Effect Of Moisture Condensation Research Articles

Effect of moisture condensation on vapour transmission through porous membranes.

Porous membranes find natural application in various fields and industries. Water condensation on membranes can block pores, reduce vapour transmissibility, and diminish the porous membranes' performance. This research investigates the rate of water vapour transmission through microporous nylon and nanofibrous Gore-Tex membranes. Testing consisted of placing the membrane at the intersection of two chambers with varied initial humidity conditions. One compartment is initially set to a high (water vapour concentration and the other low (, with changes in humidity recorded as a function of time. The impact of pore blockage was explored by pre-wetting the membranes with water or interposing glycerine onto the membrane pores before testing. Pore blockage was measured using image analysis for the nylon membrane. The mass flow rate of water vapour (ṁv) diffusing through a porous membrane is proportional to both its area (A) and the difference in vapour concentration across its two faces (, such that where K is defined as the moisture diffusion coefficient. Correlations are presented for the variation of K as a function of . Liquid contamination on the porous membrane has been shown to reduce the moisture diffusion rate through the membrane due to pore blockage and the subsequent reduced open area available for vapour diffusion. Water evaporation from the membrane's surface was observed to add to the mass of vapour diffusing through the membrane. A model was developed to predict the effect of membrane wetting on vapour diffusion and showed good agreement with experimental data.

ESTUDANDO A SUBLIMAÇÃO DE DIÓXIDO DE CARBONO

Background: The utilization of dry ice in cooling and storage units requires adjusting the intensity of sublimation due to the requirements of prudently using CO2 to maintain preset thermal conditions. Aim: When designing a carbon dioxide cycle, it is essential to consider the influence of thermal gradients on the adsorption and desorption of carbon dioxide. Methods: tests were conducted to study the production and sublimation of carbon dioxide. The testes were aimed to define the temperature relation of the dry ice sublimation period, the density of pressed СО2, and the humidity of the environment and concentration. Results and Discussion: According to the obtained test data, there was a linear relationship between the sublimation intensity and the ambient air temperature in the specified conditions. The effect of moisture condensation on the sublimation rate appeared weaker than expected, for the amount of moisture on the surface of the specimens was insignificant. The heat exchange was intensified by the fall of hoarfrost and the related surface expansion. However, much moisture froze out without reaching the dry ice surface, and the formed layer of ice formed a heat insulation surface, and the sublimation under that layer was less intensive. The direct influence of sublimation came from the pressure at which a specific specimen was formed; however, 75 kN pressure was optimal. Conclusion: Despite higher weight losses during the storage, the difference in spent energy is more critical than 90 kN. The factor no less important was the carbon dioxide storage temperature. The maximal sublimation time of a 55 g cylinder formed at 75 kN and stored at – 80°С was 135 hours, much higher than at similar parameters but at -60°С. That said, the amount of energy spent on operating a low-temperature chamber was almost identical.

STUDYING THE SUBLIMATION OF CARBON DIOXIDE

Background: The utilization of dry ice in cooling and storage units requires adjusting the intensity of sublimation due to the requirements of prudently using CO2 to maintain preset thermal conditions. Aim: When designing a carbon dioxide cycle, it is essential to consider the influence of thermal gradients on the adsorption and desorption of carbon dioxide. Methods: tests were conducted to study the production and sublimation of carbon dioxide. The testes were aimed to define the temperature relation of the dry ice sublimation period, the density of pressed СО2, and the humidity of the environment and concentration. Results and Discussion: According to the obtained test data, there was a linear relationship between the sublimation intensity and the ambient air temperature in the specified conditions. The effect of moisture condensation on the sublimation rate appeared weaker than expected, for the amount of moisture on the surface of the specimens was insignificant. The heat exchange was intensified by the fall of hoarfrost and the related surface expansion. However, much moisture froze out without reaching the dry ice surface, and the formed layer of ice formed a heat insulation surface, and the sublimation under that layer was less intensive. The direct influence of sublimation came from the pressure at which a specific specimen was formed; however, 75 kN pressure was optimal. Conclusion: Despite higher weight losses during the storage, the difference in spent energy is more critical than 90 kN. The factor no less important was the carbon dioxide storage temperature. The maximal sublimation time of a 55 g cylinder formed at 75 kN and stored at – 80°С was 135 hours, much higher than at similar parameters but at -60°С. That said, the amount of energy spent on operating a low-temperature chamber was almost identical.

Analysing the transport phenomena of novel dew-point evaporative coolers with different flow configurations considering condensation

This paper entails a study on the transport phenomena of a novel dew-point evaporative cooler with counter-flow closed-loop configuration, a potential alternative to the conventional mechanical vapor compression system. In contrast to the conventional indirect evaporative cooler and the regenerative indirect evaporative cooler, the proposed novel dew-point evaporative cooler is capable of achieving the simultaneous goals of pre-cooling, energy recovery and dehumidification. The novel dew-point evaporative cooler is classified as either type 1 or type 2 based on the relative flow direction between the water and the supply air. A two-dimensional computational fluid dynamics model is judiciously developed and employed to conduct a comparative study which evaluates the performance of the conventional indirect evaporative cooler vis-a-vis both novel type 1 and novel type 2 configurations. Experimental data is then employed to validate the predictive accuracy of the mathematical model. The distributions of both temperature and humidity ratio are plotted for a typical case of the novel cooler incorporating both flow configurations and considering the effects of moisture condensation. Additionally, the impacts made by key parameters on the cooling performance under condensation state are analyzed by using three evaluation indexes, namely, latent efficiency, wet-bulb efficiency, and enlargement coefficient. By regulating several operating parameters, the variations of the three evaluation indexes are plotted via the proposed numerical model. Key results from this study revealed that the novel type 1 flow configuration is capable of achieving the highest latent and wet-bulb efficiencies, and enlargement coefficient. It is also observed that the relative flow direction between the water and the supply air exerts a stronger influence on the cooler performance compared to the changes of the water mass flow rate.

