Applications of Dry Vapor Steam Cleaning Technique for Removal of Surface Contaminants

Chapter 17

Applications of Dry Vapor Steam Cleaning Technique for Removal of Surface Contaminants Rajiv Kohli The Aerospace Corporation, NASA Johnson Space Center, Houston, TX, USA

Chapter Outline 1 Introduction 2 Surface Contamination and Cleanliness Levels 3 Background to Precision Steam Cleaning 3.1 Steam Cleaning Principle 3.2 Steam Cleaning Systems and Equipment 3.3 Operating Considerations 4 Application Examples 4.1 Cleaning Stainless Steel Substrates 4.2 Cleaning Aircraft Ejection Seats 4.3 Cleaning Gilded Art Objects 4.4 Cleaning Mechanical Parts 4.5 Cleaning Anilox Rollers

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681 682 683 684 684 688 690 690 690 691 691 691

4.6 Cleaning Fiber Optics 691 4.7 Cleaning Electronic Components 692 4.8 Decontamination of Microbially-Contaminated Surfaces 692 4.9 Weed Extermination 693 4.10 Nuclear Decontamination 693 4.11 Food Decontamination 693 4.12 Miscellaneous Applications 695 5 Other Considerations 695 5.1 Cost Benefits 695 5.2 Advantages and Disadvantages of Steam Cleaning 695 6 Summary 696 Acknowledgment 696 References 697

INTRODUCTION

For many decades, precision cleaning in industrial applications has involved the use of a variety of solvents, many of which are deemed detrimental to the environment and are scheduled to be phased out [1,2]. In recent years, concerns about ozone depletion, global warming, and air pollution have led to new regulations and mandates for the reduction and eventual phase out of chlorinated solvents, hydrochlorofluorocarbons (HCFCs), trichloroethane, and other ozone- depleting solvents. The search for alternate cleaning methods to replace Developments in Surface Contamination and Cleaning, Volume 11. https://doi.org/10.1016/B978-0-12-815577-6.00017-7 Copyright © 2019 Elsevier Inc. All rights reserved.

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these solvents has led to the evaluation and implementation of various alternate cleaning systems [3–5 and references therein]. One such technique is dry vapor steam cleaning which has been employed in a wide variety of applications. Precision steam cleaning has been reviewed recently [6]. The purpose of this chapter is to revise and update the information in the previous review with emphasis on the applications of steam cleaning for removal of surface contaminants.

2 SURFACE CONTAMINATION AND CLEANLINESS LEVELS Surface contamination can be in many forms and may be present in a variety of states on the surface [7]. The most common categories of surface contaminants include: l l

l l l l

Particles Organic contaminants which may be present as hydrocarbon films or organic residue such as oil droplets, grease, resin additives, waxes, etc. Molecular contamination that can be organic or inorganic Metallic contaminants present as discrete particles Ionic contaminants including cations and anions Microbiological contaminants such as bacteria, fungi, biofilms, etc.

Common contamination sources can include machining oils and greases, hydraulic and cleaning fluids, adhesives, waxes, human contamination, and particulates, as well as from manufacturing process operations. In addition, a whole host of other chemical contaminants from a variety of sources may soil a surface. Cleaning specifications are typically based on the amount of specific or characteristic contaminant remaining on the surface after it has been cleaned. Product cleanliness levels in precision technology applications are specified for particles by size (in the micrometer (μm) size range) and number of particles, as well as for hydrocarbon contamination represented by nonvolatile residue (NVR) in mass per unit area for surfaces or mass per unit volume for liquids [8–10]. The surface cleanliness levels are based on contamination levels established in industry standard IEST-STD-CC1246E for particles from Level 5 to Level 1000 and for NVR from Level R1E-5 (10 ng/0.1 m2) to Level R25 (25 mg/0.1 m2) [10]. A new international standard defines the cleanliness of surfaces in cleanrooms with respect to the presence of particles [11]. It applies to all solid surfaces in cleanrooms and associated controlled environments such as walls, ceilings, floors, working environment, tools, equipment, and devices. The surface particle cleanliness classification is limited to particles between 0.05 μm and 500 μm. A new standard ISO 14644-13 has been published that gives guidelines for cleaning of surfaces in cleanrooms to achieve defined levels of cleanliness in terms of particles and chemical classifications [12]. Many of the products and manufacturing processes are also sensitive to, or they can even be destroyed by, airborne molecular contaminants (AMCs) that are present due to external, process or otherwise generated sources, making it essential to monitor and control AMCs [13]. An AMC is a chemical

