Understanding the Functional Differences Between Drag Conveyor and En-Masse Drag Conveyor

Chain conveyors whether known as Drags, Scraper, Paddle, or En-Masse all start out with a continuous chain and fixed flights operating within an enclosed trough. While each individual type functions somewhat differently, their purpose is to accelerate and move a bulk commodity from inlet to discharge at a specific rate. The word “Drag” relates to all styles; scraper, paddle or en-masse because each type of conveyance moves the chain and product forward to the discharge. The difference relates to how this is accomplished.

INTRODUCTION

The scraper/paddle style chain conveyors work on the physical push/pull principles of moving a product at the height of a tall full flight. While this is effective, it eliminates available cross-sectional area within the enclosed trough to around 50% of the physical area of the conveyor.

The capacity capabilities of each drag chain conveyor type is determined by their flight design, cross-sectional area available, chain speed, commodity characteristics, and the direction of flow from point A to point B. (Horizontal, Inclined, or Vertical.)

Full tall flights also produce excessive chain weight that requires more horsepower to merely move the chain, and increased trough wear, rather than concentrating on the amount of product to be moved.

The Efficiency of En-Masse Drag Chain Conveying

En-Masse Drag conveying works by pulling a single strand of endless chain centered in an enclosed housing with flights attached on both sides of the chain (skeletal looking) through a rectangular section of a bulk solid material. The bulk solid is conveyed by the motion imparted on the bottom surface of the trough by the chain and the shear friction between the particles of the bulk solid material conveyed.

A few of the basic principles of En-Masse conveying are;

1. A square cross section housing is the most efficient.
2. If the bulk solid becomes too deep the friction between the side walls of the housing and the bulk solid will decrease the velocity of the upper layers of the material relative to the actual chain speed. When the velocity of the upper layers of the bulk solid decreases, slippage occurs. Slippage results in premature wear, and loss of volumetric efficiency (Reduced Capacity).
3. Solid Composition
   • The BT or Flat Bar flight is primarily used for horizontal conveyors and inclined conveyors up to 15° depending on the bulk solids being conveyed.
   • The “U”-Flight, “O”-Flight, and “OO”-Flight as referenced in Figure 2 are typically used for very fluid bulk solids and greater inclines. These flights can also be equipped with filler plates for very fluid bulk solids and also to help facilitate better clean out of the conveyor housing.

With an En-Masse drag conveyor the chain is normally arranged between the end sprockets so that the lower strand runs in the trough and serves as the carrying element. However, it is possible to have two separate troughs or a split trough with an intermediate plate to divide the conveyor into upper and lower sections so that material can be actively conveyed by both strands of chain (upper & lower) in opposite directions. This makes the En-Masse drag conveyor a very versatile conveying method.

En-Masse drag conveyors are typically low speed, high torque units which equates to longer life and lower cost of operation. The lower cost of operation is the result of the conveyor being a fairly simple design, requiring less maintenance compared to other conveying types and routine maintenance is fairly simple and straight forward. The lower chain speeds reduce frictional wear issues and chain wear, require lower horsepower requirements for the conveyor, and minimizes overall power consumption.

One of the big advantages of an En-Masse drag conveyor is the high efficiency ratio of effective utilization of space used (space used for conveying) versus the actual physical size of the conveyor. In this regard, the EnMasse drag conveyor can use as much as 90% of the physical size of the conveyor for conveying. Comparably speaking a standard paddle drag conveyor can only use an approximate maximum 50% of the physical size of the conveyor for conveying. Thus, the physical size of a paddle drag conveyor will be 50% larger than an En-Masse conveyor to carry the same volume of material. Figure 3 provides an illustration of the effective utilization of space of an En-Masse conveyor compared to other common conveying types.

The En-Masse drag conveyor is not limited to conveying En-Masse, there are many applications where the bulk solids capacity that is required or the conveyor configuration isn’t practical, but in a lot of these cases the single chain strand with skeletal flights is a very good and reliable solution for conveying needs. In other words, EnMasse drag conveying is one type of operating principle of a Drag Conveyor.

