Conductive and radiative means the energy/heat transfer method, mainly there are 3 methods: heat conduction, heat radiation and heat convection.
Heat conduction is a phenomenon that occurs through the interaction of neighboring atoms and molecules, transferring their energy/heat to their neighbors. So conductive heating in a freeze dryer means: the material trays and heating plates touch each other.
Heat radiation transfer of heat by infrared waves. All matter with a temperature greater than subzero emits thermal radiation, when it hits the cooler thing, they make the molecules of the cooler object speed up (becomes hotter). Radiative heating doesn’t need matter or medium to be present, so it fits heating between 2 items that with a distance.
Because conductive heating principle: trays direct load on shelves. so, trays loading need manually. Smaller FD with less trays quantity is ok, but for large FD will hundreds trays, is not convenient.
Large FD use radiative heating. To reduce labor and save time, the trays quick loading/unloading assist by trolleys, after placed in drying chamber, trays will be suspending between 2 heating plates, because radiative heating no require 2 item touch together.