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 Continuous conveyor band dryers with single or multi-pass have, however, superseded tray drying in recent years, and this trend towards automation has obviously brought a higher degree of efficiency into modern dehydration factories and has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-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 12 trucks 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 prepared onion to be handled. This equated with 6 tons of dry onion, after conditioning. Stove dryer showing fans 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 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, 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 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 II LI- - _- 44 z y 3 v 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 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 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 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 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 The heat zones are separated by transverse baffles over and under the band sections and, in this way, temperature variations can be implemented in each zone as required, and damper control in the fan ducts also gives this facility. Lower initial inlet temperatures are normally used in the first heat zone of a band dryer, as compared with those in a tunnel dryer, because the effect of passing the hot air stream through the product, rather than over it, produces a higher rate of evaporation, and the product is, in fact, exposed to a greater degm of heat for a longer period of time in the first zone than is the trolley of trays in the first position in the tunnel dryer. Temperatures must, therefore, be very carefully controlled to avoid scorching, case hardening and pmtein denaturation, as the evaporation rate is considerably higher in the band dryer. Temperatures in the second heat zone are usually controlled at 6"- 10°C lower than in the first and, in the third zone, about 10°C lower than in the second but this varies from product to product. As with the tunnel dryer, recycling is quite normal practice, so that the hot air stream is fully saturated as it passes to atmosphere. Some products with a relatively high sucmse content tend to adhere to the band plates, at the discharge end, and require a rotating nylon brush, or mechanical scraping action, to remove them completely from the plate surface. Adhesion can, however, be minimised by applying a light coating of 'dehydrator's wax' on the band plates weekly. This is normally dispensed from a hand spray gun, and will give adequate protection for about six days. The wax is specially manufactured for the dehydration industry, and modem band dryers are fitted with a continuous wax applicator device. The speed of the conveyor is infinitely variable to suit both the product and the heat conditions, and the heat zones are thermostatically controlled once the air duct dampers have been set at the beginning of a production run. This type of dryer is ideal for a long sustained run on one product as, once drying conditions have been established, it requires very little attention. The number of operators required is no more than two - the blancher operator, to periodically check the feed level of the product on to the band, and a dryer foreman, to generally supervise the overall operating conditions of the dryer. As the capital cost of this type of dryer is high, the usual practice is to take off the product at the end of the conveyor at 10-15 percent moisture content, and transfer it into bin dryers to dry down to the final specification. In this way, the drying cycle of the conveyor band dryer can be shortened to as little as 2-3 hours, the finishing in conditioning bins normally taking a 75 further 4-5 hours. Their use permits the most efficient exploitation of the band dryer, the throughput of which would be restricted by about a third if it was used to bring the product down to final moisture specification. Very low drying temperatures and air flow are required in bin drying, as it is more of a conditioning than drying process, and it is sound economics to relieve the band dryer of this duty in the last stages. MULTI-PASS CONVEYOR DRYERS This type of conveyor dryer may be either 3 or 5 pass and, because of its multi-layer construction, is more economical with floor space than the single pass dryer. They are modular in design, and therefore can be custom-built to suit the processor's throughput capacity. There are two major European manufacturers of this type of dryer - Mitchell Dryers Ltd of Carlisle, United Kingdom, and Buttner-Schilde - Haas of Krefeld, Germany and the major features of both are explained here. A Buttner dryer at Erin Foods, Tuam, Co. Galway, factory 76 TheMitchellDryermodulesare1.8minlengthandthebeltWidth2.5m. An oscillating or apron feed is optional at the input end, and there is a stand- ard delivery section at the discharge end. A 7 module dryer would handle a wet feed input of prepared onions at 85 percent moisture of 2091kg per hour, drying to 5 percent. With bin conditioning the takeoff moisture could be 10 - 12 percent, which would, of course, shorten the drying cycle and increase the throughput. With diced potato the wet feed input would be 3470kg per hour, assuming a raw moisture of 80 percent and an end moisture of 5 percent. Again with bin drying the takeoff moisture could be as high as 15 percent. The great advantage of multi-pass dryers is that the product undergoes 4 surface changes as it is transferred from each of the five belts, and this expedites the drying cycle time. Each module has a separate hot air circulation fan, and the belts run at different speeds to take care of product shrinkage, and the through air flow can be directed upwards or downwards, usually a combination of both directions in different sections to suit the drying characteristics of the For an indirect drying system, the heat exchangers are mounted on the side of the dryer, with instant access to the motors and fans but, with the increased use of Liquid Petroleum Gas or Natural Gas firing, the products of combustion can be introduced into