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contaminant in the form of vapors or aerosols that can be organic or inorganic, and it includes everything from acids and bases to organometallic compounds and dopants [14,15]. A new standard ISO 14644-10, “Cleanrooms and Associated Controlled Environments – Part 10: Classification of Surface Cleanliness by Chemical Concentration” [16] is now available as an international standard that defines the classification system for cleanliness of surfaces in cleanrooms with respect to the presence of chemical compounds or elements (including molecules, ions, atoms, and particles). In many commercial applications, the precision cleanliness level is defined as an organic contaminant level of less than 10 μg of contaminant per cm2, although for many applications the requirement is set at 1 μg/cm2 [10]. This cleanliness level is either very desirable, or it is required by the functional use of parts such as metal devices, electronic assemblies, optical and laser components, precision mechanical parts, and computer parts.

3

BACKGROUND TO PRECISION STEAM CLEANING

Dry vapor steam is a high-temperature, low-moisture vapor (4–6% moisture). Dry vapor steam cleaning has been used in a variety of applications from household to industrial to military to medical. It is a practical and environmentallyfriendly, all-purpose nontoxic method to clean surfaces without the use of solvents. Steam can be generated by simply boiling water; however, it is the combination of temperature and pressure that makes steam cleaning effective. A steam cleaner is a highly beneficial piece of equipment for all home, commercial, and industrial applications requiring sanitizing, disinfecting, cleaning and drying capabilities in a single machine [17]. Most people are familiar with the corner car wash engine steam cleaners that remove oil, grease, and grime from the engine with a pressurized steam spray. The high heat of dry steam has the ability to emulsify grease, oil, and dirt. Other types of steam systems are used in carpet cleaning and other household applications, in which the dry steam penetrates the fibers and lifts dirt away [18–21], although in many cases, these cleaning systems employ hot water rather than steam for cleaning. The low water content means no moisture is left in carpet backing to cause mold growth. However, due to its high temperature at the nozzle, heat-sensitive surfaces, such as silks, velour, and thin plastics, must be carefully treated. The real steam cleaning systems are growing in popularity because dry steam is a nontoxic pesticide as well. The penetrating heat of steam vapor can be used to kill germs, microbes, mold and bacteria and, in some cases, disinfect without the use of chemical disinfectants, as well as for food decontamination [22–27]. Steam vapor has also been shown to be effective in killing dust mites in carpet, bedding and upholstery [19–21,28,29]. Television infomercials tout the ability of small steam generators to clean household dirt, grime, dust, and germs among other contaminants [30]. These household systems generally operate at low pressure (0.3–0.6 MPa). Large commercial steam systems

684 Developments in Surface Contamination and Cleaning

operating at higher pressure (up to 2 MPa) are utilized in industrial applications (often utilizing boilers and pressure vessels) as degreasers and for cleaning tools and large surfaces [30,31]. Medical applications call for sterilization and disinfection of surfaces, such as tools, hospital rooms, and restrooms, with steam [25]. Dry vapor steam cleaners are frequently used in hypoallergenic environments because they do not require the use of additional cleaning chemicals, which results in better indoor air quality and eliminates the need to handle or store cleaning agents. Steam has been shown to be effective in combating mold, bacteria, viruses, and other forms of biocontamination [28,29]. Steam can also be used for precision cleaning applications where contamination must be removed at a microscopic level. Due to its fluid properties as a vapor (low viscosity), steam has the ability to penetrate inaccessible areas where other fluid cleaning methods may be unsuccessful. This is especially important when cleaning printed circuit boards to penetrate blind vias and holes and to penetrate contacts and terminations to remove residual flux that may cause increased electrical resistance and corrosion.

