Comparison of different materials and types of packing for use in heat accumulators

Abstract

Comparisons are made between different materials (basalt, quartzite, quartz, dolomite, gravel, pebbles, carborundum, heat-resistant glass, high-temperature ceramics, steel, and aluminum) and different types of packings (granulated materials, spherical particles, Raschig rings, and twisted rings) for their possible use as packings in heat accumulators. It is found that within the chosen temperature range nearly all of the above materials have roughly the same average volumetric heat capacity as the granulated basalt currently used in heat accumulators and, based on existing specifications, can also be recommended for use as such packings.Heat accumulators are part of the heat recovery loop in the integrated cleaning systems (ICSs) of medium-and highcapacity air separators (ASPs).The function of the heat accumulators is to accumulate heat as the processes of cooling, pressurization, and depressurization take place in one of two alternately operating adsorbers; the stored-up heat is then removed from the accumulator as heating and regeneration take place inside the adsorber. The heat is stored in a heat-absorbing packing made of a material such as granulated basalt. The packing is heated by a regenerating gas -waste nitrogen that has first been passed through an electric heater (with a temperature 20-25°C higher than the regeneration temperature of the adsorbers). The packing is cooled by cold regenerating gas. The heat-storing packing is thus continually subjected to temperatures that fluctuate within a wide range of values (from 280 to 490 K), which means that it must meet certain service requirements.Due to the cyclic alternation of regimes involving the accumulation and removal of heat, the packing is subjected to appreciable dynamic mechanical and thermal loads. Also, most of the packing must operate at high temperatures -up to 490 K. The velocity of the gas flow in the accumulator can change by a factor ranging from three to four as the operating regime changes, while the packing's temperature can fluctuate by 160-180 K.The packing cannot be allowed to wear and break down during the operation of the ICS because the resulting dust and small pieces of packing material will enter the adsorbers during the heat-removal regime and disrupt their normal operation.In accordance with the above, the packing that is used in heat accumulators should have the following properties: high strength under dynamic mechanical loads; low abradability, depending on the material's hardness; high resistance to thermal loads. The material should also have a fairly high volumetric heat capacity in order to keep the size of the accumulator acceptable.