Vapour transfer

Figure 11.15. Relationship between point efficiency and number of liquid and vapour transfer units...

An alternative explanation concerns the existence of two equilibria. As the vapour/liquid equilibrium is disturbed by the passage of air, the concentration of dissolved compounds in the liquid phase falls, disturbing the solid /liquid equilibrium. The kinetics of transfer across this latter phase boundary are much slower than for the liquid/vapour transfer, so that the extraction of odour becomes limited by the rate of diffusion into the liquid phase. 

For control of organisms on the plant surface sufficient volatility to ensure vapour transfer is also desirable, particularly as most spray or granule applications leave a deposit in the form of discrete spots and a large proportion of the surface is effectively untreated. Approximate values may be assigned to the key properties in the light of experience... 

The optimal vapour pressure will be such as to allow some vapour transfer, but not give rise to rapid loss to the atmosphere by evaporation. This implies surprisingly low vapour pressures. The evaporative potential of the atmosphere is substantial and could easily dissipate 2 kg/ha/month of a pesticide with a vapour pressure of 10 mm Hg (28). The positive effects of vapour transfer can be exemplified by calculating the quantity of toxicant (M) taken up in time... 

Harteck (1954) measured the amount of T in the lower atmosphere before H-bomb tests had added significantly to natural activity, and found 4000 and 3 TU in hydrogen and water vapour respectively. The amount of H2 in the atmosphere has increased in recent years due to industrial production (Schmidt, 1974). Circa 1950 there were about 1.5 x 1011 kg of H as H2, compared with 1.4 x 1015 kg as H20. Thus Harteck s values for natural T correspond to tropospheric inventories of 1.8 g as HT and 13 g as HTO. Because HTO is deposited in rain and by vapour transfer to the sea, its residence time in the troposphere is only about a week, similar to that of 137Cs (Fig. 2.8), and more than 90% of the atmospheric inventory is in the stratosphere. By contrast, the residence time of HT in the troposphere is several years (Mason Ostlund, 1979), and most of the atmospheric inventory of HT is in the troposphere. It is only necessary for a small fraction of naturally produced T to form HT to account for the high specific activity of hydrogen gas. 

A simple rectifying column consists of a tube arranged vertically and supplied at the bottom with a mixture of benzene and toluene as vapour. At the top, a condenser returns some of the product as a reflux which flows in a thin film down the inner wall of the tube. The tube is insulated and heat losses can be neglected. At one point in the column, the vapour contains 70 mol% benzene and the adjacent liquid reflux contains 59 mol% benzene. The temperature at this point is 365 K. Assuming the diffusional resistance to vapour transfer to be equivalent to the diffusional resistance of a stagnant vapour layer 0.2 mm thick, calculate the rate of interchange of benzene and toluene between vapour and liquid. The molar latent heats of the two materials can be taken as equal. The vapour pressure of toluene at 365 K is 54.0 kN/m2 and the diffusivity of the vapours is 0.051 cm2/s. 

Meanwhile, measurements of water vapour permeability of woven fabrics have indicated that in the lower ranges of fabric density, the main path of water vapour transfer is through the air spaces between fibers and yarns. This covers the densities characteristic of most apparel fabrics made from staple fibers, although filament yarn fabrics may be woven to higher densities in which the kind of fiber itself in the passage of water vapour, it is necessary to account for the water vapour passage through air spaces. 

Environment Agency, Vapour Transfer in Soil Contaminants, R D Technical Report P309, Environment Agency, Bristol, 2001, in press. 

In order to compare the kinematics of both tests, a normalized time was adopted in Figure 3c. Normalized time was calculated by dividing the actual real time by the total time length of each test. The vapour transfer test was much longer than the liquid transfer one due to the lower equilibration times associated with vapour transfer. 

Experimental results have been interpreted within the framework of a double structure model, which seems to capture the essential features of observed behaviour. The proposed model is capable also of making precise quantitative predictions. This has been shown by deriving model parameters from one of the tests performed (liquid injection) and by calculating the response of the vapour transfer test. 

