The question is asking which dryer design keeps its shelves or trays physically moving while drying happens, rather than drying one fixed batch at a time.
- Vacuum dryer: Works on one static charge of material at reduced pressure and temperature. Nothing moves through it continuously, so it is a batch type.
- Tray dryer: Uses stacked, stationary trays inside a cabinet with circulating hot air. Since the trays do not travel through the unit, this is also a batch dryer.
- Spray dryer: Converts a liquid or slurry feed directly into dry powder by spraying it into hot air. There are no trays involved at all, continuous or otherwise.
- Turbo tray dryer: Its trays are mounted on a mechanism that keeps them shifting position through the heated chamber, giving continuous, uninterrupted drying as new material is loaded and dried material is removed.
The defining feature the question asks about, shelves that keep moving during drying, only applies to one design here.
So the correct answer is Turbo tray dryer.