3.1 Steam Cleaning Principle Plain water is not an effective medium for dissolving hydrocarbons such as soils and greases, but it has the capability to dissolve many inorganic compounds. By using the principle that chemical reaction rates increase with temperature, water heated to create superheated steam becomes a very effective cleaning agent for hydrocarbon compounds. The combination of moisture, heat and pressure of superheated steam provides the means for immediate removal of contaminants from a surface. The hydrocarbon compounds become less viscous at higher temperatures (some of these compounds may even melt), making it easier to remove them from the surface by wiping or vacuuming. For solid contaminants, cleaning involves primarily breaking the bond between the contaminant and the surface and the high pressure steam spray becomes an effective removal agent for solid contaminants. Furthermore, steam also exhibits the fluid properties of a gas which makes it very effective in penetrating the surface boundary layer and accessing tight spaces. The viscosity of steam at 573 K is in the range 20.279 to 20.076 μPa.s for pressures from 0.2 to 2 MPa, as compared with the viscosity of liquid water at 373 K in the range 281.74 to 282.22 μPa.s for pressures from 0.2 to 2 MPa [32].

3.2 Steam Cleaning Systems and Equipment A wide variety of steam cleaning systems have been developed and are available from different vendors [33–58]. In general, a suitable water source is used to pump a metered amount of deionized or otherwise filtered or purified water into the steam generator. The steam is produced in the generator that heats the

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feed water to high temperatures (388–428 K) to produce low pressure (only a few atmospheres), low moisture (4 to 6% water) water vapor (steam). A dispersing nozzle or atomizer may be incorporated in the steam generator which enhances steam generation because it disperses the incoming water into small water droplets and directs them onto the heated inner surfaces of the steam generation pot. The water droplets convert instantly to high pressure superheated steam. The steam which is generated leaves the steam generator by means of a conduit located in the upper half of the generator. The superheated steam is carried to a suitable external nozzle or wand assembly and can be directed as a jet spray onto the part to be cleaned. An operator-activated switch cyclically draws water into the pump to initiate the steam generation process. For smaller single tank models, a short recovery time is required for steam to be generated for each cycle. The steam exits the nozzle at a gentle pressure of 0.34–0.41 MPa, leaving surfaces dry within seconds. This is a significant difference from other high-moisture, high-pressure units, making it ideal for nearly all surfaces, even cloth. Moisture content of the steam can range from 5% (dry steam) to 99% (wet steam, a combination of steam and hot water). Dry steam systems use very little water because the vapor is created at a high temperature with very low moisture content between 5 and 6 percent water. These systems use high pressures but less water for precision and delicate cleaning tasks. A quarter cup of water produces about 1135 liters of steam. Average water use ranges from 3.78 to 19 liters per 8 hour work shift. The inherent low-moisture characteristics of dry vapor steam cleaners make them suitable for use inside buildings and residences. The steam pressure needed for the systems to obtain the heat for cleaning is produced by a boiler made primarily from stainless steel. Stainless steel offers higher safety margins and is more resistant to scaling, pitting and corrosion which can contaminate the substrate being cleaned. The part surfaces are often warmed enough during cleaning to dry quickly. However, a drying stage may be needed to prevent oxidation or corrosion of sensitive parts. Flash rusting may be a problem for some materials and preventive measures, such as the use of rust inhibitor, may be required for such parts. Two types of cleaning units are available. Single tank units have a boiler that is filled directly with water. When the machine runs empty of water, it will need to be cooled before opening the safety pressure cap and refilled with water. The larger the boiler capacity the longer is the operating time. Modern units also offer a continuous-fill system which consists of a separate non-pressurized, non-heated water tank and the boiler. This allows the machine to be refilled at any time without waiting to cool down the machine. The level of the water in the boiler is constantly detected by a special sensor. This means no downtime because there is always a constant amount of water in the boiler. The flash-heat design typically requires 6 to 8 minutes startup time, although one model has a startup time of 1 minute [36].

