If you want to make an omelet, you have to break eggs. If you want to clean parts effectively, you have to disrupt the environment near the product surface. Cleaning means accessing the part surface, removing the soil from the vicinity of the part, and keeping the soil away from the part. Read on as we discuss three techniques, flushing, ultrasonics, and cyclic nucleation, and how disruption is essential for effective cleaning.
Flushing
In flushing, one or several cleaning agents are repeatedly cycled past the component or product. It is useful for CIP (Clean in Place) applications such as reactor vessels where disassembling the product is not practical. Flushing is a logical choice for cleaning long tubes (like refrigeration coils). Systems sometimes include in-line equipment to monitor contamination in the “bath water.” The bath water can be solvent or aqueous. One limitation is that flushing systems can be ineffective due to the boundary layer. The boundary layer is the enemy of cleaning. The boundary layer forms because the velocity of liquid flowing through a tube is highest at the center but decreases rapidly near the surface of the tube. This means that flushing is ineffective right where you want to clean. Reduced velocity means less energy to dislodge contaminants; reduced velocity means less fresh cleaning agent to dissolve contaminants and to remove those contaminants from the vicinity of the surface. At the product surface, the liquid stops moving – kind of the opposite of what you hope to achieve in cleaning! The boundary effect is very pronounced in laminar flow; and the more viscous the cleaning agent, the deeper the boundary layer. Some flushing systems use compression waves to disrupt the boundary layer.
Ultrasonic Cleaning
Ultrasonic cleaning uses sound waves moving through a liquid to displace particles and thin films from surfaces. Two types of cleaning action are involved, cavitation and acoustic streaming. Cavitation involves sequential cycles of rarefaction and compression as sound waves move through liquids. Cavitation in the context of ultrasonic cleaning is different than cavitation in cyclic nucleation. To visualize cavitation, hold up a slinky and stretch out a few coils. These stretched coils illustrate rarefaction. During cavitation, vapor-filled “tears”, sometimes referred to as “bubbles”, form in the liquid. Next, push a few of the slinky coils together; this represents compression. During compression the tear implodes producing tremendous localized, high heat and force (pressure). The compression phase is very transient. Acoustic streaming converts the energy of sound to fluid flow, exerting a drag force on residue.
Ultrasonic cleaning is attractive because cavitation is omni-directional (the cleaning equivalent of “surround sound) as opposed to line-of-sight (e.g. spray cleaning). This means that, properly managed, the cleaning force accesses all product surfaces. Because of the high localized heat and forces, it is effective in penetrating the boundary layer and blasting adherent soils from the surface. While acoustic streaming also contributes to cleaning, it is unidirectional. The frequency range for ultrasonic cleaning is approximately 20 KHz to approximately 500 KHz. All other things being equal, higher frequencies mean smaller tears in the liquid, consistent with more gentle cleaning. More gentle cleaning means less product damage. However, there is a balance! At lower frequencies, cavitation is predominant. At the frequency increases, the importance of acoustic streaming increases. Because acoustic streaming is unidirectional, sample rotation may be more important at higher frequencies. Megasonics cleaning, useful in wafer fab, involves acoustic streaming.
Cyclic nucleation
To understand cyclic nucleation (sometimes referred to as cyclic cavitation), picture a teakettle. As water reaches the boiling point, it starts to form vapor bubbles, changing from the liquid to the vapor phase; there is furious bubbling. If you have ever made coffee or tea in the mountains, you have probably noticed that water boils at a lower temperature than it does near sea level. The cyclic nucleation process repeatably modulates the pressure above a liquid that is at a temperature near its boiling point, acting as if this liquid is moved suddenly back and forth between sea level, high pressure, and the top of a mountain, low pressure. During the low-pressure portion of the cycle, the liquid boils, or cavitates, starting at sharp edges or particles, nuclei.
The technique not only dislodges contamination, but flushes it away from the surface so the contamination is less likely re-deposit on the surface. The vapor phase has a much lower density than the liquid phase. The subsequent move to high pressure stops the boiling; the vapor phase collapses, leaving a void into which the liquid rushes. The turbulence of the collapsing stage disrupts the boundary layer and promotes dislodging contamination from the surface. The next low-pressure stage forcibly expels liquid from surfaces or recesses, carrying with it the dislodged contamination.
The technique is especially useful for dislodging residue from complex, ornate parts. It is one of the most effective techniques for removing soil from the interior surfaces of narrow tubing like cannula and hypodermic needles.
What’s the best cleaning method?
To achieve the best cleaning, select the optimal interaction between the product, the residue, the chemistry (sometimes several chemistries), and the physical cleaning forces. The optimal choice for your product line depends on many factors, including size, shape, and complexity of the product, materials of construction, choice of cleaning agent, soil/residue, and cleanliness requirements – how clean is clean enough.
A cleaning machine is an investment. Many cleaning processes, both aqueous and solvent based, are conducted in well-sealed systems that are often operated at reduced pressure. These systems may be offered with ultrasonic cleaning or with cyclic nucleation (sometimes called cyclic cavitation). It may not be either/or – both may be required for effective cleaning, particularly of complex parts. In a subsequent article, we plan to dive more deeply into available cleaning machines for these processes.
Resources:
Stuck and the Edge, The Boundary Layer
Barbara and Ed Kanegsberg
Volume 17—Issue 10 (Nov, 2020)
https://bfksolutions.com/stuck-at-the-edge-the-boundary-layer/
Disruption and Cleaning Action
Barbara and Ed Kanegsberg
Volume 19—Issue 2 (Feb, 2022)
https://bfksolutions.com/disruption-and-cleaning-action/