a duct at the end of the dryer and subsequently diluted by the introduction of fresh air, thence fed into the plenum chamber. Alternatively, a series of small individual burners can be used. The temperatures can be infinitely varied on the different levels at which the product is conveyed. Where in a single pass dryer three heat zones are created by a transfer module at two points in the conveyor travel, allowing a predetermined temperature in each of the the zones, the heat control in a five pass dryer can be even more sensitively predicted. Product adhesion is avoided by a built-in continuous waxing device on the belts, and hygiene is assured by a rotary brush applying hot water, detergent, etc, or a high pressure steam hose can be fitted for intermediate or continuous band cleaning. A control system, designed to achieve consistently uniform terminal moisture in the dried product is incorporated, and the whole drying operation is completely automated. One of these 7 module dryers was supplied in recent times to a major dehydrator in what was Yugoslavia (See TABLE 4.2 for performance details). product. 77 TABLE4.2 Feed Throughput Performance of the MitchellDryer 5 Pass - 7 Module DryeE Each Module is 1.8m. long. Band Width 2.5m products are Vegetables and Fruits prepared for drymg. End Moisture is calculated at Dryer Discharge, -before Conditioningin Bins. Produce cut Raw End Feed Moisture Moisture kg/Hr Apples (evaporated) 10-1 2mm slice 88 20 2051 Apples (flakes) 10xlOx2mm 88 4 1281 Apples (dice) 10xl0x10mm 88 4 1795 Cabbage (dice) 10xl0x10mm 89 5 1834 Carrots (dice) 10xl0xlOmm 90 5 2834 Celery (dice) 10xl0x10mm 96 6 1729 Garden Peas Scarified 75 7 2992 Bell Peppers 10xlOx10mm 94 5 21 59 Leeks (flakes) 1Ox10x2mm 95 6 1701 Mushrooms 5mm slice 94 5 1733 Onions (slices) 4mm slice 85 5 2091 Potato (dice) 10xl0x10mm 80 5 3470 Potato (slices) 4mm slice 80 5 2267 Swedes (dice) 10xl0x10mm 89 7 2699 Beetroot (dice) 10xl0x10mm 89 6 2038 YO 70 Green Beans Long Cut 89 6 2004 Parsnip Leaf 13xl3x12mm 80 7 1212 Parsley Chopped 86 5 973 Spinach 13xl3x2mm 94 5 1212 Apricots (evaporated) Halves 83 20 642 Pears (evaporated) Quarters 85 20 826 Prunes (Plums) Whole 83 20 642 The Computed Raw Moistures are computed and approximate and will vary according to climatic and horticultural conditions. The end moistures may be varied by conditioning to meet any specific specification. (data courtesy of MitchellDryers Ltd.) A Buttner-Schilde-Haas 5 pass 10 module dryer was observed regularly in the early 1970s in an Indian factory drying onions exclusively. 78 The manufacturers' input rating was about U3rds of the Mitchell Dryers dryer described above but under the operating conditions on site the wet input rarely exceeded lOOOkg per hour, with raw moistuE calculated at 86 percent and the end moisture 6 percent. There was no bin drying facility in this factory and the whole drying cycle averaged 7 hours. The dryer was not completely standard, in that it had no top auxiliary fans, which are now fitted on BSH dryers as standard (See TABLE 4.3). TABLE4.3 Feed Throughput Performance of Buttner-Schilde-Hass 10 Module 5 Pass Dryer: Length 28 metres: Belt Width 2.5m. Products; Vegetables Prepared for Drying. Final Moisture is calculated at Dryer Discharge; - before conditioning in Bins. Pduce cut Raw End Feed Moisture Moisture O/O YO kg/Hr Onions (slices) 4mm 87 6 1200 Carrots (dice) 10xlOx8mm 90 8 2240 Potatoes (dice) 10xlOx8mm 82 8 2798 Garlic (slices) 3.5 - 4mm 68 6 1280 Cabbage (slices) lOxlOmm 92 6 1503 (Data by courtesy of Rosin Engineering Co Ltd.) Noted Performance of the Indian Dryer Only the top belt has an independent drive, the other four having a common drive, which limited the retention time flexibility in the lower belts. With the side air flow, lower temperatures prevailed near the steam coil side, and a higher temperature near the exhaust and inlet side. This imbalance of temperature made it necessary to vary the product bed thickness as between one side of the belts and the other. The working conditions for the dryer in the 7 hour cycle were as under: Temperatures: kt zone 86 "C 2ndzone 86°C 3rdzone 75°C 4/5th zone 50°C Feed depth 28-34mm 79 Damper settings:- Exhaust Dampers (from feed end) No.1 2modules 80%open No.2 3modules 80%open No.3 3modules 70%open No.4 2modules 60%open All Modules 50% open. other belts 6hr 15 minutes total cycle 7hr. Inlet Z0,000m3 per hour exhaust 24,Wm3 per hour Inlet Dampers. Retention time: No.1 belt-45 minutes. Air volume Humidity and temperature ranges on the air inlet side of the dryer varied, according to season, as under. Relative Humidity 50 - 95% Ambient Temperature 20" - 32°C The performance of the dryer was not a criticism of the manufacturers but more of the working conditions, the non standard dryer design and the poor quality generally of the raw material. Many of the local onions were under 35mm in diameter, were ungraded when purchased, and this gave rise to problems in slicing evenly and presenting a rather impermeable bed of product on to the belts, which did not help drying conditions. A large tonnage of pduct was also lost by the absence of any dehumidification of the packing areas - an important factor when processing in the tropics where, during the rainy season, relative humidity reaches almost saturation point. This fault was subsequently rectified. A smaller version of the 5 pass conveyor dryer is the Imperial Band Dryer, designed many years ago by a prominent German engineering company who ultimately assigned the drawings, design, specifications and manufacturing rights to a Bulgarian company, who have marketed the dryer, still built in Bulgaria as far as is known, to Eastern European countries and have also exported some dryers to India, mainly for onion drying. The heating system, as illustrated in one of the Author's photographs of an onion dehydration plant in Bulgaria, tends to be oversimplified, in that it relies on an 8KW fan to draw air in from the building in which the dryer is located, across five steam tubes about 13cm in diameter, located longitudinally along the length of the dryer, each tube being opposite one of the five belts. In each heat zone the air is drawn across the product and subsequently exhausted