Patrick Alan Houghton

Upflow Boiling and Condensation in Rectangular Minichannels

When boiling or condensation occurs inside very small and non-circular channels, capillary forces influence two-phase flow patterns, which in turn determine heat transfer coefficients and pressure drop. A better understanding of the underlying phenomena would be beneficial from the perspective of optimizing the design of compact evaporators and condensers. The thrust of this study was to understand the nature of up-flow boiling and condensation heat transfer in channels with a small gap. It consisted of two parts. The first part included observation of two-phase flow patterns with refrigerant R21 in a test section containing plain fins. The shape of the channels formed between fins was close to rectangular. The test section was placed in a closed refrigerant loop, and it was fabricated with a transparent wall to allow observation of the flow. An electrically heated coil was used to introduce liquid and vapor at the needed quality into the test section. Regimes of slug, froth, annular and cell flow patterns were recognized and the areas of flow pattern were determined. The second part included up-flow boiling and condensation heat transfer measurement with refrigerant R21 in a set of vertical mini-channels consisting of plain fins. An aluminum fin pad was bonded to two dividing aluminum sheets by dip brazing. Heat was supplied to the test section from a thermoelectric module, which utilized the Peltier effect. A thick copper plate was placed between the dividing sheet on each side of the fin passage and the respective Peltier module to establish a uniform wall temperature. Heat transfer coefficient measurements were done under forced flow conditions. Data are obtained for mass flow rates of 30 and 50 kg/m2 s under both boiling and condensation modes with wall superheats ranging from 1 to 5K. The dependence of heat transfer coefficient from wall superheat was not observed both for boiling and condensing modes. It shows the primary role of evaporation from thin films in a confined space when the mass flux is small. At low vapor quality the boiling heat transfer coefficients are considerably higher than that for condensation. A high heat flux in ultra thin liquid film area near the channel corner or in the vicinity of liquid-vapor-solid contact line (after the film rupture) supports the high total heat transfer coefficient in evaporation mode. In contrast with evaporation mode, at upflow condensation mode the heat transfer coefficient is strongly dependent on vapor quality. At plug flow regime the vapor velocity determines the condensing heat transfer.Copyright

Sealed Aluminum Cavity Reactions when Submerged in Pure O 2 Reboiler Sump

Aluminum has a long history of safe service in cryogenic air separation units; however, there have been some rare instances of aluminum/O2 reactions. Aluminum ignition and propagation depends strongly on the oxygen purity, oxygen pressure, aluminum geometry, and type and energy of igniter. Over the years, the industry has experienced a particular type of aluminum/O2 reaction: cavity incidents. These incidents are characterized by the presence of a sealed cavity that is created when welding two metal items together. Such a sealed cavity can lead to aluminum ignition under certain specific conditions: (1) The sealed cavity is submerged in a bath of liquid oxygen. Over a long period of time, liquid cryogen can enter the sealed cavity through slight imperfections in the weld. (2) The sealed cavity is warmed over a short period of time, typically a few hours. (3) The liquid oxygen vaporizes, and the pressure inside the cavity builds to very high levels (potentially over 50 bars). (4) Ignition occurs in the high pressure, high purity oxygen environment, and the resulting aluminum/oxygen reaction burns through the relatively thick cavity walls. (5) The oxygen and reaction products exit the cavity through the hole, lowering the pressure and extinguishing the reaction. This paper discusses two such incidents, which have occurred since 2001. Both incidents took place in brazed aluminum heat exchanger (BAHX) reboiler support beam systems. In both cases, the BAHX reboiler was damaged, leading to a process leak, which required that the plant be repaired. In one case, the damage occurred while warming a plant. In the second case, the damage occurred during normal operation. This second case does not appear to follow the sequence of events outlined above; however, evidence is presented to support the scenario that the cavity incident occurred during a previous warming of the plant. This initial damage then impaired the normal operation of the reboiler, leading to a second hydrocarbon related reaction during normal operation. The paper discusses the potential ignition mechanisms, probable causes of the incidents, and methods to prevent future re-occurrences.