The rugged, flexible design of modern spiral conveyors can be leveraged in freezers and other cooling applications where product must be transported vertically up or down.

The primary advantage of a low-tension spiral design is that a large quantity of conveying belt can be used in a relatively small footprint while providing process flexibility.


Low-tension spiral-belt conveyor technology was invented by an engineer at Ashworth Co. in the 1960s to meet the needs of the frozen-food industry. Today, the technology has been expanded to all areas within the food industry and also is used in many non-food industries. Modern use demonstrates that its sturdy, adaptable design can be leveraged in many industries where product must be transported in a minimal amount of space. In process cooling operations, there are five major uses of spiral-belt conveyor technology:
  • Freezers.
  • Refrigerated coolers.
  • Ambient coolers.
  • Proofers.
  • Retarders.
Other applications include cookers, accumulators and basic material conveyors.

Spiral-belt conveyor technology was developed, in part, because the conventional standard - a linear conveyor - requires hundreds of feet of floor space to convey a large production throughput. Alternatives such as self-stacking spiral conveyors use successive spiral tiers supported by interlocking side links that are integrated into the belt. This design is compact: it has a lower stack height due to the lack of a rail system. However, it has limited belt widths and tier heights, as well as a fixed system diameter that may not allow optimization to accommodate physical constraints within the plant.

With a spiral-belt conveyor developed by Ashworth Co., the metal belt can be bent laterally (collapsed on the inside edge) and stacked in a helix pattern (a spiral with a constant radius) around a rotating drum. The rotating drum imparts the driving force to the inside edge of the belt, which then is transferred to the outside, or tension, edge of the belt through cross rods. The belt tension and speed are governed by the tension or outfeed drive motor. This type of system is called a low-tension spiral conveyor.

The spiral belt is driven by the rotating center drum and is supported by a continuous rail system that forms the helix, or spiral path. Spirals can be ascending, descending or twin-drum, which is a configuration with both ascending and descending spirals for products that require long dwell times or a gentle temperature change.

The primary advantage of a low-tension spiral design is that a large quantity of conveying belt can be used in a relatively small footprint. This technology has allowed food processors with a limited amount of plant space to convert from a batch to a continuous operation, thereby increasing productivity and profitability.

Initially, spirals were built to replace blast cells and linear tunnel freezers that could not handle the increased production rates and longer dwell times. After refining the low-tension system for freezer designs, suppliers and end users found other ways to apply the technology.

The first belts were made out of stainless steel. Today, the belts are available in both stainless steel and several varieties of plastic, with the dominant plastic being acetal. Belt widths range from 6 to 60", and belt surfaces can be rod-only, mesh overlay or flat-wire, depending on the needs of the plant.

The design of the conveyor system is governed by the belt-collapse factor or inside-turn radius. This factor specifies how much a belt collapses; thus, it determines the size of the drum that the belt turns around. The drum radius drum is equal to the collapse factor times the belt width. Collapse factors range from 1.0 to 3.0, with a nominal factor of 2.2. By varying the collapse factor, the designer can tailor the conveyor to meet the specific needs of each application. For a given belt area requirement, the system can be tall and thin (low collapse factor) with many tiers, or short and wide (high collapse factor) with significantly fewer tiers, depending on the size of the product being processed.

Freezers

Low-temperature freezers make up a majority of the spiral market. They continue to provide a cost-effective way to freeze large quantities of food continuously. Production rates typically range from 500 to 20,000 lb/hr, with dwell times ranging from 10 min to several hours. (Products that require shorter dwell times typically are frozen with a tunnel freezer, and those requiring longer dwell times are frozen with a variable-retention-time style freezer.)

Spiral freezer use conventional ammonia or Freon closed-loop refrigeration systems, and operating temperatures typically range from -20 to -60°F (-29 to -51°C). With the use of cryogenic gases such as carbon dioxide or liquid nitrogen, the freezing temperature can be reduced to -90°F (-68°C) or -135°F (-93°C), respectively. Cryogenic freezers primarily are used for delicate products or lower-capacity-rate production lines (typically less than 1,500 to 2,000 lb/hr). The consumable cost of the cryogen makes these freezers uneconomical at high capacity rates. Cryogenic freezers have a low capital cost but high operating cost compared to mechanical systems, which have a high capital cost but a low operating cost.

Many airflow styles are used in the spiral freezers offered by manufacturers, but they all can be classified into either a horizontal or vertical airflow pattern. Horizontal airflow blows across the product surface, and vertical blows either up or down on the product. Both designs provide adequate cooling. However, belt loading is lower in a vertical airflow freezer because an adequate open area has to be maintained. Without enough open area, an excessive pressure drop can be imparted on the fans, thereby affecting performance and decreasing airflow.

Coolers

Spiral coolers are similar to spiral freezers but are operated at a higher temperature, typically somewhere between -20 and 40°F (-29 and 4°C). The optimum cooling temperature is determined by the required final temperature and the amount of crust freezing that the product can tolerate. Colder temperatures will chill the product faster, which will make the spiral system smaller. However, using air temperatures below 32°F (0°C) will cause some freezing of the product, which could damage the product or prevent the processor from selling it as “fresh/never frozen.”

