One of the older and gentler technologies associated with drying is conveyor drying. Conveyor dryers - also referred to as band or apron dryers - are used extensively throughout a variety of industries. They have found a particular niche in the food industry for products such as pet food, fruits and vegetables, extruded snacks and cereals. The feed to a conveyor dryer needs to be reasonably well formed and robust to:
- Allow a dimensionally stable and porous bed to be formed.
- Prevent the formation of fines within the dryer, which can result in carryover to the heating system as most of these dryers operate on a recycle basis.
- Inhibiting the blocking of perforations on the belt.
This feed will take on the form of large granules, agglomerates, pellets, preformed or extruded products (pressure agglomerates), small solid particles, large solid particles and applicable agricultural products.
Conveyor dryers process at rates that are consistent with their specific applications but are on the lower end of throughput capacities in the assortment of dryer technology. This relatively low rate is a limitation primarily imposed by physical logistics and capital cost. Conveyor dryers principally are through-the-bed dryers although cross-flow and radiant units are used occasionally for specific products. Units can be directly or indirectly heated using burners (gas, LFO or HFO) or coils (steam, electrical heater banks or thermal oil).
The principle of a conveyor dryer is simple: Feed is placed on a perforated belt and hot gas is passed through the belt and feed. This principle remains the same as the technology develops. The feed is metered on the belt to create a bed, the formation of which is fundamental to the efficiency of the drying process. The belt moves and a seal between the static (stationary) and dynamic (moving) components contains the bed, preventing short-circuiting of the carrier gas. The heat source, belt, drives, feeding mechanism and primary gas movers are installed in a frame, and the entire system is insulated. Entry doors along the length and ends of the dryer provide access to the moving components. Fines collection and gas ducting (primary and recirculation) are designed for each application. It is a simple principle but with a rather complicated mechanical design. Only a few companies in the world design and fabricate what I would consider a really good conveyor dryer.
Feeding SystemThe first component of the dryer is the feeding system. Many different kinds of feeders, including vibrating pans, screws and belts, are used to feed the dryer. This is the principal method of metering the feed into the system, so providing a means to modulate and control the feed rate is important. Many of these feeders oscillate while feeding, forming the bed on the belt. Some have fishtails to spread the feed, and some feed into the center of the belt, relying on rakes, spreaders and plows to level and distribute the product across it.
The conveyor belt used in a feeding system often is a slatted belt fabricated from perforated plate with support bracing attached to roller chains on both sides. A central slide base sometimes is installed to support the belt. "Fishplates" are installed along the edges of the belt to prevent material from falling off the belt and to allow a sealing surface. A drive motor and gearbox are installed at the head of the conveyor to provide the motive force for the belt movement. In some instances, a chain link belt or screen belt with suitable edge protection and a matching drive is used.
The bed of material is conveyed into the dryer from the feed section. Each section of the dryer -- termed a zone -- is supplied with a recirculating fan, exhaust port and heat source, most commonly dedicated to the zone. Integrated steam coils, gas burners or electrical heater banks typically generate the heat in conveyor dryers. Other methods of providing energy include channeling in the carrier gas from external sources such as waste heat.
Conveyor dryers also may be multipass units in which the feed is transferred from one belt to another belt below it. These dryers usually are two- or three-pass machines with the belts stacked vertically. The multipass option allows for increased residence time for products that need it and saves floor space in areas that have limited real estate. Additionally, certain conveyor dryers such as those used in vegetable dehydration have multiple belts lined up within the dryer where the feed is transferred from one belt to another. This allows for the feed orientation to be changed, allowing new surface areas of the material to be exposed to the carrier (heat source) and for process-created variations in bulk density to be managed.
In addition to varying the feed rate and temperature profiles within the dryer, residence time also can be modulated. Increasing or reducing the bed depth while reducing or increasing the belt speed, respectively, will change the residence time. These variables allow the conveyor dryer to possess unmatched flexibility in the process-drying arena.
Air and Gas FlowYou may recall my reference to the similarity between cabinet dryers and conveyor dryers in my columns on cabinet dryers. With conveyor dryers, each zone acts in a similar fashion to a cabinet dryer. The carrier (typically air) is drawn in through an inlet duct or recycle channel into the heating compartment. The air is heated to the required temperature in this region and then distributed into the drying zone. After passing through the material bed, a high percentage of the gas is recycled, reheated and distributed in a circular motion according to requirements of the specific application.
The motive force for the air movement typically is a plug-type fan that acts as both an induced-draft and forced-draft machine. The fan will "suck" and "blow" the air through the heating chamber and into the drying chamber. The fresh air inlet is on the suction side and the exhaust on the pressure side. The inlet and exhaust are fitted with dampers that are dependent on the product and required operating conditions. This recycle improves the efficiency of the unit operation, conserving energy in the gas stream.
The gas is distributed into the drying zone above or below the belt. If from above, it passes through the product bed and the belt into the recycle channel before being reheated and recirculated. Conversely, if the gas comes from below, it passes through the belt and then the product bed before being recycled.
Most conveyor dryers have at least two zones; many have distinct, multiple zones. The airflow direction in each zone may be the same, or successive zones may have alternating or differing airflow directions depending on the materials' drying characteristics. Additionally, each zone may use a different temperature profile and control scheme. This can allow wetter products to be processed at higher temperatures, reducing the temperature in successive zones as the moisture in the product falls, thus preventing potential thermal product degradation. Conveyor dryers often have an integrated final cooling zone to cool the product to a desired temperature.
Saturated or close-to-saturated air is exhausted from the first one or two zones in either a dedicated exhaust or, more commonly, a central exhaust duct and induced-draft fan system. It is of economic advantage (depending on the operating parameters) to incorporate recuperators on the exhaust system to preheat the fresh makeup gas.
Next month, I'll look at conveyor dryer control and limitations of this type of dryer.