湿度标准及湿度计量之进展

Determination of the wet-bulb temperature at the surface of a material is the basis of one class of humidity measuring instruments, and is important in industrial applications such as dryer modelling and simulation. The psychrometer equation is a frequently used method of estimating wet-bulb temperature, and contains a psychrometer “constant”. Analysis shows that this is in fact a variable coefficient affected by temperature, pressure,radiation and conduction effects, and the identity of the gas and vapour. Radiation and conduction affect the difference between adiabatic saturation temperature and indicated wet-bulb temperature. Inconsistencies in currently recommended values for the psychrometer coefficient, including published international standards, are identified and explained. Particular problems arise when the enhancement factor is applied to vapour pressure to account for non-ideality of gases. Special considerations are also needed for wet-bulb temperatures approaching the boiling point, where the psychrometer coefficient tends to zero. Self-consistent recommendations recently published in the new British Standard BS1339 are given, which cover both the air-water system and a general vapour-gas system.     

MODELLING OF WET-BULB WETTED WITH BINARY MIXTURE

Removal of multicomponent moisture is a frequent problem in pharmaceutical and related industries. To control the process temperature of the solid and individual drying rates must be known. For nonporous (crystalline) solids of small size the problem may be reduced to simulation of a wet-bulb wetted with multi-component moisture. In the paper a simple model of the process was presented and tested using the wet-bulb of a commercial Assmann psychrometer as a test solid. Dynamics of temperature change of such wet-bulb wetted with a binary mixture of ethanol and water was measured and compared with those predicted by the model. Satisfactory agreement of the model predictions with the experiment was found.     

The evaporation of moisture from wet surfaces

The authors investigate “exact” relationships for the flux of vapour leaving a wet surface in terms of the humidity potential and wet-bulb depression. As the wet-bulb temperature rises the departure from a linear relationship becomes more marked, but the flux is still directly proportional to the humidity potential for processes at a constant wet-bulb temperature. The difference between the wet-bulb and adiabatic-saturation temperatures is shown to decrease with increasing wet-bulb temperature and vanish altogether at the boiling point. A new humidity chart, based on the “exact” relationships developed, is presented which yields a greater reading accuracy than charts of similar size hitherto produced.     

板式蒸发式冷凝器热工性能实验

对影响板式蒸发式冷凝器热工性能的主要因素——空气的湿球温度、冷凝温度、相对湿度做了实验研究。结果显示：冷凝温度和冷凝压力均随着入口空气湿球温度的升高而升高;入口空气相对湿度对压缩机功耗的影响与湿球温度对压缩机功耗的影响均比较大;对基于同一压缩系统的实验比较表明,在冷凝温度为38℃时,板式蒸发式冷凝器的热流密度比管式蒸发式冷凝器平均大30.5%;且在相同的冷凝温度下,入口空气湿球温度越低,板式蒸发式冷凝器的热流密度比管式蒸发式冷凝器大得越多。     

Empirical equations for the thermodynamic properties of aqueous sodium chloride

Equations for the thermodynamic properties of aqueous sodium chloride near its vapour pressure are presented. The equations are functions of temperature and concentration, and may be used to estimate aqueous sodium chloride's solubility, density, vapour pressure, specific enthalpy and entropy. They are valid for temperatures from 0 to 300°C, and concentrations extending to saturation with suitable accuracy. The thermodynamic equations are represented graphically and compared with published experimental data. These equations should prove to be a useful tool for modeling desalination equipment, particularly distillation processes.     

Wet-bulb temperature of binary mixtures

When binary mixtures are evaporated, a wet-bulb temperature does exist, provided that the evaporation is non-selective. The wet-bulb temperature can be calculated based on the energy balance, the kinetic equations for heat and mass transfer and the thermodynamic equilibrium. The calculated wet-bulb temperatures were found to be in good agreement with the experimental ones, obtained with mixtures of isopropyl alcohol and water.     

RATE OF DRYING AND STRESSES IN THE FIRST PERIOD OF DRYING

The paper presents a computer simulated processes and illustrate how the drying induced stresses are influenced by the rate of drying. It is shown that the moisture transport coefficient, and thus the rate of drying, depends on the thermal state of the drying material, defined by the wet-bulb temperature. Through these simulated processes one can observe the evolution of the moisture content and stress distributions during drying at constant, but in each process different, wet-bulb temperatures. A convective drying process of a bar with rectangular cross-section is considered as example, and a two-dimensional initial-boundary value problem is solved numerically with the use of the finite element method. The numerical results are visualised in spatial diagrams.     