Colder temperatures will affect the design of the evaporator system. If the suction temperature is below 32°F, the evaporator will accumulate frost, which will require a defrost cycle to clear.

Spiral freezers provide a cost-effective way to freeze large quantities of food continuously.

Ambient Spirals

Ambient spirals are designed to cool product with no external means of removing the product heat. They typically are used in bakeries and snack-food operations where the products have a lower heat load and give up their heat readily, and where dehydration is not an issue.

Because there is no evaporator to remove the heat, the dwell times are significantly longer than in a spiral cooler. However, the systems are simple and the operating cost is negligible in comparison.

Proofers

Proofers are designed to provide the specific environment to allow the yeast in bakery products to fully activate. When compared to a spiral freezer (as cold as possible with high air velocity) proofers are controlled environments that are tailored and changed for each specific product. As in all of these examples, the spiral system is needed for high-capacity, continuous production lines with a long dwell time.

The environment within the enclosure typically ranges from 80 to 110°F (27 to 43°C) and 85 to 95 percent relative humidity. The air velocity is low with approximately 10 air exchanges per hour, and the air pattern is designed to be counterflow to the product in order to control moisture.

The activation of the yeast and the dough rising is an exothermic reaction that generates heat. Because of this, proofers require either a fresh air makeup or a small refrigeration coil to control the temperature inside the enclosure. Fresh air makeup also is used to control the CO2level.

Retarders

Retarders hinder or stop the yeast activity in bread products and, if required, are placed between the proofer and the chiller or freezer. The atmosphere in a retarder operates in the range from 60 to 80°F (16 to 27°C) and 60 to 70 percent relative humidity. As with the proofer, specific product needs might require operating parameters outside of this range.

When the product enters the retarder, the yeast will still be generating heat. Because the operating temperature of a retarder is lower than ambient in most bakery plants, these systems will require a cooling coil to arrest the yeast activity. Like a proofer, the system uses low-velocity air to cool the product.

Spiral conveyor systems also are used in cookers, accumulators and basic material-handling operations. Spiral cookers fall into one of two categories. The first is a low-temperature steam oven that operates between 190 and 212°F (88 to 100°C). The second style uses a combination of steam and dry heat to generate temperatures up to approximately 350°F (177°C).

While spiral accumulators are not common, they easily can provide 10 to 20 times the accumulation area than the floor space that it occupies. The system can be configured as a double-drum system that spirals up, then down, to feed the product back to the production line. Alternatively, it can be designed as a reversing spiral, which would require a second tension drive motor and belt hold-down rails.

A spiral conveyor is suited for applications where product must be moved vertically up or down, be it product coming out of a spiral freezer or empty containers being moved from one floor of a plant to another. One example is the brewing industry, which uses spirals in the depalletizing of empty bottles. Each successive row of a pallet is loaded automatically on a descending spiral that then transports the bottles down to the production line. The benefit is that the spiral uses significantly less space than a conventional serpentine conveyor.

Sidebar
Purchasing a Spiral Conveyor

Once you have decided to invest in a spiral conveyor, you must provide the following information to the equipment supplier to ensure that the system will be sized correctly for your operation:
  • Production rate (pounds/hour or pieces/minute).
  • Product size (dimensions, weight).
  • Product orientation (random, regimented, loading pattern).
  • Product preparation (raw, parfried, fully cooked, proofed, etc.).
  • Infeed temperature.
  • Outfeed temperature.
  • Product makeup (percent moisture, protein, fat, sugar, ash).
  • Product presentation (bare-on-belt, packaged).
  • Desired run time (hours/day, days/week, weeks/year).
  • Physical plant constraints (limits to clear height or plant floor space).
Common system options include:
  • Materials of construction (stainless steel, galvanized, aluminum).
  • Desired belt width (matched to feed conveyor; optimized for available space).
  • Tier pitch (based on the maximum product height).
  • Orientation (infeed and outfeed location).
  • Direction of travel (up- or down-running).
  • Cleaning equipment (belt washer, cleaning system).
  • Airflow pattern (horizontal or vertical airflow, depending on the supplier).
  • Type of belt (metal or plastic; type of belt surface required for product).
  • Enclosure design (caulked or fully welded joints, white enamel or stainless steel surfaces).


Sidebar
Comparing Spiral Cooling Systems

Whether you’re buying a spiral freezer, retarder or other spiral-cooling system, make sure you’re comparing apples to apples when evaluating equipment from different suppliers. Look for the following specifications:
  • Production capacity.
  • Total belt area or length.
  • Design belt loading and dwell time. (Also check to see whether these criteria have been tested and documented to ensure they are achievable.)
  • Evaporator performance (capacity, suction temperature, temperature difference).
  • Air system (total velocity across product in cubic feet per minute [cfm]).
  • Cleanability.
  • Utility requirement (per unit of capacity).
  • Installation and training requirements.
  • Warranty period.
  • Proprietary parts.
  • Service networks.
  • Price.


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