Figure 4.1 Tray loading sequence for Buttner'Favorit'dryer Chat drawal of tray from drying Reintroduction of tray into second drying zone ing chamber and emptying
II LI- - _- v 44 z y 3 I Sliding the tray into the drying chamber ---I -d L
The tray resting on the lifting frame is charged with fresh product, lifted to the top position, and slid into the chamber by operating a hand mechanism. The lifting frame slides down again to the middle, ready to support the tray immediately above the intermediate heater. This tray is drawn out and moved down a little further. The product on the tray is riffled nd the tray pushed back into the chamber- this time below the heating surface. The lifting frame slides down to receive the lowermost tray of the second stack. It then moves a little way upwards and is slightly tilted so that the now dried product can be discharged. The emptied tray is moved to the bottom position and the new drying cycle commences See Fig 4.1) The drying progression, therefore starts at the top of the dryer where he higher temperature prevails, and finishes in thecool zone at the bottom, from where the tray with the dried product is drawn out and discharged. steam requirements are 600kg per hr at 7 bar for maximum throughput, th electrical load is 7KW for the heater fan and 0.7KW for the lifting device Double Through-Flow Dryer Thisis a demonstrably more sophisticated through-flow dryerdesigned by Mitchell Dryers Ltd, with twice the output of the single chamberdryer, and withless labourrequirement This is a semi continuous dryer, comprising two drying chamber each housing 10 perforated trays 3m by 2m by 150mm deep. The product filled trays travel automatically through the drying chambers at a rate commensurate with optimum drying and product quality(See TABLE 4.1) The primary drying chamber is designed for total rejection of the saturated air. The circulation fan is mounted directly above the heater batteries at the rear of the chamber and discharges the heated air into a bottom plenum hamber and the air is then directed vertically upwards through the stack of trays to the top and into the discharge hood to be ducted away to atmosphere The circulation fan in the primary unit handles 400cu m of air heated to 150C maximum. The heater has a maximum heat output of 2 350, 000 BTUs r hour when using steam at 2.72atm The second chamber air circulation is provided by a fan handling 270cu m of air. The trays are automatically advanced from the bottom to the top by four hydraulic lifting jacks, connected to the lifting frame. The trays are indexed to move into their drying position automatically At the bottom and top of the main framework there is a roller conveyor system upon which the loaded trays travel from the first chamber to the second (a)to enable an operator to examine the product at the intermediate stage of drying, and riffle over the product to effect a surface change before entering the cool chamber, and (b)on the bottom roller conveyor, to discharge
The tray resting on the lifting frame is charged with fresh product, lifted to the top position, and slid into the chamber by operating a hand mechanism. The lifting frame slides down again to the middle, ready to support the tray immediately above the intermediate heater. This tray is drawn out and moved down a little further. The product on the tray is riffled and the tray pushed back into the chamber - this time below the heating surface. The lifting frame slides down to receive the lowermost tray of the second stack. It then moves a little way upwards and is slightly tilted so that the now dried product can be discharged. The emptied tray is moved to the bottom position and the new drying cycle commences See Fig 4.1). The drying progression, therefore starts at the top of the dryer where the higher temperature prevails, and finishes in the 'cool' zone at the bottom, from where the tray with the dried product is drawn out and discharged. Steam requirements are 600kg per hr at 7 bar for maximum throughput, the electrical load is 7KW for the heater fan and 0.7KW for the lifting device. Double Through-Flow Dryer Thisis a demonstrably more sophisticated through-flow dryer designed by Mitchell Dryers Ltd, with twice the output of the single chamber dryer, and with less labour requirement. This is a semi continuous dryer, comprising two drying chambers each housing 10 perforated trays 3m by 2m by 150mm deep. The product filled trays travel automatically through the drying chambers at a rate commensurate with optimum drying and product quality (See TABLE 4.1). The primary drying chamber is designed for total rejection of the saturated air. The circulation fan is mounted directly above the heater batteries at the rear of the chamber and discharges the heated air into a bottom plenum chamber and the air is then directed vertically upwards through the stack of trays to the top and into the discharge hood to be ducted away to atmosphere. The circulation fan in the primary unit handles 4OOcu m of air heated to 150°C maximum. The heater has a maximum heat output of 2,350,000 BTUs per hour when using steam at 2.72atm. The second chamber air circulation is provided by a fan handling 270cu m of air. The trays are automatically advanced from the bottom to the top by four hydraulic lifting jacks, connected to the lifting frame. The trays are indexed to move into their drying position automatically. At the bottom and top of the main framework there is a roller conveyor system upon which the loaded trays travel from the first chamber to the second (a) to enable an operator to examine the product at the intermediate stage of drying, and riffle over the product to effect a surface change before entering the cool chamber, and (b) on the bottom roller conveyor, to discharge 71
