Adjustable Pallet Racking (APR)
Adjustable Pallet Racking is designed with single rows or back-to-back rows and multiple levels. The depth and width are defined by the unit load footprint, usually a pallet, and height is adjustable to conform to varying unit load heights. With an aisle between each row, this arrangement provides excellent pallet load accessibility but has a low storage density. The amount of aisles and single-deep density results in decreased warehouse space utilization.
Advantages: 
- Low costs
- Pallet selectivity
- Flexibility of lift trucks
- Load or unload from either direction
- Simple installation
- Full accessibility to all unit loads
Disadvantages:
- Lower space utilization
- Wide aisles
Unlike block stacking, when a pallet space is created by the removal of a load, a pallet opening is immediately available in single-deep racking. Also, because racking supports every load, stacking height is not limited by the stackability and/ or crushability of the loads, and multiple SKUs can be stacked in the same vertical column of storage space.
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- Block Stacking
Block stacking is where multiple unit loads are stored in designated floor location that is multiple levels high.
Advantages:
- Avoid cost of investment in racking
- Flexibility ( Change the storage configuration )
Disadvantages:
- Cannot sequence products
- Requires investment in lift trucks, possibly with special attachments such as side clamps or barrel clamps
- Products crushing/damage
- Floor stack height is limited by ceiling height, lift-truck mast height, or the maximum stacking weight allowed by the product and its packaging
The following are some conditions in using Block Stacking:
· Large quantities of the same SKU
· Where FIFO, lot control and expiration control are not stringent
· Acceptable safe limits
· Load stackability
· Load weight
· Pallet conditions (the wooden pallet that support block stacking)
· Floor loading restrictions
· Weather (due to corrugated softening in high humidity)
· Vehicle lift height capacity
· Crushability of the loads
· Building clear height
The loads in a block should be retrieved under a last-in-first-out (LIFO) discipline. Hence, if highly restrictive (more strict than lot or code date) FIFO requirements are in place, block stacking is particularly effective when there are multiple pallets per stock keeping unit (SKU) ,and when inventory is turned in large increments whereby several loads of the same SKU are received or withdrawn at one time.
When the loads are removed from a storage lane, a space-loss phenomenon referred to as honeycombing occurs with block stacking. Because only one SKU can be effectively stored in a lane, empty pallet spaces are created that cannot be utilized effectively until an entire lane is emptied. Therefore, in order to maintain high utilization of the available storage positions, the lane department (number of loads stored from the aisle) must be carefully determined.
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- Palletised Live Storage
Most storage equipment simply provides a static location for the housing and protection of stock, but in some cases it is found to be desirable to provide for the movement of materials held within the fixtures. Equipment which enables this movement goes under the generic heading of ‘live storage equipment’, and is particularly useful where stock rotation is of great importance.
A simple example of live storage would be a chute, where boxes of material are placed at the top of the incline, whereupon they slide to the bottom of the chute to rest against an end stop. The picking point is at the bottom of the chute, so the material which has been on the chute for the longest will be taken first, and the principle of ‘first in, first out’ will be automatically followed. New material going into stock will be placed at the top of the chute, and will take its place at the end of the queue of items waiting to be issued. Stacks of material, where issues are drawn from the bottom and replenishment takes place at the top provide another illustration of the live storage principle, examples being the column of paper cups in a dispenser, or packets of cigarettes in a vending machine.
A development of the chute idea is the use of unpowered roller conveyors arranged in sets, with an arrangement of openings resembling bins at the ends of the conveyors, where incoming material is placed, and a similar set of openings at the tails of the conveyors provides the picking face.
Advantages:
- Ensuring stock rotation
- Makes high density storage possible because the need of aisles or gangways is much reduced
- The material comes to the operator rather than the operator needing to go to the material
- Provide effective order picking regimes which automatically refill empty locations
- Provide physical separation between picking and replenishment operations.
The use of this type of live storage installation has the further advantage that, as a result of the fact that picking and replenishment take place in different locations, the movement of materials through the warehouse can take place in a continuous flow. When ordinary storage equipment is in use problems often arise as a result of issuing activities and replenishment work causing opposing flows and congestion. Quite large live storage equipment is available, enabling palletized unit loads to be stored and moved in just the same way as has been described.
Disadvantages:
- Expensive
- Space utilization is not always high
- Limited to one product type per row
- Only suitable for high throughput per line item
http://www.steelking.com/products/dynamicStorage/flow.html
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The storage equipments moved by simply sliding the equipment along the tracks, by turning a crank located at the end of each storage row, or by invoking electric motors that provide the motive power.
The space savings accrue from the fact that only one aisle is needed between all the rows of equipment.
Advantages: 
- Save storage space
Disadvantages:
- Increased the amount of time needed to access the items
Conditions:
. For very slow-moving items
. Space is very scarce and expensive
More information:
Where storage space is limited a variation on the live storage theme which is sometimes used is mobile racking.
The idea is that instead of having fixed bins with gangways between each fixture, the bins are mounted on rails or tracks. The bins are thus able to be placed close to each other, and the floor space which would otherwise be needed for gangways can be used for storage. When access to a particular bin is needed the fixtures are parted at the appropriate point by sliding them along the tracks. In the lighter type of installation simply pushing the fixtures is all that is necessary, is though most arrangements of this type have some kind of rack and pinion arrangement to make movement easier.
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Narrow aisle pallet racking allows more pallets to be stored than adjustable pallet racking. This is because the aisles are narrower, and the trucks can lift higher. Narrow aisle racking is served by specially designed trucks which do no need to turn through 90 degrees but run on a fixed path between the racks and pick up and set down on either side as required. As a result, the aisle is only marginally wider than the handling equipment which operates within the confines of the system. Goods are taken to and from the system by general purpose handling equipment which uses a pick and deposit station located at the ends of the narrow aisle. A High Rack Narrow Aisle can be as high as 20 meters tall.
Advantages:
- Good space utilization of 50% of floor space
- Placement and packing of the goods is faster.
- Trucks do not need to turn through 90 degrees
Disadvantages:
- Cost incurred in providing the required floor strength and flatness for working in high but narrow aisles
- The trucks are significantly more expensive than reach trucks
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The High Bay rack can be as high as 30 - 35 metres tall, in which the pallet movements are all depends on the stacker cranes, giving free access to all pallet locations. The racking structure may be used to support the walls and the roof of the building.
Advantges:
- good space utilisation (the use of headroom and narrow access aisles)
-narrow access aisles (as narrow as load plus side clearances)
Disadvantages:
- special fire sprinkler system may be required
- expensive to build
- inflexibility of changing design after it is built