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An innovative new heating system has been incorporated into a commercial micro precision steam cleaner [55,59]. This new heating system in the boiler cavity employs a smooth heating surface made from material with interconnecting pores that very effectively overcomes the Leidenfrost effect [60]. The Leidenfrost effect is a phenomenon in which a liquid drop impinging on a surface significantly hotter than the boiling point of the liquid immediately forms an insulating vapor layer (approximately 0.06 mm thickness at 433 K [61,62] which decreases the heat transfer from the surface to the liquid and keeps the liquid from boiling rapidly. The pores in the heating surface act as escape passages for the insulating vapor layer, which reduces the thickness of the layer and increases the contact of the droplets with the heated surface. This significantly enhances the conversion of impinging water droplets into steam compared to other systems [63–68]. Steam cleaners for precision cleaning applications can be provided with continuous steam spray capabilities without the use of boilers and pressure vessels. This is an important safety consideration since pressurized steam storage requires the use of pressure vessels that must meet strict safety requirements. Most models superheat steam for ondemand use of up to 20 minutes which is long enough for most cleaning applications. Various models are equipped with other safety features such as an audible or visual water-level indicator, automatic check valve to prevent pressure buildup, and automatic shut-off feature. Most models come with a trigger wand assembly and interchangeable nozzles, including single jet for a straight stream; fan jet for a flat spray to clean larger areas; and peripheral nozzles for hole and tube cleaning. The steam delivery system could be hand-held (pistol grip) which affords the operator direct steam control. It allows the user freedom of movement when working on small parts. By contrast, the stationary delivery systems have built-in steam nozzles which allow the operator to grasp the part with both hands for greater control. Precision steam cleaners are lightweight (6 to 25 kg) and are easily transportable wherever cleaning is required. The waste management system usually includes a cabinet, exhaust fans, spotlights and cart. Clear vinyl curtains enable the operator to see products being cleaned and avoid splashing. The waste management system has a removable drip pan to capture any contaminant residue. Steam cleaners use very little water to do large amount of cleaning, which means less waste for disposal. The typical range of specifications of commercial precision cleaning units is given in Table 17.1. For medical instrument cleaning applications, the instruments frequently need a manual pre-cleaning step to remove contaminants built up in difficult-to-access locations, primarily due to the design, shape and construction of the instruments. Often, these contaminants are not removed when the instruments are cleaned in automatic washers before reuse. Recent surveys in several hospitals in Germany indicated that between 5–15% of the surgical instruments

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TABLE 17.1 Typical Specifications of Commercial Precision Steam Cleaning Units Steam cycle time range

1–20 minutes

Voltage

100 to 240 VAC

Frequency

50 – 60 Hz

Power requirement

1700 to 3000 W

Steam pressure

0.2 to 2 MPa

Steam temperature:

373 to 573 K

Nozzle

Fixed single jet or multiple jets; customizable for a given application

Tank capacity

3 to 5 liters

External dimensions

21–38 cm
33–44 cm
20–33 cm

Weight

8–25 kg

Internal materials of construction

Stainless steel, brass, Teflon hose. Wetted brass surfaces are suitable for noncontamination-sensitive applications, such as jewelry cleaning, due to a slight risk of corrosion of brass.

System operation

Single tank for intermittent operation; twin tanks for continuous operation; foot or hand-operated pedal; pistolgrip or straight nozzle

Startup time

1–30 minutes

required a pre-cleaning step to effectively remove all residual contaminants after cleaning/disinfection for reuse of the instruments [69]. A variety of attachments and adapters (Fig. 17.1) are available for precision steam cleaning of difficult-to-clean medical instruments such as endoscopes, catheters, and orthopedic and dental handpieces [69,70]. A Luer-Lock connector [71,72] allows instruments with the respective coupling device to be connected directly to the handpiece, so that the steam spray can be guided into the rinsing channel of the instrument to clean precisely where it is required without any pressure loss (Fig. 17.2). A recent innovation involves the use of naturally occurring low zeta potential mineral to form nanocrystals in the source water [38,73]. As these crystals pass through the boiler, they gain energy from the heat. Then, when the water transforms into superheated low-moisture steam (dry steam), these energized crystals are accelerated along with the steam. This process produces an enhanced type of steam vapor that disrupts the cell membranes and has been

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FIGURE 17.1 Adapters and attachments for precision steam cleaning surgical instruments. A. Long and short hollow needles with steam vents along the side. B. Hollow straight needle attachment with steam vent at the tip. C. Hollow curved needle attachment with steam outlet at the tip. D. LuerLock adapter. E. Catheter attachments. F. Hose connection. G. Mounting pliers. H. Rack [69]. (Courtesy of Elma GmbH, Singen, Germany).

shown to kill a broad range of microorganisms after 3–5 seconds exposure to steam.

