Modelling of multiple-effect falling-film evaporators

The classic mathematical model of multi-effect evaporators operating in forward flow was combined with an accurate estimation of the overall heat transfer coefficient (HTC) in falling-film evaporators. For this correlation developed by Narayanamurthy & Sarma (1977) has been used. It was possible also to estimate the order of magnitude of the fouling factor (Rd). Finally, the design of an orange-juice double-effect falling-film evaporator (DEFFE) system was carried out according to two different strategies.     

分隔式降膜蒸发罐及其传热优势

介绍了一种新型降膜蒸发罐－－分隔式降膜蒸发罐,并通过对降膜蒸发罐的糖液沸点升高和蒸发罐传热系数的分析讨论,阐述了分隔式降膜蒸发罐的传热优势。     

牛奶降膜蒸发传热膜系数的研究

本文简要回顾了前人对牛奶降膜蒸发传热膜系数的研究成果;在牛奶降膜蒸发实验中,考察了液膜流动、汽相流动、传热条件及牛奶物性等因素对牛奶降膜蒸发传热性能的影响;测定了牛奶降膜蒸发传热膜系数,导出了计算该系数的Nu准数关联式,为牛奶降膜蒸发设备的设计及优化提供了可靠依据。     

竖管降膜蒸发器的性能研究

为了给竖管降膜蒸发器的设计提供依据，对降膜蒸发器的总传热系数的计算进行了讨论。分析了采用螺旋型分布器时的压力降。通过实验对所确定的公式及算图进行了验证。     

新型碱回收蒸发设备——管式降膜蒸发器

管式降膜蒸发器是一种新型制浆造纸废液回收处理蒸发设备,它具有结构简单、投资低、传热效率高、加热面易清洗、耗电量低、出液浓度高、操作弹性大且结构牢固、容易维修、使用寿命长、不易结垢等特点。以管式降膜蒸发器组成的管式降膜蒸发站已被广泛应用于草浆黑液、苇浆黑液、竹浆黑液及木浆黑液等碱回收项目中。     

自然循环自动清洗式高效节能蒸发器的核心理论研究

严重影响现有蒸发器效益的焦点是高能耗的强制循环泵和加热面盐垢的周期性停车清洗。自然循环自动清洗式蒸发器能够满意地解决这两个问题 ,为企业创造相当可观的效益。以动力温度概念和不平衡汽化过程机理为基础的自然循环推动力理论研究 ,不仅得到有很大意义的计算模型 ,并且给出了自然循环推动力有效强化的思路 :设计较大截面、足够深度、出口部呈渐扩形的沸腾室来提高汽化的平衡度、降低出口动能损失 ;必须采用安装有像螺旋齿带那样能够低流速下自转、高效强化传热、较低流体阻力的自动清洗元件的短管加热室 ,以便增大动力温度 ,又使自然循环总阻力较低。按此动力学理论设计的自然循环自动清洗式蒸发器顺利地获得中试成功。     

基于不确定性理论的乳品降膜蒸发器设计

基于蒙特卡洛不确定性方法,研究了乳品三效降膜蒸发器中参数和模型两类不确定性对设计结果的影响。结果表明,与参数不确定性相比,模型不确定性导致的蒸发器传热性能变化更为显著。通过综合考虑参数不确定性与模型不确定性的影响,提出模型集成法以降低模型不确定性,提高模型预测可靠性。基于不同的设计概率选取蒸发器传热面积,可为不同设计条件和不同操作条件下的蒸发器设计和运行提供理论依据。     

板式蒸发器设计应注意的几个问题

板式蒸发器的最大优点是体积较小,占用空间小;与管式降膜式蒸发器一样在负压下蒸发,属于低温蒸发,其主要形式有升膜式、升降膜式及降膜式三种,降膜式在国内还不多见。对升降膜板式蒸发器存在的问题及设计时应注意的事项进行了阐述。     

