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Tray dryers (Practical Action Technical Brief)
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== The Semi-continuous Tray Dryer == Semi-continuous cabinets were developed by Practical Action in order to overcome some of the disadvantages of the batch system. In a semi-continuous cabinet a lifting mechanism allows all of the trays except the bottom tray to be lifted. It is thus possible to remove the lowest tray as soon as the product is dry. The mechanism then allows all the trays to be lowered (now tray 2 is at the bottom of the stack). This leaves a space at the top of the stack to load a tray of fresh material. Two types of lifting mechanism are available both of which activate four movable fingers that lift the second tray upwards. One design is operated by a handle which is pulled downwards. The other design, developed in Sri Lanka, has been found more suitable for use by women and here the lifting mechanism is a car screw jack which, on winding up, lifts the four fingers. The advantages/disadvantages of this system are: [[File:tray dryers tray lifting mechanism .jpg|thumb|Figure 4: The tray lifting mechanism on a semi continuous tray dryer.]] * over-drying is avoided<br> * product quality is higher<br> * fuel efficiency is considerably increased<br> * a higher daily throughput is possible<br> * the cabinet is however more expensive to construct<br> * labour costs are higher due to loading and unloading trays at regular intervals<br> * in order to maximise output 24 hour working is recommended<br> Typical semi-continuous dryers are shown in Figure 3, which shows the lifting mechanism and the gap above the lowest tray that is ready for removal. Detail of lifting fingers<br> # External lifting mechanism # Main lifting lever pivot support # Vertical lifting rods # Lifting fingers # Trays with angle iron edge bearers # Inner wall of drying cabinet # Lifting finger pivot support
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