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Types of Distribution Centers
There are three different types of distribution centers:
- Conventional - Material movement is performed by people and mobile equipment
- Mechanized - Material movement is assisted by mechanized, conveyance and sortation systems
- Automated - Material movement is performed in part or in full by machines or robotics.
Regardless of the type of warehouse operation being designed, the basic building block of any material handling system is typically the pallet. It is critical to document the size, dimensions, type, variety, and quality of pallets being stored and handled. Similarly, it is important to develop accurate operational profile information including product variety, movement and inventory projections to a design year time horizon (usually 5 to 7 years into the future). This information is required to determine the type of racking systems, material handling systems, and order picking technologies that will be the most cost effective for the new distribution center.
Operating Aisles
Operating aisle widths are a function of: the type of mobile equipment being used; the size of pallets being handled; and the need for vehicle passing within aisles.
- Very narrow aisles (VNA) are typically in the range of 72" wide. They require the use of specialized turret trucks for pallet storage and handling; and/or order selector trucks for picking cases from vertical storage locations. Because the aisles are very narrow, one vehicle works in the aisle which imposes a constraint on how the facility operates. This operating environment is often deployed in buildings where the requirement for storage density is a very high priority; or when the variety of products being distributed exceeds the number of pick facings available at floor level.
- Narrow aisles are typically in the range of 108" to 132". This operating environment is suitable for electric forklift trucks (e.g. stand-up, reach trucks). Aisle widths can be designed to allow two forklifts to comfortably pass within the aisle to support flexible operating requirements. These are the most common facilities deployed in North America.
- Wide aisles are typically 156" or higher. These facilities tend to use sit-down counterbalance lift trucks for heavy load handling or to enable a single vehicle to perform all unloading, storage, material movement and loading activities. These operations are common in full pallet environments such as in manufacturing or cross dock operations.
Pallet Storage Systems
There are multiple pallet racking and storage options available, including:
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Storage System
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Description
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Static Shelves and Case Flow Racks
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Storage media typically used to provide picking locations for less than full case SKUs. These can be freestanding units positioned on a floor or mezzanine; or they can be tightly integrated into floor level positions within racking systems.
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Bulk Floor Storage
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Pallets stored in deep lanes; vertical stacks are usually 2 - 4 pallets high depending on pallet stability.
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Selective Pallet Racks
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Fixed racking systems allowing 1 to 2 pallets of storage depth.
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Cantilever Racks
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Specialized racking systems to store longer unit loads that may or may not be palletized.
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Stackable Racks
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Specialized racking systems that also serve as storage containers and that can be vertically stacked on top of one another to hold products such as fabric rolls or tires.
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Drive-in Racks
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Multiple pallets of depth whereby the lift truck drives into the rack to access the pallets.
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Push-Back Racks
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LIFO product rotation of up to 6 pallets in storage depth.
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Flow Through Racks
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FIFO production rotation for multi-pallet deep storage.
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Shuttle System Racks
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Many pallets of depth whereby pallets rotate in FIFO sequence through the use of a robotic pallet shuttle carrier.
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Size and Shape of Facility
In the perfect world, a "greenfield" distribution center is a new facility that is built to specification. This means that the optimal material handling system is designed as a first step. Then the building is designed as an envelope that wraps around the material handling layout. Having said this, the most common situation is that the material handling system needs to be designed around the building because the building has already been leased or acquired. In the latter case, it is important to keep building columns from being exposed in operating aisles; and to minimize the loss of storage capacity caused by building columns being situated in the racking system.
In general, it is a good idea to maximize the height of a building when designing a new distribution center because it is generally less expensive to go up then it is to go out. Conventional forklift trucks now lift as much as 505" (42') which is much higher than the 27' - 35' clear stacking heights that most modern facilities have been constructed to. Today, automated distribution centers in North America are typically constructed between 60' - 110' in height and this goes up to 150' in Europe.
Dock Design
Perhaps the most common attribute of a stressed distribution center is a facility that has been designed with insufficient dock space and depth. The symptoms of this phenomenon are typically overcrowded dock conditions and operating aisles that are congested with pallets as a means of alleviating insufficient dock space. Both of these symptoms are easy to diagnose and both are the cause of significant loss of operating efficiency.
The dock may look like an expensive piece of real estate that is underutilized but in reality it is the most essential part of the building. The dock is the infeed and outfeed point of the building and must therefore provide adequate space for all required operations including, but not limited to receiving, inspection, quality assurance, pallet building/lumping, sortation, cross docking, flow through, checking/auditing, staging, load consolidation, and loading activities.
Docks are typically (but not always) configured such that receiving and shipping operations are distinct or apart. Keeping receiving operations together in one dock area helps to security and supervision of incoming goods controlled in one area. The same logic applies for shipping operations. The question of whether to set up separate receiving and shipping docks on opposite sides of the complex is an operating strategy that is favored by some distributors. Because this strategy requires adequate space to provide trucking access both sides of the building, it requires more land and a higher capital investment in concrete and asphalt in the yard. Suffice to say that this strategy is more common with facilities where it makes logical sense to flow product through the complex from receiving through to shipping.
Mechanized and Automated Distribution Centers
Warehouse operations that utilize more complex conveyance, sortation, packaging and/or automation systems are becoming more commonplace, especially in facilities that have high throughput volumes and/or labor intensive operations.
