Spray nozzles must be selected based on minimum and maximum flow rate of the ingredient and minimum and maximum particle size of any suspended solids that might be in the liquid. It is always better, whenever possible, to use hydraulic pressure to atomize the liquid rather then an air assist atomization nozzle. An air assist nozzle has more of a tendency to cause the product to become air born, and using compressed air requires more energy.
Method of Application
After you have chosen a method to control the ingredients that are going to be applied, you need to choose where and how these ingredients are going to be applied. The most common methods of application are:
- Spray in a screw, ribbon, cut flight, paddle, conveyor
- Spray in a rotating drum or reel
- Spray using a spinning disk
- Spray into a screw conveyor
Spray nozzles (fig. 4) can be added to a screw conveyor to apply liquids to extruded product before load out or packaging. Screw conveyors provide very little mixing action or retention. If you cut the flights or substitute ribbons for solid flights then you get better mixing action. If you put paddles between the ribbon flights that are pitched to throw product back in the opposite direction of conveying, then you have even better mixing action and retention.
The problem with this type of system is that the spray nozzles apply liquid on a relatively small portion of the product and count on the mixing action of the conveyor to disperse liquid onto the rest of the product. This can cause spotty coverage where some of the product has a great deal of liquid applied and others have very little. One way to alleviate this problem is to drop the product into a plenum or weir with spray nozzles prior to entry in a mixing conveyor.
Spray into a rotating drum (fig. 5): The food and pet food industries quite often use a rotating drum with a variable angle of tilt on the drum. The product to be coated is discharged into the interior of the drum, which commonly has raised flights on the inside to tumble the product.
The advantage of this type of system is that retention of the product can be adjusted. Also, the drum eliminates pinch points that can cause product breakage, so it is very gentle. The disadvantage of this type of system is that the drum is open on two ends and mist from the spray action can migrate out of the drum and settle on the surrounding equipment. As with the mixing conveyor, the spray contacts a small portion of the product and the tumbling action of the drum is used to intermix the liquid. This type of system can also benefit by putting the spray nozzles into a plenum prior to entry into the drum.
Spray using a rotating disk (fig. 6): All of the systems previously mentioned use spray nozzles. Spray nozzles have a tendency to clog when applying liquids with suspended solids. Rotating disk applicators use two spinning disks to apply liquid to extruded products. Dry material is spun from a low RPM rotating disk and falls in a 360-degree curtain around a second disk. Liquid enters through the central bore of the dry disk shaft and drops onto the second disk spinning at a high RPM. The liquid is atomized into a fine mist that is driven by centrifugal force into the surrounding curtain of dry product. This results in a uniform coating of liquid without the need for spray nozzles.
This technology differs greatly from the traditional method of spraying, which sprays a relatively small portion of the product and depends on the mixing action of a drum or conveyor to further distribute the liquid. Since the rotating disk applicator applies liquid as the product is falling past the liquid disk, you are assured of having some liquid on all of the particles that pass by the liquid disk.
Many of these systems incorporate a mixing conveyor, however, the purpose of this conveyor is more for retention of the product in order to give the liquid time to absorb into the product. Since the system does not require spray nozzles, any liquid capable of flowing through a half-inch pipe is going to be able to flow through the machine. This means that the user has a great deal of flexibility in the selection of ingredients. Liquids that could previously not be used in coating systems because the high percentage of suspended solids clogged the spray nozzles easily pass through the spinning disk applicator. Since these machines are totally enclosed, you eliminate the overspray sometimes associated with open-ended coating systems. The top material disc spins at low RPM so the system is very gentle. The amount of liquid that the machine can apply really depends on the hardness of the product and the porosity.