to atmosphere. The volume of air from this fan is adequate on the top three belt chambers but tends to lose velocity towards 80 the two bottom belts, where the final drying takes place. This has been known to give rise to scorching, even at low temperatures, and it appeared to the author that the dryer needed some auxiliary fan power to overcome this problem. The rated wet prepared onion input is about 500kg per hour but it was observed that this figure was rarely sustained. Two Favorit through-flow tray dryers in the same factory gave a much more reliable 500kg per hr input for the pair, admittedly with a little more labour content, but a better quality end-product was usually obtained. BIN DRYERS These are used for conditioning dried pduct which has passed through the primary dryer - either a conveyor band dryer, Stove or Through- Flow dryer, where the product may leave at 12 to 15 percent moisture. The function of the bin dryer is to apply a low temperature air stream through a plenum chamber at the bottom of the bin, permeating a relatively deep bed of product, and conditioning this to 5 - 7 percent moisture. The bins are lm wide and 2m long and allow up to 0.6m of working depth. The product is contained on a perforated base which allows a through draught of low velocity air at 50" to 60°C. Several bins are normally required to allow for a conditioning time of 4-1Ohr, and are coupled up to a central air distribution duct supplied by a single fan and heater which may be either steam or electrically powered. Such units, therefore, offer a valuable intermediate storage facility between discharge from the primary dryer and subsequent packing or bulk storage. The Mitchell Dryers engineering specification for a ten-bin condition- ing unit is as follows: The system comprises a main air circulating fan connected to a steam heater battery and a duct incorporating 10 separate docking points for the mobile bins, which are moved from the main dryer on castors. The air circulation fan is a backward curved centrifugal type with 6KW totally enclosed fan cooled driving motor. The fan is designed to handle a total volume of 283cu m of air at 15°C and, to enable the volume of air to be reduced if all ten bins an? not used at the same time, an adjustable damper, fitted to the fan air inlet, is provided. To maintain a maximum drying air temperature of 65'C in the system a mild steel gilled tube air heater battery is provided. The heater tubes are enclosed in a mild steel flanged casing bolted into the distribution duct, and the heater is designed for a pressure of 5.4atm. The main circulating duct is manufactured in 16 gauge galvanised 81 mild steel, flanged and braced down its length to eliminate vibration. This duct is taped to ensun? a constant air velocity and volume to each of the bin docking points. Ten mobile bin docking points are provided on to which the loaded bins would be clamped prior to commencement of conditioning. Each feeder point is fitted with an air control gate for sealing when the bin is removed, or at any time when the duct docking points are not being utilised. To control the heated air temperature a thermostatic controller is provided, complete with probe and adjuster, steam control valve and strainer. A dial-type thermometer is fitted for visual indication of air temperature. The bins are fabricated from 16 gauge galvanised mild steel sheets suitably braced and of robust welded construction, and each is fitted with one pair of fixed and one pair of swivel ball and roller bearing castors. The base of the bin forms an air distribution plenum chamber to match up with the feeder points on the distribution duct. The top surface of the chamber, which forms the base of the product container, comprises a perforated stainless steel diffuser screen and product support. The top of the bin is open for charging of product. At the opposite end of the bin to the air feed point a discharge gate is fitted to assist manual emptying after completion of conditioning. The bins are normally lifted and tilted into the hopper feeding the screening plant by an electrically powered box-tipper. The box tipper is of the type also used for emptying raw produce received in boxes or crates from the farm into the bulk feeder on the vegetable processing line. AIR LIFT DRYERS These, in the main, fall into three categories: (a) the Pneumatic Ring dryer, (b) the Thermal Venturi dryer, and (c) the Fluidised Bed dryer. Whilst these dryers are used in many fields other than the drying of foodstuffs, their main function in the vegetable dehydration plant is the secondary drying of potato granules. As will be described in the next chapter, this type of dryer performs a secondary drying operation, rather than functioning as a primary or complete dryer in itself. The Pneumatic Ring Dryer, as the name implies, comprises a qua= sectioned metal duct, usually elliptical in shape, with a connecting duct to the heat source at a suitable point in the elliptical ring, and an entry point for the product adjacent to it. Apowerful fan draws the hot air at relatively high velocity around the ring duct, carrying the product in suspension through one or more complete cycles into a circular shaped manifold, whence it is a2 The Rosin ring dryer deflected into a cyclone that discharges the air to atmosphere and separates the product, through a rotary valve, for the next processing stage. The Thermal Venturi Dryer performs the same function, except that, instead of a closed ring system, a powerful hot air stream is directed upwards through a cylindrical jet section, picking up the product and lifting this at a high velocity through a tower shaped duct into a diffuser section, where the diameter increases to slow down the air stream. At the top of the diffuser the product impinges on a conical deflector and, separating itself from the aimtream, falls down an outer tower with a collector chute at the bottom, where it discharges. The moist air passes out of the top of the tower to atmosphere. Another type is operated by two balanced fans - one at the foot of the venturi column where the heat source is located, and the second at the head of the column, which draws product into cyclones and discharges air to atmosphere. A vertical drying column may be upwards of 19m tall but, where the height of the building precludes this being installed, a 'serpentine' duct is equally effective. At the product entry point on these dryers, static air conditions apply, and the granulated material can be metered in freely from a vibratory trough conveyor. Both the pneumatic ring and the thermal venturi are most suitable for drying granular material, of 35-40 percent moisture at inlet, down to 10-15 percent at outlet. This performance and capability is well suited to the potato granule process, where the initial moisture in the potato is taken up by the adding back of dry 'seed' potato powder, prior to the resultant blend entering 1. 