor/blower assembly The inlet of the pump is fitted with

a two-stage secondary filter The purpose of this filter is to prevent

Figure 7.15 Silo fill line by railcar (vacuum).

fines which may have passed the pellet screen from entering thepump.

Note: As a matter of routine maintenance, this filter should be inspected frequently.

The pump inlet also has a manual vacuum relief which, in the event

of a material line blockage, will allow air to enter the system and vent damage to the system

pre-The loader control panel, which may be mounted on the unit or at

a remote point, contains motor starters, timers, high- and low-levelswitch lights and, on multiple silo systems, a selector switch toenergize the proper high-level switch for automatic operation.When the operator wishes to change from loading one silo to anoth-

er, the flex-hose connections to the material and vacuum line of thenew tank must be changed and the tank selector switch on the con-trol panel must be positioned to the new silo The system is thenready to run

A variation of the vacuum system just described utilizes a rotary lock on the bottom of the hopper in place of the flapper assembly Theairlock seals the vacuum in the hopper while, at the same time, allowsthe material to be discharged into the silo The primary advantage ofthis system is the fact that it conveys continuously rather than inintermittent batches Its drawbacks are higher cost than the flapperdischarge, airlocks tend to be a high-maintenance item located on top

air-of the silos, and the tendency air-of some materials to break up or smearwhen passed through the rotor

Figure 7.12 shows a typical combination negative-positive pressurevacuum system Vacuum from the pump draws material into either acyclone separator or filter receiver The pellets are passed through arotary airlock and enter the blower discharge airstream, which is atpositive pressure The air/material mixture is transferred to the silosvia stainless-steel flex-hose and fill lines Combination units tend to

be high-capacity systems used primarily with multiple silo systems.One advantage is that they require no equipment whatsoever on top

of the silo, only a simple fill line All maintenance is performed atground level

7.5.4 Transfer rates

As with any type of pneumatic conveying system, the transfer ratedepends upon material characteristics and distance Railcar unloadinghas the added complication of always being a high-lift situation, any-where from 40 to 70 ft vertical A 25-hp unit with a 4-in conveying linemoving polyethylene pellets approximately 100 ft horizontal and 60 ftvertical will maintain an approximate throughput of 10,000 lb/h

Auxiliary Equipment: Material Handling 7.31

Note: Final hookups to railcars and silo fill lines are always plished with flex hose These connections should be kept as short and straight as possible, otherwise the conveying rate will be severely impaired.

accom-Precautions must be taken to avoid cross-contamination Fill linescan be equipped with locks, as described previously in the truck deliv-ery section, or interlocking proof switches can be fitted to the fill lines

so that the hose connection must agree with a selector switch setting

on the control panel in order for the transfer to proceed

7.5.5 Railcar connections and accessories

Figure 7.16 shows various details and layouts of commonly used car discharge systems The cars themselves are divided into variouscompartments, each of which has its own outlet These outlets can beadjusted for the air/material ratio and perform the same function as avacuum tray adaptor on a dryer hopper Three basic accessories arerequired for unloading:

The adaptor is equipped with a female quick disconnect fittingwhich is the piece that makes the connection to the flex hose

The air inlet filter fits on the far side of the discharge and preventscontaminants from entering the conveying airstream from that point.The hatch on the compartment must be empty and open to allow air

to take the place of the material being withdrawn Contamination isprevented by placing the hatch filter over the opening

Most railcar unloading installations are equipped with a manifoldarrangement This is a transfer tube that runs parallel to the sidingand equipped with “Y” laterals and disconnect fittings The discon-nects are all capped, except for the one used for drawing material fromthe railcar The use of a manifold in a system allows for variations

Figure 7.16 Railcar takeoff.

which occur in railcar spotting, while minimizing the use of flex hosewhich has a much higher flow resistance.

