Abstract
A heat-treatment installation with a plurality of transport lines disposed one above the other, trays with two wheels and a latch, lifts for lifting a tray from a lower transport line to another upper transport line, rails on which the trays travel, and transmission belts with counter-latches cooperating with the latches of the trays where, at each lift, each rail has a gate arranged to open when a tray is raised and to close when the tray is above the rail.