Regardless of how the raw metal is made into a tube or pipe

Regardless of how the raw metal is made into a tube or pipe, the manufacturing process leaves a significant amount of residual material on the surface. Forming and welding on a rolling mill, drawing on a drafting table, or using a piler or extruder followed by a cut-to-length process can cause the pipe or pipe surface to become coated with grease and can become clogged with debris. Common contaminants that need to be removed from internal and external surfaces include oil- and water-based lubricants from drawing and cutting, metal debris from cutting operations, and factory dust and debris.
Typical methods for cleaning indoor plumbing and air ducts, whether with aqueous solutions or solvents, are similar to those used for cleaning outdoor surfaces. These include flushing, plugging and ultrasonic cavitation. All of these methods are effective and have been used for decades.
Of course, every process has limitations, and these cleanup methods are no exception. Flushing typically requires a manual manifold and loses its effectiveness as the flush fluid velocity decreases as the fluid approaches the pipe surface (boundary layer effect) (see Figure 1). Packing works well, but is very laborious and impractical for very small diameters such as those used in medical applications (subcutaneous or luminal tubes). Ultrasonic energy is effective at cleaning external surfaces, but it cannot penetrate hard surfaces and has difficulty reaching the interior of the pipe, especially when the product is bundled. Another disadvantage is that ultrasonic energy can cause damage to the surface. The sound bubbles are cleared by cavitation, releasing a large amount of energy near the surface.
An alternative to these processes is vacuum cyclic nucleation (VCN), which causes gas bubbles to grow and collapse to move liquid. Fundamentally, unlike the ultrasonic process, it does not risk damaging metal surfaces.
VCN uses air bubbles to agitate and remove liquid from the inside of the pipe. This is an immersion process that operates in a vacuum and can be used with both water-based and solvent-based fluids.
It works on the same principle that bubbles form when water starts to boil in a pot. The first bubbles form in certain places, especially in well-used pots. Careful inspection of these areas often reveals roughness or other surface imperfections in these areas. It is in these areas that the surface of the pan is in more contact with a given volume of liquid. In addition, since these areas are not subject to natural convective cooling, air bubbles can easily form.
In boiling heat transfer, heat is transferred to a liquid to raise its temperature to its boiling point. When the boiling point is reached, the temperature stops rising; adding more heat results in steam, initially in the form of steam bubbles. When heated rapidly, all the liquid on the surface turns into vapor, which is known as film boiling.
Here’s what happens when you bring a pot of water to a boil: first, air bubbles form at certain points on the surface of the pot, and then as the water is agitated and stirred, the water quickly evaporates from the surface. Near the surface it is an invisible vapor; when the vapor cools from contact with the surrounding air, it condenses into water vapor, which is clearly visible as it forms over the pot.
Everyone knows that this will happen at 212 degrees Fahrenheit (100 degrees Celsius), but that’s not all. This happens at this temperature and standard atmospheric pressure, which is 14.7 pounds per square inch (PSI [1 bar]). In other words, on a day when air pressure at sea level is 14.7 psi, the boiling point of water at sea level is 212 degrees Fahrenheit; on the same day in the mountains at 5,000 feet in this region, the atmospheric pressure is 12.2 pounds per square inch, where the water would have a boiling point of 203 degrees Fahrenheit.
Instead of raising the temperature of the liquid to its boiling point, the VCN process lowers the pressure in the chamber to the boiling point of the liquid at ambient temperature. Similar to boiling heat transfer, when the pressure reaches the boiling point, the temperature and pressure remain constant. This pressure is called vapor pressure. When the inner surface of the tube or pipe is filled with steam, the outer surface replenishes the steam necessary to maintain the vapor pressure in the chamber.
Although boiling heat transfer exemplifies the principle of VCN, the VCN process works inversely with boiling.
