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Cold-chain Storage Types

We frequently see confusion in use of the terms that describe or relate to different types of cold storage spaces. This article defines the basic categories of environment-controlled storage systems used in food supply chain.  
Size and Capacity - are distinctly different, yet correlated

STORAGE SIZE: Is the volumetric measure of a holding space of a transport or storage chamber. As a general norm, this is a static measure in cubic metres of the space. The actual mass of goods held in a holding space varies - because the volume to mass ratio of the cargo handled in cold-chain differs depending on the density of the produce, the packaging used, the storage/stacking system used, space design, etc. The storage size is only a factor of the total handling capacity of a cold store.

STORAGE CAPACITY: Is the total throughput of the goods passing through a holding space in a given time period. In other words, this is the handling capacity, and is a multiple of the storage size and holding cycle. In case of a weekly holding cycle, the capacity is derived from "storage size x 52 weeks" or a 52x multiple of its holding size to arrive at its annual operating capacity. Therefore, the final handling capacity of a static space will depend on the product being handled - whether inventory is fast moving or long term.

HOLDING CYCLE: The period of time a specific good is held in a storage or transport chamber. Also called inventory turnover, it is a longer period in case of products like potato, spices or apples and few days in case of tomato, milk, litchi, etc. The handling capacity of a space is in multiples of its size and the holding cycle or rotation of the inventory held. Storage Size x Holding Cycle = Capacity.

For example, a cold store designed with 1000 tons space, will have a capacity of 52,000 tons if the inventory has a weekly turnover. In effect, the size of a cold store is only a factor of its handling capacity also called operational capability. 

It is essential to understand and measure infrastructure capacity in terms of end-use and not merely as size of space created. This will minimise unnecessary infrastructure investment overruns, promote proper end-use and capacity utilisation.

The types of storage space can be categorised by their end-use. Not all cold storage capacity is designed for the same type of use. Understanding the principle uses of cold store spaces will make it is easier to appreciate and harmonise concepts.

COLD STORAGE (BULK storage): A large environment controlled warehousing space with multiple chambers, intended for bulk storage of perishable produce. The chambers are designed to cater to long duration storage of the goods. The inventory should be held to serve as a buffer to smoothen the supply gap arising due to episodic production, and the intention is to stabilise & sustain the supply lines. This type of cold store is normally developed close to farm-gate, for a selective set of crops, to hold a single harvest for the remaining year. The Storage capacity is normally equal to the static Storage size (1 storage cycle per annum), as it has one holding cycle per annum. The common examples are cold stores designed for potato, apple, spices, pulses, seeds. In case of potato, spices, pulses, seeds, no precooling is required. Produce like potato, spices, pulse, etc., on exiting such cold stores may not require onward cold-chain interface. Bulk cold stores are also used as stores of raw material, for captive use of processing factories.

COLD STORAGE (Distribution HUBS): An environment controlled distribution centre designed for short-term storage of produce so as to function as a logistics platform for frequent handling and throughput of packed (ready-to-retail) goods. These cold stores types are part of the uninterrupted farm-to-market supply chain - key to effective distribution of perishable produce and built at the front-end of the cold-chain. These are designed to have multiple temperature zones (frozen, chilled, mild-chill) and the storing methodology must facilitate frequent put-away and pick-up activities. These stores function like railway platforms or airports, as consolidation and distribution gateways for adjoining market centres. These storage hubs are used as perishable cargo centres, at sea ports, as city distribution centres, etc. and essential for maintaining the integrity of the cold-chain. The usable capacity is a multiple of the static size (eg. a daily turnover in case of perishable cargo centres or 12 to 24 storage cycles per annum in case of bi-monthly turnover for chicken or ice cream).

COLD ROOM (STAGING): An insulated and refrigerated chamber, that works as a transient staging space, and is necessarily attached to a Pre-Cooling Unit. This component is typically installed at farm-gate (village level) as part of a modern pack-house, and temporarily stores preconditioned fresh produce, awaiting transport link to a distribution point (a cold store close to market). Conjoined with pre-coolers, a staging cold room serves to free the pre-cooling room for undertaking the sequential batch load cooling of the next lot of incoming freshly harvested produce. The dimensions created are such as to hold a maximum of two or three days batch load from the pre-cooler unit, to buffer for any delay in arrival of the connecting transport. This is used for nearly all perishable horticulture and floriculture crops. The usable capacity is a multiple of the static size (eg. 3 storage cycles per week - goods are not stored longer than 2 to 3 days).

REEFER TRUCKS: Road transport vehicles with a fixed insulated body equipped with active refrigeration designed for environment controlled carriage of temperature sensitive produce. These are effectively cold rooms on wheels - mobile storage spaces. The refrigeration on long haul trucks is powered through integrated diesel driven motors, independent of the main truck engine. In case of small vehicles, the use of direct drive systems linked to the vehicle engine (like a car AC unit) or battery powered refrigeration is the norm. Electric power option for use when parked is also possible. The usable capacity is a multiple of the static size (eg. 52 storage cycles per annum - on average one round trip per week).

REEFER CONTAINER: An insulated container with in-built refrigeration, which unlike the fixed body of reefer trucks, can be removed & relocated from its wheeled carrier or chassis (trailer). Commonly used for multi-modal activities where rail-road-sea-air movements are involved in the distribution chain. The equipment is designed to source electric power from a separate generator (power-pack) which is independent of the reefer container. Grid electricity can also be connected when stationed on-site for temperature controlled storage pending subsequent logistics operations. Capacity calculation is similar as in case of reefer trucks, depending on round trips.

 -Pawanexh Kohli (2015), NCCD Newsletter (ISSN 2395-4515)