Porous boiling surfaces

Figure 7-15 Linde porous boiling surface—mechanism of operation. PHYSICAL PROPERTIES...

Suction involves either vapor removal through a porous heated surface in nucleate or film boiling, or fluid withdrawal through a porous heated surface in single-phase flow. 

Bankoff [343] suggested that heat transfer coefficients in film boiling could be substantially improved by continuously removing vapor through a porous heated surface. Subsequent experimental work [344, 345] demonstrated that coefficients could be increased by as much as 150 percent, provided that a porous block was placed on the surface to stabilize the flow of liquid toward the surface. Wayner and Kestin [346] extended this concept to nucleate boiling and found that wall superheats could be maintained at about 3 K (5.4°F) for heat fluxes over 300,000 W/m2 or 95 x 103 Btu/(h-ft2). This work was extended by Raiff and Wayner [347], The need for a porous heated surface and a flow control element appears to limit the application of suction boiling. 

M. Fujii, E. Nishiyama, and G. Yamanaka, Nucleate Pool Boiling Heat Transfer from Micro-Porous Heating Surfaces, in Advances in Enhanced Heat Transfer, J. M. Chenoweth, J. Kaellis, J. W. Michel, and S. Shenkman eds., pp. 45-51, ASME, New York, 1979. 

Webb, R. L., "Nucleate Boiling On Porous Coated Surfaces", Heat Transfer Engineer-... 

Diazomethane is easily decomposed by rough surfaces for this reason glass apparatus with scratches and also porous pot ( boiling stones ) should not be used. 

A Type II isotherm indicates that the solid is non-porous, whilst the Type IV isotherm is characteristic of a mesoporous solid. From both types of isotherm it is possible, provided certain complications are absent, to calculate the specific surface of the solid, as is explained in Chapter 2. Indeed, the method most widely used at the present time for the determination of the surface area of finely divided solids is based on the adsorption of nitrogen at its boiling point. From the Type IV isotherm the pore size distribution may also be evaluated, using procedures outlined in Chapter 3. 

Ca.ustic Corrosion. Concentration of caustic (NaOH) can occur either as a result of steam blanketing (which allows salts to concentrate on boiler metal surfaces) or by localized boiling beneath porous deposits on tube surfaces. 

The three principal concentration mechanisms postulated as being responsible for on-load corrosion processes by Mann are dry-out, concentration in crevices, and concentration in porous deposits. (Clean boiler tube surfaces on which high-pressure water is boiled under forced convection do not develop concentration factors of more than about two.)... 

Decontaminate clothing with steam or by boiling. Twenty percent alcoholic caustic soda is effective on materials, but may damage it. Porous surfaces, such as earth, are very difficult to decontaminate. 

To test for volatility in steam, heat a small sample of the substance to boiling (porous pot ) in a test tube with about 2 c.c. of water and hold the bottom of a second test tube containing some ice in the issuing vapours until a drop of water has formed on the cold surface. A turbidity in the drop indicates that the substance is volatile with steam. 

There is also another topological limit on C H ratio. Physisorption proceeds only in the monolayer above the boiling point of hydrogen, and the adsorption must follow the Langmuir isotherm. Hence, the storage capacity depends on the pressme of Hj gas above the carbon surface at a fixed temperatme. It is also greatly limited by available SSA of carbon phase as provided by porous stmctme. 

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