7.5.6 Surge bins

Resin storage silos are almost always located outdoors and, in manycases, at a considerable distance from the ultimate point of use ofthe material in the plant The use of intermediate surge bins (seeFig 7.17) enhances system flexibility in these cases by providing thefollowing:

1 An internal distribution point closer to the resin use point

2 A simple method of transferring material to drums or boxes

3 In cold weather, resin conveyed to the surge bins will come to roomtemperature, thereby avoiding delivery of cold resin to processingmachines

In addition, surge bins can be useful for inventory control purposes

If equipped with conveying systems using load cell–weigh chambertechnology, as shown in Fig 7.18, all material brought into the plant

Surge bins.

is weighed and totalized This greatly simplifies the tracking of resinusage patterns over a given period of time.

7.6 Blending Systems

Because of economic considerations, an increasing volume of resin isshipped in the natural or uncolored state Color must be added atthe plant level before the material is fed to the processing machine.This, along with the fact that most processing operations producescrap which must be reintroduced into the material stream, has led

to numerous systems for blending these various ingredients

In their simplest form, all blenders contain the same elements:

1 Individual feed hoppers to contain each ingredient, typically virginresin, regrind, and color pellets

2 Metering devices, such as a feed screw, vibrator tray or

air-operat-ed slidegate, to regulate the flow of each ingrair-operat-edient

3 A mixing section to homogenize the batch before leaving theblender

4 A control section ranging from simple speed controllers to cated microprocessors or loss in weight blenders

sophisti-Auxiliary Equipment: Material Handling 7.35

Figure 7.18 Weigh chamber.

Blending systems fall into several broad categories which will be cussed in the next several subsections.

dis-7.6.1 Volumetric blenders

Volumetric units rely on different speed settings on individual dient feeders to provide different proportions of each material Anynumber of ingredients can be metered this way and they are quite costeffective; however, feed devices must be carefully calibrated for eachindividual ingredient If a feed hopper runs empty or experiences par-tial flow due to a bridging condition, the controls cannot sense this con-dition, and an improper mixture can result Accuracy of volumetricunits is typically in the 2 to 3% range, which means that if a signifi-cant number of time-weight samples are taken from a single feeder,they will cluster within 2 to 3% of the set point

ingre-7.6.2 Gravimetric blenders

Gravimetric units offer more consistent and accurate performance

at a very small premium The simplest gravimetric blenders are called batch weighing systems which meter ingredients, one at atime, into a weigh chamber attached to a load cell The load cell cutsoff one feeder when its required weight is reached and then calls forthe next ingredient and so on The only calibration that is required

so-is for the load cell itself, which usually takes less than 1 min Ifbridging or an empty feed hopper occur, the load cell will sense thisshort weight and indicate an alarm condition, thereby minimizingthe chance of incorrect mixes reaching the process Accuracy ofgravimetric units is typically greater than 1% of the batch weightsize

7.6.3 On-the-press blenders

Either volumetric or gravimetric units can be mounted directly onthe feed throat of a processing machine This arrangement brings allequipment and controls directly to the machine where it is easilymonitored by the operator This method is typically used where aplant has numerous machines, each using a different material-colorcombination The main drawbacks to this approach are difficultaccess to the equipment on some large presses or extruders and thedowntime required for cleanout when material changes are made.Examples of machine-mounted and central blenders are shown inFig 7.19

7.6.4 Central blenders

One blender can be mounted on a floor stand and feed the same rial to a number of processing machines In such cases, this reducesthe number of blenders required and has the additional benefit of min-imizing cleanout on the machine for color changes

mate-The choice of blender configuration depends on numerous factorswhich must be evaluated at the plant level The following is a briefsummary of different configurations widely used in the industry:

1 Dual ratio receiver–color pellet feeder This arrangement uses a

proportional valve arrangement to load virgin and regrind materials

As the processing machine screw rotates and material flows into thefeed throat, the color is metered in by the feeder The feeder typicallyhas a variable speed drive to allow different feed rates for different col-oring requirements

2 Dual-feeder arrangement By incorporating two separate feeders,

one for regrind and one for color concentrate, a more precise control ofproportions can be achieved

Note: When using either the single- or dual-feeder arrangements described here, it is advisable to mount the riser section on all process- ing machines so that the feeder units can be relocated if necessary.