Evaluation of corrosion resistance of materials under conditions of moisture condensation in the presence of carbon dioxide

The paper investigates aspects of the development of corrosion processes under conditions of moisture condensation in the gas phase in the presence of carbon dioxide, which lead to the formation of local damage. The authors developed and tested a methodology for conducting steels corrosion testing The causes of the formation and the corrosive effect of moisture condensation on steel under conditions of carbon dioxide corrosion at gas production facilities are analyzed. It was found that at elevated temperatures, when the temperature difference is higher, more moisture condenses on the surface of the steel, which leads to an increase in the rate of both general and local corrosion by 2–3 times, compared to room temperature. The increased localization of corrosion processes under conditions of moisture condensation and the presence of CO2 makes the depth index of steel corrosion much higher than the general corrosion rate. When assessing the corrosiveness of environments with condensation of the aqueous phase, the rate of corrosion associated with the depth of the observed corrosion damage should be taken into account. According to the test results, it was determined that samples from the weld compared with the sample from the main body of the pipe differ in the degree of localization of corrosion in conditions of moisture condensation.

When superhydrophobic coatings are icephobic: Role of surface topology

Among different types of anti-icing coatings, superhydrophobic coatings have attracted considerable attention due to their water repellency and low heat-transfer rate. However, condensation on superhydrophobic surfaces at low temperatures usually causes an increase in ice adhesion because of the induced wetting of micro- and nanostructures. By tuning the weight ratio of surface-modified nanoparticles to unmodified ones, five superhydrophobic coatings with different structural features at the microscale were developed. Ice-adhesion strength and ice-nucleation temperature were studied, together with the effect of moisture condensation on ice adhesion. It was found that the ice-adhesion strength and icing temperature of these coatings do not necessarily follow the same order among these surfaces because of different mechanisms involved. Surface roughness is inadequate to describe the necessary surface features that critically affect the anti-icing behavior of the coatings. Detailed topology/geometry has to be considered when designing icephobic coatings. Superhydrophobic coatings can be adopted for icephobic applications once the surface topology is carefully designed.

Effect of moisture condensation on long-term reliability of crystalline silicon photovoltaic modules

Moisture condensation (MC) can occur in photovoltaic (PV) modules in hot and humid climates, and the resulting water droplets can cause more areas of corrosion. Therefore, in this study, MC history of PV modules exposed to Miami climate (FL, USA) has been derived employing corresponding meteorological data. The duration of MC versus temperature of PV module (Tmodule) was calculated over 1year. Furthermore, five types of accelerated tests were conducted to develop a MC-induced degradation prediction model. The thermal activation energy, 0.4524eV, was calculated. The Brunauer–Emmett–Teller (BET) model was used to predict the degradation rate. The accumulated degradation rate of a PV module exposed to the accelerated condition of MC was 1.45times greater than that of damp heat (DH). The effect of encapsulant materials on the frequency of MC and accumulated degradation rate over 1year were calculated in the Miami climate.

97/01666 The effect of moisture condensation on the spontaneous combustibility of coal

Coal oxidation at low temperatures is the heat source liable for the self-heating and spontaneous combustion of coal. This phenomenon has imposed severe problems in coal related industries. Attempts to understand this phenomenon by previous researchers have provided significant progress. It is well-known that coal oxidation at low temperatures involves oxygen consumption and formation of gaseous and solid oxidation products. This process is majorly influenced by temperature, oxidation history of coal, coal properties, particle size distribution of the coal, etc. The current understanding of the phenomenon of self-heating and spontaneous combustion of coal is discussed along with the different experimental and numerical models established to predict self-heating characteristics of coal. This paper focuses on the global position of the study carried out by academics, research institutes and industries on spontaneous combustion of coal and coal mine fires. Within this framework, the generally used spontaneous combustion techniques to predict the spontaneous combustion liability of coal were evaluated. These techniques are well-known in their usage, but no specific method has become a standard to predict the spontaneous combustion liability. Further study is still needed to indicate a number of impending issues and to obtain a more complete understanding on the phenomenon.

The effect of moisture condensation on the spontaneous combustibility of coal

The reaction-diffusion approach developed in previous work is extended to consider the effects of moisture evaporation and condensation on the rate of oxidation of coal. For a single isothermal particle, pseudo-steady-state balances on moisture and oxygen permit calculation of the effect of different levels of coal-bound moisture on the rate of oxidation. These species balances are combined with an energy balance to determine the particle temperature history during evaporation and condensation. It is shown that partial wetting of coal or condensation of moisture exerts two competing influences on the overall rate of oxidation. On the one hand, a portion of the coal fills up with liquid moisture; in this region, the rate of oxidation becomes negligible, since the oxygen has to dissolve in the moisture before it can gain access to an active site on the coal surface and the solubility of oxygen in water is low. On the other hand, condensation also leads to the release of the latent heat of vaporization. This heat effect raises the temperature of the particle and increases the rate of oxidation in the dry region of the coal. The relative magnitude of the rates of these competing influences determines whether the potential for spontaneous combustion is abated or enhanced. Based on the insight provided by the model, a simple criterion is derived to determine conditions under which condensation of moisture on coal can lead to spontaneous combustion. Field observations have consistently indicated that moisture condensation can enhance the potential for spontaneous combustion. This model is however thought to be the first mathematical description which links the coal-moisture interaction parameters (which can be determined independently) to the potential for spontaneous combustion.