Evaluating the Paddle Drag Conveyor Operation

The standard Paddle drag conveyor is typically designed with two endless strands of chain with flights “Paddles” connected in some fashion in between and perpendicular to the two endless chain strands at a common spacing. These conveyors operate by dragging or scraping a bulk solid through a trough utilizing the flight “paddle” to provide the movement. Unlike the En-Masse style drag the amount or volume of bulk solid moved by the paddle drag conveyor is dependent on the cross sectional area of the flight “paddle” itself and the chain speed. Typically, chain speed is slow and capacities are generally small. Because Paddle drag conveyors have two strands of endless chain the horsepower consumption increases significantly due to the chain weight and friction created between the cross flights and the housing bottom plate. Another possible area of concern related to the dual strands of chain is the overall conveyor length; however, the advantage of a higher chain pull due to the dual strands is somewhat offset. This is because there is twice the chain weight compared to a single strand of chain. A majority of horsepower consumption used in a Paddle drag conveyor is due to the chain and paddles.

Chain tension is also an important factor regarding the length of a Paddle drag conveyor. Chain tensioning a dual strand of chain versus a single strand of chain can be problematic because it can be very difficult to obtain equal length between sprocket centers in both strands and have equal tension in both strands because it is almost impossible to have an equal wear rate in both strands. As running time increases this elongation differential is acerbated. If a Dual strand conveyor becomes jammed with a foreign object or has some other upset condition the tendency is for the flight “Paddle” to be bent near the center, this causes the two chain strands center distance to be reduced and therefore cause tracking issues at the sprockets. Typically a bent flight will not travel well through the conveyor housing, particularly at the terminal sprockets. If the chain tensioning device adjustment distance is completely used then a chain link(s) must be removed to once again allow for tensioning adjustment. In a dual strand chain system both strands must have the exact same amount of links removed.

The Versatility and Strength of Drag Chain Conveyor Design

Typical chain used in drag conveyors comes in many style, sizes, shapes, and construction. The majority of chain used is made from either stamped steel parts, cast, or forged. Stamped steel chain is typically used for lighter duty applications such as agriculture. Cast and forged chain is typically used for medium to heavy duty applications. Industry standard for medium to heavy-duty is a drop forged link style chain made from CrMn steel alloy.

The material, coupled with the micro structure formula provides for an extremely high tensile strength and hardness on the casing. The external hardness, coupled with a more malleable core, produces increased chain life, wear resistance, durability, and economic value.

The Right Drag Chain Operation

En-Masse drag conveyors are designed to carry a maximum volume of material within the available housing structure. These conveyors are most effective in horizontal and low-incline applications where the rise is less than the angle of repose of the material. Materials with granular particles which tend to cling together or have a higher coefficient of friction between the particles will accept higher inclines. Extremely free-flowing or more fluid materials may be limited to a 6° to 15° incline. For elevating material at angles higher than their angle of repose, “U”, “O”, or “OO” flights can be used to help reduce material slippage. Also, filler plates installed on the flights at a determined spacing can be utilized to capture material that is back flowing.

The right functional operation of a drag chain conveyor should be determined by factors of the material and movement requirements of a given application.

Conveyors may be selected for horizontal, straight incline, or with one or two bend sections.

Drag Chain Provides Optimized Material Handling

The operational nature of drag chain conveyors affords use in a wide range of material handling applications. Understanding the differences in operating conditions between En-Masse and Drag Chain helps with proper size and selection of conveyor options to achieve required capacities within infrastructure and elevation parameters.

The efficient utilization of space in the En-Masse drag chain conveyor makes it the most cost-effective option based on physical size and power requirements. When choosing a Drag Chain Conveyor knowing material and application details will help a conveyor manufacturer size and select the equipment options that are best suited for the individual application.

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Manually shoveling sand to fill cope and drag boxes (casting flask) for sand casting is a physically demanding process for mold makers, who on average shovel 20 tons of casting sand weighing 100 lbs. per cubic foot into 70-80 molds per day. This demanding operating condition creates a high level of physical strain on the mold maker’s body. The result of this strain was a multi-million-dollar-per-year workers compensation obligation for an aluminum foundry. The financial impact not only hit the company’s bottom line but slowed overall worker efficiency. Company leaders knew that keeping workers safe and healthy was not only important for their people and profits but also for their customers. They knew if they could improve productivity through process improvements, they could reduce lead times and improve customer satisfaction, which in turn would continue to grow their business. After many searches on Google and conversations with various materialhandling companies, foundry management discovered Hapman.

