23 HEAT EXCHANGERS, PARTICULARLY AIR COOLERS SUCH AS USED IN CONNECTION WITH COMPRESSED AIR IN REFINERIES, ETC., OIL COOLERS SUCH AS USED IN COOLING LUBRICATING OIL, QUENCHING OIL, FUEL OIL, ETC., WATERCOOLERS FOR JACKET WATER FOR ENGINES AND THE LIKE; AIR AND GAS COOLERS FOR USE IN CONNECTION WITH COMPRESSORS, BOOSTERS, OR BLOWERS; GAS AND STEAM CONDENSERS