3.3 Operating Considerations The first consideration when utilizing vapor steam in precision cleaning applications is the quality of the water used. Tap water contains many contaminants including minerals, silicates, and organic matter, so it is unsuitable for precision cleaning applications. Use of high purity filtered, deionized or distilled water is absolutely essential for such applications. Another consideration should be whether the parts being cleaned can withstand the temperature and pressure of the steam spray. Precision steam cleaners can be provided with a variety of outlet pressures and temperatures. Steam saturation quality, supply of wetter or dryer steam, and steam pressure can be adjusted for specific applications for contaminant removal. Cleaning units have steam pressures ranging from 0.14 MPa to over 2 MPa, while temperatures can range from 373 K to 573 K. The temperature decreases very rapidly in the steam vapor plume. Heat generated is minimal for electronic components such as circuit boards. By controlling the distance from the steam outlet nozzle, temperature-sensitive parts can be cleaned without exposure to unnecessarily high temperatures. Recent testing on aluminum alloy 7075 strips (0.5 mm thick
24.5 cm wide
48.3 mm long) has shown the temperature of the surface was

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FIGURE 17.2 Steam cleaning of medical instruments with different adapters. (a) Straight bore needle. (b) Hollow curved bore needle. (c) Direct connection with the Luer-Lock interface [69]. (Courtesy of Elma GmbH, Singen, Germany).

between 311 K and 318 K ten seconds after removal of steam from the surface [40]. The maximum temperature of the surface was 253 K at 25 mm steam exit distance from the surface to as high as 372 K at 0.64 mm from the surface [74]. Condensed water can penetrate and/or damage joints, seals, and bonded areas, and limits the usefulness of steam vapor cleaning. Preventive measures, such as air blowers, may be required to remove residual moisture. Steam vapor cleaning is optimal when individual components are removed or disassembled from the hardware as part of the normal task. However, disassembly can be time- and labor-intensive. Additives, such as detergents, alcohol, or saponifiers, are generally not added to the water as most additives will break down at the high temperatures needed to make steam. Often these additives are sprayed on the parts prior to steam cleaning. Caution should be used in operating steam cleaners since surfaces, nozzles and substrates can become very hot during cleaning operations. Steam pressure is also a safety consideration. Personal protective equipment (safety glasses, gloves, appropriate work clothing) should be employed. Operators must be trained properly to use the steam cleaning equipment efficiently and safely.

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Often, the equipment is employed in an enclosed cabinet to capture the contaminants and to minimize worker exposure to debris from the part being cleaned.

4 APPLICATION EXAMPLES Dry vapor steam precision cleaning can be used in various applications, such as: l

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l l

Removing water-based (no-clean) flux from circuit boards and other soldering applications Cleaning fiber optics and other optical components Cleaning hybrid circuits prior to wire bonding Cleaning medical devices and instruments Cleaning dental implants Cleaning and degreasing aerospace tools and parts Cleaning military hardware such as weapons, automotive parts, electronics, ground support equipment, and other gear Cleaning automotive components to remove grease and grime Cleaning jewelry and gems

4.1 Cleaning Stainless Steel Substrates The effectiveness of micro precision steam cleaning has been demonstrated on contaminated 304 stainless steel substrates [75]. Ten 0.1 m2 plates were precleaned and individually contaminated with approximately 8–16 mg/m2 of dust and drilling lubricant as determined by weighing. The samples were dry steam cleaned for approximately 2 to 3 minutes and reweighed. The remaining contamination ranged from 1 to 2 mg/m2; the cleaning efficiencies were between 75 and 93%, with an average cleaning efficiency of 86%. Visual inspection of the cleaned surface showed uniform removal with no evidence of visible contamination on the surface.

4.2 Cleaning Aircraft Ejection Seats The prevailing method of cleaning aircraft ejection seats in the U.S. Navy employs an organic solvent such as isopropyl alcohol. This method of cleaning the ejection seat and/or ejection seat components is extremely labor-intensive and generates significant quantities of hazardous waste for disposal. An alternate cleaning method was evaluated using a wet steam cleaning system [76]. The steam vapor cleaning process is found to be optimal when individual components of the ejection seats were removed or disassembled for cleaning as part of the normal task. This ensured that water was not trapped, but disassembly was time- and labor-intensive. There was concern about deterioration or damage to the O-ring seals, but no damage was observed; however, the steam cleaning process did remove the lubricants on the seals and had to be replaced.

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Cleaning Gilded Art Objects

Recently, cleaning of three gilded objects has been investigated: fire gilded copper pipe; a leaf gilded spire; and a festive bonnet made of fibers wrapped by chemically gilded silver wire [77]. Micro precision steam cleaning was successful in removing surface contamination, although some surface damage was observed. This is not unexpected when the surface coating layer is very thin, as in the case of the gildings which were only 10 μm thick.

