The experiments with saturated steam as a medium for cleaning surfaces were the subject of the last episodes in the series. Now follows a description of the tests, test results and the evaluation of technological parameters for successful surface cleaning.
Removal of dirt films with superheated steam on external regular and irregular surfaces
The following section explains the effective cleaning of wire, sheet metal, shaped profiles and irregular surfaces using superheated steam. Cleaning systems using superheated steam technology working in practice show how the theory is put into practice. Calculations and diagrams for selecting the technological parameters are shown.
Overview of the tests and test results
As a result of their production technologies, the sheets and shaped profiles usually have lubricants and very fine metal chips as foreign particles on the base material surface. As a result of very high pressures during production, the foreign particles are highly statically charged at a very small distance from the base material surface, i.e. they have a high binding energy. Neutralizing the surface charges therefore requires higher voltages, i.e. still in the mV range. We are talking here about 100 to 200 mV higher DC voltages. The required discharge voltages depend on the base material. In the range of 2 to 3.5 V DC voltages, the charges are safely discharged galvanically. If pure galvanic cleaner solutions are used, 300 to 500 mV voltage of the solution to the V2A tank wall should be sufficient.
In this work, various tests were carried out with superheated steam. Steam at 200 °C was heated electrically at 8.5 bar overpressure in the steam boiler and overheated by 200 to 400 °C using a hot air gun. In addition, DC voltages of 2 to 3.5 V DC or 2 to 3.5 V AC were connected as cathode and anode to the base material.
The foreign particles were removed in all tests, with differences in the duration of the cleaning process. The most effective surface cleaning was found with drawn wires, up to 13 drawing steps with drawing soaps and finest drawing chips soiled up to 15m/sec drawing speed.
The cleaning of smooth, finely rolled sheets was the most difficult. Here, the reliable separation and transportation of the separated foreign matter were the limits of the cleaning results. Only additional heated compressed air with up to 4 bar overpressure and/or cross-flowing liquid with injector/strong suction make the hot steam technology possible. Flat nozzles arranged one behind the other in a fan-shaped arrangement, comparable to snow plows on a wide, snow-covered highway, make it possible to use this technology. The removal of the dissolved foreign particles and oils sets the limits of the cleaning speed.
Tests with a relative speed of 15 to 35 m/s with drawn wire show that the separation of the foreign particles (drawn soaps) from the surface of the base material takes place, but the transport from the area of separation is not sufficient. The nozzles block within 20 to 30 seconds, up to welding, i.e. wire breakage.
The dwell time of the surface areas in the HP nozzle area results from the following equation:
vWire = 15 m/sec with wire diameter
of dwire = 1.2 mm <1>
Nozzle gap - distance of the ring nozzle = 0.05 mm, with ring nozzle diameter 4 mm, nozzle area = 0.05mm x 3.14 x 4mm = 0.628mm²
Assuming a layer thickness of the drawing soaps of 0.2 mm thickness, this results in 0.628mm2 x 0.2mm x 15000/0.05 = 37 680mm3/sec of HP steam cleaning foreign particle volume per mm wire length.
It is therefore understandable that the amount of foreign particles removed is the limit value of the process. The optimization of faster removal was not the subject of this work, but is being pursued further in the development department of the system manufacturer.
What is essential for this work is the unambiguous statement that, based on the cleaning results, the separation of the foreign particles from the base material surface occurs reliably at a relative speed of up to 35 m/sec.
The dwell time of the surface segment for the cleaning period is calculated as follows:
15000 mm : 1 sec = 0.05 mm gap length HP nozzle : t sec<2>
→ 0.05 mmsec / 15000 mm =t sec = 3.3x10-6sec
It is therefore sufficiently certain that chemical reactions do not play a role, i.e. the addition of electrolytes or acids is insignificant for the cleaning result. Tests on this statement were carried out before HD cleaning with a wire dipped in phosphoric acid; no measurable changes were determined.
It is known that wire surfaces are cleaned by electrolysis in an acid bath. Cleaning is carried out at 20 volts pulsating alternating DC voltage in an acid solution. The detached foreign particles are rinsed off in the volume flow via filtration. Here too, the transport quantities of the foreign particles are the limit of the relative speed.