the product, clean the trays and recharge them before they re-enter the rma The trays are emptied pneumatically by an air-hose which lifts the product into a hopper, thence feeding into the conditioning bins. The air is discharged through a cyclone. If more than one unit is installed, the pneumatic emptying device can be connected to a common duct, providing discharge points from several dryers TABLE 4.1 THROUGHPUTPERFORMANCES OF THEMTCHELL DRYERS THRUFLO DOUBLE TRAYDRYER Thruflo drying units may be used where an intermediate level of production is required. They have been designed to give a good degree of automation to batch drying and to provide the facility for a staged drying e as used in conveyor band drying, thus improving efficiency and output as well as providing a high quality product These semi continuous dryers employ a through circulation of drying air and comprise two drying chambers each housing 10 perforated trays measuring 3m by 2m by 150mm deep. There are transfer zones between the two compartments where the trays are loaded and emptied, and also the facility to agitate the material on the trays part way through the drying cycle, which helps ensure more even drying The product filled trays travel automatically through the drying chambers at a rate commensurate with optimum drying and pr TYPICAL PREPARED FEED RATES A 310kg/hr M shrooms 40kg/h Beans, French 500 570 Carrots 630 Potatoes Cabbage Pe Cauliflower 630 Parsley 330 630 630 Leek 53
the product, clean the trays and recharge them befoE they re-enter the primary chamber. The trays are emptied pneumatically by an air-hose which lifts the product into a hopper, thence feeding into the conditioning bins. The air is discharged through a cyclone. If more than one unit is installed, the pneumatic emptying device can be connected to a common duct, providing discharge points from several dryers. TABLE 4.1 7HROUGHF'DPERFORZ"CES OFl7EMITcKELL DRYERS THRWLQ DOUBLETRAYDRYER Thruflo drying units may be used where an intermediate level of production is required. They have been designed to give a good degree of automation to batch drying and to provide the facility for a staged drying technique as used in conveyor band drying, thus improving efficiency and output as well as providing a high quality product. These semi continuous dryers employ a through circulation of drying air and comprise two drying chambers each housing 10 perforated trays measuring 3m by 2m by 150mm deep. There are transfer zones between the two compartments where the trays are loaded and emptied, and also the facility to agitate the material on the trays part way through the drying cycle, which helps ensure more even drying. The product filled trays travel automatically through the drying chambers at a rate commensurate with optimum drying and product quality. TYPICAL PREPARED FEED RATES Asparagus 3lOkg/hr Mushrooms 54Okg / hr Beans,French 500 " Onions 570 " carrots 630 " Potatoes 710 " Celery 650 " Peas 470 " Cabbage 450 " Peppers 600 " Cauliflower 550 " Parsnips 630 " Cloves 630 " Parsley 330 " Ginger 630 " Swedes 630 " Garlic 570 " Spinach 270 " Leeks 530 " 72
CONVEYOR BAND DRYERS- SINGLE PASS Reference has been made, in Chapter 2, to this type of dryer, and an ideal size unit for medium scale operation is a dryer 30-40m in length, with a conveyor width of 2.5-3m The conveyor band dryer is used in many industries outside food dehydration, and has been standard equipment in the textile, chemical and tobacco industries for many years. Lucerne and other silage is also dried by this method, and some of the first band dryers developed for food products owed much in their design to the experience the engineers had gained in The dryer normally has three heat zones, each of which is served by an individual fan drawing hot air from a common heat source. The latter can either be a seriesof steam batteries or a heat exchanger mounted on a coal or oil furnace. both indirect methods or the air stream can be from a direct source, such as gas or LPG The hot air stream is ducted underneath the interlocking perforated conveyor plates, which make up the continuous band, and the drying ai passes through the perforations in the plates, and thmugh the mass of product which is being conveyed at a controlled depth along the length of the dryer. The plates, running the full width of the band are about 23cm wide, and are made from perforated stainless steel plate. The perforations can be either 4mm round holes or 4mm squareones at 6. 4mm centres to give adequate open ama through which to pass the hot air stream. Air flow can alternate in an upward or downward direction as drying proceeds. The 23cm wide plates connect at either side with a 23cm pitch chain, which carries the band over the drive and free sprockets at either end of the Right: Continuous band dryer for vegetables with eloped by th company of Proctor
