4 Dryers y rying of vegetables is still the most widel here are several options open to the dehydrator as to the type of dryer that can be used for this purpose. In the early days of the industry, tunnel and stove dryers were in general use. Designs varied widely but all of them involved the use of shallow trays upon which the material for drying was spread to a depth of 25-40mm. The tray loading and unloading involved a fairly high labour content but, in spite of this, many factories throughout the world are still using this method Continuous conveyor band dryers with single or multi-pass have however, superseded tray drying in recent years, and this trend towards automation has obviously brought a higher degree of efficiency into modern hydration factories and has substantially reduced the labour content of the Cabinet dryers are still useful, however, for pilot runs, and for specialised products where a high level of throughput is not desired or possible. This type of dryer is, therefore, described in this chapter, as it could well fill some special requirement, albeit not in the context of the main roduction line STOVE AND CABINET DRYERS Stove dryers are ideal for small to medium levels of Production, and are a smaller version of the tunnel dryer, in that they operate a system of tray drying, the trays being racked on mobile trucks. The air-flow, however, is
4 Dryers Air drying of vegetables is still the most widely used methodand there are several options open to the dehydrator as to the type of dryer that can be used for this purpose. In the early days of the industry, tunnel and stove dryers were in general use. Designs varied widely but all of them involved the use of shallow trays upon which the material for drying was spread to a depth of 2540mm. The tray loading and unloading involved a fairly high labour content but, in spite of this, many factories throughout the world are still using this method. Continuous conveyor band dryers with single or multi-pass have, however, superseded tray drying in recent years, and this trend towards automation has obviously brought a higher degree of efficiency into modern dehydration factories and has substantially reduced the labour content of the operation. Cabinet dryers are still useful, however, for pilot runs, and for specialised products where a high level of throughput is not desired or possible. This type of dryer is, therefore, described in this chapter, as it could well fill some special requirement, albeit not in the context of the main production line. STOVE AND CABINET DRYERS Stove dryers are ideal for small to medium levels of production, and are a smaller version of the tunnel dryer, in that they operate a system of tray drying, the trays being racked on mobile trucks. The air-flow, however, is 65
cross-flowand introduced at the side of the dryer rather than at the end The trucks may be in a single line down the length of the dryer or in double mws, side by side Access doors are fitted to both ends of the dryer and the design is modular so that the drying compartment can be extended to meet throughput demands, within certain limits stoves and 12 trucks in double formation, ie, 6 pairs side by side. The stoves were used in tandem, the firststove providing the hot zone, then a space was provided for the two end trucks to be removed for riffling'over the semi dried onion on the trays, prior to moving both trucks into a second stove operating at a lower temperatuxe As two trucks moved from the hot stove to the cool stove, two freshly loaded trucks were entered into the hot stove and two trucks wereremoved at the end of the cycleat the cool end. The total installation, therefore, comprised six stoves divided into three pairs, in other words, three dryers each with hot and cool drying zones The trucks carried 30 trays measuring 13mmby 813mmby 5lmm deep and each tray was loaded with 5kg of prepared onion. A 30 minute cycle was used for each pair of stoves, and in 24 hours 4 complete charges per dry unit of two stoves enabled some 43 tons of preparedonion to be handled This equated with 6 tons of dry onion, after conditioning Stove dryer showing fans and heater loures
cross-flow and introduced at the side of the dryer rather than at the end. The trucks may be in a single line down the length of the dryer or in double mws, side by side. Access doors are fitted to both ends of the dryer and the design is modular so that the drying compartment can be extended to meet throughput demands, within certain limits. An onion dehydration installation regularly visited in Egypt utilised 6 stoves and 12trucks in double formation, ie, 6 pairs side by side. The stoves were used in tandem, the first stove providing the hot zone, then a space was provided for the two end trucks to be removed for ‘riffling’ over the semi dried onion on the trays, prior to moving both trucks into a second stove operating at a lower temperatuxe. As two trucks moved from the ‘hot‘ stove to the ‘cool’ stove, two freshly loaded trucks were entered into the hot stove and two trucks were removed at the end of the cycle at the cool end. The total installation, therefore, comprised six stoves divided into three pairs, in other words, three dryers each with hot and cool drying zones. The trucks carried 30 trays measuring 813mm by 813mm by 5lmm deep and each tray was loaded with 5kg of prepared onion. A 30 minute cycle was used for each pair of stoves, and in 24 hours 4 complete charges per drying unit of two stoves enabled some 43 tons of preparedonion to be handled. This equated with 6 tons of dry onion, after conditioning. Stove dryer showingfans and heater lourves 66
