外文翻译---干燥：囊括科学之技巧
附录一中文译文
干燥：囊括科学之技巧
——面对商业障碍和复杂的关系设计
干燥器
经过半个世纪以来，湿料-泥浆和溶液的干燥仍佣尽科学之技巧。干燥技术的发展受商业障碍和传统所阻挡，但影响更大的是在多种干燥物质中固-液-气关系的复杂性和它们的一些不同的属性。
干燥应用极广，例如食物、药品、集合物、矿物质、回收品以及大量的有机物和无机物都需要干燥。由于干燥之后的湿气各不相同，即使相同的物质也很少能表现出一致的干燥特性。
因此干燥器的设计不基于干燥理论原理，而是建立在实验测试或过去经验的数据的基础上。这些数据有助于确定干燥系统的尺寸或者估计已有的大小。使用的方法可以是粗略的估计，也可以使用公式进行细致的分析。相比与机械设计，干燥器的设计是一个过程性的设计，这是至少150个制造商使用的200多个干燥参数所共有的属性。
在化学工业中，连续、开放或直通的直接干燥器应用最广泛，主要类型有：喷雾式，闪蒸式，流化床式、滚筒式、回转式以及盘式干燥器。即使在每一同种类型中，大小和设计大不想同，但基本的干燥原理都大致一样。水是大部分干燥中的成分，然而在封闭系统中，也可以干燥其它的液体。
干湿表长期都用于估计干燥器进展程度，也需要用算术来解决。用一个实例来说明如何用公式法得出结果。为了减轻工作量，我们把公式输入到计算机的程序中，不过最后结果局限于输入的数据。
图1包含关键元素——一种典型，小型的直通设计的系统。热空气或其他气体直接地与湿物料接触，湿分被气体所蒸发。这实例选择在海拔2000英尺的状态下，用天然气作燃料。处理量是每小时270磅，进口含水量是55%，出口含水量是5%。表1中给的空气温度可进行简化选择，得出一个小的温度范围就可以。如果知道干燥其内部的湿球温度，就可以通过查表得到气体的湿度。
图1：一个简单的直通式直接干燥系统
要知道外部参数，就需要做一个更复杂的测量，但是这样能减少不知道或被忽略的影响，提高准确率。
干燥器内外气体温差使干燥能够进行，温差越大，每磅空气能干燥的水分就越多。这样就能减少空气和能量的使用。然而，由于物料的热稳定性，干燥器里面的最高温度总受限制。
而且，由于高温的应用，这限制将会扩大，并影响建筑材料的温度-应力值关系。干燥器出口允许的最小输出量依赖于期望产品的湿含量和其他特性，有时也与设备壁的物料垢有关。
对于加热器，入口气体的湿含量通过实验和粗略的使用相对湿度或其他措施来测量，然后用饱和湿度与焓之间的关系式算出结果。干湿表非常实用，几乎能用于每一种情况。假定干球/湿球温度为60/50F，从表中就可以得到1大气压下，每磅干空气中含水0.0055磅，这与实际数据足够接近（比海拔2000英尺下实际高0.0004）。空气的焓和湿体将在下文中计算。两量一起计算能减少错误，提高效率。
在燃料的燃烧过程中，由于产生水分，夹入到热空气中。其含水量可通过表2种经验数据得出。在表中，通过各个低温和高温不同的值，表示出了温差线的斜率，是与湿含量成反比。必要的话，可用插值法计算值。
斜率乘以温差就能得到湿含量的增量，如：
Mf = Sl (T2 - T1)
= 2.30 x 10-5 x (260 - 60)
= 0.0046
总湿含量，磅/磅干空气
M2 = M1 + Mf
= 0.0055 + 0.0046
= 0.0101
加热器出口空气的含湿量，磅/磅干空气
注意到湿含量是用每单位重的干空气的含水量来表示。同样也可用于焓和湿体的表示。例如，焓单位是Btu/lb干空气。这些比率使关系得到统一，简化了计算。
空气的焓湿干气及其湿气的热量的总和。特别地，在200F到1000F干空气的热量从0.24升到0.25，所以在这我们可以用一个值0.24来进行计算个点的值（C1=C2=C3）。然而，水蒸气的焓随温度的变化变化很大。它的经验式是：
Wi = 1061.8 + 0.433 Ti + 0.00004 Ti2
W1 = 1087.9
W2 = 1177.1
W3 = 1123.2
enthalpy of water vapor, Btu/lb
空气的焓的计算式是：
Hi = Ai Ci Ti + Ai MiWi
空气和湿气的焓，
我们定的是1磅干空气，因此Ai=1.0,代入这三个点，变成：
H1 = C1 T1 + M1 W1
= 0.24 x 60 + 0.0055 x 1087.9
= 20.4 Btu/lb dry air