83 the ring or thermal venturi dryer. The latter receives this blend of moist granulated material with 35-40 percent water content, and agglomeration is avoided because the product becomes immediately airborne as won as it enters the dryer. Evaporation is extremely rapid, up to 65 percent of the water content of the blend being removed in one or more orbits of the ring, or in the vertical lift area of the venturi. It is obvious, however, that these dryers are for specialised use and are not adaptable for a wide range of vegetables. Fluidised Bed Dryers are often installed as secondary dryen, following a ring or venturi dryer. Such a dryer consists of a rectangular box or trough with a porous ceramic base. Hot air is blown into a plenum chamber, below the porous base, and through a layer of granulated product, which is fed into one end of the upper dryer chamber. The air stream is controlled, through the ceramic base, at sufficient velocity to fluidise granular material, until it has the characteristics of a liquid, and moves from the feed to the discharge end at a steady flow rate, finally falling over a weir at the exit point. The capability of this type of dryer, when used as a 'finisher', will be a reduction in moisture of 7-10 percent. That is, granules leaving an air lift dryer at 12-15 percent moisture will be finished in the fluidised bed dryer at 5-8 percent moisture, as desired. ROTARY DRYERS The Rotary or Louvre Dryer is another simple type of secondary dryer, often used as a 'finisher' for granulated products. It comprises a drum, rotating on trunnions with an annular gear around the periphery of the drum and a pinion drive. The inside of the drum is fitted with louvre vanes, on the inner circumference, designed to turn the product as it passes through from the feed end. A fan and steam battery provide the heat source at the feed end, and the air stream flows concurrently with the turbulent product, which is agitated by the louvres and the slow helical movement of the drum. At the discharge end of the dryer the product passes into a cyclone, which disperses the air and drops the dry material through a rotary valve. This cyclone is sometimes fitted with a manifold and several collecting stockings, which Ieceive any fine material carried over by the air stream. Collecting stockings are sometimes frowned upon where fine powders are concerned, as the latter may possibly create an explosion hazard, and Local Authorities may insist that cyclones handling these products be exhausted to atmosphere. The evaporative capability of this type of dryer is limited, and the evaporative duty will equate with that of the fluidised bed dryer, ie, about 7- 10 percent reduction in moisture when used as a secondary dryer in granule 84 production. These dryers are not particularly successful as a drying medium for larger particles, such as dice, as the tumbling action tends to distort the shape of the cut, and the dwell time in the drum is normally insufficient to effect any significant degree of evaporation when compared, for example, with the belt trough dryer. VACUUM DRYERS The Vacuum Dryer is not widely used for vegetable dehydration but has special applications, such as the drying of pharmaceutical products, plasma, sera, etc. It is used in America for the dehydration of citrus juices, apple flakes and for heat sensitive fruits and products where the ascorbic acid retention factor is important. The lower drying temperatures used under conditions of vacuum, and the shorter drying cycle, reduce the product's susceptibility to 'browning', denaturation, protein damage and the loss of highly volatile constituents. It has the disadvantage of being a batch system, and the equipment is costly. The vacuum shelf dryer is heavily constructed to withstand high vacuum conditions, and the ancillary plant - vacuum pumps, injectors and condensers - also involve a high installation and operating cost, in relation to the capacity of this type of dryer. This tends to confine the use of the dryer to high value raw materials, or products requiring reduction to extremely low levels of moisture without damage. The heat source can be in the form of a steam or hot water jacket around the exterior of the shell of the cabinet, or heated shelves inside the cabinet, upon which trays of product are placed. Some vacuum dryers have been fitted with dull emitter electrical rod elements to supplement the heat transmitted by the outer steam or water jacket. The construction of the shell of the dryer has to be robust enough to operate at a vacuum of 0.5 to 4 millibars, although such conditions more usually apply when vacuum drying is incorporated with Accelerated Freeze Drying, which is described later. Continuous vacuum dryers, for the dehydration of fluids, fruit juices, purees, etc, comprise a continuous stainless belt passing over rollers through a cylindrical vacuum vessel, and the product is spkayed on to the belt, sometimes with the addition of a foaming agent. Heated platens, or infra red heaters over and under the belt provide the heat source, and the product is removed from the belt at the end of the cycle by doctor knives, and ejected through an airlock valve. Where solid products are processed in this way trays are used and the 85 exit and entry method used means this again is a partial batch system - one tray has to be entered as one is =moved, through airlocks, and consequently operators have to be constantly in attendance for this simple task. An enlarged version of this dryer uses