Heater protective tube for molten metal holding furnace

A molten metal holding furnace, the molten metal holding furnace comprising:

• a furnace body including a bottom wall, a ceiling wall, and a side wall extending between the bottom wall and the ceiling wall to form a molten metal containing space defined by the bottom wall, the ceiling wall, and the side wall, the furnace body including at least one insertion hole formed to extend through the side wall or the ceiling wall, the at least one insertion hole having an inside tapered cylindrical portion and an outside non-tapered cylindrical portion, wherein the inside tapered cylindrical portion is closer than the outside non-tapered cylindrical portion to the molten metal containing space;
• and
• a heater protection tube including a heat generator, the heater protection tube being inserted in the at least one insertion hole,
• wherein the heat generator generates heat for holding a molten metal contained in the molten metal containing space at a predetermined temperature,
• wherein the heater protection tube includes a distal tapered cylindrical portion and a proximal non-tapered cylindrical portion, the distal tapered cylindrical portion corresponding to the inside tapered cylindrical portion and the proximal non-tapered cylindrical portion corresponding to the outside non-tapered cylindrical portion,
• the heater protection tube is configured so that it can be mounted in the side wall with the distal tapered cylindrical portion located at the inside tapered cylindrical portion and with the proximal non-tapered cylindrical portion located at the outside non-tapered cylindrical portion, and
• wherein the distal tapered cylindrical portion of the heater protection tube has a discontinuously tapering outer diameter such that the outer diameter increases from inside to outside of the furnace.