H2 = 74.3
H3 = H2 = 74.3
直接干燥经常叫做等热干燥，也就是干燥器出口处热量总和与入口处相等，空气所带的热量提供干燥时必要且隐藏的热量，而整个系统中总热量不发生变化。大多数简单的干燥装置近似于等热干燥。
因为假定是等热干燥，在2和3点的焓相等，这样不用的量和物料平衡就能把3点的湿量算出来。现通过输出量就能计算出3点的M3：
干燥器出口时的湿含量，磅/磅干空气
如果出口气体的湿球温度已知，就能避免作等热的假设，在干湿表总能读出口端的湿含量。举例来说，假设侧的湿球温度湿98F，3点的湿含量就是0.0332磅/磅干空气，从最后的结果可以看出，热量效应使流速提高约13%。
能引起等热操作的偏差的因素如下列举：
供给的热量
通过内部环热传导物料的热量
结晶或者反应固体热量
辐射—对流热损失
产品带走的热量
各方面空气的流入（如冷却）
这些因素的计算方法在“过程干燥练习”或其它文章中阐述。在实际中，它们对等热干燥的影响是：1、2条为提高，3、4条为可能提高也可能降低，其它都为降低。
尽管产量是最后考虑的，但与流速和装置费用相关的是干燥速度。实际上所有的直接干燥器中，仅在供给端含湿量非常低时，固体流速影响结果。下式就是将产量转化成干燥速度
干燥速率，磅/小时
干空气的流速，Ai，是用于计算干燥器出口量，因为没有什么余漏或其它流速，故也可以用于计算所有三点。干燥器进口端和出口端湿量的差值。
干燥器出口气体流速，磅/分钟
空气的质量流速必须转换成体积流量，对于大多数干燥器，容器的尺寸大小取决于出口端的体积流量。体积流量也决定着部件的大小，例如产品收集器，鼓风机，烟窗，通风管，以及任一种节约热量的交换机和吸热装置。规范入口装置的大小，例如加热器和入口空气过滤器，也是依赖于气流流量。
气流流量受操作压力的影响，操作压力可以根据当地海拔高度计算，一般在500到3000英尺，但也有可能高于5000英尺，计算压力可参考式9和式10，湿体可以从式10种得出。
Hg = 29.921 - 0.001078 x El + (8)
1.44559 10-8 x El2
= 29.921 - 2.156 + 0.0578
= 27.823 mmHg
压力, mmHg
Pt = 14.696 Hg/29.921
= 13.67 压力, psia
湿体，Cu ft/lb 干空气
Fi = Ai x Vi (11)
F1 = 191.6 x 14.21 = 2,723
F2 = 3,798 F3 = 3,296
空气流速，cu ft/min(acfm)
对于一个合适绝热的容器，热损失大约为热负荷的0.2%至0.6%，当进口与出口端之间的温差非常小或这是该台干燥器有大的漏洞或其他特别的散热装置时，采用大的等热损失可能是背离其道。更好的方法是给每个可能成为系统的阻挡物价一个适当的余量。例如加热器、鼓风机和产品收集器。干燥器将来的发展就应当按这方向发展。
计算气流流速时须根据干燥器的种类进行调整，但流速不能太高以至于能吹动太多的物料，也低于是物料流动和传输的极限速度。这些极限都是决定空气速率的量，范伟伟从输送干燥机的低于100ft/min到一些闪蒸干燥器的高于5000ft/min。
热负荷，也称热量传递速率，与加热器出口与进口空气焓的差值有关。
Qt = A1 x 60 x (H2 - H1)
= 191.6 x 60 x (74.3 - 20.4)
= 619,634
热负荷， Btu/hr
安装费用与体积流量有很大的牵连，另一方面，操作费用主要有热负荷决定，在大多数情况下，约为总能量花费的85%。
另外一个值得考虑的问题，干燥器的出口端影响产品的性质，而且还有清理问题，其最重要的是饱和的近视程度。像气体流量与压力有关，通过测量湿球、露点温度和相对湿度来求出。计算上述任意参数，都须用到饱和湿度，Ms，但所有这些参数在一张干湿表中很轻松地找到。
一些干燥界的专家喜欢选择等热饱和率作为参数，这在等焓条件下，与饱和湿度相区别的空气湿度。对于出口端，在干湿表种菜的饱和湿度市0.0452磅/磅干空气：
等热饱和率的封闭循环系统（对于非水流体）有类似的计算，除了所有的进入干燥器的湿气肯定被压缩，在加热器中，干燥气体时循环的。使用回收系统更复杂，尤其是带有在经验中经常发生的漏洞。
参考文献
[1]Cook, E. M., Chem. Eng., April, 1996.
