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.
Low-Boiling Solvents
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.
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.
A related issue pertains to drying the parts after cleaning. The latent heat of vapourization measures the energy required to shift a chemical from liquid to gas phase. Aqueous cleaning systems will use more energy both to heat the cleaning agent above the boiling point of water and to dry water from cleaned parts than a system that uses a solvent with a lower latent heat of vapourization, lower specific heat and a lower boiling point. Again, modifications such as air knives are available for aqueous systems, but these increase energy consumption.
By the Numbers
Given that water has to work harder than solvents to accomplish the same cleaning task, it’s informative to compare the energy consumption of the two types of systems. But before we do, there is one important caveat: Cleaning systems come in myriad sizes, configurations and capabilities. Different soils and part geometries can dramatically affect the cleaning efficiency of a given system. As they say in the automotive ads, your mileage may vary.
In general, most solvent cleaning processes tend to be vertical, moving parts up and down in the cleaning system. Entirely self-contained, vapour degreasers usually are single-box machines about the size of a large desk or kitchen table. For example, a typical medium-sized vapour degreaser has outer dimensions of about 60×30 inches, with two cleaning tanks that are 10 inches wide, 12 inches long and 10-12 inches deep. Such a machine typically will use 3-5kw of electricity per hour of operation.
In contrast, aqueous processes tend to be horizontal, moving the parts through a series of dip tanks. Aqueous cleaning systems have typically a 50-60% larger footprint than vapour degreasers of the same capacity, simply because of the need for more tanks, larger pumps, blowers, filters and so on. These machines typically use 8-10kw of electricity per hour of operation.
Many aqueous systems enhance cleaning with ultrasonics, adding a further 200-500W of power consumption per tank. But whereas most vapour degreasers have only one tank fitted with ultrasonics, aqueous cleaning systems generally have three or more. So, ultrasonics adds roughly another 1-2kw of energy consumption to aqueous systems.
Auxiliary Equipment
Aqueous systems are not single-box designs. Auxiliary equipment required in addition to the basic three- to five-tank washing and rinsing system includes a deionized (DI) water system, some type of dryer and a wastewater treatment system. Each auxiliary process has its own energy requirement.
A typical aqueous batch system has one wash tank and two or three rinse tanks that require 2-5gpm of DI water. The DI system also needs to heat the water to operating temperature; typical aqueous systems cannot tolerate large influxes of cold water. Assuming a cleaning temperature of 140°F, the deionizer will need at least 2-3kw of power simply for purification and heating and more for the pumps and support equipment.
At the other end of the system, parts need to be dried. Infrared heaters, blowers, turbo-blowers and air knives are used. On a typical aqueous cleaning system, any of these drying approaches can easily use 5kwh. That number could double on a bigger machine simply because of the larger motors, fans and compressors required.
Wastewater treatment is a very complex issue because of the wide variety of processes and options available. In addition, some plants have suitable facilities already in place, so the extra energy consumption of waste treatment for a cleaning process may not be significant.
However, if the primary use of the water treatment system is to support the aqueous cleaner, then the energy costs and footprint of the waste treatment system are opportunities for savings. Assuming the system needs to process 5gpm of wastewater, even the most frugal waste treatment system is going to use 3-5kw of power. In general, aqueous cleaning systems always add cost and burden to plant treatment facilities.
Each of these auxiliary systems also adds heat to its surrounding environment, increasing the load on plant HVAC systems. The differences here are striking: the vapour degreaser mentioned above will add about 82,000 Btu/hr of heat to the room in which it is operating, while the aqueous system will add nearly 300,000 Btu/hr. The aqueous system also will add approximately 15lb (roughly 2 gal) of water into the plant air of the plant every hour, which will need to be removed by the HVAC system.
One last consideration is stand-by power draw – that is, use of electricity to keep the machine ready for operations. In order to minimise solvent losses, vapour degreaser refrigeration units should be kept operational at all times. This generally requires 0.5-1kw/hr of electricity. But at many companies, the heaters on aqueous cleaning systems are never shut down because of the long delay in reheating the water. This means the system uses 2-5kw of electricity, hour after hour, day after day, even when not in use.
Mike Jones MicroCare Corp., USA PF-online Acknowledgements Inputs from Wayne Mouser of Forward Technologies, Jon Harmon of Branson Ultrasonics, Art Gillman of Unique Equipment and Bill McCormick of Tiyoda-Serec Corp.