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

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 ho rizontal orientation. A demountable transfer line has been designed an d fabricated allowing experiments under varying jacket vacuum conditio ns. The experiments made were LN(2) carrying pipe exposed to atmospher e LN(2) carrying pipe enclosed by jacket at varying vacuum levels LN(2 ) carrying pipe enclosed by jacket filled with carbon dioxide at atmos pheric pressure. When the liquid line was enclosed by an outer jacket with air at atmospheric pressure, the interspace pressure fell approxi mate to 100 mbar below the atmospheric pressure, as the line cooled to 77 K. Under these conditions, the liquid mass flow rate was higher (a nd the cool-down time less) than that of a liquid line directly expose d to atmosphere. Experiments have been performed with coarse vacuum (8 00-1 mbar), medium and high vacuum (10(-1)-10(-5) mbar) in the jacket for different LN(2) supply dewar pressures. For coarse vacuum conditio ns, liquid mass flow rates were high as compared to high vacuum condit ions for 200-300 s after the start of cool-down, but the flow rate dec reased on reaching the steady state. Improvement in jacket vacuum resu lted in increase of the steady state liquid mass flow rates. Thus, it is necessary to optimize the level of jacket vacuum for a particular a pplication. Experiments were made with the interspace filled with CO2 at atmospheric pressure. During LN(2) transfer, CO2 condensed to a vac uum of about 10(-3) mbar. The liquid fraction mass flow rate was initi ally very low before the CO2 condensed. After CO2 condensation the liq uid fraction mass flow rate increased becoming comparable to that of a vacuum insulated transfer line. Copyright (C) 1996 Elsevier Science L td