[2]Cook, E. M., and H. D. DuMont, "Process Drying Practice," McGraw-Hill, NY,1991.
[3]Jorgensen, R. (ed.), "Fan Engineering," 8th edn., Buffalo Forge Co., Buffalo, NY, 1983.
[4] Mujumdar, A. S. (ed.), "Handbook of Industrial Drying," Marcel Dekker, NY, 1995
[5]Perry, R. H., and D. W. Green, "Perry's Chemical Engineers' Handbook," 7th edn., McGraw-Hill, NY, 1997.
附录二外文资料原文
Drying: As Much Art As Science
Designing dryers in the face of commerical barriers and complex relations
By Edward M. Cook, Energy Saving Consultants, Boynton Beach, Fla.
After a half century, the dry ing of wet solids, slurries and solutions is still as much art as science. Advances in dry ing technology are held back partly by commercial barriers and traditions, but more by the complexity of solid–liquid–vapor relations in the myriad substances that can be dried and their many different properties.
Materials that need dry ing cover a wide range of foods, pharmaceuticals , polymers, minerals, wastes, and a host of organic and inorganic chemicals. Even similar substances rarely exhibit identical dry ing characteristics, because reluctance to release moisture varies widely.
Thus dry er designs are based, not on theoretical concepts, but on data from pilot tests or from past experience. This data helps size new dry ing 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 dry er variations available from at least 150 manufacturers.
Continuous, open or once-through direct dry ers are the most widely used in the chemical industry. The main types are spray, flash, fluid-bed, rotary, conveyor and tray dry ers. Sizes and designs differ greatly, even within each type, but the basic
dry ing principles are common to all. Water is the 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 dry er, and are even resorted to with the equation method. We'll show with an example how equations yield results. Setting the 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 example chosen is at 2,000 ft elevation, and natural gas is the heat source. Feed is 270 lb/hr, 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 temperature at the dryer inlet helps find the air moisture. Fig. 1. A simple once-through direct drying system
Knowing 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 difference between dry er 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 maximum temperature at dry er inlet is limited, however, by the heat sensitivity of the solids.
And, for high-temperature applications, the limit may be the expansion and temperature–strength 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 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 bulb/wet bulb of 60/50oF, a chart drawn for one atmosphere gives
0.0055 lb/lb dry air, which is close enough (at 2,000 ft it is actually 0.0004 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 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 added. For instance:
Mf = Sl (T2 - T1)
= 2.30 x 10-5 x (260 - 60)
= 0.0046
combustion moisture, lb/lb dry air
M2 = M1 + Mf
= 0.0055 + 0.0046
= 0.0101
air moisture out of the heater, lb/lb 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 in Btu/lb 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 0.24 under 200oF to 0.25 at 1000oF, so here we can use a value of 0.24 for all stations (C1 = C2 = C3 = 0.24). Water vapor enthalpy, however, is very temperature sensitive. An empirical polynomial for it is:
Wi = 1061.8 + 0.433 Ti + 0.00004 Ti2
W1 = 1087.9
W2 = 1177.1
W3 = 1123.2
enthalpy of water vapor, Btu/lb
The equation for air enthalpy is:
Hi = Ai Ci Ti + Ai Mi Wi
enthalpy of air and its moisture, Btu/lb dry air
But our basis is one pound of dry air, thus Ai is 1.0, and for the three stations, Eq. 4 becomes:
H1 = C1 T1 + M1 W1
= 0.24 x 60 + 0.0055 x 1087.9
= 20.4 Btu/lb dry air
H2 = 74.3
H3 = H2 = 74.3
Direct dry ing is often called adiabatic dry ing, meaning that the total heat at the outlet to the dry er is the same as at the inlet. The sensible heat in the air supplies the needed latent heat of evaporation, and no change occurs in the total heat in the system. Most simple plant dry ing applications are close to adiabatic conditions.
Because adiabatic dry ing is assumed here, the enthalpies are the same at stations 2 and 3. This allows station 3 moisture to be calculated without a heat material balance.We can now calculate M3 by rearranging Eq. 4 for the outlet,
station 3:
moisture at dry er outlet, lb/lb dry air
We can avoid making the adiabatic assumption if the wet bulb temperature in the outlet air is known. Then the outlet moisture can be read on a psychrometric chart. If, for example, the wet bulb measured 98oF, the moisture at station 3 would be 0.0332
lb/lb dry air, and the final calculations would show that heat effects increase the airflows and heat load by about 13%. The conditions that can cause deviation from an adiabatic operation are listed are:
? Heat in the feed;
? Heat added to the material by internal heat
exchanger;
? Solid's heat of crystallization or reaction;
? Radiation–convection heat loss;
? Heat in the product; ? Leaks;
? Miscellaneous air inflows (as for cooling).