3 Machine-mounted blenders By placing a volumetric or

gravimet-ric blender directly on the feed throat of a machine, direct control isachieved over all ingredient feed rates As with any press-mountedsystem, careful consideration must be given to allow access for main-tenance, calibration, and cleanout

Auxiliary Equipment: Material Handling 7.37

Figure 7.19 Basic blenders.

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 hopper must be mounted there instead

In these cases, floor mounting the blender either at the machine or insome central location is appropriate Floor mounting the blender alsoleads to the possibility of using one unit to feed several machines run-ning the same material

Numerous other variations are possible, some of which are shown inFig 7.20 As with most equipment selections, the correct choicedepends on specific plant operating patterns and may result in differ-ent types of blending systems for different applications

5 Loss-in-weight blenders Figure 7.21 shows a schematic

arrange-ment of a loss-in-weight blender These units differ from batch weightsystems in that each individual feed hopper with its feeder is mount-

ed on a load cell As material is metered out of the hopper, the load cellsenses the loss in weight of the hopper and adjusts the feeder speed tomaintain correct proportioning Since all feeders run simultaneously,higher throughput rates are possible with loss-in-weight units Theyare used mostly in sheet and film extrusion applications and are ofteninterfaced with extruder or downstream drives to maintain precisegauge control

7.7 Regrind Systems

Every plastic processing operation produces scrap material either erated directly from the production operation, such as runners andsprues or extrusion edge trim, or from reject finished parts Whateverthe source, economics dictate that the scrap be reintroduced into thematerial stream The first step in the process is to grind the parts into

gen-a pgen-article size smgen-all enough to be mixed with virgin pellets gen-and flowthrough the various blenders, dryers, and loaders upstream of theinjection press or extruder

Figure 7.22 shows a simple upright or “beside-the-press” scrapgrinder and identifies key elements of the unit

Factors to consider when selecting a grinder are

1 Part size, shape, and wall thickness These factors have a directbearing on unit size, rotor, knife, and screen configuration

2 Material Hard or abrasive materials require one set of featureswhile very soft materials may require their own option

3 Throughput

Many grinder variations exist, each tailored for a specific type of

Figure 7.20 Blender options.

scrap material The most common of these are discussed in the ing subsections.

follow-7.7.1 Simple upright grinders

This type of grinder has an arrangement similar to that shown in Fig.7.22 It is usually positioned alongside an injection or blow moldingmachine for immediate reprocessing of runners, sprues, bottle trim, orimproperly formed parts When selecting an upright grinder, consider-ation must be given to the method of feeding material into the unit.This can be done manually by an operator or via sprue picking robots

or conveyor belts

7.7.2 Auger feed grinders

These units are typically used in injection molding applications Theauger trough is situated directly under the mold and catches runnernetworks as they fall from the mold The auger pulls the runners intothe cutting chamber where they are ground Because of their low pro-file, they do not have a large storage capacity for ground material andmust be continuously evacuated (see Fig 7.23)

Figure 7.21 Loss-in-weight blender.

Figure 7.22 Upright tangential feed grinder.

7.7.3 Central grinders

Where press-side grinding is not practical, the scrap is brought to acentral grinder capable of handling material from numerous process-ing machines These units are considerably larger than press-sideunits and often incorporate infeed conveying systems and large evacu-ation blowers to accommodate their high throughput

7.7.4 Edge trim and web grinders

Extrusion operations generate scrap in the form of thin strips of rial from the edge of sheets or webs of material after thermoformingoperations Grinders for these applications typically incorporate pow-ered feed rolls to assure consistent infeed of scrap material

knife tolerances are required, very small screen openings to assurereasonable bulk density of the regrind, and high-volume evacuationsystems both to draw material through the chamber and provide acooling action to prevent melting.

