Aqueous cleaning normally delivers quality results but aqueous systems tend to be large and complex. Vapour degreasers are usually smaller and simpler and that simplicity enables them to deliver a more consistent and headache-free cleaning process. A properly designed and maintained vapour degreaser can be more budget-friendly, parts-friendly and planet-friendly than an aqueous cleaning system of comparable capacity.
Firstly, the term vapour degreasing describes a type of cleaning system based on solvents with boiling points of 90-170°F. Unlike water, these solvents also have low viscosities, low surface tension ratings, high densities, low specific heat and low latent heat. Used in properly configured equipment, they can deliver affordable, fast, reliable, safe and environmentally acceptable cleaning.
To begin cleaning, solvent is placed in the machine’s boil sump and heated using electric heating elements, hot water coils, steam coils or a heat pump unit. The boiling solvent produces a clear and dense vapour that rises into the chamber above and gradually displaces air there. This is the vapour blanket and it helps contain the liquid solvent inside the machine.
Eventually, the vapours rise to the first set of refrigeration coils – the primary condensing coils – which chill them and condense the solvent back to its liquid state. This condensate drips into a trough that wraps around the internal circumference of the machine. The solvent then moves through a water separator because some airborne humidity from above the vapour blanket also condenses.
At this point, the distilled solvent is routed into the rinse sump. Because the rinse sump already is filled with clean solvent, the addition of more solvent causes it to overflow and spills solvent back into the boil sump. This purging maintains a constantly clean rinse sump. It also concentrates any contaminants into the boil sump for easy removal and maintenance.
How does this differ from cleaning with water?
When it comes to industrial cleaning there are better solvents than water. To overcome some of water’s natural chemical limitations, aqueous cleaning processes need more complex systems or use additives to make it a passable cleaning agent.
One issue stems from surface tension – the measure of the wetting ability of a liquid. The lower a liquid’s surface tension, the more easily it will flow across a substrate as well as around and under parts to be cleaned, creating more opportunities for cleaning to occur. In general, high surface tension equates to poor cleaning: If you can’t wet, you can’t clean.
Water has the highest surface tension of any popular cleaning agent, necessitating addition of surfactants to lower surface tension of aqueous cleaning formulations. Other additives such as detergents (alkaline formulations are common) are also used to boost the cleaning power of the water mixture, especially to help remove organic contaminants.
Even with these additives, the surface tension will still be higher than that of any modern solvent. Indeed, surfactants and detergents can actually be considered contaminants since it is sometimes difficult for the rinse water to remove them completely.
The second troublesome factor is viscosity. Water may seem thin compared with, for example, motor oil, but in comparison with modern solvents, water is the molasses of cleaning agents. A thick fluid like water resists flowing into tiny crevices or apertures and so is a less effective cleaning agent than thinner solvents.
This manifests itself with occasional “spotting” on cleaned parts. Additives and high-pressure sprays can improve system performance, but in general, a solvent with low viscosity is going to out-clean water simply because the low-viscosity material can get into and out of places that water cannot.
The third problem area with water is its density. When it comes to cleaning, heavier is better: many applications feature insoluble particulates that must be removed. A solvent with low surface tension and low viscosity, combined with high density, can more easily remove particulates from substrates and keep them suspended in the solvent. From there, it is a simple for a properly-designed filtration system to remove the particulate from the solvent.
In either type of system, cleaning usually is enhanced if the solvent is warm. The energy required to raise the temperature of a liquid is called its specific heat. It takes a lot of energy to raise the temperature of water, but low-boiling solvents have a lower specific heat and work at lower temperatures which reduces energy consumption.