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1、外文翻译-面对商业障碍和复杂的关系设计枯燥器 Drying As Much Art As ScienceDesigning dryers in the face of commerical barriers and complex relationsBy Edward M Cook Energy Saving Consultants Boynton Beach Fla After a half century the drying of wet solids slurries and solutions is still as much art as science Advances in d
2、rying technology are held back partly by commercial barriers and traditions but more by the complexity of solidliquidvapor relations in the myriad substances that can be dried and their many different properties Materials that need drying cover a wide range of foods pharmaceuticals polymers minerals
3、 wastes and a host of organic and inorganic chemicals Even similar substances rarely exhibit identical drying characteristics because reluctance to release moisture varies widely Thus dryer designs are based not on theoretical concepts but on data from pilot tests or from past experience This data h
4、elps size new drying systems or evaluate existing ones The determinations may be rough estimates or detailed analyses using equations It is process design as contrasted with mechanical design which is mostly proprietary for each of the more than 200 direct dryer variations available from at least 15
5、0 manufacturers Continuous open or once-through direct dryers are the most widely used in the chemical industry The main types are spray flash fluid-bed rotary conveyor and tray dryers Sizes and designs differ greatly even within each type but the basic drying principles are common to all Water is t
6、he liquid in the great majority of cases but other liquids can be handled in closed systems Psychrometric charts are a time-honored means of estimating the process conditions of a dryer and are even resorted to with the equation method Well show with an example how equations yield results Setting th
7、e equations into computer programs reduces the work involved but final accuracy is limited by the input data Fig 1 contains the key elements a typical minimum system of the once-through design Hot air or other gas directly contacts the wet material evaporates the liquid and carries off the vapor The
8、 example chosen is at 2000 ft elevation and natural gas is the heat source Feed is 270 lbhr and the moisture contents are 55 water in the feed and 5 in the product The air temperatures are given in Table 1 and were chosen to simplify the operation and confine it to a small area Knowing the wet bulb
9、temperature at the dryer inlet helps find the air moistureFig 1 A simple once-through direct drying systemKnowing it at the outlet requires making a more difficult measurement but it can eliminate the effects of the unknown or ignored nonadiabatic factors increasing accuracy Air temperature differen
10、ce between dryer inlet and outlet drives the process A greater difference allows more water to be evaporated into each pound of air This minimizes the use of both air and energy The imum temperature at dryer inlet is limited however by the heat sensitivity of the solids And for high-temperature appl
11、ications the limit may be the expansion and temperaturestrength relations of the materials of construction The minimum allowed at the dryer outlet depends on the desired product moisture and other properties and sometimes on material build-up on the equipment walls The moisture in the supply air to
12、the heater could be found by trial and error using relative humidity or some other measure with equations for saturated moisture and enthalpy But a psychrometric chart is far more practical and is used in nearly all cases At dry bulbwet bulb of 6050oF a chart drawn for one atmosphere gives 00055 lbl
13、b dry air which is close enough at 2000 ft it is actually 00004 higher Enthalpy and humid volume of the supply air will be calculated later Doing them all at the same time is more efficient and less error-prone Combustion of fuel contributes moisture to the airflow and the amount can be found using
14、the empirical data in Table 2 For low and high temperatures it lists the slope of the line of temperature difference burner outlet minus inlet plotted against the moisture added When necessary you can interpolate to get intermediate values Slope times the temperature difference gives the moisture ad
15、ded For instanceMf Sl T2 - T1 230 x 10-5 x 260 - 60 00046combustion moisture lblb dry airM2 M1 Mf 00055 00046 00101air moisture out of the heater lblb dry air Note that moisture is expressed as a unit weight of dry air The same convention is used for enthalpy and humid volume For example enthalpy is
16、 in Btulb dry air These ratios unify the relationships and simplify the calculations Air enthalpy is total heat in the dry air and its moisture as given in Eq 4 Specific heat of dry air varies from 024 under 200oF to 025 at 1000oF so here we can use a value of 024 for all stations C1 C2 C3 024 Water vapor enthalpy however is very temperature sensit