CONVEYOR BAND DRYERS - SINGLE PASS Reference has been made, in Chapter 2, to this type of dryer, and an ideal size unit for medium scale operation is a dryer 30-40m in length, with a conveyor width of 2.5-3m. The conveyor band dryer is used in many industries outside food dehydration, and has been standard equipment in the textile, chemical and tobacco industries for many years. Lucerne and other silage is also dried by this method, and some of the first band dryers developed for food products owed much in their design to the experience the engineers had gained in grass drying. The dryer normally has three heat zones, each of which is served by an individual fan drawing hot air from a common heat source. The latter can either be a series of steam batteries or a heat exchanger mounted on a coal or oil furnace, both indirect methods, or the air stream can be from a direct source, such as gas or LPG. The hot air stream is ducted underneath the interlocking perforated conveyor plates, which make up the continuous band, and the drying air passes through the perforations in the plates, and thmugh the mass of product which is being conveyed at a controlled depth along the length of the dryer. The plates, running the full width of the band are about 23cm wide, and are made from perforated stainless steel plate. The perforations can be either 4mm round holes or 4mm square ones at 6.4mm centres to give adequate open ama through which to pass the hot air stream. Air flow can alternate in an upward or downward direction as drying proceeds. The 23cm wide plates connect at either side with a 23cm pitch chain, which carries the band over the drive and free sprockets at either end of the conveyor. Right: Continuous band dryer for vegetables with oscillating feed arrangement developed by the parent company of Proctor Dalgleish
The blanched vegetables are delivered on to the feed end of the dryer by various methods. One is an inclined chute at about 45, with an adjustabl levellingplate running across the full width of the band to control the depth of material passing underneath it. Another loading device is an oscillating boom swinging across the width of the band and delivering the material in an even swathe at a prescribed depth The depth to which the dryer is loaded will vary according to the type of vegetablebeing dried, the size to which the material is cut and the general permeability of the bed. For example, strips of root vegetable will dry at 11- 12cmdepth, whereas 9. 5mm cubes can rarely be dried on a deeper bed than 8-10cm Cabbage, which tends to mat and create a high resistance to the ai stream, may have to be reduced to a 5cm bed depth. A single pass conveyor band dryer is, on this account, not so suitable for drying cabbage but successfulresults are obtained with a multiple-pass dryer, which comprises a seriesof bands, each transferring the product to the conveyor immediately beneath it at the end of each pass. In this way the product benefits from a surface change in relation to the air stream, which facilitates drying to a very significantdegree A surface change can be effected in a single pass dryer by fitting a rotating shaft with metal tines, that just clear the band sections but rake through the product. This is fitted about one third of the way along the length of the conveyor, and such a device rotates at about 100rpm. A second pin rake may be fitted at a further distance along the conveyor. 4-stage single pass dryer
The blanched vegetables are delivered on to the feed end of the dryer by various methods. One is an inclined chute at about 45°, with an adjustable levelling plate running across the full width of the band to control the depth of material passing underneath it. Another loading device is an oscillating boom swinging across the width of the band and delivering the material in an even swathe at a prescribed depth. The depth to which the dryer is loaded will vary according to the type of vegetable being dried, the size to which the material is cut and the general permeability of the bed. For example, strips of root vegetable will dry at 11- 12cm depth, whereas 9.5mm cubes can rarely be dried on a deeper bed than 8-10cm. Cabbage, which tends to mat and create a high resistance to the air stream, may have to be reduced to a 5cm bed depth. A single pass conveyor band dryer is, on this account, not so suitable for drying cabbage but successful results are obtained with a multiple-pass dryer, which comprises a series of bands, each transferring the product to the conveyor immediately beneath it at the end of each pass. In this way the product benefits from a surface change in relation to the air stream, which facilitates drying to a very significant degree. A surface change can be effected in a single pass dryer by fitting a rotating shaft with metal tines, that just clear the band sections but rake through the product. This is fitted about one third of the way along the length of the conveyor, and such a device rotates at about 100rpm. A second pin rake may be fitted at a further distance along the conveyor. 4-stage single pass dryer 74