APV Dryer Handbook - Umbc

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10 Direct heating is used extensively in industrial drying equipment where much higher thermal efficiencies are exhibited than with indirectly heated dryers. This is due to the fact that there are no heat exchanger losses and the maximum heat release from the fuel is available for the process. However, this method is not always acceptable, especially where product contamination cannot be tolerated. In such cases, indirect heating must be used. With forced convection equipment, indirect heating frequently employs a condensing vapor such as steam in an extended surface tubular heat exchanger or in a steam jacket where conduction is the method of heat transfer. Alternatively, systems which employ proprietary heat transfer fluids can also be used. These enjoy the advantage of obtaining elevated temperatures without the need for high pressure operation, as may be required with conventional steam heating. This may be reflected in the design and manufacturing costs of the dryer. Furthermore, in addition to the methods listed above, oil- or gas-fired indirect heat exchangers can also be used. In general, dryers are either suitable for batch or continuous operation. A number of the more common types are listed in Table 1, where an application rating based on practical considerations is given. In the following review, some of the factors likely to influence selection of the various types are discussed for particular applications. BATCH DRYERS It will be apparent that batch operated equipment is usually related to small production runs or to operations requiring great flexibility. As a result, the batch type forced-convection unit certainly finds the widest possible application of any dryer used today. The majority of designs employ recirculatory air systems incorporating large volume, low pressure fans which with the use of properly insulated enclosures, usually provide thermal efficiencies in the region of 50 to 60%. However, in special applications of this type of dryer that call for total air rejection, this figure is somewhat lower and is largely related to the volume and temperature of the exhaust air. Capital investment and installation cost are relatively low. The use of fan systems minimizes both power requirements and operating costs. In contrast, labor costs can be high.
OUTPUT (LBS/24HRS) 8000 6000 4000 2000 4 THRUFLO UNITS 3 THRUFLO UNITS TYPE ‘C2’ 2-TRUCK EXTRUDED MATERIAL TYPE ‘C2’ 2-TRUCK FILTER CAKE 2 THRUFLO UNITS 1 THRUFLO UNIT TYPE No. 2 2-TRUCK FILTER CAKE 0 0 5 10 15 20 25 30 NUMBER OF TWO-TRUCK UNITS Figure 2. Comparative performance curves for Thruflo and conventional units In such a plant, the drying cycles are extended, with 24 to 45 hours being quite common in certain cases. This is a direct result of the low evaporative rate, which normally is in the region of 0.15 to 0.25 lb/ft2hr. (0.7 to 1.2 kg/m2h). Following the recent trend and interest shown in preforming feedstock with regard to the design of extruding and tray-filling equipment for de-watered cakes – it is now possible to obtain the maximum benefit of enhanced evaporative rates by using through-air circulation dryers when handling preformed materials. Figure 2, shows how a high-performance dryer can produce 1950 lbs (890 kg) of dried material in a 24 hour period, at a terminal figure of 0.5% moisture (when handling a preformed filter cake having an initial moisture content of 58%). The significant improvement in performance can be seen from the curve, where the Figure 3. Thruflo dryer 3000 2000 1000 OUTPUT (KGs/24HRS) 11
Page 1: Improving Process Profitability...C
Page 5: I N T R O D U C T I O N The drying
Page 8 and 9: 8 POWDER TYPE D R Y E R S Fine Free
Page 12 and 13: 12 corresponding number of conventi
Page 14 and 15: 14 Two particular types are the dou
Page 16 and 17: 16 particular application and the m
Page 18 and 19: 18 whether there is any need to re-
Page 20 and 21: 20 material comes into contact with
Page 22 and 23: 22 together since both share simila
Page 24 and 25: 24 circulation. Alternatively, and
Page 26 and 27: 26 Each of the types available are
Page 28 and 29: 28 At this time Pulse Drying techno
Page 30 and 31: 30 E F F I C I E N T E N E R G Y U
Page 32 and 33: 32 DRYER FEED MOISTURE CONTENT (% W
Page 34 and 35: 34 DRYING COLUMN SCREW FEEDER PARAM
Page 36 and 37: 36 way a dryer can be turned down i
Page 38 and 39: 38 3. The wetted particles sink int
Page 40 and 41: 40 TEMPERATURE (°C) 150 100 50 0 4
Page 42 and 43: 42 flow patterns, residence time di
Page 44 and 45: 44 As indicated by this partial lis
Page 46 and 47: 46 FEED HP PUMP COMPRESSED AIR AGGL
Page 48 and 49: 48 S P I N F L A S H D R Y E R S fo
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Page 54 and 55: 54 The drying chamber would be main
Page 56 and 57: 56 COST BENEFITS In comparison to a
Page 58 and 59: 58 S P R A Y D R Y E R S SPRAY DRYE
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60 inwards towards the centerline o
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62 a high speed by a spinning disc.
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64 internal bowl diameter, radial d
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66 variations, some plants utilize
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68 Condensation deposits can freque
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70 HIGH FAT PRODUCTS If the powder
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72 INLET TEMPERATURE The powder coo
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74 POWDER DENSITY The density of sp
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76 chamber wall, tending to scour a
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Local Office APV Separations Produc
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