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 ower 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 degree 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 protein denaturation, as the evaporation rate is considerably higher in the band dryer. Temperatures in the second heat zone are usually controlled at 6 10C 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 sucrose 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 dehydrators waxon 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 modern 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
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 specification. Very low drying temperaturesand 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 he prc There are two major European manufacturers of this type of dryer MitchellDryers Ltd of Carlisle, United Kingdom, and Buttner-Schilde-Haas of Krefeld, Germany and the major featuresof both areexplained here A Buttner dryer at Erin Foods, Tuam, Co. Gahvay, factory
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
The Mitchell Dryermodules are 1.8minlengthand the belt width 2.5m. Anoscillating orapron feed is optional at the inputend, and there is a stand ard delivery section at the discharge end. A7 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 take-off 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 take-off 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 ncreased 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 enum chamber. Alternatively, a series of small individual burners can be The temperatures can be infinitely varied on the different levels at hich the product is conveyed. Where in a single pass dryer three heat zones re created by a transfer module at two points in the conveyor travel, allowing a predetermined temperature in each of the three 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 wate 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)
TheMitchellDryermodulesare1.8minlengthandthebeltWidth2.5m. An oscillating or apron feed is optional at the input end, and there is a standard 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
TABLE 4.2 Feed Throughput Performance of the Mitchell Dryer5 Pass-7Module Dryer Each Moduleis 1.8m. long Band Width 2.5m Products are Vegetables and Fruit prepared for drying End Moisture is calculated at Dryer Discharge, - before in bi Produce Raw Moisture Moisture kg/hr Apples(evaporated) 10-12mm slice 88 2051 Apples(flakes) 10x10x2mm88 %244 Apples(dice) 10x10x10mm88 1795 Cabbage(dice) 1010x10mm89 Carrots(dice) 10x10x10mm90 2834 (dice) 1010x10mm 1729 Garden Peas Scarified 75 2992 n Be Long cut 2004 Bell Peppers 10x10x10mm94 2159 10x10x2mm 95 1701 Mushrooms 5mm slice 1733 Onions (slices) 4mm slice 85 2091 Potato(slices) 4mm slice 80 2267 Parsnip Leaf 13x13x12n 80 567656555577655 1212 Beetroot (dice) 10x10x10mm Parsley 86 973 1212 Apricots(evaporated) Halves 642 Pears(evaporated) Quarters Prunes(Plums) Whole 83 642 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 fication data courtesy of Mitchell Dryers Ltd. A Buttner-Schilde-Haas 5 pass 10 module dryer was observed regularly in the early 1970s in an Indian factory, drying onions exclusively
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