This system was labour intensive but suited conditions in Egypt where labour was plentiful One factor to recognise with any system using trucks and trays is the necessity of providing a good hardened floor surface, treated against acid and alkaline attack and the wear and tear of truck wheels passing over continuously. It is a good precaution to lay steel tracks flush with the floor surface, where the trucks pass through the stoves in the drying compartments, and extend them out to the loading and unloading areas, and the riffling space between the dryer heat zones The cabinet dryer is essentially a small batch tray dryer, suitable for any product being dried on a pilot scale, or small production level They are usually of 10 or 20 tray capacity, each tray measuring 813mm by 406mm by 30mm deep. The trays are supported in the cabinet on angle brackets at the sides spaced 75mm apart, with one tray per level in the 10 tray dryer and two at each level in the case of the 20 tray unit. The heat source may be steam or electricity and is located at the side of the drying compartment-a fan provides a cross-flow of drying air. TUNNEL DRYERS emicontinuousa orporate the tray drying technique of the stove dryer on a semi continuous basis. They can be designed to give a viable commercial throughput and, as stated at the beginning of the chapter are still used in America and Europe, in some of the older factories As the name implies, the drying chamber consists of one, two or sometimes three tunnels, rectangular in section, up to 12m in length, with a loading aperture sufficiently large to allow the entry of trolleys, which carry the drying trays in racks up to 1. 8m in height. The trays are racked in pairs on each shelf of the trolley. Double Tunnel Dryers This type of dryer was developed in the UK in 1940 for a programme of vegetable dehydration under the auspices of the Ministry of Food. A typical double tunnel unit comprises awet tunnel and a 'dry tunnel running Parallel. The dryers set up by the ministry were some 10 7m in length, and the heat source was gilled steam tubes with the fans positioned aft, so that air was drawn through the heater bank and blown through the tunnel The trolleys entered the wet tunnel sideways, locating on a track fitted with a pusher device. In the first position, the trolley was sufficiently far away from the fan to permit adequate diffusion of the hot air stream to avoid ScOre
This system was labour intensive but suited conditions in Egypt where labour was plentiful. One factor to recognise with any system using trucks and trays is the necessity of providing a good hardened floor surface, treated against acid and alkaline attack and the wear and tear of truck wheels passing over it continuously. It is a good precaution to lay steel tracks flush with the floor surface, where the trucks pass through the stoves in the drying compartments, and extend them out to the loading and unloading areas, and the riffling space between the dryer heat zones. The cabinet dryer is essentially a small batch tray dryer, suitable for any product being dried on a pilot scale, or small production level. They are usually of 10 or 20 tray capacity, each tray measuring 813mm by 406mm by 30mm deep. The trays are supported in the cabinet on angle brackets at the sides spaced 75mm apart, with one tray per level in the 10 tray dryer and two at each level in the case of the 20 tray unit. The heat source may be steam or electricity and is located at the side of the drying compartment - a fan provides a cross-flow of drying air. TUNNEL DRYERS Tunnel dryers incorporate the tray drying technique of the stove dryer on a semi continuous basis. They can be designed to give a viable commercial throughput and, as stated at the beginning of the chapter, are still used in America and Europe, in some of the older factories. As the name implies, the drying chamber consists of one, two or sometimes three tunnels, rectangular in section, up to 12m in length, with a loading aperture sufficiently large to allow the entry of trolleys, which cany the drying trays in racks up to 1.8m in height. The trays are racked in pairs on each shelf of the trolley. Double Tunnel Dryers This type of dryer was developed in the UK in 1940 for a programme of vegetable dehydration under the auspices of the Ministry of Food. A typical double tunnel unit comprises a ’wet’ tunnel and a ‘drf tunnel running parallel. The dryers set up by the Ministry were some 10.7m in length, and the heat source was gilled steam tubes with the fans positioned aft, so that air was drawn through the heater bank and blown through the tunnel. The trolleys entered the wet tunnel sideways, locating on a track fitted with a pusher device. In the first position, the trolley was sufficiently far away from the fan to permit adequate diffusion of the hot air stream to avoid scorching the product. 67
