EFFECT OF VACUUM ON THE MASS FLOW CHARACTERISTICS OF A HORIZONTAL LIQUID NITROGEN TRANSFER LINE

摘要

This paper presents a study of the effect of vacuum level on the mass flow characteristics of a liquid nitrogen transfer line held in the horizontal orientation. A demountable transfer line has been designed and fabricated allowing experiments under varying jacket vacuum conditions. The experiments made were LN(2) carrying pipe exposed to atmosphere LN(2) carrying pipe enclosed by jacket at varying vacuum levels LN(2) carrying pipe enclosed by jacket filled with carbon dioxide at atmospheric pressure. When the liquid line was enclosed by an outer jacket with air at atmospheric pressure, the interspace pressure fell approximate to 100 mbar below the atmospheric pressure, as the line cooled to 77 K. Under these conditions, the liquid mass flow rate was higher (and the cool-down time less) than that of a liquid line directly exposed to atmosphere. Experiments have been performed with coarse vacuum (800-1 mbar), medium and high vacuum (10(-1)-10(-5) mbar) in the jacket for different LN(2) supply dewar pressures. For coarse vacuum conditions, liquid mass flow rates were high as compared to high vacuum conditions for 200-300 s after the start of cool-down, but the flow rate decreased on reaching the steady state. Improvement in jacket vacuum resulted in increase of the steady state liquid mass flow rates. Thus, it is necessary to optimize the level of jacket vacuum for a particular application. Experiments were made with the interspace filled with CO2 at atmospheric pressure. During LN(2) transfer, CO2 condensed to a vacuum of about 10(-3) mbar. The liquid fraction mass flow rate was initially very low before the CO2 condensed. After CO2 condensation the liquid fraction mass flow rate increased becoming comparable to that of a vacuum insulated transfer line. Copyright (C) 1996 Elsevier Science Ltd [References: 6]