HOW TIME BETWEEN CLEANINGS AFFECTS PERFORMANCE AND SUCROSE LOSSES IN ROBERT'S EVAPORATORS

In U.S. sugarcane factories, multiple‐body evaporator stations mostly consist of Robert's‐type calandria (rising film) evaporators that are simple, robust and easy to operate, but are susceptible to scaling and require periodic cleanings. Scaling reduces heat transfer, but its effect on sucrose losses is unknown. Therefore, a comprehensive study was conducted at a U.S. factory to determine the effects of time between evaporator cleanings on overall sucrose losses and evaporator performance. The factory operated two Robert's pre‐evaporators in parallel, and three sets of triple‐body Robert's evaporators in parallel. Each body was usually subjected to a 9‐day cleaning cycle, and two cleaning cycles were studied in early and midseason, respectively. Gas chromatography was used to determine sucrose losses asΔ% glucose/% sucrose ratios. A wide range of 0.21–1.42% total sucrose losses to acid hydrolysis were measured across the station, which were affected by seasonal changes in clarified juice quality. Losses in the pre‐evaporators were up to 98% of the total station losses in early season but decreased in midseason, and were more a function of temperature, heating surface, °Brix (% dissolved solids), scaling and pH than retention time (R_t). Scaling had a dramatic effect on increasing sucrose losses, because it causes increases in R_ts as a result of decreased heat transfer coefficients and flow rates, and rises in the heating juice temperature to partially compensate for the reduced heat transfer in scaled tubes. For both pre‐evaporators, as time between cleanings increased from 1 to 8 days, the amount of sucrose hydrolysis increased. In general, hydrolysis occurred in the first and second evaporator bodies only when scale had built up, i.e., >6 days after the last cleaning, and became worse until the next cleaning. The contribution of sucrose losses to economic losses is described.     

DTDC蒸脱机热风风网节能降耗的技术改造

植物油加工企业的节能降耗是维持其持续发展的重要保证。在原有的400 t/d浸出生产线基础上,将一蒸、二蒸的溶剂蒸气出口与蒸发冷凝器之间增加风冷换热器,充分利用其蒸发器出来的二次溶剂蒸气所含的热量给干空气加热,通过传质、传热降低DC湿粕中的水分及温度,既减少了蒸发二次溶剂蒸气冷凝所需的冷却水用量,又省去原有冷凝回水加热热风的热量,有效降低了植物油厂产品加工吨料成本。     

Modelling of heat transfer in a climbing-film evaporator: Part I

The absence in the literature of precise performance details and of any relationship between the heat transfer, the operating conditions and the physical and dynamic properties of the liquor fed to a climbing-film evaporator led us to develop a mathematical model to evaluate the performance of such an evaporator.The theory based on heat and mass balance equations is presented in this part of the paper: this model is validated by application to a pilotscale single-tube evaporator in part II. In part III the model is used to predict the performance of bigger industrial evaporators.     

EVAPORATION IN A HORIZONTAL THIN FILM SCRAPED SURFACE HEAT EXCHANGER

Literature surveys have revealed inadequate information on heat transfer characteristics during evaporation in straight-sided horizontal thin film scraped surface heat exchangers (SSHE). the evaporation of water, concentration of milk and dehydration of cream (30–40% total solids) at different rotor speeds, number of blades, flow rates and temperature differentials were studied in 108 tests with objectives to develop a predictive equation for overall heat transfer coefficient and to study its variation with regard to above parameters in the light of proposed mechanism governing fluid flow, film formation and heat transfer. Data were processed in HCL System-4 computer to fit in quadratic form by method of least squares. Experiments were conducted at higher temperatures compared to those encountered in milk evaporators. the information would be useful in designing SSHE for processing several Indian dairy products.     

