Heat transfer studies in a climbing film evaporator: Part I. Heat transfer from condensing steam to boiling water

Hard water was concentrated using steam heating in a single tube climbing film evaporator, of length 9 ft. and external diameter 1 inch. The heat flux Q/A in BTU/hr. sq. ft. was found to be related to the feed rate of the water M lb./hr. and its temperature T_i °F by the equation Q/A = ψ M^0.6 + 90.3 (T_b – T_i) C_p where ψ is a graphically read function of the temperature difference driving force between the condensing steam and the boiling water. The data fit the above equation with a standard deviation of 2.6%. The empirically derived equation for heat flux was also found to represent the data for other aqueous solutions with reasonable accuracy.     

Heat transfer in condensation and boiling in a tubular film evaporator

The results of a study of heat transfer in condensation and boiling in a tubular evaporator with smooth and rough surfaces under the gravity flow of a water film are presented. Relationships are derived for calculating a heat-transfer coefficient, and the effect of helical roughness on heat transfer is revealed.     

3万t/a镍升膜蒸发装置的改造

我厂有两套升膜蒸发装置,其中一套升膜蒸发器的加热室和蒸发室材质为不锈钢,设计产能为3万t/a ;另一套升膜蒸发器的加热室材质为镍,蒸发室上部材质为不锈钢,底部为不锈钢衬镍,设计产能为2万t/a 。两套装置的进料和成品冷却共用一套系统。生产出来的成品主要通过两个换热器降温冷     

Heat transfer in a rotating disk evaporator

Experiments were carried out in a rotating disk evaporator with and without a wiper on the evaporation side. The tests were carried out with copper and with monel disks. Overall heat transfer coefficients were obtained directly from the measurements. In addition, evaporation heat transfer coefficients were calculated and the results were correlated. The effectiveness of the wiped film evaporator was clearly manifested in the larger heat transfer coefficients that were obtained.     

Heat transfer in turbulent flow on a horizontal tube falling film evaporator. a theoretical approach

A simplified theoretical approach for the prediction of evaporation (no-nucleate boiling) heat transfer coefficient in a horizontal tube falling film is proposed. The correlation is derived from an analysis of the thermal boundary layer under the assumption of turbulent flow regime and taking into account the thermal developing region. A diagram to evaluate $\text{h}$(μ²/ρ²gk³)^{$\text{1}\text{3}$} as a function of Re and Pr is proposed.A regression analysis of the numerical computations allows us to provide a dimensionless formula $\text{h}$(μ²/ρ²/gk³)^{$\text{1}\text{3}$} = 0.046 Re^0.18Pr^0.47 valid in the ranges Re = 1500–5000 and Pr = 1–5.     

板式升膜蒸发器在离子膜烧碱中的应用

介绍了升膜蒸发的原理及影响传热量的因素。重点介绍了板式升膜蒸发器在离子膜烧碱蒸发系统中的应用情况及蒸发装置的特点。     

关于提高降膜蒸发器镍管使用寿命的探讨

通过计算镍管的强度极限及失稳破坏条件,分析了片碱引进工程进口设备降膜蒸发器出现中心镍管过早损坏的原因,提出以国产镍管替换进口材料以及提高降膜蒸发器中心镍管使用寿命的措施。     

Modelling of flow in a wiped film evaporator

In a wiped film apparatus, the hydrodynamic conditions of the flow have a direct effect on the film thickness and on the residence time of the molecules and thus on the effectiveness of mass and heat transfer. Measurements of the residence time distribution (RTD), for different stirring velocities, allowed to propose a model for the flow as a series of perfectly mixed cells in parallel with a series of perfectly mixed cells exchanging mass with a dead zone.     