In this work, high-pressure steam with an electrical charge of 3 volts DC at a current of 0.08 amperes was used to safely clean the component surface of foreign particles. The experiments carried out are described below.
Technological parameters for cleaning any freely accessible component surfaces
The cleaning of cast aluminum surfaces, gray cast iron surfaces, rolled sheets and rolled, profiled sheets was carried out using high-pressure steam.
Cast aluminum
Die-cast parts removed from the machining process, contaminated with cooling lubricants and various chips, are cleaned using full jet high-pressure steam nozzles and flat high-pressure steam nozzles. The clear distance between the nozzle slot steam outlet and the surface of the base material is between 3 and 7 mm. The feed rate of the HP nozzle to the surface is between 30 and 150 mm/sec. The HP steam is generated in the steam generator at 200 °C at 8 bar. The feed water is slightly acidic from the water circuit, i.e. condensed purified steam from previous cleaning. The nozzle is heated to temperatures between 200 and 400 °C using a hot air nozzle. The HP nozzle is operated with and without an electrical connection of 2.5 volts DC.
Significant in all tests: In every test, the surface was cleaned of foreign particles. Cleaning at 200 °C and a voltage of 2.5 volts DC was the most effective. The mixture with heated compressed air was required in every case to transport the dissolved foreign particles. The alloy was essential for the experiments, i.e. Si and Mg contents in the aluminum were significant for the coloring of the base material surface in gold or black, the latter due to strong oxidation of the aluminum. Up to temperatures of 180 °C on the base material surface, the color and structure of the base material remained unchanged. The electrical charge was of little different effect up to these parameters.
If the base material is connected as a cathode, the diffusion of aluminum molecules becomes visible after 2 to 3 minutes, the surface becomes porous and material erosion begins.
The following temperature parameters must therefore be selected for these aluminum base materials, depending on the alloy: HP steam from the steam generator at 200 °C at 8.5 bar and post-heating of the HP nozzle by means of a hot air blower up to 250 °C air temperature. Depending on the layer thickness of the foreign particles and the type of cooling lubricant, a feed speed of up to 35 m/sec must be selected for cleaning. The local maximum surface heating of the base material is essential; embrittlement and discoloration must be excluded. Monitoring the parameters is absolutely essential in series production. If the relative speed is reduced to zero, the temperature input must be reduced immediately, i.e. a quick stop of the HP nozzle and a quick stop of the hot air heating in the direction of the base material surface to be cleaned would be required. The cleaned dry surface is metal-clean, i.e. the surface oxidizes as soon as it exits the high-pressure steam area. As the surface is reliably dry, there is little or no electrolytic oxidation. Only the humidity in the environment starts the oxidation. The cleaned Aluninum surface appears bright and radiant. Casting pores and hairline cracks are freed from grinding dust and foreign particles, i.e. hairline cracks are visible and pores are permeable to air.
Gray cast iron, cast steel
Cast parts removed from the machining process, contaminated with cooling lubricants and various chips, are cleaned using full jet HD steam nozzles and flat HD steam nozzles. The clear distance between the nozzle slot steam outlet and the surface of the base material is between 3 and 7 mm. The feed rate of the HP nozzle to the surface is between 30 and 150 mm/sec. The HP steam is generated in the steam generator at 200 °C at 8 bar. The feed water is slightly acidic from the water circuit, i.e. condensed purified steam from previous cleaning. The nozzle is heated to temperatures between 200 and 400 °C using a hot air nozzle. The HP nozzle is operated with and without an electrical connection of 2.5 volts DC.
Significantly in all tests, the surface was cleaned of foreign particles in every test. Cleaning at 400 °C and a voltage of 2.5 volts DC was the most effective. The mixture with heated compressed air was required in every case to transport the dissolved foreign particles. The alloy and the surface roughness were essential for the tests. If no heated ambient air is blown to the cleaning surface, the HP vapor is extracted from the cleaning area at a negative pressure of -200 mbar. Adjacent ambient air flows into the cleaning area, i.e. the flowing air also removes the dissolved foreign particles from the base material surface. A mechanical particle filter is installed directly in the work gun in the extraction area, where the "heavy" particles are separated. Light foreign particles are moved towards the condenser together with the vapors and filtered there. Purified and slightly acidic condensate flows back into the collection tank.