Below are some informal guidelines in the North American context to illustrate when a distribution center may be a candidate for mechanization or automation. It is important to note that there are no hard and fast rules on this topic because the drivers for automation are often only partially related to the issues discussed below.
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Metric
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Value
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Distribution Center is a Candidate For
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Direct labor associates
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> 50+ per operating shift
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Mechanization or Automation
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Fully loaded direct labor wage rates
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> $30.00/hour
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Semi-automation or automation
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Order picking transaction volumes (Pieces/Cases)
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Pick > 15,000 order lines/shift
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Mechanization/Sortation
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Pick > 50,000 order lines/shift
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Mechanization/Sortation
Semi-automated Picking Systems
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High volume refrigerated or freezer full pallet environment
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Ship in excess of 300 pallets/hour
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High Bay Warehouse/ASRS Automation
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Store in excess of 10,000 pallets
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SKU Variety
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> 80,000 - 100,000 parts
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Semi-automation for slow moving SKUs
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Mechanization and Automation Options to Consider
There are an unprecedented number of options for mechanization and automation available to distribution companies today.
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Material Handling System
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Description
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Case/Tote/Unit Conveyor systems
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Multiple types of conveyors are available to horizontally and/or vertically transport loose units, cases, or totes from one point to another within the distribution center as a means of reducing travel time.
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Pallet Conveyor Systems
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Multiple types of pallet conveyor systems are available to horizontally transport pallets between locations in a distribution center.
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Automatic Sortation Systems
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Multiple sortation systems exist to enable the sortation or products or orders into discrete entities prior to shipping/ These include tilt tray sorters; bombay sorters; conveyor sortation systems (crossbelt, sliding shoe, quad sorter, diverter).
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Carousel Systems
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A mechanized goods-to-man system typically used to provide storage for loose pieces. Carousels are usually configured in 2 to 3 pods whereby an operator picks from one pod while the other pod(s) rotate.
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A-Frame Dispensing Systems
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Automated picking / dispensing machines that tend to be used in distribution operations that have a high volume of order lines associated to smaller products that have a uniform profile such as pharmaceuticals or recorded media. Dispensers automatically pick product to a belt for each customer order and dispenser stacks must be replenished by operators.
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Robotic Goods to Person Systems for Less Than Full Case Applications
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Systems that utilize robotic vehicles to transport the goods to the operator which eliminates the travel time associated with storage, retrieval and picking operations. In some cases, a tote or case is presented to the operator for picking. In other cases, a pallet or bin shelf is presented to an operator’s work station. Contemporary solutions include Kiva Systems, Swisslog Autostore, Dematic Multishuttle / RapidPick system, Knapp OSR Shuttle, and Viastore’s Viapick solution amongst others.
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Automated Full Case Picking Systems
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Full blown automated case picking systems that require no operator involvement involve the integration of multiple material handling systems including case and pallet conveyors; ASRS pallet systems; high-speed miniload ASRS systems; automated pallet building and shrink wrapping systems; etc. Automated case picking systems may or may not involve the use of an intermediate material handling such as a tray. Companies that are making important advances in this area include Witron, SSI Schaefer, Symbotic, Nedcon and Vanderlande Industries.
Many modularized solutions exist for homogeneous pallets where SKU variety is low and there is product uniformity such as can be found in the food and beverage manufacturing industry amongst others. These solutions are capable of both case and layer picking.
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Shuttle Cart Systems
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Typically deployed in high bay warehouses. Used in place of an AS/RS to automatically move pallets within racking systems. Each level in each aisle of the racking system is served by a shuttle cart that is powered by a busbar. The parent shuttle cart transports the pallet to a storage lane and then a child shuttle cart departs the parent cart to store the pallet into the lane to enable multiple depth of storage positions. Shuttle cart systems are now being introduced to perform a similar function for case handling.
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Automated Storage and Retrieval Systems (ASRS)
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Automated pallet storage and retrieval systems that can move materials up to 110’ to 150’ in height. Multiple options exist including single and 2-deep pallet storage systems; ASRS combined with push-back storage systems; ASRS machines equipped with turret forks; aisle-changing ASRS systems whereby one crane works in multiple aisles; ASRS systems with a shuttle device whereby the shuttle can transfer pallets into a multi-pallet deep storage location; & single and double mast ASRS machines capable of transporting pallets in pairs.
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Early Supplier Involvement (ESI)
Because mechanized and automated systems tend to be designed specifically for each project, it is a good strategy to leverage the skills of material handling suppliers/experts to validate and fully develop the options being considered. This concept is referred to as ESI or Early Supplier Involvement and it is an important element of successfully developing an optimized distribution center. Historically, the mainstream practice has been to involve equipment vendors at the time that a request for quotation is issued. We believe that this is a mistake, especially when material handling technologies become more complex. Involving competing suppliers early in a warehouse design project allows a firm to understand the human capital strengths that potential supplier-partners can bring to the table and this is ultimately as important as the equipment itself.
Marc Wulfraat is the President of MWPVL International Inc. He can be reached at +(1) (514) 482-3572 Extension 100 or by clicking here. MWPVL International designs distribution centers and material handling systems and can help your firm evaluate the most appropriate solutions for your manufacturing or distribution operation.
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