Heat and mass transfer in non-isothermal absorption of gases in falling liquid films Part II: Theoretical description and numerical calculation of turbulent falling film heat and mass transfer

A mixing length turbulence model is presented, which was used for the numerical calculation of turbulent falling film heat and mass transfer coefficients. Numerous proposals of different authors for the prediction of eddy transport coefficients are discussed. Good agreement between calculated heat and mass transport coefficients and experimental data was achieved by the combination of Hubbard, Mills and Chung's proposal for the calculation of eddy transport near the film surface with a modified form of van Driest's, the latter being for the eddy transport near the solid wall. Furthermore, the dependence of the turbulent falling film heat transfer coefficients on the Prandtl number is discussed. It is shown that, besides the Prandtl number, a further dimensionless group must be considered in order to describe the effect of liquid properties on heat transfer.     

Heat transfer mechanisms in gas fluidized beds. Part 3: Heat transfer in circulating fluidized beds

Part 1 of this contribution reported on the effects of system properties on heat transfer between heating or cooling surfaces and bubbling fluidized beds. This investigation produced four correlations which define the respective maximum heat transfer. Part 2 of this study suggests that the heat transfer between exchanger surfaces and bubbling fluidized beds depends on superficial gas velocity, expressed as dimensionless excess gas velocity. The present paper shows that heat transfer coefficients in circulating fluidized beds can be predicted by evaluation of a state diagram, which combines three dimensionless groups: Nusselt number, Archimedes number and a dimensionless pressure gradient. A comparison of coal combustion experiments with own cold model measurements indicates that the radiative component of heat transfer coefficients is only evident at very low dimensionless pressure gradients.     

伞板式降膜蒸发器传热性能的实验研究

针对专利设备伞板式蒸发器,建立了一套实验装置。以水和5%~40%的碳酸钾水溶液为工作介质,对实验装置的传热性能进行了实验研究,分析了影响伞板式降膜蒸发器传热的主要因素,并且得到了伞板降膜蒸发侧无因次传热系数的实验关联式。通过对液膜破裂的实验研究,证明了该结构的蒸发器在小流量情况下液膜不易破裂的特性。     

伞板式降膜蒸发器传热性能研究

对伞板式降膜蒸发器进行了合理的结构设计,并对此蒸发器的传热性能进行了实验研究。结果表明,伞板式降膜蒸发器在低温差下仍能保持较高的传热系数,得出了伞板降膜蒸发侧无因次传热系数的实验关联式并与竖直光滑管的传热性能进行了比较。同时对伞板式蒸发器进行了液膜破裂的实验研究,得出了液膜破裂条件的实验关联式并与竖直管的破断条件进行了比较,从而得出了此蒸发器在小流量下,液膜分布均匀不易破裂等特性。     

Heat transfer property of refrigerant-oil mixture in a flooded evaporator: The role of bubble formation and oil retention

We examined the effect of oil retention on the heat transfer performance of a shell-and-tube-type evaporator which had 26 inner tubes and was filled with the refrigerant R-134a. The refrigerant was boiled on the surface of the inner tubes in the evaporator, while chilled water circulated through these tubes. An experimental apparatus was designed to measure both the pressure and temperature profiles at the inlet and outlet of the flooded evaporator. Four windows were installed for observing the operation of the flooded evaporator. A series of experiments were carried out under the following conditions: the refrigerant saturation temperature, 5 °C; refrigerant inlet quality, 0.1; heat fluxes from water to the refrigerant, 5–7 kW/m². The concentration of the oil retained in the refrigerant was then varied up to approximately 10% to observe the effect on the heat transfer performance of the flooded evaporator. Increasing the oil content (i.e., increasing the concentration up to a maximum of approximately 10%) in the refrigerant R134a did not lead to any appreciable reduction in the overall heat transfer coefficient of a flooded evaporator with multiple-innertubes. When the oil concentration in the refrigerant was approximately 10%, the heat transfer degradation in the case of the flooded evaporator with multiple-inner-tubes was approximately 11%, which was found to be much smaller than the heat transfer degradation in the case of a flooded evaporator with a single-tube (26–49%). This observation suggested that the oil retained in the refrigerant did not significantly deteriorate the heat transfer performance of the flooded evaporator, presumably because the presence of tube bundles promoted forced convection by agitating bubbles.     