4.4

Cleaning Mechanical Parts

A new facility has been designed and built that is equipped with modern precision cleaning equipment, including micro precision steam cleaners [78]. The company has incorporated micro precision steam cleaning in the new facility for precision cleaning of mechanical parts and assemblies. These parts can range in size from a single threaded fastener to large composite structures. Materials that are processed include optical glass, ceramics, composites, metals and various high performance coatings. First pass yield of 98% has been achieved in the new facility with annual savings of US$1 million, part of which is attributable to the use of micro precision steam cleaning.

4.5

Cleaning Anilox Rollers

A system has been designed for steam cleaning of anilox inking rollers used in flexographic printing processes [79]. This system can be used to safely and easily remove dried ink and foreign other materials from the surface of anilox rollers using one or more jets of high pressure steam (0.34–4.13 MPa) at a temperature of up to 450 K. The steam quickly wets the dried ink and may also melt and or soften other non-water soluble residue materials embedded within the cells of the anilox roller, making them easier to remove. The high pressure of the steam creates a high velocity jet that blasts steam into the cells, causing the ink and other residue materials to be ejected out of the cells. The system also includes a means for spraying liquid water to rinse the surface of the roller. For nonwater-based inks, a surfactant or degreaser can be sprayed on the surface before steam cleaning.

4.6

Cleaning Fiber Optics

Recently, a novel four-stage automated process has been developed to efficiently clean and dry fiber optic endfaces in high density multiferrule connectors [80]. The system incorporates a micro precision steam cleaning as the first process step, followed by CO2 snow cleaning for final precision cleaning. The parts are dried in flowing heated ionized air. All process steps are carried out with a single multistage programmable cleaning system. It has been used to

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successfully demonstrate the removal of various contaminants, including fibers, dust, oils and other residues with better than 90% efficiency.

4.7 Cleaning Electronic Components A method has been proposed using both water and steam for removing particle residue from chemical mechanical planarization of semiconductor wafers [81]. This method can be used for other applications that require super-clean surfaces, such as manufacture of liquid crystal display products and magnetic disks for hard drives.

4.8 Decontamination of Microbially-Contaminated Surfaces Steam cleaning of surgical instruments and medical devices has also found increasing acceptance in hospitals worldwide, primarily due to the ability to reach inaccessible locations and the effectiveness of steam cleaning in destroying microorganisms. In 2005, the University of Washington in Seattle, Washington tested a steam vapor system in restrooms at the Odegaard Undergraduate Library, which is used by as many as 15 000 students daily, and reported labor savings and hygienic improvements compared with traditional cleaning methods such as mechanical scrubbing with detergent solutions [82]. Cleaning time was reduced from 46 minutes to 42.5 minutes with the steam vapor system, which can be significant time savings over a cumulative period. Custodians attributed the time savings to less squatting and stooping to clean hard-to- reach places behind toilets, and under sinks and urinals. With no residual water on floors, drying time also was reduced. Steam cleaning resulted in cleaner fixtures and surfaces. Steam-cleaned faucet handles and towel dispenser handles, for example, were more than 10 times cleaner than those cleaned with traditional methods. Steam vapor cleaning also eliminated workers’ exposure to chemicals. The effectiveness of precision steam cleaning to decontaminate microbially-contaminated surfaces was investigated recently [25]. Dried films of different microorganisms (Escherichia coli, Shigella flexneri, Enterococcus faecalis, Salmonella enterica, Staphylococcus aureus, Pseudomonas aeruginosa, Candida albicans, Asperigillus niger, Bacteriophage MS2, and Clostridium difficile) were deposited on porous clay tiles that are more representative of surfaces found in the health care and food service industries. The initial concentrations of the microorganisms ranged from 1.08
104 to 4.83
106 CFU per test surface (colony forming unit (CFU) is a unit of measurement for viable bacteria numbers). The test surfaces were disinfected with a portable steam cleaner for 0.5, 1.0, 2.0 or 5 seconds. Individual colonies were counted and the results recorded as CFU per test surface. The results showed the diverse array of microorganisms tested at high concentration was

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completely destroyed very rapidly. For example, the initial concentration of salmonella and E coli of 9.75
104 and 1.14
106 CFU, respectively, was reduced to 3.16 and 5.98
103 CFU after 2 second exposure and to 0 CFU after 5 seconds exposure to steam. Similar studies in Italy showed the efficacy of precision steam cleaning in destroying six bacteria strains (E. coli, S. aureus, S. faecalis, Bacillus cereus, Saccaromyces cerevisiae, and Pseudomonas aeruginosa) deposited on porcelain tiles, Teflon, and stainless steel substrates, after 5 seconds exposure to steam [83,84]. Several other studies have shown that steam cleaning results in significant reductions in the microbial load and elimination of biofilms in hospital and veterinary facilities [85–90].