The blanched vegetables, racked on 50 trays per trolley, remained in the first position for 25min, then a second trolley was moved in, the pusher gear moving the first one to the second position in the tunnel. Thereafter, nd at the same 25min interval, further trolleys entered and all moved progressively down the wet tunnel, concurrently with the air flow At the end of the wet tunnel, each trolley emerged and, after turning through 180entered the dry tunnel. Here the trolleys met the air stream in counter flow, the hot air fan in the dry tunnel being positioned at the opposite end, ie, alongside the wet tunnel fan. The progression through the dry tunnel was at the same 25min interval, and the whole drying cycle varied from 6hr to 7hr, according to product and weight of tray loading. These dryers, in the main, were used for the dehydration of potatoes, cabbage and carrots for use by the services but after the War, many were used for a wide variety of vegetables for commercial distribution The inlet temperatures to the drying section of each tunnel of this typ are thermostatically controlled and typical operating temperatures for root vegetables are as follows: Wet Inlet:99°104℃ Dry Inlet:65°71℃ The wet tunnel outlet temperature will be in the range 57"-60C The air flow is controlled by louvres over the tunnels and it is possible to recycle 50-75 percent of the air, by louvre adjustment, before discharging it Recycling tends to slow down the drying cycle but this is usually done in the interests of economy and of restricting the demand on the fans The capacity of this type and size of dryer, when drying potato cubes or strips at 6kg tray loading, was of the order of 250kg of dry product per hour, according to the British Ministry of Food statistics over the period when these dryers were in general use in the UK for supplying the Services The wet tunnel fan had a rating of 1416cu m per min and the dry tunnel fan, 991cu m per min The construction of the tunnel walls uses engineering bricks with 28cm cavity external walls to lessen radiation losses. Both inlet and exit doors are of the counterbalanced lifting type, suitably insulated. Each tunnel has an overhead recirculation duct with louvres as previously described Access doors are provided in the fan chambers for servicing purposes The drying trays are ideally constructed of non corrosive metal angle with stainless bottom mesh, and the trolleys are of a size to fit neatly into the tunnel section, so that the air stream passes uniformly across the trays, and
The blanched vegetables, racked on 50 trays per trolley, remained in the first position for 25min, then a second trolley was moved in, the pusher gear moving the first one to the second position in the tunnel. Thereafter, and at the same 25min interval, further blleys entered and all moved pmgressively down the wet tunnel, concurrently with the air flow. At the end of the wet tunnel, each trolley emerged and, after turning through 180' entered the dry tunnel. Here the blleys met the air stream in counter flow, the hot air fan in the dry tunnel being positioned at the opposite end, ie, alongside the wet tunnel fan. The progression through the dry tunnel was at the same 25min interval, and the whole drying cycle varied fmm 6hr to 7hr, according to product and weight of tray loading. These dryers, in the main, were used for the dehydration of potatoes, cabbage and carrots for use by the Services but, after the War, many were used for a wide variety of vegetables for commefiial distribution. The inlet temperatures to the drying section of each tunnel of this type are thermostatically controlled and typical operating temperatures for root vegetables are as follows: Wet Inlet: 99' 104'C Dry Inlet: 65 o 71 "C The wet tunnel outlet temperature will be in the range 57" - 60'C. The air flow is controlled by louvres over the tunnels and it is possible to recycle 5@75 percent of the air, by louvre adjustment, before discharging it to atmosphere. Recycling tends to slow down the drying cycle but this is usually done in the interests of economy and of restricting the demand on the fans. The capacity of this type and size of dryer, when drying potato cubes or strips at 6kg tray loading, was of the order of 25okg of dry pduct per hour, according to the British Ministry of Food statistics over the period when these dryers were in general use in the UK for supplying the Services' nqukments. The wet tunnel fan had a rating of 1416cu m per min and the dry tunnel fan, 991cu m per min. The construction of the tunnel walls uses engineering bricks with 28cm cavity external walls to lessen radiation losses. Both inlet and exit doors are of the counterbalanced lifting type, suitably insulated. Each tunnel has an overhead recirculation duct with louvres as previously described. Access doors are provided in the fan chambers for servicing purposes. The drying trays are ideally constructed of non corrosive metal angle with stainless bottom mesh, and the trolleys are of a size to fit neatly into the tunnel section, so that the air stream passes uniformly across the trays, and 68