Effectively and reliably moving foundry sand is a challenge. The sand carries a high-moisture content and does not move easily when conveyed. Initially the foundry tried a Helix^® Flexible Screw Conveyor; however, even with modifications and flow aids, the right combination of auger type and speed could not be achieved to move the sand. After numerous discussions, site visits and extensive material testing, it was clear the solution to this material-handling challenge would be a conveyor engineered to fit the exact requirements of the application.

INDUSTRY

Metal

Manual Labor Eliminated

The result of the testing and trials was a Single Strand Top Carry Drag Chain Conveyor with integral hopper. The engineered material-handling assembly allowed front-end loaders to empty foundry sand into the hopper as the drag conveyor reliably metered the sand into the molds. This process eliminated the need for the slow, back-breaking process of manually shoveling sand into molds.

Manually shoveling sand to fill cope and drag boxes is physically demanding for mold makers.

The redesigned mold-filling process begins as prepared moist foundry sand is introduced into the hopper by a small skid steer loader. The dense material is screened as it enters the hopper to eliminate aluminum tramp metals or larger pieces from entering the process. To provide consistent material flow to the conveyor, the hopper is constructed with an electro-polished stainless steel finish, a pin-style agitator, and a vibrator to keep material from bridging. The drag chain moves the sand in a consistent, metered flow to the discharge point where the mold maker controls the filling of the casting flask.

The redesigned mold-filling process begins as prepared moist foundry sand is introduced into the hopper by a small skid steer loader. The dense material is screened as it enters the hopper to eliminate aluminum tramp metals or larger pieces from entering the process. To provide consistent material flow to the conveyor, the hopper is constructed with an electro-polished stainless steel finish, a pin-style agitator, and a vibrator to keep material from bridging. The drag chain moves the sand in a consistent, metered flow to the discharge point where the mold maker controls the filling of the casting flask.

The Hapman Drag Chain Conveyor features a design that offers minimal maintenance requirements and abrasion-resistant construction. The Hapman drag chain is engineered in a CrMn alloy and drop-forged construction. The chain is machined, carburized and case-hardened for strength and durability. The bolted UHMW polyurethane flight design reduces the possibility of sand sticking on the flights while also resisting wear and providing a quiet operation.

Increased Production, Lower Operating Costs

The foundry started with one Hapman Drag Chain Conveyor and was so pleased with its performance that it added 14 more. Now all of the foundry’s mold-filling lines feature a Hapman Drag Chain Conveyor system. This has increased production capacity by more than 30 percent and resulted in a 100-percent return on investment in less than 6 months. Automating the foundry sand transfer process also significantly lowered ergonomic risk factors and eliminated the daily physical strain on mold makers, which has significantly decreased costs. Because the foundry is now able to take on more jobs and deliver shorter turnaround times, overall business has increased considerably. The Drag Chain Conveyor, along with all Hapman equipment, comes with an exclusive Performantee^®, a true performance guarantee that ensures the equipment achieves the specific results it was designed and manufactured to deliver.

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A large oil company sought to expand refinery operations at one of their global locations. The expansion required an effective way to handle the filter cake at a new refinery. The high-volume of filter cake coupled with the system redundancy required by the producer meant numerous conveyors would be needed to meet specifications. In addition, the corrosive environment required conveyor design to provide reliable, long-term, 24/7 operation.

INDUSTRY

Chemical

ENGINEERED MATERIAL HANDLING SYSTEM

The filter cake handling process involved automatically taking the cake from the discharge of each of four filter cake presses and transferring the material to two multiple hearth furnaces. Figure 1 is a 2D schematic of the filter cake processing layout for a single system; the entire system had 4 identical units totaling 64 individual conveyors. The requirements of the material handling system involve effectively moving filter cake from the filter presses to a series of multiple hearth furnaces (MHF) where the material is used to fuel the burning process.

