In a vacuum dehydrator material to be dehydrated is deposited as a thin film on the underside of an endless steel conveyer band 36 which moves past a number of electric radiant heaters 51, 52 in a cylindrical vacuum chamber 21, the band being supported on two drums 34, 35, the former being water-cooled and driven by a motor 40 while the latter is heated by steam and is arranged as a tensioning and centring drum. For these purposes drum 35 is mounted for movement towards and away from the cooling drum 34 and is also tiltably mounted and provided with means (see Group XXX) for automatically adjusting the position and or tilt of the drum to maintain the tension and lateral position of the band within predetermined limits. Should these limits be exceeded, means is further provided for switching off the driving motor 40 and reducing the heating effect of the heaters 51, 52, as described below. The material is fed to the band by a feed drum 40 which is carried by a pivoted plate 282, Fig. 7, and dips into a trough 47 containing the material in paste form. A roller 48 guides the band over the drum 46 so that a layer of material is received on the under face of the band. The thickness of this layer is determined by the spacing between the band and the feed drum 46, this being adjusted by manually rotating a shaft 292, Fig. 7, which adjusts the position of a stop 297 with which a projection 284 on the plate 282 engages. A spring 289 urges the projection 284 against the stop 287 so that drum 46 can move downwardly should a lump of the material come between it and the band. After traversing the heaters 51, 52 on the lower run, the heating drum 35, and further heaters 51, 52 on the upper run the material is cooled as it passes around the cooling drum, the cooling being further improved by the provision around the chamber 21 of a water cooling jacket 297 which prevents heat reaching the discharge end of the chamber. The material is discharged after cooling by a doctor blade 53 which is adjustably mounted by a slotted holder 333, Fig. 14, on a mounting plate 330 which is secured to a sleeve 327 and cooled by water circulating through a chamber 332. An alternative arrangement for cooling the blade is also described (Fig. 15, not shown). The blade 53 is oscillated transversely of the band 36 by a motor 227 driving an eccentric and linkage. The blade 53 may also be swung away from the band 36 by water-operated cylinders 363 adapted to cause rotation of the sleeve 327. In place of the blade 53, a wire brush 348, Fig. 8, may be used, this being mounted on spring blades 355 and provided with a pole piece 358 which is acted on by an electromagnet 356 through which an intermittent current is passed to cause vibration of the brush. The material discharged by the blade 53 or brush 348 falls into a transverse trough 305 along which it may be propelled in either direction by a screw 306 driven by a reversible electric motor 308, the arrangement being such that the material can be delivered through either of two outlets 311 as desired to be received in a container 312. In operation, as one container 312 is being filled a previously filled container 312 is being replaced at the other outlet by an empty container. Cleaning and clearing devices. Material which may have found its way on to the inner surface of the band beyond the discharge position is removed by a scraper 58 of V-shape which is carried pivotally by arms 61 pivoted to brackets 62 on the chamber 21. Two cylinders 76 are connected in parallel for simultaneous operation to tension the band, or to relieve tension in the band, by respective conduits 80, 192, Fig. 16, leading from water reservoirs 181, 181 respectively. Air under pressure is adapted to be supplied to either reservoir to tension or relieve tension in the band. The supply of air to reservoir 181 is via a flow-restricting valve such that should tension in the band become excessive pressure-sensitive switches 186, 187 in a control circuit for motor 40 which includes a starter switch 209 operated by a solenoid 218 in the control circuit. Upon opening of this circuit the switch 209 opens and through opening of its contacts 223 locks out both this control circuit and a further control circuit comprising two solenoids 261 normally maintaining closed switches 258 controlling saturable reactors 248, 254 in the supply to the heaters 51, 52. As a result the current to the heaters is reduced. The current to the heaters is also reduced if the vacuum in the chamber 21 should weaken since vacuum-operated switches 257 in series with switches 258 are then opened. In addition, the motor 227 vibrating the blade 53 is stopped since its starter switch 235 is also in circuit with the contacts 223 of switch 209. The control circuit which includes pressure-sensitive switches 186, 187 also includes switches 264, 265, 266, 267 <PICT:0850077/III/1> <PICT:0850077/III/2> <PICT:0850077/III/3> <PICT:0850077/III/4> <PICT:0850077/III/5> <PICT:0850077/III/6> disposed at the belt edges in association with the drums 34, 35 and which are normally closed but which open if the belt wanders excessively on the respective drum.