7.7.6 Combination shredder/granulators

For very large parts or extremely high volumes, a two-stage der/granulator process is often used The first stage consists of multi-ple low revolutions-per-minute rotors which tear the parts into smallerpieces which then are fed into the grinder section

shred-7.7.7 Noise and safety considerations

By their very nature, scrap grinders generate very high noise levels.Sound treatments consisting of special acoustic enclosures to reducenoise levels are available for most units All grinders must be equippedwith safety switches and interlock circuits to assure that personnelcannot be exposed to dangerous conditions while maintaining or clean-ing these units

7.7.8 Grinder evacuation systems

Manual removal of regrind material is inefficient, dangerous, andmessy; therefore, most grinders are equipped with evacuation blowers

In addition to continuously removing material, they have the addedadvantages of drawing a large quantity of air through the cuttingchamber which helps cool the material, thus reducing degradation

7.7.9 Size classification and fines removal

For elutriator systems, mechanical screen separators, and cyclone arators, various devices to perform the classification and removalfunction, include eludriator systems, mechanical system separators,and cyclone separators Once parts have been ground, there is often aneed to remove dust or oversize slivers of material from the regrindprior to its reintroduction into the process Removal of fines can beaccomplished by means of elutriation or air scalper systems which areshown 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

heating and drying of plastic resins can be important to the ture of a consistently acceptable product that meets quality require-ments The processor must pretreat plastic resins in strict adherencewith the manufacturer’s recommendations.

manufac-Plastic resins may be either hygroscopic or nonhygroscopic, ing on whether or not the resin will absorb or adsorb moisture.Nonhygroscopic resins collect moisture only on the pellet surface(adsorption), making it easy to remove Hygroscopic resins, however,collect moisture throughout the pellet (absorption), making itsremoval more difficult Wherever it is found, the presence of moisturepresents potential problems from cosmetic surface blemishes to seri-ous structural defects

depend-Figure 7.24 Elutriator style fines separator.

Typical nonhygroscopic resins are PE, PP, polystyrene (PS), andPVC Fillers such as talc, calcium carbonate, carbon black, and woodflour, when added to nonhygroscopic resins like PE or PP in sufficientquantity, can make the resin behave like hygroscopic resins.

Surface moisture on nonhygroscopic plastic pellets can be

effective-ly removed using oneffective-ly heated ambient air, but removal of moisture collected within hygroscopic pellets requires dehumidified heated air

Auxiliary Equipment: Material Handling 7.45

Figure 7.25 Vibrating bed fines separator.

Either system requires that the resin be exposed to adequate airflowheated to the proper temperature for the time prescribed by the resinmanufacturer The equipment required for these two types of dryingsystems varies considerably in terms of cost and complexity (see Figs.7.26 and 7.27).

7.8.1 Drying system parameters

In order to properly dry hygroscopic resins, the drying system mustprovide the following parameters:

1 Process airflow The volume of air passing through the drying

vessel (ft3/min) must be sufficient to transfer enough heat (Btu/h) toraise material to its proper drying temperature Most resin manufac-turers recommend airflow of 1 ft3/(minlbh); therefore, a dryer for 250lb/h of ABS should have an airflow of approximately 250 ft3/min

2 Process air temperature Different materials require different

drying temperatures for efficient moisture removal Drying tures vary widely among resins, with nylon requiring only 140 to160°F, while PET may require temperatures as high as 350°F

tempera-3 Low dewpoint air In order to extract moisture from deep within

a pellet, the process air must have a very low dewpoint Critical

RETURN

INSULATED DRYER HOPPER

WITH HEATER

DHD — STYLE DEHUMIDIFIER DRYER

High-temperature material drying system.

Figure 7.27 Multimachine central dryer.

applications such as molding PET bottle preforms may necessitatedewpoints as low as 50 or 60°F.

4 Residence time Given proper airflow, temperature, and dewpoint,

it still takes time for the moisture to be extracted from the resin, fore, the drying vessel must have sufficient capacity to allow the mate-rial to be exposed to drying conditions for the recommended time.Times can vary from 1 to 2 h for removal of surface moisture from PE

there-to 5 there-to 6 h for PET

As an illustration of drying requirements, consider an injectionmolder running 200 lb/h of ABS Material manufacturers recommendthe following:

