Process Cleaning Magazine
© 2013
AMT-The Association For Manufacturing Technology
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PC FEATURE
There are many types of drying systems, but all seek the same result—removal of liquids (generally water) from part surfaces both external and internal. Just as cleaning agents are chosen based on the particular soils to be removed as well as the substrate from which they are removed, selecting a drying process should be based on multiple factors. These considerations include which cleaning agents are used (aqueous, semi-aqueous, or solvent); materials the part is made of (metals, plastics, composites); part geometry and final drying requirements (still-wet, dry “to the touch,” or a specific high-level dryness requirement); production line configuration (batch process, continuous line, or assembly line); and operating costs and capital expenditures. The following is a brief overview of many of the options availabe in the world of drying.
Non-Evaporative Drying Methods
COMPRESSED AIR
Generally considered to be best for small-scale drying of parts, a compressed air system is composed of several major sub-systems, including the compressor, prime mover, controls, treatment equipment and accessories, and the distribution system. The compressor is the mechanical device that takes in ambient air and increases its pressure. The prime mover powers the compressor with a minimum of 10 psi. Controls serve to regulate the amount of compressed air being produced. The treatment equipment removes contaminants from the compressed air, and accessories keep the system operating properly. Distribution systems are analogous to wiring in the electrical world—they transport compressed air to where it is needed. Compressed air storage can also serve to improve system performance and efficiency.
PROS: Many times, a compressed air system with sufficient pressure is already in place, so capital investment can be quite low. Spot drying with compressed air blowoff nozzles is a good option for blind holes and other complex configuration; however, care must be taken to avoid contamination of the part from oils and condensation present in the air line.
CONS: While this method of drying can be used for parts with a large surface area or complex configurations, it is less efficient at those applications and can become quite costly to operate. Also, protective goggles are required when working with compressed air. Noise levels are high with all compressed air installations and may require hearing protection equipment for employees working in the area.
BLOWERS
A blower, or more specifically, high-velocity air blowoff, is best used when properly scaled between 0.5 and 5.0 psi and directed through nozzles or air knives (see next entry).
PROS: The most efficient application is for fairly large parts with simple surface geometries. Blowers are more advantageous when part cleanliness is critical because there is less risk for condensation in the blower system than in the compressed air system, and because blowers with sealed bearings will not introduce oils into the air lines, as can happen with compressors.
CONS: Blowers are less effective for smaller parts or complex geometries, or for applications with higher throughput rates. One of the downsides of working with blowers is that they have considerable maintenance requirements. Filters typically must be changed at least monthly, belts quarterly, and bearings annually. On-site spares are required in order to minimize downtime. Heating elements can be added to the system for increased surface drying, but this increases capital expenditures as well as operating costs. Noise is also a problem with blowers, and air must be recirculated or exhausted from the system in line with specific safety requirements.
AIR KNIVES
As the name suggests, the air knife creates a “blade” of pressurized air from an internal chamber containing a series of holes or slots for release of said air. The air velocity creates an impact on the surface of a product ranging from a gentle breeze, to greater than 40,000 ft/min, to dry or clean with no actual surface contact. Typical applications include drying bottles and cans after filling and rinsing, printed circuit boards following the conveyorized wash to remove solder paste and flux, and metal castings after automatic machining.
PROS: Blow-off can be used for drying when it’s impossible to hand-dry the part, and the high velocity air can clean critical parts that may be sensitive to touch. This method can also deliver heated or cooled air to a surface or create an invisible air barrier to separate heated or cooled environments from one another in industrial applications such as continuous metal heat treating ovens, cold process or dust containment for the entrance to clean rooms. It is fairly easy to add to a line process for batch handling of parts as the air knives can usually be installed anywhere along the production line. The air source can be compressed air or a blower system.
CONS: The cons of air knives are similar to those listed under blowers. Parts with complex unexposed areas cannot be dried with an air knife.
CENTRIFUGAL DRYERS
Centrifugal dryers work like a clothes washer’s spin cycle—parts are placed in the basket, the lid is closed, and rapid spinning lets the liquids roll off the parts. Liquids can be collected for reuse or disposal. Some of these machines are also equipped with a forced-air system that adds an extra drying measure by blowing heated air across the parts chamber while it spins. The combination of both drying techniques can offer faster drying times than either technique alone.
