Spray Cleaning

On a sunny Sunday afternoon, the helpful people from Roto-Rooter visited the kitchen of the Cleaning Lady and the Rocket Scientist. They used aqueous spray cleaning to unclog our kitchen drains. Their actions were inspirational. 

Spray cleaning is widely used in industrial, precision, and critical cleaning. Spray can be used in cleaning, rinsing, and drying. Spray ranges from a low pressure fine mist to a high pressure directed jet strong enough to remove paint. Spray cleaning involves the momentum of the impacting media and solvency of the surface soils. The media associated with spray can be liquid, solid, or gas. Liquid spray can involve water, aqueous cleaners, or solvents. Examples of solids include blast media, ice, or dry ice. Gas media includes shop air, purified air, or specific purified gases. 

Spray is line-of-sight
A cabinet washer with spray nozzles, an in-line system for cleaning electronics assemblies that shoots jets of cleaning agent or rinse water onto assemblies, and a spray ball cleaner in a mixing container or reactor used in pharmaceuticals are all line-of-sight cleaning. Understand the limitations of any line-of-sight process. One of the most important things to understand about a system of fixed spray nozzles is that it doesn’t turn corners. The spray must “see” the surface to be cleaned.

Cabinet washers
In a cabinet washer, water jets of cleaning agent and sometimes of rinse water are aimed at the parts to be cleaned. A cabinet washer can be effective for general cleaning. However, manufacturers may try to save money by using a cabinet washer to clean parts that would be more effectively cleaned with an immersion process. For example, it is very difficult to clean the internal surfaces of tubes, especially bent tubes, with a cabinet washer. There have been some proposals to carefully fixture each part prior to cleaning and perhaps to repeat the process after repositioning the parts. The approach depends on having people pay scrupulous attention to managing the cleaning process. When production goes up, cleaning quality tends to go down. Cleaning should not require so much human intervention! 

In-line spray systems
In-line cleaning is popular for removing “no-clean” (better termed “low-residue”) flux. Electronics assemblies are placed on a conveyor belt and are sequentially washed, rinsed, and dried. If you haven’t worked with an in-line cleaning system, picture Lucy and Ethel in the mid-century comedy series squirting whipped cream on top of pies as they move along a conveyor belt. Electronics cleaning is a bit like that, except with an automated spray system. One problem is that all in-line systems depend on appropriate timing. During production crunches, there is a tendency to speed up the conveyor belt. While the spray nozzles may not convey the comedic exasperation of Lucy and Ethel, poor cleaning and decreased yields are likely to result.

Another common problem with in-line electronics cleaning systems is that designers achieve immense satisfaction by putting 20, 30, or 40+ pounds of stuff into a 10-pound bag, and then decreasing the dimensions of that 10 pound bag. One response to incomplete cleaning with in-line spray is to repeatedly clean the electronics assemblies, repositioning the boards each time. The approach is not as effective as one would hope for. At best, it is tedious and time-consuming. In response to these limitations, some electronics assemblers have moved to immersion batch cleaning or have adopted systems that include both spray in air and a flood chamber. 

In-line spray-in-air systems have also been successfully used for metals cleaning; the parts are fixtured or racked a bit like Christmas ornaments and travel through wash, rinse, and drying steps. In such systems, orientation of the parts relative to the spray is crucial. It is also difficult to clean highly complex, convoluted parts.

CIP
Tanks and reactors are often cleaned using spray balls. This is an example of a clean-in-place (CIP) system. A spray ball is covered with holes, each of which is a nozzle; spray is directed throughout the tank, the way light bounces off of a disco ball. A spray ball may be static, or it may rotate to achieve better coverage and more effective cleaning (this achieves the total ‘80s vibe!). Spray balls are used in many tank cleaning operations. In critical cleaning applications, a spray ball is used for mixing and reaction vessels in pharmaceutical applications. 

CIP is cleaning in situ. Single-use spray cleaners are used in CIP processes for large structures that are difficult to disassemble and that may contain electronics assemblies. Solvent-based are typically preferred because they are self rinsing, and evaporate rapidly with minimal residue. If the electronics must remain live during cleaning, flammability is a concern.

Plumbing
The people who fixed our clogged sinks used a CIP system, because it is impractical to rip apart an entire plumbing system. The nozzle used to clear our sink drains was somewhat flexible and “bi-directional” to push through the heavy soils that occlude the drain and at the same time to remove surface soils. The nozzle directs high pressure water ahead to clear the path of debris while a higher volume of water is directed back and toward the surface to dislodge and flush soils near the surfaces (1). 

Fixturing
The direction of the spray steam and the orientation of the product surface to be cleaned are factors. Nozzle design, nozzle placement, hand held spray wands and rotation of the parts basket can be employed to be sure the spray contacts the appropriate surface. 

How much force?
In the February 2022 issue of Clean Source (2), we outlined many methods to apply Action or Force in a cleaning process. To be effective, a spray system must not only wet the surface but must also provide sufficient momentum to dislodge the soils. We often think of materials compatibility in terms of reactivity with the chemical cleaning agent. The force of the cleaning spray can itself represent a kind of incompatibility. The jet used by Roto-Rooter is great for removing greases, roots, and errant children’s toys from heavy metal pipes; but it can be too much of a good thing. We have seen examples where the metal part was dented by the force of the spray, or where components were propelled off electronics assemblies or critical sub-assemblies. Avoid murder of the product due to “blunt force trauma!”

Maintenance
Finally, our experience with plumbing malfunctions is a reminder that maintenance is a must –that includes nozzle maintenance. A clogged nozzle cannot readily reach the surface. Nozzle alignment is essential. Nozzles that are out of kilter may spray properly, but not in the direction of the parts to be cleaned. 

References

1. Roto-Rooter water jet https://www.youtube.com/watch?v=XiEgDsB5C80 

1. E. Kanegsberg and B. Kanegsberg, “Disruption and Cleaning Action”, Clean Source, February 2022. https://bfksolutions.com/disruption-and-cleaning-action/