Sintering does not usually involve chemical reactions, and the driving force is a reduction in the surface area and the associated reduction in surface energy. This driving force can be illustrated for a flat surface that contains a spherical promberance and a similar spherical depression. The vapour pressure over a curved surface is related to the vapour pressure over a flat surface by the Kelvin equation. This shows that the vapour pressure over a protuberance will be greater than the vapour pressure over the flat surface and will increase as the radius of the curved surface decreases. Similarly, the vapour pressure over a depression will be less than the vapour pressure over a flat surface. When a solid is heated, vapour transfer of matter will take place from a protuberance to a depression, and the surface will tend to become flat. 

Similar to conventional cut and sew sportswear, the conventional sports shoe upper is often made of various materials to address the mapping of the required physiological requirements such as heat and vapour transfer and also moisture management. In addition, materials of varying rigidity and strength are also normally mapped onto the sport shoe upper to provide required combination of support and flexibility. These various materials and other components could be joined with an adhesive or seams. 

The main property of the outer shell is to provide a protection against the outside weather conditions, mainly rain, snow and wind. In order to avoid excessive sweat accumulation, the outer layer should allow moisture transfer from the body to the environment. Such waterproof, windproof and breathable fabrics (WBF—mainly membranes and coatings) have been on the market for more than 30 years. The waterproof properties and, at the same time, the water vapour transfer are either achieved with a micro-porous or a hydrophilic structure, or a combination of both technologies (bi-component WBF). Waterproofness and breathability are contradictory requirements and, therefore, a compromise has to be found between protection and comfort properties. This compromise is usually achieved by adapting the porosity and thickness of such WBF layers. 

Ruckman, J.E., 1997. Water vapour transfer in waterproof breathable fabrics part 3 under rainy and windy conditions. Int. J. Cloth. Sci. Technol. 9, 141-153. 

Kayserilioglu, B. ., Bakir, U., Yilmaz, L., Akka , N. Use of xylan, an agricultural byproduct, in wheat gluten based biodegradable films mechanical, solubility and water vapour transfer rate properties. Bioresour. Teehnol. 87, 239-246 (2003)... 

Fig. 2.3 Water vapour transfer through microporous (a) and hydrophilic (b) membranes 1, water vapour molecules 2, polymer molecular chains 3, active hydrophilic groups ...

Hong et al. suggested two approaches for measuring moisture vapour transfer ... 

From the water vapour permeability point of view, textile materials behave in two different ways first, materials in which moisture vapour transfer takes place predominately by diffusion through air spaces betweeu yams and fibres, following Pick s law. The measured water vapour permeability values are independent of the measuring conditions. Woven, nonwoven, and semipermeable membrane laminates fall into this category. Second are textile material composites that contain a layer of hydrophilic membranes and which behave quite differently. In particular the rate of diffusion through the hydrophilic manbrane is dependent on the test conditions, such as concentration of water vapour in the layer or relative humidity. ... 

In a series of investigations, " it was found that the vapour transfer behaviour of different types of product are quite different under steady state, raining, windy and rainy and windy conditions. In general, wind increases while rain decreases the water vapour transfer rate of fabrics. Breathability of most of the fabric ultimately ceases after long exposure to prolonged severe rainy conditions. 

To clarify the principles and mechanisms of water vapour transfer by diffusion in waterproof breathable fabrics for clothing, experiments using a simple glass dish were carried out under steady state conditions with and without a temperature gradient in the climatic chamber. It was found that both vapour pressure and natural convection... 

Hong K, Hollies NRS, Spivak SM. Dynamic moisture vapour transfer through textiles part I clothing hygrometry and the influence of fiber type. Text Res J 1988 58 697. 

Gibson PW. Factors influencing steady state heat and water vapour transfer measurements for clothing materials. Text Res J 1993 63(12) 749-64. 

Rossi RM, Gross R. Water vapour transfer and condensation effects in multilayer textile... 

The results of this study demonstrate the varying physical differences between the selected wound dressings assessed arxl average results are shown in Table 1. Two of the dressing types showed very low WVTR (Water Vapour Transference Rate) and air permeability. The Carbopad VC specimens showed very low/minimal w permeability, exhibiting an occlusive characteristic this presented problems during the test to evaluate the efficiency of odour adsorption and was therefore with-drawn from this test. 

Wound dressing Carboflex WVTR Water Vapour Transference Rate (g m Z4hr ) 4160 Air Permeability (cm sec ) 28.21 Fluid handle Absorption test (ml 100cm ) 42.98 Average efficiency of odour adsorption (time in min to ISmim ET2NH2) 20 19... 

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