Fluid sprinkling – spraying – and diffusing – With heating or cooling means for the system or system fluid – Heating means
Reexamination Certificate
2000-03-10
2001-10-09
Scherbel, David A. (Department: 3752)
Fluid sprinkling, spraying, and diffusing
With heating or cooling means for the system or system fluid
Heating means
C239S135000, C392S396000, C392S399000, C122S040000
Reexamination Certificate
active
06299076
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to cleaning systems in which a jet of heated steam is directed onto a workpiece to remove contaminants.
DESCRIPTION OF THE PRIOR ART
It is known to direct steam onto a workpiece to rid it of contaminants. In certain systems of the prior art the steam is developed by pumping water into the internal cavity of a “steam pot” or generator. An integral heater raises the temperature of the cavity walls and the resultant heat transfer raises the water temperature to a critical point characterized by conversion or “flashing” of the water into steam.
The flashing of water into steam is adversely affected by a phenomenon known as the Leidenfrost effect, in which water globules or droplets dance or skitter across the hot cavity walls. The lack of sufficient contact between the levitated water droplets and the heated surfaces reduces thermal conduction of heat to the water.
Empirical studies show that the heating of a droplet of water to a certain temperature develops an insulating vapor layer under the droplet after it impacts with the heated surface. This forms a vapor layer or pressure field at the bottom of each droplet. The pressure between the droplet and the heated source builds to the point that the droplet moves away from, or is levitated above the heated surface, i.e. the weight of each droplet is buoyed by the pressure field beneath the droplet. This results in only intermittent contact of the droplet with the heated surface and delays complete vaporization of the droplet.
Certain prior art studies investigated the effect of cavity surface roughness on the fluid dynamics of the vapor layer under the levitated droplets. One such study was directed to surface roughness characterized by prominent rectangular, semi-cylindrical or triangular surface irregularities. The study suggested that impingement of the droplets against such irregularities reduced the thickness or rate of flow of the vapor layer, and thereby promoted more continuous contact between the water droplets and the cavity walls.
For example, U.S. Pat. Nos. 4,414,037 and 5,471,556 teach the formation of irregular surfaces by etching or grooving of the cavity surfaces. This was apparently intended to enhance collision of the water droplets with such surfaces to enhance their flashing into steam. The '037 patent describes such surfaces as non-uniform internal surfaces formed by overnight chemical etching as deep as possible, in the order of {fraction (1/32)} inch to ⅛ inch. The '556 patent describes the surfaces as having a plurality of ridges and grooves, the height and depth of which vary substantially randomly, and a typical depth in the order of 0.030 inch to 0.050 inch.
Prior art Pat. Nos. 2,652,645; 3,218,141; and 3,721,802 also teach container flashing surfaces with rough or irregular features.
However, the formation of cavity wall surfaces into randomly configured surface irregularities is time consuming, technically demanding and yields randomly variable results. In contrast, the present invention does not adopt prior art techniques that involve impingement of the water droplets against irregular cavity wall surfaces. Instead, there is impingement against relatively smooth or uniform surfaces formed of material having interconnecting pores or passageways. These are operative to carry away vapor and thereby reduce the thickness of the vapor layer under the droplets.
SUMMARY OF THE INVENTION
The cavity walls are typically made of relatively high heat conduction material such as an aluminum alloy which is machined or otherwise fabricated to provide surfaces adapted to accept a coating or layer of porous non-corrodible material such as stainless steel.
Although all of the wall surfaces of the cavity can be coated, coating only the lower half of the cavity has been found to provide good results.
The coating is preferably very thin and made of stainless steel, although other materials can be used if desired for particular applications.
It is theorized that the small interconnecting pores which characterize the porous coating serve as escape passages for the heated vapor which is generated beneath the water droplets when they impinge upon the heated surfaces within the cavity. The heated water droplets are apparently levitated or supported by the heated vapor layer. With the pores serving as escape passages for this vapor layer, the thickness or pressure of the layer supporting the water droplets is reduced and the droplets then move into closer thermal relationship with the heated cavity walls. This greatly improves the conversion or flashing of water droplets into steam compared to the systems of the prior art.
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“Leidenfrost Temperature—It's Correlation for Liquid Metals, Cryogens, Hydrocarbons and Water” Baumeister/Simon May 1973 Journal of Heat Transfer.
“On the Effect of Surface Roughness on the Vapor Flow Under Laidenfrost-Levitatatech Droplets” Prat/Schmitz Jun. 1995 vol. 117 Journal of Fluids Engineering.
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Bellegarde Eugene L.
Sloan Jeffrey E.
McLellan Joseph F.
Nguyen Dinh Q.
Scherbel David A.
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