1 Process airflow An airflow of 1 ft3/(minlbh) is recommended;therefore, a 200-ft3/min dryer is needed

2 Process air temperature A temperature of 160 to 180°F is

recom-mended

3 Dewpoint A dewpoint of 30 to 40°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 arerequired

A desiccant is a material with a natural affinity for moisture The

most commonly used desiccant is a synthetically produced crystallinemetal, alumina-silicate, from which the water of hydration has beenremoved, permitting it to adsorb moisture When used in a dehumidi-fying dryer, the desiccant eventually becomes saturated; however, it

can be renewed through a process called regeneration, which is

accom-plished by heating the desiccant to drive off the collected moisture.After a cooling period, the desiccant is again able to adsorb moisture,making it ideal for use in drying systems

The typical dehumidifying drying system consists of the process airfan, dehumidifier, an electric or gas-fired air heating system, a controlsystem, the drying hopper, and a process air filter The dehumidifieritself includes the desiccant regeneration system

Single rotating bed. A single desiccant-coated honeycomb wheelrotates slowly, exposing part of the wheel to process air, part to regen-eration, and part to cooling prior to returning to the process

Multiple indexing bed. This is usually a three-bed arrangement, withone bed on process and one regenerating while the third is being cooledprior to going on line to the process.

Twin stationary beds. One bed is on process while the second is beingregenerated and then cooled prior to going on to process (see Fig 7.26).The twin stationary bed dehumidifier is simple, with relatively fewmoving parts, making it easy and inexpensive to maintain The initialinvestment is usually lower than the other designs

Selecting a bed design. Systems utilizing the twin-bed design are themost widely used, followed by the multiple indexing beds, and then thesingle rotating bed All of these systems are designed to provide a con-tinuous supply of dry air Both the multiple indexing bed and singlerotating bed work very effectively, although they tend to be mechani-cally complex, costly to maintain, and usually involve a higher initialinvestment

Machine mounted. In many applications, the drying hopper is

mount-ed directly on the femount-ed throat of the processing machine This ment eliminates conveying of material once it has been dried Severaldisadvantages of this method are

arrange-1 Tall stack-up height, which may be a problem because of low ings or overhead cranes

ceil-2 Lost production time due to hopper cleanout and predry whenmaterial changes are made

3 Requires one dryer per machine, which may be inefficient if theplant is running a limited number of materials requiring drying.These disadvantages of machine-mounted dryers have led to theincreasing popularity of remote-monitored or central dryers

Remote mounted. Putting dryer hoppers on floor stands at a remotelocation from the processing machines has several advantages:

1 Head space requirements are minimized

2 Multiple machines can be serviced by central drying systems,increasing energy efficiency and reducing changeover time Becausematerial must be conveyed after it is dried in a central drying sys-tem, this movement is often accomplished by using dry air supplied

by a separate dehumidifier Additionally, the conveying lines can bepurged of material by means of appropriate valves and controls

Auxiliary Equipment: Material Handling 7.49

The starting point for any evaluation of central drying systems isthe gathering of material throughput information by defining thenumber of materials requiring drying and accurate estimates of poundper hour throughput of each of the materials This information leads

to specifications regarding number of drying hoppers, size of hoppers,and cubic feet per minute of central dryers Future growth in through-put 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 mable logic controllers which monitor process parameters such astemperature, airflow, dewpoint at numerous points in the system, andalso monitor various machine functions such as heater amps, valvepositions, and alarms for diagnostic and troubleshooting purposes.The most advanced dryers include energy-saving features, such asregeneration, based on process dewpoint versus time and an ability toprotect material from degradation by reducing drying temperaturewhen throughput is reduced or temporarily stopped

program-7.9 Loading Systems

When designing a plantwide conveying system, throughput rates andconveying distances are two of the most important criteria to evaluate.Figure 7.28 represents a set of curves corresponding to systemthroughputs over a range of distances for conveying systems starting

at 3-hp units with 2-in-OD lines to 25-hp pumps with 4-in-OD lines.This chart should be used as only a rough guide to system throughput,and the following notes regarding its use are important:

1 For multiple-station vacuum sequencing systems, you must use thetotal throughput for all loading stations serviced by a single pump

2 The distance shown on the X axis of the graph represents total

equivalent feet of the conveying system and is calculated as follows:

Equivalent feet  H  (E15)  (V2)  (F3) where H horizontal material run

E number of 90° elbows in the run

V vertical material run

F flex-hose runWhen evaluating multistation systems, it is best to use the longestmaterial run in the system

3 Figure 7.28 applies to pellet conveying systems If powder is beingconveyed, the expected throughputs are approximately one-thirdlower than those indicated

Figure 7.28 Loading system throughput curves.