4.9

Weed Extermination

One novel application is weed control with steam which has an interesting fringe benefit of sterilizing the soil [30,39,56,91]. Steam heats the plant environment very efficiently and allows for a sudden increase in the temperature of the plant tissues, preventing transpiration by the plant. As a result, the plant is completely destroyed. In practice, the energy input is nearly half that of gas flame weeding technique. Wet steam is an efficient, ecological, and economical method of weed extermination and is in use worldwide.

4.10

Nuclear Decontamination

The Kelly steam vacuum decontamination system has been successfully used in commercial nuclear facilities to decontaminate rooms, fuel pool walls, large components and other large and/or smooth surfaces [92–94]. The superheated water stream (523–573 K) is delivered via a handheld wand that can incorporate different spray nozzles. The water flashes to steam when it contacts the surface being cleaned. Decontamination rates are 30–45 m2/hr. The main advantage of this system is that the spray head is enclosed within the integrated vacuum recovery subsystem that simultaneously traps and collects dislodged contaminants, steam and water droplets, thereby significantly reducing airborne contamination and personnel exposure to contaminated waste.

4.11

Food Decontamination

The potential of steam condensation at above and below atmospheric pressure has been evaluated as a food decontamination method for red and white meat, meat products, vegetables, herbs, and fruits [95–97]. Pressurized steam decontamination is very effective as very high rates of temperature rise at the surface of the samples can be achieved by condensing steam under pressure. Surface

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damage can be avoided by immediately vacuum cooling after treatment. However, any pressurized steam/vacuum cooling system is likely to be expensive and be difficult to automate for high volume processing. On the other hand, an atmospheric steam decontamination system has a much simpler design and mode of operation. Steam is continuously fed into the top of a vessel with an open bottom. As the steam fills the vessel, it displaces any air and because it is lighter than air it remains in the vessel with a slight spillage from the open bottom. The food to be decontaminated is inserted through the open bottom. After the set treatment time has been reached, the steam is shut off, the food is removed and immediately cooled. With heat-sensitive foods, decontamination may have to be carried out at temperatures lower than 373 K. In a recent study, a commercial high-powered, portable dry steam cleaner was evaluated for the efficacy of removal of residual food substances and elimination of food pathogens (S. aureus subsp. aureus (ATCC® 6538), Listeria monocytogenes (ATCC 19111), Campylobacter coli (ATCC 33559), and S. enterica subsp. enterica) in food substances from stainless steel surfaces [98]. The results of the study demonstrate that dry steam cleaning significantly reduced the pathogens from stainless steel after an 8-second cleaning. The residual food particles were no longer visible after treatment. A combined vacuum–steam–vacuum process has been developed to eliminate microbial contaminants from freshly processed foods such as chickens [99–101]. The process exposes solid food product to vacuum, then to steam, and then back to vacuum. Saturated steam is used to take advantage of the very large latent heat of condensation relative to the sensible heat (amount of energy needed to decrease the temperature of steam independent of phase changes) transferred due to temperature difference in cooling superheated steam. Field tests with the processing unit achieved 1.1–1.5 log kill of inoculated E. coli and 1.2 log kill of Campylobacter on freshly processed chicken using 3 cycles and 411 K saturated steam with a total process time of 1.1 seconds. The results were validated by numerical analysis of heat transfer from the surface into interior pores of the food by conduction. A new decontamination technique for meat and poultry has been successfully tested on a commercial scale [102,103]. This technique, known as SonoSteam®, is based on simultaneous treatment of the meat surface with a combination of steam and ultrasound [104–108]. Ultrasound enhances the killing effect of steam by efficiently removing the protective laminar air boundary layer on the meat surface. Testing of naturally contaminated broilers showed an average reduction of 2.51 log10 units (CFU/ml) of Campylobacter on the surface and no visual changes of the carcasses, which is as good as or better than currently used decontamination methods. An authorized sensory panel at the Danish Veterinary and Food Administration concluded that broiler carcasses treated with SonoSteam® were acceptable for purchase. These conclusions were based on organoleptic differences (smell, skin/meat consistency, texture, and color) of treated versus untreated carcasses.

