See Cooling Unit (How it works) to see what a perk tube does. The purpose of this page is to show the difference between the old style perk tube and the new style perk tube (more accurately: new style boiler) shown in "Cooling Unit (How it works)". The old style perk tube was simply a pipe with the element holder and the chimney welded directly to it for heat transfer. It didn't have the benefit of an outer pipe to isolate it from the heat sources.
The old style perk tube worked flawlessly when the refrigerator was operated properly. It did, however, tend to plug up sooner than the new style boiler in adverse situations like being run out of level. It should be noted that the new style boiler only helps slow perk tube problems, not prevent them.