Selective cleaning process. Bubble generation is a selective process aimed at clearing certain areas. Removing all the air reduces atmospheric pressure to 0 psi, which is vapor pressure, causing steam to form on the surface. Growing air bubbles displace liquid from the surface of the tube or nozzle. When the vacuum is released, the chamber returns to atmospheric pressure and is purged, fresh liquid filling the tube for the next vacuum cycle. Vacuum/pressure cycles are typically set to 1 to 3 seconds and can be set to any number of cycles depending on the size and contamination of the workpiece.
The advantage of this process is that it cleans the surface of the pipe starting from the contaminated area. As the vapor grows, the liquid is pushed to the surface of the tube and accelerates, creating a strong ripple on the walls of the tube. The greatest excitement occurs at the walls, where steam grows. Essentially, this process breaks down the boundary layer, keeping the liquid close to the high chemical potential surface. On fig. 2 shows two process steps using a 0.1% aqueous surfactant solution.
For steam to form, bubbles must form on a solid surface. This means that the cleaning process goes from the surface to the liquid. Equally important, bubble nucleation begins with tiny bubbles that coalesce at the surface, eventually forming stable bubbles. Therefore, nucleation favors regions with high surface area over liquid volume, such as pipes and pipe inside diameters.
Due to the concave curvature of the pipe, steam is more likely to form inside the pipe. Because air bubbles easily form at the inside diameter, vapor is formed there first and quickly enough to typically displace 70% to 80% of the liquid. The liquid at the surface at the peak of the vacuum phase is almost 100% vapor, which mimics film boiling in boiling heat transfer.
The nucleation process is applicable to straight, curved or twisted products of almost any length or configuration.
Find hidden savings. Water systems using VCNs can significantly reduce costs. Because the process maintains high concentrations of chemicals due to stronger mixing near the surface of the tube (see Figure 1), high concentrations of chemicals are not required to facilitate chemical diffusion. Faster processing and cleaning also results in higher productivity for a given machine, thus increasing the cost of the equipment.
Finally, both water-based and solvent-based VCN processes can increase productivity through vacuum drying. This does not require any additional equipment, it is just part of the process.
Due to the closed chamber design and thermal flexibility, the VCN system can be configured in a variety of ways.
The vacuum cycle nucleation process is used to clean tubular components of various sizes and applications, such as small-diameter medical devices (left) and large-diameter radio waveguides (right).
For solvent-based systems, other cleaning methods such as steam and spray can be used in addition to VCN. In some unique applications, an ultrasound system can be added to improve the VCN. When using solvents, the VCN process is supported by a vacuum-to-vacuum (or airless) process, first patented in 1991. The process limits emissions and solvent use to 97% or higher. The process has been recognized by the Environmental Protection Agency and the California District of South Coast Air Quality Management for its effectiveness in limiting exposure and use.
Solvent systems using VCNs are cost effective because each system is capable of vacuum distillation, maximizing solvent recovery. This reduces solvent purchases and waste disposal. This process itself prolongs the life of the solvent; the rate of solvent decomposition decreases as the operating temperature decreases.
These systems are suitable for post-treatment such as passivation with acid solutions or sterilization with hydrogen peroxide or other chemicals if required. The surface activity of the VCN process makes these treatments fast and cost effective, and they can be combined in the same equipment design.
To date, VCN machines have been processing pipes as small as 0.25 mm in diameter and pipes with diameter to wall thickness ratios greater than 1000:1 in the field. In laboratory studies, VCN was effective in removing internal contaminant coils up to 1 meter long and 0.08 mm in diameter; in practice, it was able to clean through holes up to 0.15 mm in diameter.
Dr. Donald Gray is President of Vacuum Processing Systems and JP Schuttert oversees sales, PO Box 822, East Greenwich, RI 02818, 401-397-8578, contact@vacuumprocessingsystems.com.
Dr. Donald Gray is President of Vacuum Processing Systems and JP Schuttert oversees sales, PO Box 822, East Greenwich, RI 02818, 401-397-8578, contact@vacuumprocessingsystems.com.
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