trolleys on rails, which enter and leave the vacuum chamber in much the same way as in a tunnel dryer, although, in this instance, an airlock has to be provided to contain the vacuum. All these systems can only be described as quasi-continuous, and throughput in relation to capital cost is very limited, a relatively large unit not having more than 50kg of product output per hour. Mitchell Dryers have recently introduced a refined version of a continuous vacuum dryer by using 10 conveyor belts in a vacuum chamber. The Band Dryer consists of a vacuum chamber, 11.7m long 2.5m in diameter, (standard size for 10 belts but available in alternative lengths) with ten belts passing over platens heated by steam or hot water, mounted one above the other and running longitudinally along the chamber. The feed, a viscous paste or slurry fed by a pump under pressure, is continuously distributed across the bands at one end by nozzles. The paste or slurry, distributed in an even layer across the band, dries by conduction during its passage over the heated plates into a form with a honeycombed structure. Centring and controlling the lateral movement of the bands during drying is controlled from outside the dryer either manually or automatically. The product leaving the bands at the discharge end is broken into short lengths by a guillotine, before falling into a coarse breaker producing pieces less than 15mm. Product discharge from the vacuum chamber is by double hopper system operated intermittently. The advantages of the system are (1) the form of dried material can be varied considerably by alterations to the vacuum, the feed solids of the material and the hot plate temperature; (2) The increase in mass and heat transfer driving forces enables materials with a high Esistance to diffusion to be dried; (3) Toxic materials can be dried without health hazards; (4)The risk of oxidisation is reduced; (5) Cooling of the material can easily be provided; (6) Labour is reduced to a minimum and maintenance costs are low. Product Possibilities Chocolate crumb, yeast extract, vegetable extracts, meat extracts, starch, herbal extracts, liquorice extracts, gland extracts, glucose, syrups, resins and chemicals, malted milk, malt beverages, malt extract, molasses, patent foods, hydrolysed protein, soya extract, tea and coffee extracts, fruit juices, dye extracts, Shellac and gelatine, pharmaceuticals and food enzymes. 86 The capacity of a standard 10 band dryer operating on a milk product is a production rate of lOOOkg per hr with a capability of 150kg per hr evaporation. Pilot plant facilities for clients’ materials are available at the manufacturer’s works in Carlisle, UK where all preliminary tests can be carried out. FREEZE DRYERS Accelerated Freeze Drying (AFD) is widely practised in America, and is economically viable there on account of the large scale of operation, and the magnitude of the distribution outlets. It is viable with high cost materials, such as meats, poultry, shell-fish and certain non food specialities. It must be appreciated that the process is in two stages: fnxzing, followed by high vacuum drying. Both systems are, by themselves, expensive to operate and, as the original kze dryer plants were based on the batch system, this again added to the operating costs, as throughput was relatively small. Continuous plants are now available but the capital cost is very high. Reduced to its simplest terms, AFD is a method of drying a pduct whereby the water content is first converted to ice and then changed into vapour without passing back through the water phase. In consequence, since ice has a greater volume than water, the freezing of the product causes it to expand, under pressure of the formation of ice within itself. This expansion is not followed by equal contraction as the ice sublimates into vapour under conditions of vacuum, therefore the stretching of the capillary system in the product is permanent. This assists rapid rehydration and the product texture is very uniform. It could be argued that, following this expansion of the capillary system, much of the flavour and nutrient of the food leaches out into the cooking water and is lost. This is an argument used by critics of the method, who also refute the claim that the cost of evaporating water under vacuum requires less heat or calories per pound evaporated than at atmospheric pressure. The vacuum dryer, used in conjunction with the freezing plant, can be the batch type, as illustrated, with heated platens to carry the trays of product. Alternatively, infra red heaters can be used over the product, or, when meat is being dehydrated in slab form, heated metal spikes can be pressed into the latter. Refrigerated condensers are used in some plants to remove the water vapour from the chamber befoxe this enters the pump. Defrosting has to take place between drying cycles to ensure a passage for the evacuated air. Alternatively, the vapour can be absorbed by chemical agents. 87 The Autec freeze dryer illustrated is a batch-type with a shelf area of 36sq m which can be loaded with about 630kg wet weight of product. The drying cycle for the plant is determined by the nature of the product, and ranges from 8 to 24hr. The vacuum level is from 4 to 0.5 millibars and the vapour condenser temperature is minus 30°C. The dry output on chicken is calculated at 210kg per 24hr with an input of 630kg of prepared meat. Rossi & Catelli, of Parma, Italy, manufacture a larger batch unit with an input capacity of 3125kg of prepared produce per day for a single chamber unit, which, on the assumption that the total solids of the product is 20 percent, would produce 625kgs. of dried product per 24 hours. For larger volume production two or more units could be used as a battery, allowing more efficient utilisation of the services ancillary to the freeze drying opera tion, thereby reducing costs. The procedure with this type of plant is as follows: The raw produce is prepared, cleaned, washed, cut, blanched and cooled and then stored in cold moms at the required temperature. In a separate cold room there is an automatic filling plant when? the product is fed into trays, which are made of extruded and anodized aluminium. The filled trays, each