P-RC APMP制浆废水沸腾传热系数的研究

研究了P-RC APMP制浆废水的沸腾传热系数,实验在强制循环蒸发器上进行,考察了热通量、流体流速、流体浓度对P-RC APMP制浆废水沸腾传热系数的影响。结果表明,在一定的热通量下,随着流体流速的增加,P-RC APMP制浆废水沸腾传热系数增加;浓度不同,流速对传热系数的影响不同,浓度越大,流速对传热系数的影响越大。     

料液的沸点升高对蒸发器换热面积的影响

溶质的存在可使溶液的蒸汽压较纯水的的低,从而使沸点升高。不同性质的溶液在不同的浓度范围内沸点上升数值是不同的,浓度越高沸点升高也越大。沸点升高会导致蒸发器的换热面积增大,在计算蒸发器换热面积时是不能忽略的。仅以RNJM03-6300型三效降膜式蒸发器为例加以阐述。     

干、湿工况下蒸发器计算机仿真的理论基础

详细介绍了制冷时干、湿工况下蒸发器盘管空气侧、制冷剂侧的传热系数相关计算公式；同时给出了湿工况下析湿量的计算方法，为蒸发器计算机仿真建立了理论基础。     

Subject: 'Skim milk' Technology of skimmed milk evaporation

The paper seeks to show how the requirements of skimmed milk concentration are met in modern evaporators and how the type of plant used has developed over the years, the falling film tubular evaporator now being virtually standard. Very economical operation is now possible with sophisticated multiple-effect plants with thermocompression, and further economies are possible with mechanical vapour recompression. The importance of adequate wetting of heat transfer surfaces is discussed with regard to length of run and achievable concentration, and methods of maintaining correct wetting conditions are illustrated. Preheating is an important feature of such evaporators for both economy and control of final powder properties, and the methods adopted are described, Finally, the control of modern skimmed milk evaporators is considered briefly.     

ENERGY CONSUMPTION IN A CONCENTRATED ORANGE JUICE PLANT

The consumption of electricity and thermal energy for frozen concentrated orange juice (FCOJ) and citrus pulp pellets (CPP) was determined in a plant. Thermal energy accounted for 90% of the total energy consumption in the plant and its consumption for CPP exceeded that for FCOJ. The kilocalories of thermal energy per kg of water evaporated (KKWE) in the evaporators increased as the feed rate of single strength juice was decreased. At the design evaporation capacity, the steam efficiency of two tubular evaporators and two plate evaporators was found to be 0.85N and 0.82N, respectively; N being the number of effects of the evaporator. Fouling of the waste heat evaporator was a major reason for the high energy consumption in the CPP unit.     

机械蒸汽再压缩系统的流程模拟及性能研究

为使得整个机械蒸汽再压缩(MVR)蒸发系统的稳定性、节能性更好,借助Aspen Plus软件学习版,根据MVR运行原理,构建了MVR性能分析模型,并通过改变MVR节点的参数,模拟研究了蒸发量、补充新鲜蒸汽量与进料温度、蒸发压强的关系;蒸发量、沸点与进料浓度(NaCl质量分数)、蒸发压强的关系;蒸发器换热量、COP与蒸发压强、压缩机压力升的关系。通过分析模拟结果得出:适当减小压缩比,可以提高蒸发系统的COP;原料液应该加热到沸点后,再进入蒸发器中进行换热;从节能效果看,MVR蒸发系统更适合在低温低压下运行;在蒸发前,应对浓度较大的原料液进行适当的稀释。     

Modelling of heat transfer in a climbing-film evaporator: Part II

The equation developed theoretically in part I for the liquid zone is validated from experimental data obtained with a pilot size single tube glass climbing-film evaporator.From experiments with the feed entering at the boiling temperature the heat transfer coefficient in the boiling zone is calculated.The validity of the proposed model is demonstrated for sucrose solutions of different concentrations and for tomato juice.     

降膜蒸发器的节能设计

降膜蒸发器是一种高效蒸发设备，传统的设计是通过增加效数、利用热泵和水力喷射器的结构形式。在相同效数基础上，采用热泵、冷凝器、真空泵、凉水塔与水环真空泵系统节能达到节能降耗的目的．