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Miscellaneous Applications

Precision steam cleaning is used to remove adhesive, flux, oil, grease, fingerprints and other contaminants from optics and electronic components, automotive components, various components in the defense industry, jewelry and gems, medical instruments and dental implants, household appliances, clogged drains, and building structures. Steam can be used on both the interior and exterior of a car or boat to leave a streak-free surface for subsequent treatment. Cleaning brick, stonework, concrete, mortar and grout of mold, spores and grime is fast and mess-free, without the use of dangerous herbicidal chemicals. It will even remove grease and motor oil from the driveway and garage. It also leaves streak-free, fingerprint-resistant window panes [33–58]. Very recently, steam treatment has been employed to boost the catalytic activity of Pt– CeO2 to clean automobile emissions [109]. Industrial and governmental organizations worldwide that are involved in precision cleaning are increasingly adopting precision steam cleaning for their pre-cleaning and/or their final cleaning operations for many different applications. The U.S. Department of Defense (DoD) has recommended the adoption of steam cleaning in all service branches as a viable alternative to solvent cleaning of components and parts such as electronics, weapons, printed circuit boards, and other items. Significant cost savings have been realized at different DoD installations by employing precision steam cleaning [35,36]. Other examples of applications and case studies of steam cleaning are given by the various commercial vendors of cleaning systems [38–57].

5

OTHER CONSIDERATIONS

Some other items need to be considered with respect to dry vapor steam cleaning.

5.1

Cost Benefits

Significant cost savings can be achieved by steam cleaning compared to solvent cleaning. For example, the total annual operational costs to clean 120 guns per day at a DoD installation were estimated to be approximately US$217 000 using a portable steam cleaner compared with approximately US$660 000 for solvent cleaning for estimated savings of US$443 000 [35,36]. Similar savings of US$308 000 were achieved by steam cleaning of aircraft ejection seat frames and components compared with solvent cleaning [76].

5.2

Advantages and Disadvantages of Steam Cleaning

5.2.1 Advantages l Steam cleaning eliminates the use of solvents. l Steam cleaning reduces the amount of hazardous waste and hazardous air emissions generated compared to solvent degreasing.

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l

l l l

l l

l l

l l l

l

l l

The wastewater stream is generally compatible with conventional industrial wastewater plants. The technique has low implementation cost utilizing simple equipment. Steam cleaning provides solvent cost savings. Steam cleaning is ideal for removing grease, oil, flux, adhesive, fingerprints, and other contaminants. The cleaning systems have low water usage. Safe, clean design of the cleaning systems generates superheated steam on demand. There is no high-pressure steam storage. Cleaning units feature lightweight, portable design for point-of-use applications. Auto shut-off feature safeguards the heater. Steam can reach otherwise inaccessible areas and spaces. Steam cleaning enables elimination and control of the biofilms that resist typical disinfectants It is an effective technique as a manual pre-cleaning step for medical devices. Steam cleaning can be applied very selectively and precisely. Steam cleaning is very handy and fast.

5.2.2 Disadvantages l Steam cleaning is not recommended for temperature- or moisture-sensitive parts. Rusting or surface oxidation may occur. l There is a risk of electrostatic discharge (ESD) when cleaning electronic components. l Damage to joints, seals, and bonds can occur from residual moisture.

6 SUMMARY Dry vapor steam cleaning employs superheated steam to remove surface contaminants. It is a low-cost, effective method for precision cleaning and for decontamination of microbially- contaminated surfaces. This method uses very low volumes of water and is a viable alternative to solvent cleaning in diverse applications such as cleaning of metal surfaces, electronic components, gilded art objects and anilox rollers, decontamination of microbially-contaminated surfaces, disinfection and sterilization, weed extermination, radioactive decontamination, and elimination of pathogens in food.

ACKNOWLEDGMENT The author would like to thank the staff of the STI Library at the Johnson Space Center for help with locating reference articles.

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DISCLAIMER Mention of commercial products in this chapter is for information only and does not imply recommendation or endorsement by The Aerospace Corporation. All trademarks, service marks, and trade names are the property of their respective owners.

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