MEASUREMENT OF HUMIDITY ABOVE TEMPERATURE OF 100°C WITH A DOUBLE-WICK WET BULB PSYCHROMETER

A need exists for a simple device to measure the psychrometric properties of high temperature and high humidity air. To meet this need, a small laboratory double-wick psychrometer was developed and tested which showed good results to 175°C dry bulb temperature and 100°C wet bulb temperature. Based on the results of the laboratory unit, a portable prototype psychrometric system was designed and built. The overall design provided quick measurement and indication of wet bulb temperature, dry bulb temperature, humidity, and enthalpy of a high temperature air stream.     

PREDICTION OF PSYCHROMETRIC PARAMETERS USING NEURAL NETWORKS

Abstract

A neural network approach was used for the prediction of the psychrometric parameters in a non-iterative manner. Neural network models were developed for the each of the three main variables - dry bulb temperature, wet-bulb temperature and relative humidity - as a function of the other two variables. Models were also developed for the prediction of the dew point temperature using the dry bulb and wet bulb temperatures, and for the saturation vapor pressure as a function of the dry bulb temperature. The sensitivity of the neural network performance to the form of input and output variables employed was also investigated. The prediction accuracy of the neural network models were found to be very good, with errors less than 4%.     

A CALCULATION METHOD OF OBTAINING DESORPTION ISOTHERMS OF BEECH (FAGUSMOESL4CA) WOOD

ABSTRACT

This paper presents the specific methods to obtain sorption isotherms. Sorption isotherms were first obtained experimentally for 20°C and 100°C and then, by using the presented method, isotherms were also obtained for 50°C and 80°C. The same treatment can be applied in finding any isotherm sorption in the temperature range between 20°Cand 100°C.

The experiments were carried out on beech (Fagus moesiaca) and sample dimensions were 3×3×3cm. Based on the results of the experiment, by regression analysis, the most suitable analytic formula was proposed. It relsites temperature and air humidity with the equilibrium moisture content in wood. Experimental results were compared with psychrometric tables and Bramhall's formula for desorption curves with good agreement. This proves that the method presented in this paper is precise and efficient to decrease the lime needed to obtain sorption isotherms experimentally.     

PROTOTYPE HIGH TEMPERATURE/HIGH PRESSURE KILN FOR THE EVALUATION OF WOOD DRYING SCHEDULES

ABSTRACT

A new laboratory kiln was developed and built to perform over a very wide range of drying conditions. For example, the dry bulb temperature can vary from 30°C to 150°C and the dew point can be adjusted between 20°C and 130°C. Obviously, with such a high level of dew point, pressures over atmospheric pressure may be induced inside the chamber. For this reason, the kiln has been designed to withstand pressure of up to 3 bars. This kiln can also perform vacuum drying.

A programmable controller allows the temperature levels to be maintained within ± 0.2°C. Because the whole kiln can be heated only through the agitated water present at the bottom of the kiln, the load temperature can be increased up to 130°C in saturated conditions, without any change of moisture content.

The kiln has various sensors attached and is capable of withstanding severe conditions (high temperature, saturated vapour and elevated pressures). At present, air and water temperatures as well as temperature at different locations within the board can be collected during the drying process. A load cell and pressure gauges are also available. The first tests performed using this equipment are presented at the end of the paper.     

EFFECT OF TEMPERATURE AND RELATIVE HUMIDITY ON THE DRYING RATES AND DRYING TIMES OF GREEN BELL PEPPERS (CAPSICUM ANNUUM L)

ABSTRACT

Green bell pepper dices were dehydrated at different dry bulb air temperatures (55°, 60°, 65°, 70° and 75°C) and relative humidities (15, 20, 25, 30, 35 and 40%). The effects of temperature and relative humidity (RH) on the drying rates and drying period of diced green bell peppers were determined. Drying rate curves were characterized by a short induction (heating) phase followed by a falling rate period. Near constant rate drying was observed only at 55°C at 15% RH and at 65°C (15% RH). Drying rates generally increased with increasing temperatures and decreasing RH. The effect of temperature on the drying rates became less pronounced with increasing RH Drying rate maxima at 70°C and RH of 15, 20 25 and 40% exceeded those at 75°C, possibly due to case-hardening.     