PROS: Centrifugal drying works well for small parts in a bulk process because those parts are harder to dry with typical convection or blower dryers. Units are available in a range of sizes, from benchtop models to accommodate very small or delicate parts, to large stand-alone units suitable for mid-size parts such as fasteners, connectors, and circuit boards. This method costs less to operate than other drying methods, particularly in terms of energy consumption. Noise level is much lower than most other methods.
CONS: There is some debate about how effective this technique is at drying complex internal configurations. For such parts, a combination of techniques will likely prove most effective at drying. Not easily conveyorized, centrifugal drying requires manual loading and unloading and room for operator error in overloading baskets. Delicate parts prone to scratching need special consideration or modifications for use with this process.
VIBRATORY/ROTARY/TUMBLE DRYERS
These processes involve shaking, turning or tumbling the parts to cast liquids off. Then, via the drainage process, liquids can be recovered for re-use.
PROS: This is one of the less expensive options to install and maintain.
CONS: Vibratory drying may not be particularly effective as a stand-alone method.
DISPLACEMENT DRYING
For water-wet parts, this process uses solvents to displace the water on the parts, rather than evaporating it, resulting in parts that are spot-free, dry, and free of contaminating residue. Also called solvent drying or solvent dewatering, displacement drying submerges parts into a hydrophobic solvent containing a surfactant, which separates the water from the parts. Since these solvents are much heavier than water, they go beneath the water and simply float it away. This step is followed by a rinse in the solvent vapors. Some typical applications include aerospace components, medical devices and jewelry.
PROS: This simple process can be highly effective on complex internal configurations when the cleaning requirement is so stringent that absolutely no water spots can remain on the parts. Energy consumption is low, and parts emerge clean, dry and at room temperature.
CONS: Not as effective for parts that trap water. Also, the process is typically top-loading, not conveyorizable, and requires a solvent management program.
VACUUM DRYERS
In vacuum drying, heat is supplied by contact conduction or radiation (or microwaves) while the produced vapor is removed by the vacuum system. Because manual handling is required, the cost-effective use of vacuum drying for spot parts is dependent on the production rate, with high production rates requiring a more-easily automated drying method than can be attained manually.
PROS: Generally used as the last step of a combination of drying processes, vacuum drying works well for removal of the “last little bit” of moisture from flat parts and sheet goods.
CONS: May not be an effective method to remove moisture from complex internal configurations such as deep, small diameter and blind holes.
Evaporative Drying Methods
AIR AND GAS DRYERS
Steam or electric heat options are available whereby large volumes of air move through a large air path to dry parts. Platform oscillation plus airflow is another option to speed drying time.
PROS: Hot air drying systems are often touted as efficient and economical; available in top-, front-, roller- and conveyor-loading configurations so they can be added right into the production line process. They are available in many sizes and can be custom-tailored to specific applications.
CONS: Hot air drying is an inefficient process. It takes a lot of energy to heat the air. Many of these systems port hot air exhaust out of a venting system. Greater efficiency and energy cost savings can be gained by choosing a model with a recirculating air system. Recirculated air on these systems is filtered and reheated before being directed back into the parts chamber to avoid parts contamination.
FORCED AIR DRYING WITH HEAT
Adding heated air to the forced air circulation speeds the drying time but increases energy consumption.
PROS: If waste heat from another process can be effectively harnessed into the drying chamber, the energy cost will not be much higher than that of the forced air without heat process, and the drying process will be considerably faster.
CONS: Introducing heat into the drying equation creates a new set of potential problems: Heat can bake contaminants onto surfaces; it can cause parts to become too hot to handle in the next production step; it can swell parts such that they fall outside tolerable specs, and it can damage some heat-sensitive substrates.
RADIANT HEAT
Radiant heat drying typically involves use of an infrared (IR) heat source to dry small amounts of surface moisture (the “last little bit” of moisture) after another drying method has been used.
PROS: Easy to conveyorize, radiant dryers can be very efficient as a second phase of the drying cycle for parts that need to be dried more than just “to the touch.” They’re also quiet.
CONS: If the first drying method is insufficient to remove contaminants, the radiant heat step can actually bake those contaminants on to the part surface. Radiant heat usually is not a good standalone drying option because of the high cycle times and energy costs it requires when used as a primary means of drying.
FORCED AIR DRYING WITHOUT HEAT
Forced air drying involves moving volumes of air through an enclosed chamber to speed the ambient evaporative process.
PROS: It is quiet and requires minimal start-up equipment. Easily conveyorized, the process uses less energy than other drying methods.
CONS: Without heat, forced air drying is not typically sufficient for complex surfaces, stacked parts, or applications where finished surfaces need to be drier than “to the touch.”