does not by-pass round the edges of the trolley. Reference has been made to the 25min cycle from trolley entry in tunnel drying, and this cycle has to be maintained at all times. If, for example, there is a delay in tray loading due to lack of product from the blancher, or when the system is being run down, an empty trolley must be moved into the tunnel, or a series of empty trolleys, if necessary, so that the preceding loaded trolleys move in their proper sequence through the drying cycle Three Tunnel Dryer This dryer is a variation on the double tunnel system, and comprises two wet tunnels, with the dry tunnel running down the middle. Trolley entry is at the ends of the wet tunnels, and not by sideways loading as in the case of the double tunnel dryer. The fans and heaters are mounted on the top of the dryer, and the air stream is deflected downwards by louvres on to the trolleys in the first position. The air flow is parallel or concurrent with the trolley movement, and the interval of loading is usually reduced to 20min; each wet tunnel A and B accepting a trolley alternately at this time interval Thus, the product remains in the first position for 40min, before being moved into the second position by the second trolley On reaching the end of the wet tunnel, the trolleys again move alternately from A and B into the middle dry tunnel, thereby shortening the duration of travel in the latter, as here the trolleys resume the 20min cycle This system is equally as effective as the double tunnel but in making a choice of the type of dryer to install, the dehydrator must give full consideration to the labour content inherent in the quasi-continuous tray drying system, as against the completely continuous system offered by the Through Conveyor Band Dryer THROUGH-FLOW BATCH DRYERS The Buttner Favorit batch dryer has been used widely for many er in overseas factories for the drying of vegetables and fruits-especially where labour has been plentiful and cheap. It was used where small to intermediate levels of production were required It comprises a single drying chamber holding either 8 or 10 trays, 3m by 2m by 150mm deep. The trays are charged on an operating stand in front of the drying chamber. The tiltable tray lifting frame slides up and down between lateral tubular supports, with five arresting points controlled by limit switches, so that the progress of the electrically lifted trays can be pre- set A horizontal battery of heaters separates the trays in the drying chamber in two stacks
does not by-pass round the edges of the trolley Reference has been made to the 25min cycle from trolley entry in tunnel drying, and this cycle has to be maintained at all times. If, for example, there is a delay in tray loading, due to lack of product from the blancher, or when the system is being run down, an empty trolley must be moved into the tunnel, or a series of empty trolleys, if necessary, so that the pding loaded trolleys move in their proper sequence through the drying cycle. Three 'hnnel Dryer This dryer is a variation on the double tunnel system, and comprises two wet tunnels, with the dry tunnel running down the middle. Trolley entry is at the ends of the wet tunnels, and not by sideways loading as in the case of the double tunnel dryer. The fans and heaters are mounted on the top of the dryer, and the air stream is deflected downwards by louvres on to the trolleys in the first position. The air flow is parallel or concurrent with the trolley movement, and the interval of loading is usually reduced to 20min; each wet tunnel A and B accepting a trolley alternately at this time interval. Thus, the product remains in the first position for 40min, before being moved into the second position by the second trolley On reaching the end of the wet tunnel, the trolleys again move alternately from Aand B into the middle dry tunnel, thereby shortening the duration of travel in the latter, as here the tmlleys resume the 20min cycle again. This system is equally as effective as the double tunnel but in making a choice of the type of dryer to install, the dehydrator must give full consideration to the labour content inherent in the quasi-continuous tray drying system, as against the completely continuous system offered by the Through Conveyor Band Dryer. THROUGH-FLOW BATCH DRYERS The Buttner 'Favorit' batch dryer has been used widely for many years in overseas factories for the drying of vegetables and fruits -especially where labour has been plentiful and cheap. It was used where small to intermediate levels of production were required. It comprises a single drying chamber holding either 8 or 10 trays, 3m by 2m by 150mm deep. The trays are charged on an operating stand in front of the drying chamber. The tiltable tray lifting frame slides up and down between lateral tubular supports, with five arresting points controlled by limit switches, so that the props of the electrically lifted trays can be pn?- set. A horizontal battery of heaters separates the trays in the drying chamber in two stacks. 69
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