with the same quantity of product are automatically conveyed and stacked on trolleys. Two or more trolleys (see illustration) are thus loaded and kept in the cold storage room. The freeze drying cycle commences by loading the trays through the open der of the vacuum chamber. A mechanised rail track conveys the loaded trolleys to the loading point, and a chain mechanism, integral to the trolley pushes simultaneously on to the heating plates of the chamber. This takes only a few minutes and is contmlled by one operator. Product door Refrigerated vapour Condenser Healing/co Figure 4.2 Diagrammatic vapourflow chart 88 The parameters for the temperature of the condenser, the degree of vacuum and the heating gradient of the plates in the chamber are all set at the commencement of the operation, and are automatically controlled by a programming device for the duration of the cycle, which ends when the Right: Batch freeze dying plant - tray loading product into the freeze-drying chamber on to heated platens Below: Batch freeze drying plant- end of tray washing plant - followed by automatic transfer of trays into the cooling room 89 product reaches the pre-determined end-moisture. The services are disconnected and the vacuum in the chamber is broken. The back door of the plant is opened and the trays with the dry pduct are pushed out simultaneously again by the chain mechanism provided in the chamber onto another trolley situated in a room with conholled low humidity atmosphere. the plant is then free to start another cycle. A single operator is required, and his direct intervention is necessary only at the end and begining of cycles, which normally follow one another at 15-20 minute intervals. For the major part of the cycle time, the function of the operator is limited to occasional supervision of the control instruments and he is therefore available for other duties. nay Emptying In the low humidity room there is an automatic tray emptying device again programmed electronically From the trolley each tray is automatically taken in turn and taken over to the storage tank, where it is overturned, shaken by a vibratory and scraping mechanism to ensure complete emptying, and sent out of the dry room. The empty trays are then taken up automatically and stacked on a storage rack awaiting cleaning, or they can be taken immediately and put through the washing, sterilising and cooling machine, then transfed into the cold mom ready to be filled again with the frozen product. All these handling operations are automatic, and no labour is invofved except for supervision of the equipment. Service requirements are as follows: Electricity per 24 hours 5OOOKW Fuel per 24 hours Wkg Water at 18°C per 24 hours 750~3 Operators 1 per shift. Filling trays 2 per shift. The plant is designed to freeze-dry any pduct under pressure of up to 50 microns, if necessary In conclusion, this method of drying cannot be recommended unequivocally for all vegetables but is viable for meat, shell fish, some fruits and high cost vegetables, such as asparagus, mushrooms, etc. DRUM DFWERS Drum Dryers also have specialised applications, and are used in the main for the drying of potato flakes and bananas. Milk and tomatoes are 90 almost exclusively spray dried now. Drum dryers can either be single or double drum units. Each drum is heavily constructed, and capable of withstanding an internal steam pressure of up to 7atm. Its diameter is lm to 1.8m, and the length can be anything between 3m and 4.5m in standard machines. The principle of drying by this method is that the liquid, or semi liquid, is coated on to the surface of the drum, which rotates slowly and, in the course of about 300' of one revolution, the moisture in the product is flashed off and the dry material is peeled off the drum surface in flake form by a series of doctor knives. In the case of a double drum dryer, the two drums are sited paraUe1 and in close juxtaposition to each other. The feed is usually from a trough, ool rf - 0; ' 4 , I ,-. ~ \ { j , ~.. -- .-._ .______ I I .@" A BW ' !' NIP FEED- Simplest type of feed suitable for milk and many other such materials FEED ROLL - Suitable for glutinous materials such as starch and flour. C D DOUBLE APPLICATOR ROLL- For heat- sensmve materials. The materml IS in contacl with the hot roll for the minimllm poss,ble rime. SPLASH FEED - Especially useful for materials with a high rate of sedimentation. ( i .~ -' ..~. I I E F @ \ L DIP FEED ~ Used for certain suspensions of solids usually wtth recirculation of material m the lray MULTIPLE APPLICATOR ROLLS ON SINGLE DRUM DRYER- Used lor increasing film Ihkckness for cereais. potaio llakes. etc. Figure 4.3 Typical feed arrangement developed by Richard Simon G. Sons Ltd for their drum dyers 91 situated above and running longitudinally between the two drums, which rotate away from one another, one clockwise and the other anticlockwise. Feed rollers, also running along the periphery of the drums, rotate and even out the feed material to a uniform thickness on the drum surface. There can be three to five of these feed rollers on each drum. The steam inside the drums, at 7atm, produces intense heat on the surface, flashing off the moisture in the few seconds' travel between the feed position and the doctor knives at the lower part of the drums. The dried material is taken off in the form of a fine curtain, which falls into a scroll conveyor and is reduced, by the action of the latter, to flake or coarse powder form. With a single drum dryer, the feed is usually at the top, where the drum passes through a shallow trough of the feed material, and the doctor knives are located to meet the dried material at a point where the drum has moved through about 300° of one revolution. The drying time for potato flakes with this system is about 20sec, the raw material at the feed point being 80 percent moisture, and the dry material 6-7 percent. In the case of both double and single drum dryers, the water vapour is extracted through a hood over the drums, either by natural flow or fan assistance. Typical drum dryer for potato fIakes - input: 2.5T/Hr of prepared potatoes. Dry output: 550kg/Hr 92 FOAM MAT DRYERS Foam drying is a system used in America for citrus fruit