The material handling system starts with dual strand drag conveyors at the discharge of each of the four filter cake presses and transfers the filter cake material to either of two collection conveyors. The collection conveyors then feed the filter cake into either of two main elevating conveyors (#1). After the filter cake is elevated 150 feet at a 40° incline the filter cake material is transferred into the main horizontal conveyors (#2). The main conveyors deliver the filter cake material to either of two filter cake surge bins. Dual strand metering drag conveyors located at the bottom of the surge bins controlled by VFD’s then deliver a determined rate of filter cake material. The rate is determined for optimum burn efficiencies to either of two transfer conveyors via a diverter chute which then feed the two multiple hearth furnaces. The furnaces burn the filter cake as a reclaimed energy source. The furnace requires the pressed material to meet specific standards for dryness and material consistency. If the filter cake contains too much moisture or is too dense, the burn process will be less efficient. To achieve optimum processing within the MHF, an inspection process at the discharge of the filter cake press allows for off-spec material to be discharged separately. The off-spec material required a separate hopper and conveying system to effectively and automatically transfer the unusable material to a storage container.

To achieve the material transfer rates of the system and to provide reliable operation in a corrosive environment the oil producer, working with a general contractor looked to the Hapman drag chain conveyor as the optimum selection for the project.

A HIGH-VOLUME APPLICATION REQUIRES THE RIGHT CONVEYOR

The cake trough, collecting, and the main dual-strand conveyor have a required capacity of 53,000 lb/hr (24,000 kg/hr). The cake bin conveyors each holds a material movement rate of 11,023 lb/hr (5,000 kg/hr). The drag conveyors were selected with dual strand and cross flights to achieve the required capacity with a material bulk density of 575Kg/M3. The exclusive Hapman CrMn forged steel alloy chains have a core hardness of 300400 BHN and are machined and carburized for a case of 550-650 at .030” to .040” effective depth (Figure 2). This configuration, coupled with the thickness, height, width, and spacing of the flights minimized chain pull and mitigated unplanned downtime risk. To support longevity in a 24/7 refinery operation the housing was constructed of a Ferritic Stainless Steel.

CONVEYOR RELIABILITY INCREASES PRODUCTIVITY

A refinery, like many large scale chemical and mineral processing facilities, rely on the 24/7 operation of the conveyors to carry bulk material. This is true of raw materials for process or process by-products for recycle and/or reuse. When a conveyor goes down unexpectedly because of design failure the productivity of the facility is compromised, and workers and the company are exposed to unnecessary risk. Large-scale production facilities have many confined space environments, hazardous material components, and dangerous utilities and operations that when exposed for emergency downtime and repairs have the possibility for injury or extended outage. Engineering and selecting the best conveying technology in the design phase for a given application is critical to longevity of equipment and productivity stability.

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CHALLENGE

Selecting a conveyor for a bulk material handling application is not always as straight forward as one might think. Moving material consistently, at a rate that is inline with production requirements, and in a manner that does not contribute negatively to the plant operating environment; such as dusting or increased maintenance, can be a challenging endeavor for process engineers and procurement personnel.

INTRODUCTION

Often conveyors are miss-applied because of the overall plant desire to keep the number of unique pieces of equipment to a minimum while also adhering to spending limits. These factors are critical to consider when selecting a conveyor for a material handling application; however, they should not be the first, or the only, evaluation metrics used.

The most effective way for a processor to evaluate all of the best options for a material handling application is to consider each material movement requirement from a blank starting point. Looking at an application with the mind-set of “What is the best way to satisfy our material conveying need”, will put processors in a better position to make the right equipment selection prior to purchase. Taking this stance, there are several key considerations to evaluate for conveyor selection they are; material, operation, environment, envelope, cost, and history.

MATERIAL

There are several important characteristics that make up the complete material definition, which should be understood in their entirety. Some of these are dynamic and can influence or may be influenced by one or more of the other characteristics, so it is always recommended to look at them together.