Drying Kinetics of Pinus Rqdiata Boards at Elevated Temperatures

ABSTRACT

From the numerical results of a rigorous, mathematical model, the rate–o–drying curves for high–temperature seasoning of Pinus radiata board show that the drying process can be divided into three periods. Based on the mechanistic picture of moisture movement and differences in the movement in each period, this paper simplifies the model by using the concept of the haracteristic drying curve.

In heartwood drying, the first period is very short (5–15 minutes), hence it can be neglected, whereas for sapwood, the first period is substantial with the drying rate being taken as constant in the analysis. Since the physical characteristics of the second and third drying periods are different, separate characteristic drying curves have been developed for each period. It has been found that common normalised drying curves are able to describe the results generated from the earlier model for normal range of kiln conditions over the examined ranges in dry–bulb temperatures of 1 10–140°C, wet–bulb temperatures of 70–90°C and air velocity of 3–7 m s^-1. Therefore. the derived simplified model can directly be used to calculate kiln–wide variations in moisture content and drying rate.     

Characterization of a Spray Drying System for Soymilk

ABSTRACT

Soymilk slurries were spray dried at various combinations of inlet air temperature (1255. 265, and 275°C), feed rate (0.38 and 0.57 Lpm), and atomizer speed (119,300 and 26,800 rpm). Monitored dryer parameters included dry and wet bulb temperaturea of ambient and exhaust air, drying chamber's inlet and outlet air temperatures, product and feed temperatures, velocity pressure at exhaust pipe, atomizer speed, and feed rate. Weather data was included in the analysis.

Collected data was used to estimate the state of the outlet air using psychrometrics and assuming adiabatic saturation. Equations to predict outlet and product temperatures were developed. Average thermal and eveporative efficiencies were 65.9 and 78.4%. respectively.     

THIN-LAYER DRYING CHARACTERISTICS OF ROUGH RICE AT LOW AND HIGH TEMPERATURES

Abstract

Thin-layer drying characteristics of rough rice were determined at temperature ranging from 11.8 to 51 °C and for relative humidities ranging from 37.1% to 91.3%, with initial moisture contents in the range of 24.7 to 41.6% dry basis. An oven, a self contain air conditioning unit, recently developed in Japan, was used for this experiments. With this apparatus, very smooth drying curves were obtained. The data of sample weight, and dry and wet bulb temperatures of the drying air were recorded continuously throughout the drying period for each test. The drying process was terminated when the moisture content change in 24 h was less than 0.2 % d.b. (weight change was less than 0.05 g). The final points were recorded as the dynamic equilibrium moisture contents.

The drying data were than fitted to the Page model. The model gave a very good fit for the moisture content with an average standard error of 0.294 % d.b? Both the drying parameters, K and N, are function of drying air temperature and relative humidity. The effect of variable initial moisture content was also described effectively by the empirical Page model. The drying time employed had a large effect on the K and N values. The results presented here, over typically 5 day drying, will be useful in the long term moisture transfer process occurring during ventilated storage.     

Determination of activation temperature of glass bulb sprinklers using a thermal liquid bath

A thermal liquid bath was used to determine the activation temperature of a wide range of temperature rated (57–182°C) glass bulb (3 and 5 mm diameters) sprinklers using water and glycerine. An optical switch and a thermocouple were installed adjacent to each sprinkler (within 10 mm) to record the activation of each glass bulb in the liquid bath having a uniform temperature distribution (±0.5°C). All the tests for determining the activation temperatures of glass bulb sprinklers were conducted in the liquid bath using a 0.277°C/min rate of temperature rise. Based on a heat transfer analysis, this rate is within the maximum allowable rate of rise of water and glycerine temperatures, which allows the glass bulb temperature to closely follow the liquid temperature. The sprinkler activation temperature was evaluated in terms of percentage rating, which is expressed as the percent variation of the average activation temperature, as measured in the liquid bath, from the nominal rated temperature. Sprinklers (93°C rated or lower) activation temperatures in water were consistently within ±3.5% of rating. In glycerine, the activation temperatures of sprinklers rated between 93 and 182°C performed within 3.5% of their rating. Copyright © 2005 John Wiley & Sons, Ltd.     