juices. It can be extended to any non particulate foods in puree or slurry form. The plant consists of a continuously moving stainless steel belt, which passes through a heating chamber. Steam condenses on the underside of this belt, and heated air passes over the top. Fruit juice, in concentrated form, is pumped into a foam generator and the foamed material is distributed evenly over the surface of the belt at an average thickness of 0.05in. After exposure to the heat, the concentrate dries to 1.5 percent moisture and then passes through a cooling section. Here it crystallises and, at the end of the belt, a doctor knife removes the crystals, which are conveyed to the packing station. Operating costs using this method are claimed to show a considerable reduction when comparwl with vacuum drying. The recently improved technology, as demonstrated in modem spray dryers, may eventually take precedence over foam mat or vacuum drying of fruit concentrates. SPRAY DRYING IN THE DAIRY AND FOOD INDUSTRY Introduction Basically, spray and fluidized bed processing are well known techniques for production of powders in the dairy and food industry. However, advanced pre-processing of feed products, peculiar rheological properties and delicate thermal sensitivity of materials have prompted the development of new, tailor-made processes and controls. Powder manufacturers demand increased throughput and flexibility of spray and fluid bed equipment for production of specialty products, and the end users require an increasing control of functional properties. Particle size and bulk density must be well defined. Dustiness, dispersibility and flowability, are required to be controlled within specific limits, too, all in order to ensure adequate added value of the produced products. Last - but not least - producers, consumers and legislators require careful considerations in respect of health, hygiene, safety and environment. During the past 20 years considerable development of the spray drying and fluid bed technology has met these demands in a number of ways of which a few examples will be given. 93 Demands The most important demands made by the dairy and food powder producers of an up-to-date spray drying plant are: 0 Flexibility , i.e. it must be possible to produce a large variety of products in the same plant. 0 The plant must produce high quality powder with specific physical/chemical, functional and bacteriological properties i.e. dustless, stable, defined density, etc. APV Anhydro Spray Dyer installed in the milk Marketing Board Creamery, Kendal, for the drying of mi& products 9 It must be possible to have long production periods of the plant between cleaning, i.e. without the occurrence of deposits of any significance in the process equipment. Fast and effective cleaning of the plant must be possible. Automatic control of the plant, with small deviations in relation to selected parameters and automatic start, stop and cleaning. 0 Low energy consumption and low operation costs. Development In short, the most important development has taken place within the following fields: One-stage dryers have been replaced by two or multi-stage drying plants. Atomization and agglomeration techniques have been improved. 0 Load on environment and energy consumption have been reduced. 0 Hygiene and CIP cleaning have been improved. 0 Control and automation have become sophisticated. Today, spray drying is a flexible process with many possibilities of controlling the operating parameters in order to obtain the desired powder quality. Powder production consists of a number of different processes, e.g. homogenization, concentrate preheating, atomization / agglomeration, drying, lecithination, rewet agglomeration and final drying/cooling which all may be controlled in one single dryer system. One process Many functional powder properties, e.g. wettability, dispersibility and viscosity, are closely related to the method of agglomeration, the process of forming porous clusters of single particles. Agglomeration can take place by return of fine particles to the atomizer or to the fluid bed, by means of particle impingement around the atomizer nozzles or by rewet agglomeration in the fluid bed. Instant properties of agglomerated fat-filled powder, required for reconstitution at temperatures below the melting point of fat, are achieved by lecithination in an external fluid bed prior to powder cooling. Drying and agglomeration are performed in one process in the spray bed dryer, a spray dryer with a fluid bed integrated into the bottom of the chamber. Developed for dustless powders in the late seventies, this system 95 has since then been sophisticated for widespread use in the dairy and food industry. In the spray bed dryer a concentrated feed product is atomized by a system of pressure nozzles or by a centrifugal atomizer into a downward stream of hot air. Moist powder particles collide with dry particles in the upper part of the dryer chamber and form porous agglomerates that fall down by gravity into the fluidized bed. The fluidized bed acts as a second drying zone and as a classifier in which very fine particles are removed from the formed agglomerates. The spent drying air exits at roof level, and entrained fine particles are separated in cyclones and can be re-injected into the wet atomization zone around the nozzles, in order to form porous agglomerates here, too, in a second agglomeration process. Figure 4.4 shows a computer generated simulation of gas stream lines and gas velocity profiles above the integrated fluid bed in this process. Axial velocities FiPre 4.4 Profile Of a spray bed dryer showing air--ow streamlines and vertical air velocity 96 with external fluid bed with rewet for two-stage agglomeration for production Fig y of infant formulas bed dryer ure Spra 4 .5 As a matter of nature, the amount of recirculated fine particles, the formation of agglomerates and the process of drying in the integrated fluid bed are not independent variables. Therefore, an optional nozzle with water or feed may be placed above the powder in an external fluid bed which performs a fully independent third agglomeration process directly in the bed, for example to make instant whole milk powder. Here too fine individual particles are blown off continuously, which makes the produced powder dustless with a desired and uniform particle size, selected within a broad range. The external fluid bed acts at the same time as an after-dryer and cooler. The spray bed drying plant is compact with short powder conveying ducts and only one cyclone system, from where the powder can be recycled either to the internal fluid bed, to the external fluid bed or to the atomizer. Flexibility The mentioned principles of agglomeration in the spray bed dryer system make it extremely flexible in respect of production of controlled and unique functional powder properties: 0 The fraction of fine particles recirculated for agglomeration can be controlled separately. 