1. Name of the material (many materials are known by more than one name)
   • An Esoteric or Trade Name (such as SnoMelt)
   • Generic Name (such as salt) or Chemical formula, which is defined & called by its primary ingredient or ingredients (sodium chloride, NaCl).
2. Material form or state in which it must be handled
   • Solids (free flowing or semi free flowing)
     • i. What is the loose bulk density? (lbs/ft³, g/cc, kg/m³)
     • ii. Loose Bulk Density is commonly interchanged confused with Specific Gravity but should not be. They are very different. Specific gravity is the weight per given volume of a substance, usually in its most natural, concentrated, unreduced solid form (no air pockets or interstitial spaces between particles). Loose Bulk density is the weight per given volume of the material in its reduced, free-flowing or semi free-flowing state.
3. Solid Composition
   • Powder
   • Prill
   • Granule
   • Pellet
   • Fiber
   • Flake
4. Particle Size
   • Symmetrical Solids are usually described in terms of their ability to pass through a screen of a certain size.
   • Asymmetrical Solids are usually described in terms of their geometric dimensions, minimum and maximum.
5. Flowability – Flowability is perhaps the most important characteristic of any solid material to understand, yet it lacks a single standardized method of measurement across industrial disciplines.
   • Most often it is defined in somewhat arbitrary terms as Very Free Flowing, Free Flowing, Average Flowability or Sluggish.
   • Angle of repose is also frequently used as an indicator of flowability. It is simply the natural angle or rate of incline that is observed when material is metered from a single discharge point without vibration or any effort to settle or distribute the accumulating mound.
   • Some use a 1 to 4 or 1 to 10 indexing system in an attempt to quantify the flowability.
   • Recommended reading for anyone wanting to better understand flowability co-written by James K. Prescott and Roger A. Barnum of Jenike & Johanson and published in the October 2000 issue of Pharmaceutical Technology. Downloadable from the Jenike & Johanson website at: http://info.jenike.com/technical-papers/on-powder-flowability
   • If the material is known or suspected to be a challenge because of its flowability, it may be best to send a sample to the equipment supplier for review.
6. Abrasiveness
   • Like flowability, abrasiveness lacks a universal standard definition. However there are two material factors that tend to determine the abrasive quality. First is the material hardness, which is measurable and can be defined using the Mohs scale or Vickers scale. Secondly, is the particle shape. A hard particle having no abrupt edges (a sphere) is less likely to abrade a contact surface than one with sharp edges. Most often, abrasive materials are known to plant personnel and experience tends to be the most valuable measure. In other words, if a material has been found to erode other plant process equipment, certain parallels can be drawn and educated assumptions made as to the way the material will effect proposed equipment.
7. Material Temperature
   • Usually defined as a range (min. to max.)
8. Moisture Content
   • Usually described as a percentage by weight.
   • Can provide an important clue as to flowability or cohesiveness.

Note: One of the most common misconceptions with regard to materials and their various handling characteristics is that they can be obtained from MSDS (Material Safety Data Sheets). However, this is rarely the case. As the name implies, the document is a declaration of any health, safety or environmental concerns and while it may offer some vague clues, it seldom enables a sufficient understanding of the way a material is likely to behave while being handled, stored or processed.

Note: There is an excellent resource printed on a web site called http://www.tankconnection.com/ which goes to extensive effort to quantify the most important characteristics for hundreds of common solids, assigning each a code and providing a convenient code chart for interpreting a material’s unique character profile. That chart may be found at http://www.tankconnection.com/products/understanding-dry-bulk/material-characteristics/

OPERATION

Simply stated, operation is that definition of function and performance that you need to satisfy. There are certain aspects of the operation that should include some detail and clarity.

Fundamentally, moving material though a process falls into two main categories. It is important for processors to understand the difference to ensure the right conveyor is ultimately selected.
– Conveying
– Feeding