新的氨水体系状态方程

本文提出一个以三参数立方型方程为基础的氨水体系状态方程。由同时拟合蒸汽压、汽相体积和液相体积确定的三个参数都是温度的函数。混合物的混合法则考虑了三分子相互作用对引力的贡献。四个交互作用参数处理成压力的线性关系后,在压力小于21atm下预测汽液平衡和热力学性质达到满意的结果。     

Correlation of vapour pressure,temperature, and latent heat of vaporisation

The vapour pressures of six liquids have been measured in the approximate range 0.05 to 200 torr and correlated with that of benzene by a method published in an earlier paper. The relationship so obtained which is linear for non-associated substances and is valid from critical conditions to a vapour pressure at least as low as 0.1 torr presents a reliable method of extrapolation of vapour pressure data. It may also be used (at vapour pressures below ? 100 torr) to evaluate the entropy and latent heat of vaporisation of a liquid from even scant vapour pressure data.     

Simulation of sodium boiling using a water model: voiding and re-entry processes

Experiments are described in which boiling was initiated in uniformly superheated water in tubular and pin-cluster geometries, producing voiding into a sub-cooled plenum. By suitable choice of system temperature and pressure, sodium vapour growth rates and liquid metal boiling characteristics were simulated. The processes of voiding, condensation in the plenum and re-entry were studied as functions of thermal and geometrical conditions. Dispersed two-phase flow regimes and a lack of coherent re-entry were observed in the pin-cluster, contrasting with the single-bubble “chugging” which occurred in tubular geometry.A simple theory for the void growth, in which the vapour energy and liquid momentum equations are coupled, is derived; this gives good agreement both with the present experiments and with other published experimental data for different fluids and geometries. From the observations of the bubble dynamics in the sub-cooled plenum an empirical model of this process is presented which also gives good agreement with experiment over a range of conditions.     

干球温度和湿球温度对湿式空冷器冷却能力的影响

依据湿式空冷器测试标准建立了性能测试试验台,试验研究了干球温度和湿球温度对湿式空冷器冷却能力的影响。结果表明,当湿球温度一定时,干球温度对湿式空冷器的影响较大;因此,设计湿式空冷器时不能只考虑湿球温度,不考虑干球温度。同时分析了湿球温度与湿空冷管程进水温度的温差对湿式空冷器冷却能力的影响。     

On the Determination of the Surface Temperature of an Evaporating Liquid

Some methods for calculating the wet-bulb temperature are compared. These methods are based on different forms of equations relating the heat and mass transfer in evaporation of a liquid into a gas flow. It is hypothesized that the deviation from the similarity between heat and mass transfer is described by the first power of the Lewis number, for which the parameters of the gas mixture are calculated on the phase-transition surface. In this case, the heat-transfer coefficients calculated from the enthalpy and temperature gradients of the gas–vapor mixture are equal to each other. The wet-bulb temperature calculated using this approach is in satisfactory agreement with experimental data.     

Rapid approximate vapour—liquid equilibrium calculations

-Methods are presented for effectively noniterative approximate methods for calculating vapour—liquid equilibria in the three main variations: specified vapour fraction, specified enthalpy and specified temperature together in each case with a given system pressure. Accuracy is typically around 1% at a first estimate for near ideal systems. Adjustable parameters in the approximate model used may be refitted to increase accuracy to any degree required.     

A SIMPLE THERMOELECTRIC PSYCHROMETER FORMEASUREMENT OF EQUILIBRIUM HUMIDITY

This paper describes a simple thermoelectric psychrometer that measures equilibrium humidity of a sample in a small vessel. The psychrometer consists of one thermocouple and detects the wet-and dry-bulb temperatures using the same measuring junction. The detector consists of a K-thermocouple, 0.1mm in diam., and the surface of the junction is covered with a thin fiber material. When the fiber is moistened by water, the junction acts as a wet bulb. As the moisture in the fiber is lost, the temperature of the junction rises rapidly to that of the dry bulb. The psychrometer constant is derived from equi-dew-point lines drawn up experimentally and is found to be 0.00074°C-1 Data of the equilibrium humidities observed for saturated sodium chloride solution and some foods are shown.     

A SIMPLE THERMOELECTRIC PSYCHROMETER FORMEASUREMENT OF EQUILIBRIUM HUMIDITY

This paper describes a simple thermoelectric psychrometer that measures equilibrium humidity of a sample in a small vessel. The psychrometer consists of one thermocouple and detects the wet-and dry-bulb temperatures using the same measuring junction. The detector consists of a K-thermocouple, 0.1mm in diam., and the surface of the junction is covered with a thin fiber material. When the fiber is moistened by water, the junction acts as a wet bulb. As the moisture in the fiber is lost, the temperature of the junction rises rapidly to that of the dry bulb. The psychrometer constant is derived from equi-dew-point lines drawn up experimentally and is found to be 0.00074°C-1 Data of the equilibrium humidities observed for saturated sodium chloride solution and some foods are shown.     