If the base material is switched as the cathode, the diffusion of molecules becomes visible after 2 to 3 minutes, the surface becomes porous and material removal begins. The feed rate of the cleaning process must be selected depending on the layer thickness of the foreign particles and the type of cooling lubricant. The local maximum surface heating of the base material is essential. Embrittlement and discoloration must be ruled out. It is essential to monitor the parameters during series production. If the relative speed is reduced to zero, the temperature input must be reduced immediately, i.e. quick stops of the HP nozzle and the hot air heating in the direction of the base material surface to be cleaned would be necessary.
Alternatively, it is possible to quickly increase the distance between the HP steam nozzle and the surface.
The cleaned, dry surface is metal-clean, i.e. the surface oxidizes as soon as it exits the high-pressure steam area. As the surface is safely dry, there is little or no electrolytic oxidation. Only the humidity in the environment starts the oxidation. The cleaned surface appears bright and radiant. Casting pores and hairline cracks are freed from grinding dust and foreign particles, i.e. hairline cracks are visible and pores are permeable to air. Graphite dust in the surface area is removed. Especially with gray cast iron, graphite diffuses from the core area of the base material. Cleaned gray cast iron components are gray and coated with graphite 2 to 3 hours after cleaning. If you wipe the cleaned surface with a cloth, graphite and rust-brown powder will be visible on the cloth. Cast steel is bright after cleaning and, depending on the air humidity, electrolytic oxidation begins through "rust film formation".
Rolled flat steel sheet, non-perforated and perforated
The statements made under points a and b also apply to these tests. Differences were measurable in the physical behavior of the flows. The perforations in the flat sheets have a significant influence on the distribution of the HP vapor flows as a reflection surface. Turbulence and free passage as well as undefined reflection surfaces alternate. This results in irregularly cleaned areas of the perforated surface. If the metal sheet is not perforated, the jet angle of the HP nozzle to the surface determines the cleaning quality. Here too, the transport of the dissolved foreign particles depends on the flow conditions. The flatter the inflow angle of the HP nozzle to the surface, the higher the injection of surrounding air, i.e. the greater the air flow drawn in. If a compressed air nozzle with heated air of up to 400 °C is connected to the HP nozzle, a surge is formed, a wave of foreign particles. This is moved along the surface towards the outer edge. Once there, it travels away from the surface and enters the environment in an undefined manner. This solution is not desirable. For this reason, targeted extraction using negative pressure down to -300 mbar with vapor extraction was used. This technology had its limits in terms of relative speed; above certain film thicknesses of the foreign particles, the kinetic energy to entrain the particles/oils was no longer available. As the consistency of cooling lubricants varies, it was determined in tests that surface cleaning must be tested for the required parameters in each case. Sheet metal surfaces with a length-to-width ratio where the length is greater than the width (strip material) can be cleaned easily and safely all the way around. The limit for HP steam cleaning is the requirement for foreign particle transport from the surface of the base material.
In tests, sample cleaning was carried out in the series production of aluminum strip materials up to 120m/min, or 2m/sec. The surfaces had test values of max. 2 % C content. According to the company, the cleaning result was comparable to evaporation in the oven. The surface temperatures were kept below 190 °C.
Profiled open and closed continuous profiles made of chrome/manganese/nickel alloyed sheet steel, V2A and aluminum sheet
The tests were divided into two sections, freely accessible outer surfaces and cavities and hollow surfaces (inner surfaces of tubes). Special tests were carried out with perforated square profiles made of V2A and aluminum sheet, used as glued spacers in thermal panes between the glass panes.
As a rule, profiles are manufactured endlessly, i.e. they are cut to length after profiling. It is therefore common practice to degrease the endless profile surface before cutting to length and to clean it of fine cutting chips that have formed before and during profiling. In addition, there are traces of welding. Cut profile lengths are cleaned again of cutting chips and cooling lubricants after cutting.
High-pressure steam cleaning or aqueous cleaning can be used for cleaning. HP steam cleaning is effective for cleaning cut-to-length profiles before delivery. The HP nozzle is only suitable for the profile contour to a limited extent. As a rule, a ring nozzle with a 3 to 5 mm gap to the profile contour is sufficient.