Effect of moisture on the oxidation behavior of ZrB2

Oxidation studies of ZrB₂ were performed under wet air and dry air conditions at 1200°C, 1400°C, and 1500°C for 1, 4, and 10 h. Compared to dry air, the presence of water vapor was found to enhance the oxidation kinetics by a factor of 7 to 30, depending on the temperature. Thermodynamic calculations suggested that water vapor promoted the formation of additional volatile species such as boric acid (HBO₂), in addition to boria (B₂O₃) produced in dry air, which increased the evaporation rate of B₂O₃. Compared to dry air, the presence of water vapor leads to more rapid evaporation of boria and the transition from parabolic oxidation kinetic behavior (ie, rate controlled by diffusion through boria) to linear (ie, underlying ZrB₂ is directly exposed to the oxidizing environment) at shorter times and lower temperatures.     

DRYING '80 VOLUME 1: DEVELOPMENTS IN DRYING

ABSTRACT

The purpose of this research was to investigate strategies for papaya glacé drying in tunnel. To evaluate the optimum conditions of drying, corresponding mathematical models were also considered. The criteria set for this study included low drying time, low specific energy consumption and acceptable qualities of papaya glace. The results obtained from the model of batch tunnel drying were in good agreement with the experimental ones. From the mathematical models, it was found that the optimum conditions of the first stage of drying of papaya glace (3.1×7.8×1.4 cm) were drying temperature of 70°C, specific air flow rate of 12 kg/h-kg dry papaya glace (velocity of 1.25 m/s) and air recycle ratio of 70%. In the second stage of drying papaya glace (0.98×0.98×0.98 cm), it was found that the optimum drying conditions were: drying air temperature of 55°C, specific air flow rate of 10 kg/h-kg dry papaya glace (velocity of 0.6 m/s) and air recycle ratio of 80%. Ambient air temperature and relative humidity were 30°C and 70% respectively.     

THE MEASUREMENT OF PARTICLE TEMPERATURE IN A FLUID1ZED BED DRYER

ABSTRACT

A probe for particle temperature measurements in fluidized bed dryers is described in this work. The probe, consisting of a thermistor placed inside a turning cup, was tested in a pilot scale dryer, where particle temperatures were monitored during batch drying tests. The measurements were validated by comparison with readings made in collapsed beds. Temperature readings were given a physical interpretation, as surface particle temperature, or average particle temperature, depending on the drying regime. The dynamic response of the probe was also analyzed. As an application, it is shown how panicle temperatures, along with local wet bulb and dry bulb air temperatures, can be used to monitor and control the drying process. The simplicity of construction, installation and associated method makes the probe attractive for both research and industrial purposes.     

Cr2AlC high temperature oxidation under dry and wet air: Understanding of the oxidation mechanism

Cr₂AlC is one of the most interesting MAX phase compositions to operate under aggressive environments up to 1400 °C, but the effect of humidity at high temperatures is still poorly investigated. Cr₂AlC high-temperature oxidation tests under controlled humidity are carried out on fined-grained samples for 40 h in the [1000–1400 °C] temperature range and compared to a previous study performed on single-crystals, fine and coarse-grained samples oxidized under dry air. Oxidation products are characterized and mass gain curves are analysed using both cubic and paralinear oxidation models to extract kinetics and characterize gas emission during the oxidation tests. These experimental data are supported by thermodynamic calculations in order to propose a quite complete oxidation mechanism for Cr₂AlC.The nature and the arrangement of the oxidation products formed during wet and dry air oxidation tests are similar - a continuous Al₂O₃ layer and a Cr₇C₃ sublayer with pores- which demonstrates that the water vapor does not affect significantly the high-temperature oxidation of Cr₂AlC. The acceleration of the oxidation process during wet air tests is correlated to the higher partial pressure of gaseous species produced in the presence of humidity which is associated with the chromium carbide’s volatilization in the form of carbon oxide.     