1. Conveying is simply moving material (or materials) from one or more pick up points and delivering materials to one or more drop points. The rate at which this is accomplished is usually fixed and the delivery time, while important, fits comfortably within a minimum and maximum range. Conveyors are most often used as a refilling device for surge hoppers, feeders or process equipment.
   • Specifying the operation of a conveyor consists of:
     • Defining the amount of material that needs to be moved, and the window of time within which it must be moved.
     • In cases where there may be multiple discharge points, it is necessary to know the demand at each drop point.
     • Determining what condition will initiate a refill, stop a refill and in the case of multiple discharges, developing suitable logic (or sequence of operation) to establish refill priority, so that the process is not inadvertently starved.
     • When considering a single conveyor for multiple materials, it is imperative to know whether cross contamination will be an issue.
2. Feeding generally is much more time sensitive and process critical in terms of the amount of material delivered. Material is normally received from a single source and delivered to a single drop point. Because of the precision with which material needs to be delivered, delivery rates usually vary, either to coincide exactly with a continuous fluctuating demand or slowing to creep up on a batch-complete set point, for example.
   • Defining the operation of a feeder consists of determining whether the feeder will be required to deliver in discrete batches or deliver on a non-stop, rate-controlled basis.
     • If batching, it is important to know the amount that must be delivered, the time in which it must be delivered and what level of accuracy will suffice (usually defined as a +/-percentage of the target weight). It is also important to know the idle or at-rest time of the feeder between batches.
     • If feeding continuously, the rate needs to be defined, as well as the accuracy (usually defined as a +/- percentage of the instantaneous rate).

Note: One of the most frequently occurring mistakes with regard to conveyor or feeder sizing comes about as a result of confusing material usage with instantaneous demand. For example, while a process may consume 1,000 pounds per hour of material, the conveyor or feeder may need to deliver at a rate that is 12 times higher if the batch needs to be satisfied in 5 minutes, for example. Note: Most automatic means of feeding and conveying are sized according to the equipment’s volumetric capacity. However, the material demand is most often understood and communicated in terms of its weight. Therefore, a reliable value for the bulk density is necessary in order to calculate the volumetric requirement so that the equipment can be properly sized.

ENVIRONMENT

There are a number of environmental factors that must be considered in the proper selection of equipment. Some of these may combine with material characteristics to cause or exacerbate handling concerns, while others may necessitate added health and safety counter measures.

1. Some environmental factors may include:
   • Open sources of ignition
   • The potential for a flammable or explosive atmosphere
   • Corrosive vapor
   • High humidity
   • Temperature
   • Vibration
   • Pressure or vacuum (at inlet, discharge or both)

ENVELOPE

The application of one device versus another quite often comes down to envelope- how much room is available to install the equipment or device. This appears straight forward and yet this is one of the most frequently omitted pieces of information from inquiries.

1. When considering new equipment, particularly when some portions of the system already exist, take care to consider the following:
   • What will the new equipment receive from?
     • What is the discharge elevation of the upstream equipment?
   • What will it discharge into?
     • What is the inlet elevation of the downstream equipment?
   • What is the centerline distance between the proposed inlet and discharge?
   • Is the proposed route in a straight line or must it turn a corner or change elevation more than once to avoid an existing plant feature?
   • How much width/depth is available to accommodate the new device or equipment?
   • What is the ceiling height?
   • Are there any other layout considerations such as the need to temporarily locate pallets of material, etc. or allow maneuvering room for a fork truck?

COST

Some consider it taboo to have any up front discussion with suppliers about budgets and yet there is no denying the role cost plays in the feasibility of every project. Understandably, the justification formula is different from one business to the next. There are initial costs and long term costs. Some companies more heavily weight the long term cost of ownership in their justification calculation, focusing more on reliability, reduced energy consumption and maintenance. While others more heavily weight the initial investment. When inquiring about potential solutions, therefore, it is in your interest to have the ability to speak with potential suppliers about these factors so as to obtain from them, as soon as possible, whether a particular approach is likely to gain any traction or whether to consider another approach entirely. Considering how lean companies are and how busy everyone is, it can save precious time and energy to have some of this dialog up front.

HISTORY

When replacing existing equipment, the importance of service history cannot be overemphasized. Where reliability has been an issue, understanding the difficulties experience with existing equipment can provide important clues that enable the incorporation of suitable countermeasures. Simply changing brand names is no guarantee that you will end up with a more reliable, longer lasting piece of equipment. It is far better to provide details in regard to service history and ask the vendor to explain how the proposed equipment will provide a more satisfactory result.

SUMMARY

The proper selection and sizing of a conveyor is critical to successfully meeting production goals. The key factors that should be evaluated when choosing a conveyor are; material, operation, environment, envelope, cost, and history. Taking the time upfront to understand these factors and gather the most accurate data possible related to each component will ultimately make the selection of conveying technology the best for your specific needs and offer the greatest return on investment.

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