Whenever possible, distances between various system elements,such as surge bins, blending stations, and processing machines, should

be kept as short as possible given the site specifics

From the equivalent feet calculation, it is evident that each elbowequals 15 ft of horizontal run; therefore, the number of els in a systemshould be held to a minimum In addition, the elbows should have acenterline radius of at least 6 times the diameter of the conveying line

If possible, 10 times the diameter is recommended

Excessive distances on the vacuum side of the system should also beavoided Placing vacuum pumps close to the loading stations theyserve can greatly increase the efficiency of a system Additionally, theuse of oversize vacuum lines can reduce losses through that portion of

a system

7.9.1 Transfer line layout

Figure 7.29 shows the material transferred to the individual machines

by individual material lines The alternate method would be to utilizecommon material lines With common lines, you have one line permaterial source rather than one per use point The common line is fit-ted with Y laterals at each use point, and they are selectively connect-

ed to the vacuum hoppers The Ys not in use must be securely capped

or the vacuum will be lost through the openings One advantage to acommon material line system is the fact that each line is always dedi-

(a) Individual material transfer lines; (b) common material transfer lines.

cated to the same material, thereby eliminating the possibility ofcross-contamination.

It is also possible to distribute the vacuum in a multiple-station tem using either common or individual lines Common vacuum linesare fitted with Ts at each station on which the vacuum sequencingvalves are located The common vacuum line has been almost univer-sally adopted because of the savings in material and installation labor

sys-7.9.2 System hardware and installation

The principal point to remember when discussing material

distribu-tion systems is the fact that material lines cannot be run the way

ordi-nary plumbing, compressed air, and electrical conduits are installed.Because you are moving particles of material in an airstream, everyattempt must be made to keep the runs as straight as possible with aminimum of turns Long lengths of flex hose should also be avoidedbecause of their negative effect on conveying rates

Almost all conveying systems installed in resin transfer systems lize thin-wall aluminum tubing and elbows The principal sizes are

uti-112-, 2-, 212-, 3-, and 4-in OD The tubing is available in 10- and 20-ftlengths, with the latter preferred because fewer couplings arerequired

7.9.3 Material line elbow selection

Most wear in resin transfer lines occurs at elbows where the materialimpinges on the interior surface with maximum force Various materi-als are used for conveying line elbows as the following table illustrates:

Aluminum Nonabrasive material such as PE, PP, soft

PVCs Stainless steel Moderately abrasive materials such as PC,

PET, and rigid PVCs Ceramic-lined steel and glass elbows Extremely abrasive materials such as 30 to

40% glass-filled nylons

When extremely soft materials are being conveyed, the use of elbowswith internal spiral grooving is recommended The grooving has a ten-dency to lessen the impact of pellets on the el’s interior and also breaks

up streamers into smaller flakes which are less prone to form ages downstream

block-Elbows are of the long radius type to assure smooth flow istics The elbow centerline dimensions are listed in the followingtable:

character-Auxiliary Equipment: Material Handling 7.53

Tube OD, in Elbow centerline radius, in

Connections between tubing and flex hose are accomplished withdisconnect fittings which are also illustrated in Fig 7.30

Figure 7.31 shows a bulkhead-style switching station which is quently used where multiple lines are to be brought in from silosthrough a block or masonry wall The unit consists of an inner and out-

fre-er aluminum plate mounted ovfre-er the wall opening Tube stubs are

weld-ed to the inner plate which is fittweld-ed with female disconnect fittings.Once the lines are inside of the plant there are various ways of sup-porting them The most common method involves the use of Uni-Strutbracket and clamps which are shown in Fig 7.30 These bracketsalong with their accessory items can be used to fabricate crossovers tospan tracks or roadways and to suspend tubing from roof trusses,columns, or other supports in the plant

With the material and air-return lines thus run to the area of thevacuum hoppers, the final connections are generally made with flex

Conveying line hardware.