Calculations that include these items can be found in "Process Dry ing Practice" or other texts. Their influence on operations is: items 1 and 2 improve, item 3 can improve or worsen, the others worsen.
Although product rate is the ultimate concern, it is the evaporation rate that relates to airflow and equipment costs. In virtually all direct dry ers, the solids rate affects the results only when feed moisture is very low. The equation for converting
product rate to evaporation rate is:
evaporation rate, lb/hr
The flow of dry air, Ai, is calculated for the dry er outlet, but is the same for all three stations because there are no leaks or other airflows. The moisture difference is between dry er inlet and outlet.
airflow at dry er outlet, lb/min
The weight flowrate of air must be converted to volumetric flow (acfm). For most dry er types the acfm at the outlet determines the size of the vessel. It also governs the size of downstream elements, such as product collector, blower, stack, their connecting ductwork, and any heat-saving exchangers and exhaust-treating equipment. Sizing the upstream equipment, such as heater and inlet air filter, is based on the acfm at those points.
The acfm is affected by the operating pressure, which can be computed from the plant elevation, which is typically at 500 ft to 3,000 ft, but may be over 5,000 ft. The pressure is found from Eq. 8 and Eq. 9 and the humid volume from Eq. 10.
Hg = 29.921 - 0.001078 x El + (8)
1.44559 10-8 x El2
= 29.921 - 2.156 + 0.0578
= 27.823 mmHg
pressure, mmHg
Pt = 14.696 Hg/29.921
= 13.67 pressure, psia
humid volume, cu ft/lb dry air
Fi = Ai x Vi (11)
F1 = 191.6 x 14.21 = 2,723
F2 = 3,798 F3 = 3,296
airflow rate, cu ft/min (acfm)
Heat loss is generally between 0.2% and 0.6% of the heat load for a properly insulated vessel. Applying a large arbitrary heat loss can be especially distorting when there is a small temperature difference between inlet and outlet or for a dry er with large leaks or other extra heat requirements. A better practice is to add appropriate safety margins to components that might be system bottlenecks, such as heaters, blowers and product collectors. Future expansion plans can be handled in the same way.
The calculated airflow may have to be adjusted, depending on the type of dry er. Airflow cannot be so high as to carry off too much product, nor be dropped below a minimum for fluidizing or conveying the solids. These limits are a function of the air velocity and they range from a low of under 100 ft/min for conveyor dry ers to over 5,000 ft/min for some flash dry ers.
The heat load, also called the heat transfer rate, requires the enthalpy difference between the heater outlet and inlet.
Qt = A1 x 60 x (H2 - H1)
= 191.6 x 60 x (74.3 - 20.4)
= 619,634
head load, Btu/hr
Installation cost is largely a function of the volumetric airflow. Operating cost, on the other hand, depends mainly on heat load, and in most cases is about 85% of total energy costs.
Another consideration, most important at the dry er outlet, where it may affect product properties and add to clean-up problems, is the proximity to saturation. Like the acfm, this is a function of pressure. Measures used to identify it are wet bulb and dew point temperatures and relative humidity. Calculating any of these involves iteration to determine saturation moisture, Ms, but all are easy to find using a psychrometric chart.
A measure preferred by some dry ing experts is adiabatic saturation ratio, which is the air moisture divided by the saturation moisture at the same enthalpy. Saturation moisture found on a psychrometric chart for our outlet condition is 0.0452 lb/lb dry air:
adiabatic saturation ratio Closed-cycle systems (as for nonaqueous liquids) have similar calculations, except that all moisture that enters the dry er must be condensed,
and the dry ing gas is returned (saturated) to the heater. Partial-recycle systems are more complex, especially with the leakage that often occurs in actual experience. BIBLIOGRAPHY
Cook, E. M., Chem. Eng., April, 1996.
Cook, E. M., and H. D. DuMont, "Process Dry ing Practice," McGraw-Hill, NY, 1991. Jorgensen, R. (ed.), "Fan Engineering," 8th edn., Buffalo Forge Co., Buffalo, NY, 1983.
Mujumdar, A. S. (ed.), "Handbook of Industrial Dry ing," Marcel Dekker, NY, 1995 Perry, R. H., and D. W. Green, "Perry's Chemical Engineers' Handbook," 7th edn., McGraw-Hill,
NY, 1997.