Even square box profiles can be safely cleaned with ring nozzles.
Rectangular box profiles are cleaned using ring nozzles with integrated full jet nozzles for the HP steam line in the direction of the profile surface. Examples of the various solutions are shown below.
The inner surfaces of open or perforated profiles are degreased, but the cleaned foreign matter is only removed from the interior to a limited extent. Additional heated compressed air nozzles are required here. Extraction of the dissolved foreign particles at -300 mbar can also be used in combination with vapor extraction systems. For example, perforated spacers made of V2A material must be run at a surface temperature of 200 °C so that the oil evaporates to reduce friction during profiling. If the profiles are made of aluminum, the permissible surface temperatures are required to ensure that the surface structures are preserved.
In open profiles, the flow lines must be determined so that the flow direction of the suction does not end in a dead water zone. The relative velocities from the running direction of the profiles and the direction of flow of the HP steam from the ring nozzle must be observed. If the directions are opposite, this can often result in walls of dissolved foreign matter, which then block the cleaning process. If the surface is severely undercooled after the profile has passed through the annular gap of the HP nozzle, water ball droplets are formed. These are highly electrically charged and have a very high surface tension, comparable to the mechanical properties of mercury balls. Blowing off with up to +500 mbar warm, dry air separates or rolls the water droplet balls from the component surface. This is then safely dry.
Plastics
As part of this work, plastic surfaces were cleaned in test trials using high-pressure steam. Systematic test series were not carried out. Therefore, only the basic application possibilities of high-pressure steam technology can be determined here. Further test series should be carried out if the applicability is of interest.
The key parameter for the application is the surface temperature of the base material. This is strongly dependent on the plastic material.
Teflon or Murthfeld green, for example, are very different materials, so the parameters of the cleaning process are also completely different in high-pressure steam technology. Rubber-like materials differ even more from other plastics in terms of the required parameters. The cleaning of surfaces using high-pressure steam was not systematically investigated in this study due to the scope of the investigation and the lack of demand in the industry.
Minerals and glasses
No investigations into the cleaning of minerals and glass using high-pressure steam were carried out as part of this work. It is known from practical work that cleaning processes with dry steam are possible. Wet steam cleaning agents are also possible, but leave a dried "water film" of condensate in the dry phase. The cleaning of cut silicate panes is possible using high-pressure steam.
Thoughts and drafts on cleaning methods for freely accessible component surfaces using superheated steam
The cleaning of freely accessible component surfaces using superheated steam is described in the following points in this paper. The continuous removal of the neutral, detached dirt film is essential for the success of cleaning in all the processes examined. Superheated steam alone is not able to reliably remove dissolved dirt particles from the component surface. The air flow from the environment, generated by the effect of the injector of the high-pressure steam nozzles or by medium-pressure air blowers with 50 to 300 mbar air pressure, is sufficiently reliable as a transport carrier. This air is accelerated in the HP steam nozzle and carries the loosened dirt particles in the air flow to the collection point. An optimum solution for extraction is the combination of vapor extraction from the cleaning point on the component surface. The vapors enriched with dirt particles are conveyed towards the collection tank by injectors and condense there. The dirt particles now dissolved in the water are safely filtered by the mechanical filtration in the circuit of the steam generator's filling pump.
The HP steam nozzle should have a distance of 3 to 7 mm to the component surface, not more than 30 mm. From a distance of 30 mm, the dry steam cools down to the temperature of wet steam, i.e. the steam becomes visible and has water droplets that remain on the component surface as water beads. If water droplets remain on the cleaned component surface, islands of residual dirt films form and the water hardness becomes visible. Drying then requires additional energy, heat or a stream of dry air.
The practical use of variants of this cleaning technology proves that the dirt film is removed from the component surface without the addition of chemical cleaners. After cleaning, the component surface is free of cleaning agents and the base material is directly adjacent to the environment. There is then the possibility of oxidation or renewed contamination with foreign particles.
External cleaning of thread-rolled V2A tubes directly after leaving the rolling machine. The "thread" outer jacket is cleaned in the two silver tubes of the box at a steam temperature of 450° C. Rolling oil and chips are 100% removed, the cleaned surface is dry and ready for shipment. The steam vapors are normally trapped in the box and are extracted there for condensation