0 The velocity of the fines returned can be controlled within a wide range so as to obtain a defined site and relative velocity of collision between particles and droplets in respect of desired agglomeration. 0 An adjustable direction of the nozzles allows for a controlled collision and agglomeration of droplets. 0 Rewet-nozzles allow for agglomeration and an optional lecithination of fat-rich powders in the external fluid bed. Further it has been shown that by such control of the return of fines to both atomizer and external fluid bed it is possible to agglomerate with addition of much less water than what has hitherto been possible. Further control and improvement of the quality of agglomerates may be performed in a multistage counter current air classifier in which - by means of a rising gas stream - introduced particles are separated into two size fractions. The coarse fraction of a predetermined particle size is withdrawn as the agglomerated product. The undersize fraction is entrained in the exit gas stream from the classifier and is returned to the atomization 9 Figure 4.6 Spray bed dryer with nozzles, integratedjuid bed, air classifier and with fines return for production of agglomerates with low dust content 99 zone as fine particles for formation of agglomerates. Figure 4.6 depicts an illustration of the classifier from which fine particles are returned to the atomization zone while agglomerated particles are withdrawn from the valve at the bottom. In this way it is possible to produce agglomerates with a low dust content and excellent fluid-mechanical properties. Some products are very difficult to dry and tend to stick to the chamber wall. This can be prevented by an optional rotating arm which blows compressed air through slits against the wall. The rotating arm gives a prolonged period of time between cleaning intervals of the spray bed dryer, and makes possible the drying of some products which previously were regarded as unsuitable for spray drying. Hygiene Detachable insulation cladding is becoming a market standard in order to enable inspection of the chamber walls from the outside with the purpose of detecting leakages or micro-cracks, Fig. 4.7. Such panels combine the Figure 4.7 Easily detachable insulation panels for inspection of the chamber wall against leakages or micro-cracks 100 advantage of an air gap between chamber wall and insulation and the totally encapsulated cladding, thus ensuring the best possible conditions for protection against bacterial growth. The insulation panels may be made of foamed-up polyurethane covered on both sides with stainless steel. Conclusion The spray bed dryer offers a unique possibility of flexible production of high quality, tailor-made powder products -including concentrates and products with high contents of fat and properties according to customers' wishes and to international standards, such as: Ice cream mix Whey permeate Lactose Yoghurt Whey protein concentrate Infant formulas Milk protein contentrate Caseinate Fat-filled whey Cheese Coffee whitener 101 Tho Stage Spray Dryer During drying the atomised particle experiences two separate phases. The first is the constant drying rate period when the unbound moisture is removed. Surface moisture is rapidly evaporated, and so is the moisture within the particle, which can move by capillary forces quickly to the outside. The second period is the falling rate when bound moisture diffuses to the surface for evaporation. The rate of diffusion decreases with the moisture content and so the time required to remove the last few percent of moisture takes a major part of the residence time with a single stage dryer. High outlet temperatures, are needed to give a sufficiently large driving force to remove this moisture and this adversely affects the thermal efficiency of the process. Figure 4.9 TWO Stage Spray dVeY I02 Three-stage spray dryer The Anhydro three-stage dryer, consisting of a spray dryer with integrated fluid bed and an external fluid bed, is the most flexible system for production of a large variety of agglomerated and non-agglomerated dairy and food products. The atomization principle can be either nozzle or centrifugal. The internal fluid bed normally works as second stage dryer, and the external fluid bed as third stage dryer and cooler or, for products with high fat- content, as cooler only. Figure 4.10 Three stage spray dnjer 103 Performance Comparison Table 4.4 shows an example of typical figures for the 3 systems with an evaporation capacity of 2000kg per hr. Drying Process Unit Single Two Three S tage Stage S tap Powder production kg/h 1980 1980 1980 Solids in feed % 48 48 48 Inlet temperature to spray dryer 'C 210 230 240 Outlet temperature from spray dryer 'C 100 85 78 Heat consumption MJoule/h 8811 7453 71 14 Water evaporation total kg/h 2000 2000 2000 Water content from spray dryer 46 3.5 6.5 9 Electric consumption kW 167 151 164 Cooling consumption MJoule/h 205 90 63 Operating hrs. per year 6Ooo 6ooo 6Ooo Heat costs (0.03 E/MJoule) f/Yr 379,436 320,940 306,360 Electric costs (0.04 f/kWh) E/Yr 40,080 36,240 39,360 Cooling costs (033 f /MJoule) E/Yr 68,600 30,066 21,000 Energy costs per Yr f 488,120 387,246 366,720 Investment costs, total E 666,700 766,700 833,400 Investment costs (deprec. 20 years, interest 10% per year) E/Yr 75,000 90,000 96,700 Energy and investment costs f/Yr 563,120 477,246 463,386 Index (energy + investment cost) 100 85 82 TABLE 4.4 Comparatize operating wsts in spray dying I04