STP1561

    Pressure Dependence of Aluminum Ignition in Gaseous Oxygen and Possible Ignition Mechanisms in Brazed Aluminum Heat Exchangers

    Published: Nov 2012


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    Abstract

    The ignition of aluminum foils in gaseous oxygen was experimentally tested using a diode laser as the energy source, which provided a well-controlled, accurate, and reproducible method of ignition. The tests were conducted under different conditions in terms of oxygen pressure, oxygen purity, aluminum thickness, and gas velocity. The aluminum foils tested were between 0.2 mm and 0.45 mm thick, a range typical of fins contained in brazed aluminum heat exchangers (BAHXs) used in air separation units (ASUs). The experimental apparatus was composed of a pressure vessel in which a single aluminum test sample was placed. The vessel contained an optical window that allowed a short laser pulse of known power to be applied to the aluminum sample. The energy dose was systematically varied in order to identify the threshold ignition energy, defined as the point at which the probability of aluminum combustion with propagation beyond the laser spot was 50 %. The experimental results show that O2 pressure has no significant effect on the ignition energy of aluminum over the pressure range tested (10 bar to 120 bar). This conclusion holds for both standard commercial grade purity O2 (99.8 %) and high purity O2 (99.99 %), as well as for gas velocities higher than typically encountered in ASU BAHXs. Heat conduction calculations indicate that aluminum ignition occurs when the laser spot temperature reaches the melting point of the passivating oxide layer (about 2200 K to 2300 K). The heat conduction model accurately explains the dependence of the ignition energy on the aluminum sample thickness. These test results have been used in assessing the risk of ignition of BAHXs used in high pressure oxygen service in ASUs.

    Keywords:

    oxygen, ignition, flammability, aluminum, air separation unit


    Author Information:

    Crayssac, Frédéric
    CRCD, Air Liquide R&D,

    Rostaing, Jean Christophe
    CRCD, Air Liquide R&D,

    Werlen, Etienne
    Air Liquide E&C Cryogenics Standard Plants,

    Sun, Lianming
    Air Liquide Engineering,

    Houghton, Patrick
    Air Products and Chemicals, Inc., Allentown, PA

    Kleinberg, William
    Air Products and Chemicals, Inc., Allentown, PA

    Coste, Frédéric
    PIMM Laboratory CNRS, Arts & Métiers ParisTech, Paris,


    Paper ID: STP20120006

    Committee/Subcommittee: G04.01

    DOI: 10.1520/STP20120006


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