空气与水逆流直接接触热质交换模型计算及与实验比较

According to general equations deducted from a model of heat and mass transfer of air and water in direct contact counter flow reported in the author‘s previous work, the outlet parameters of air and water counter flow through a paper wet pad were calculated by using the MATLAB-software. Comparing the calculation results with experiment data of prior researchers, the deviation of calculated outlet air wet bulb temperature from experimental outlet air wet bulb temperature was less than 10%, the deviation of calculated outlet water temperature from experimental outlet water temperature was less thang 9%, and the deviation of calculated outlet air dry bulb temperature from experimental outlet air dry bulb temperature was less than 10% when NTUh was more than 0. 6. The deviation of calculated outlet air dry bulb temperature from experimental outlet air dry bulb temperature was more than 10% when NTUh was less than 0. 6. The model calculations were in good agreement with experiments and the general equations could be used to design evaporation cooling equipment, such as wet pad and so on.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

ABSTRACT

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.     

中小型玻璃纤维增强塑料冷却塔标准中几个参数的试验验证

通过试验,验证了GB7190.1-1997玻璃纤维增强塑料冷却塔第1部分:中小型玻璃纤维增强塑料冷却塔标准中湿球温度、进塔水温及相对湿度3个参数对冷却塔冷却效率的影响,试验数据表明,湿球温度、相对湿度及进塔水温对冷却塔的测试结果基本上没有影响;对冷却塔的进水温度的要求还可以适当放宽到31.5℃。     

THIN LAYER AND DEEP BED DRYING OF LONG GRAIN ROUGH RICE

ABSTRACT

The paper presents new data for thin-layer drying characteristics of Thai long grain rough rice measured under various conditions of drying air temperature (35 to 60?°C), drying air relative humidity (30 to 70 % ) and the initial moisture content of rough rice (20 to 40 % dry basis). Empirical equations were developed using the instantaneous weight, the weight loss and drying time, with temperature, relative humidity and initial moisture content of rough rice as the independent variables. A computer program was developed to simulate the deep-bed drying process. The thin-layer drying equation developed before was used in the computer simulation. Experimental data from the fixed bed dryer were compared with the results from the calculation.     

CONVECTIVE DRYING OF BANANA,GUAVA AND POTATO PIECES : EFFECT OF CYCLICAL VARIATIONS OF AIR TEMPERATURE ON DRYING KINETICS AND COLOR CHANGE

ABSTRACT

Pieces of banana, guava and potato were dried in a two-stage heat pump dryer capable of precise control of air humidity with predetermined cyclic variations of air temperature entering the drying chamber. The air temperature variations tested were : a cosine, a reversed cosine and three different square wave profiles with peak-to-valley variations from 20^°C to 40^°C. The cycle time was about 60 minutes with drying time of approximately 300 minutes. The drying samples were placed on trays in a thin layer. With appropriate choice of temperature-time variation, it is possible to reduce the overall color change while maintaining high drying rates.     

EXPERIMENTAL ANALYSIS OF THE DRYING KINETICS OF A FOOD PRODUCT

ABSTRACT

A laboratory system allowing for the characterization of the thin-layer drying kinetics of olive bagasse at relatively high temperatures was designed and constructed. The system, which permits a wide range of operating velocities and temperatures, up to 700°C, allowed the weight loss of the sample to be monitored continuously. Constant drying conditions were maintained using an on-line computer. The drying conditions investigated in this study included combustion products of atmospheric air and propane with dry-bulb temperatures ranging from 125°C to 250°C, relative humidity lower than 1% and gas velocities ranging from 0.5 to 2.0 m/s.     

Surface temperature of acoustically levitated water microdroplets measured using infra-red thermography

A 58 kHz acoustic levitator was fitted with an infra-red thermography camera to examine the drying behaviour of water microdroplets at various drying-air temperatures. The evaporation rate was greater with larger initial droplet size at otherwise identical drying-air temperature (T_da). Measurement of droplet aspect ratio indicate that this is caused by differing acoustic field strengths. The measured droplet surface temperature in dry air showed no dependence on initial droplet size, but deviated from the wet bulb and also from the droplet temperature predicted by acoustic levitation theory. The degree of deviation of drying rate from that predicted by the d²-law using the wet bulb was dependent on T_da. Use of measured droplet surface temperature instead of the wet bulb gave, however, good agreement with the d²-law in dry air. No substantial effects of acoustic field streaming on drying rate could therefore be seen, even at the sound pressure levels of 106-165 dB used. Interpretation of evaporation rates of acoustically-levitated droplets requires therefore the measured droplet surface temperature.