Heat transfer from a horizontal smooth tube to a laminar falling film

For the case in which the liquid, freely falling between two adjacent horizontal tubes, forms a liquid sheet, Rogers (1981) gave an analysis of the heat transfer from the tube to the liquid. The flow within the film was assumed to be laminar. Some of the results obtained by Rogers are extended and discussed in the present paper. In particular, a simple relation is given for the angle θ_d, at which the thermal boundary layer intersects the film surface. This relation provides an easy means for calculation of the heat transfer coefficient and for determination of the range of parameters in which Rogers' analysis is applicable.     

Heat transfer in a fluidized bed. Part II. Interpretation of the heat transfer coefficient on the basis of solids movement

In Part I the influence of the thermal driving force on the coefficient for heat exchange with a fluidized bed was described.The extent of this was clearly related to the kind of powder fluidized, i.e. powders which exhibited dense phase expansion were strongly influenced as opposed to those which did not exhibit dense phase expansion. The latter yielded heat transfer coefficients which could be reasonably predicted by a theoretical model based on a combination of solids movement and unsteady-state heat conduction, using well-known models from the literature. The slight influence of the driving force on the experimentally determined heat transfer coefficients could also be anticipated when accounting for the temperature-dependent thermal properties of gas and solids.For powders exhibiting dense phase expansion, prediction with the proposed model proved to be impossible. Here, the solids behaviour seems to be governed by random movements which are difficult to relate to fluidizing parameters. Tentative calculations suggested that the bubble frequency may be a useful correlating parameter.     

Heat transfer and gas holdup studies in a bubble column: Air-water-sand system

Air-holdup and heat transfer coefficient data are reported for the air-water and air-water-sand system as a function of air velocity in the temperature range 297-343 K as measured in a 0.305 m diameter bubble column operating in semi-batch mode and equipped with either a five- or seven-tube bundle. A 65 μm average size sand powder is used at concentrations of 5 and 10 mass percent in the slurry. Available correlations of gas holdup and heat transfer coefficients are examined on the basis of these data. These are found inappropriate and inadequate for representing these experimental data. Gas holdup data are well represented by an approach based on Nicklin's (1962) work, and heat transfer data are adequately represented by a simple semi-empirical expression. Accurate experimental data on additional systems are needed to develop a reliable heat transfer theory particularly for process representation at temperatures higher than ambient.     

Heat and mass transfer in multicylinder drying: Part II. Analysis of internal and external transport resistances

The present study investigates two models—two limiting cases—for the internal heat and mass transfer in a multicylinder dryer: the first model assumes that complete redistribution of heat and moisture in the thickness direction occurs instantaneously. The second model assumes that moisture transfer occurs only by vapor diffusion and that heat transfer takes place by conduction and condensation. The two models are written in dimensionless form, and applied for the same set of standard paper machine data. For basis weights below approximately 0.05 kg d.s./m² the results are so close that the first, simple model is sufficient. A higher limiting basis weight—approximately 0.16 kg d.s./m²—was previously indicated by comparison of model predictions, assuming the simple model, to extensive sets of machine data from four different paper machines.     

Heat transfer mechanisms in gas fluidized beds. Part 1: Maximum heat transfer coefficients

This contribution presents the prediction of maximum heat transfer coefficients in bubbling fluidized beds, which takes into account thermal and fluid-dynamic properties of particulate material and fluidizing agent. The analysis suggests that heat transfer between heating or cooling surfaces and bubbling fluidized beds consists mainly in a particular manifestation of convective heat transfer. Another feature is an appropriate modelling of the particle convective component leading to a two-phase Prandtl number.     

影响降膜蒸发器中成膜原因分析

阐述了降膜蒸发器的成膜过程,分析了影响碱液成膜的原因包括:碱液极限最低进料量和浓度,热流强度和降膜管上的分布器垂直度.指出若想使碱液在降膜管内均匀成膜,首先必须保证降膜管上的分部器垂直且不能松动,其次要控制好极限进料量和极限浓度,还要保证稳定的蒸汽压力和蒸汽流速,以及对高压蒸汽进行增湿,利用蒸汽的汽化潜热来保证稳定的热流强度.