VAPOUR LIQUID EQUILIBRIA IN STRONG ELECTROLYTE SOLUTIONS BY THE CORGC EQUATION OF STATE

The capability of chain-of-rotalor group contribution equation of state (CORGC EOS) in prediction of vapour-liquid equilibria for electrolyte solutions is discussed. A new approach is considered in which salt is considered as a single component and therefore, there is no ion in the solution. The interaction parameters between water and 50 salts are determined and reported. The parameters are temperature dependent and thus, vapour-liquid equilibria calculations can be achieved at moderate temperature. The percent average absolute deviation for prediction of vapour pressure and osmotic coefficient for 50 aqueous solutions are equal to 0.89 and 16.7, respectively. Comparison between the calculated results and experimental data has been performed.     

A 1-D non-isothermal dynamic model for the thermal decomposition of a gibbsite particle

A 1-D mathematical model describing the thermal decomposition, or calcination, of a single gibbsite particle to alumina has been developed and validated against literature data. A dynamic, spatially distributed, mass and energy balance model enables the prediction of the evolution of chemical composition and temperature as a function of radial position inside a particle. In the thermal decomposition of gibbsite, water vapour is formed and the internal water vapour pressure plays a significant role in determining the rate of gibbsite dehydration. A thermal decomposition rate equation, developed by closely matching experimental data reported previously in the literature, assumes a reaction order of 1 with respect to gibbsite concentration, and an order of −1 with respect to water vapour pressure. Estimated values of the transformation kinetic parameters were k₀ = 2.5 × 10¹³ mol/(m³ s) for the pre-exponential factor, and E_a = 131 kJ/mol for the activation energy. Using these kinetic parameters, the gibbsite particle model is solved numerically to predict the evolution of the internal water vapour pressure, temperature and gibbsite concentration. The model prediction was shown to be very sensitive to the values of heat transfer coefficient, effective diffusivity, particle size and external pressure, but relatively less sensitive to the mass transfer coefficient and particle thermal conductivity. The predicted profile of the water vapour pressure inside the particle helps explain some phenomena observed in practice, including particle breakage and formation of a boehmite phase.     

Thermodynamic properties of the benzene and n-heptane system at elevated temperatures

The total vapour pressure and the composition of the liquid and vapour phases of the benzene and n-heptane system have been experimentally determined under equilibrium conditions at 110, 125, 140, 155, 170, 185, 200 and 215 °C. The corresponding composition of the vapour phase was also estimated using an expression for the liquid phase activity coefficient as a function of the composition of the liquid phase. Several expressions were compared and it has been found that the three-constant Redlich and Kister equation gave the best fit of the experimental vapour pressure data than the other equations. Comparison between the calculated and measured vapour compositions has shown a discrepancy of y_meas,—y_calc. of the order of 0.02 mol fraction or 10 times the experimental tolerance. Nevertheless, the experimental values were reasonably thermodynamically consistent. The low values of the liquid phase activity coefficients indicated that the behaviour of this particular system was not far from ideality. Also, it was found that the degree of non-ideality decreased with the increase in temperature. The behaviour of the vapour phase was shown also to be nearly ideal. The excess Gibbs free energy of mixing was calculated, and it was found to be non-symmetrical with mole fraction. On increasing the temperature of the system, the liquid composition corresponding to the maximum G^E tended to shift to a higher benzene concentration, whilst at a fixed liquid composition, G^E initially decreased and then remained stationary or increased again. This was thought to be due to differences in the molar volumes of the two components.     

PCHE内轴向导热对局部换热性能的影响研究

通过改变印刷电路板式换热器（PCHE）通道壁厚、直径以及冷热侧CO₂进口温度差,数值分析了层流条件下,轴向导热对PCHE内超临界压力CO₂流动换热的影响。结果表明,考虑轴向导热时,局部热通量沿程变化更加平缓,冷侧对流传热系数在小于拟临界温度区域变小,而热侧对流传热系数在小于拟临界温区域增大;轴向导热仅对热效率峰值附近区域有所影响,其使得峰值略有增大且向低温侧移动;传热熵产主要发生在高温区域,轴向导热可有效降低局部传热熵产。增大壁厚和直径可增大轴向导热的影响,而增大进口温差,轴向导热的影响变化不大。     

Thermal Conductivity of Titanium Carbide, Zirconium Carbide, and Titanium Nitride at High Temperatures

The thermal conductivity of titanium carbide, zirconium carbide, and titanium nitride was measured by a variety of techniques in the temperature range 200° to 2000° C. The titanium nitride results are a factor of four higher than the previously published values at 1000°C and show a positive temperature coefficient for the thermal conductivity rather than the negative coefficient previously reported in other work. Recent thermal conductivity data on Tic confirm earlier data obtained by the writers which differed from previous literature values. Contemporary theory fails to explain the temperature dependency of the phonon and/or electron contribution to the heat conduction in the refractory interstitial compounds.