Air entry pressure

The desirable properties for the authors equipment are air entry pressures greater than 750 p.s.i., stability (many plastic films deteriorate with time), low air permeability of the wetted film, availability, and low cost. Cellophane seems to come closest to satisfying these requirements. 

Figure 6 shows the evolution of gas pressure at the injection point and the gas flow rate at the outflow point. It can be observed that after a period of gas pressure increase corresponding to the desaturation of the injection layer, gas starts to penetrate in the medium. Figure 7 shows degree of saturation for both the fracture and the rock matrix. Gas flow takes place through the fracture because permeability is higher than in the matrix and because desaturation is easier than in the matrix. Due to fracture aperture, permeability increases and the air entry pressure decreases. 

Primary zone size is important with regard to efficiency and limits also. Within practical limits, a larger primary zone cross-sectional area will provide the best performance 138). Possible reasons arc lower velocities, less wall impingement by fuel, larger zone of low velocity, and less wall quenching of chemical reactions. The best axial distribution of open area of a combustor will depend on required operating conditions, the pressure loss characteristics, and the shape of the air entry ports. It will also depend on fuel-injection and fuel-volatility characteristics, as these factors will affect the amount of vapor fuel present at any location. If proper burning environment is to be obtained, these factors must be matched, and compromises in performance must be expected. 

The void ratio at air entry eae is a function of the load pressure P and the void ratio eaeo at air entry for P = 0, according to... 

The extent of the residual shrinkage, eae — o, is related to the swelling pressure at air entry according to... 

For a hydrophobic porous material with contact angle greater than 90°, the APc is >0 and depends on the liquid surface tension and the membrane pore size. As an example, considering water-air-polypropylene system, one can calculate that for a dry membrane with a pore size of 0.03 pim (30 nm) the critical entry pressure of water is more than 300 psi (>20 bar). 

The relationship between i/j and the water content of a porous medium is also determined empirically Fig. 3-23 shows typical ijs — 0 curves. When is zero, the porous medium is at its saturated moisture content, which is equal to porosity (assuming that there is negligible trapped gas). The pressure head can become somewhat negative before air enters (and water leaves) some soils the value ijja at which air begins to enter is called the air entry value. Water leaves the matrix as i/j (defined as pressure head) becomes more negative than ifsa as water leaves the soil, the hydraulic conductivity also decreases. 

Laboratory measurements of capillary entry pressures are commonly performed on Hg-air systems. To calculate maximum hydrocarbon column heights, mercury-air capillary pressure data must first be converted to hydrocarbon-water pressures, using the following equation (Watts, 1987) ... 

Our database indicates mercury-air capillary entry pressures for siltstones between 20 and 30 MPa (equivalent to a 400-7(X) m column of 30° API oil sealed at 2.5 km depth), and 45-55 MPa for mudstones (900-1200 m oil columns). For shales, these... 

A rock sample is considered with an initial permeability assumed to be lO m, both in the porous medium and in the element that will contain the fracture. Capillary pressure to start desaturation (air entry value) in the porous medium is assumed to be 0.3 MPa. 

Size reduction operations take place within a tented area, within the conditioning area, which is provided with a Tedak ventilation/filtration system. Where feasible the more efficient plasma arc size reduction methods are used rather than mechanical techniques to minimize operator dose. Glovebox removal, size reduction, and waste disposal operations are performed by operators wearing air-fed, pressurized, completely isolated suits (Windscale Suits). Entry into the Windscale Suit areas of FL3 is via a Windscale Suit recirculatory decontamination shower facility. 

The System 80+ Standard Design Control Room Ventilation System provides continuous pressurization of the room to prevent entry of dust, dirt, smoke, and radioactivity originating from outside the room. Filtered outdoor air for pressurization is taken from either of two locations such that a source of uncontaminated air is available. Each intake location is monitored for radioactivity, toxic gases and products of combustion (see CESSAR-DC,... 

Pressure in air phase at air entry surface of cake on actual filter installation. Pa... 

Pressure in air phase at air entry surface of cake. Pa Pressure in air phase at air exit surface of cake. Pa Threshold pressure or vacuum. Pa Barometric pressure. Pa... 

Air entry occurs if excessive negative pressure condition arises in RGH. 

FIGURE 3.23 Relationship between water (moisture) content (0) and pore water pressure (ij/) in a certain unsaturated soil. Pore water suction is the negative of pore water pressure, and is thus a positive quantity in unsaturated porous media. As in the case of the K-6 relationship (Fig. 3.21), the ij/-9 relationship is influenced by whether the soil was most recently wetted or dried. As porewater pressure is decreased, desaturation begins at a pressure called the air entry value, indicated by the dashed line. 

Feng [86] and Feng et al. [87] developed novel nanofibrous membranes for seawater desalination by air-gap MD. The PVDF nanofibrous membranes were characterized by SEM, AFM, and DSC, measurement of LEPw (liquid entry pressure of water), equilibrium contact angle, and particle separation. It was found that the pore size of the PVDF nanofibrous membrane was around 1.5 pm. The equilibrium contact angle of some nanofibrous membranes was above 120°. Feng et al. [86] attempted for the first time to use ENMs for desalination by MD. PDVF nanofibrous membrane could produce potable water (NaCl concentration <280 ppm) from a saline water of NaCl concentration 6 wt% by air-gap MD (AGMD). This new approach may eventually enable the MD process to compete with conventional... 

Total enclosure may be in the form of a room with grilles to facilitate air flow this functions as a hood and operates under a slight negative pressure with controls located externally. Entry is restricted and usually entails use of comprehensive personal protective equipment. Ancillary requirements may include air filters/scmbbers, atmospheric monitoring, decontamination procedures and a permit-to-work system (see page 417). 

Even when the building as a whole is maintained under positive pressure, there is always some location (for example, the outdoor air intake) that is under negative pressure relative to the outdoors. Entry of contaminants may be intermittent, occurring only when the wind blows from the direction of the pollutant source. The interaction between pollutant pathways and intermittent or variable driving forces can lead to a single source causing lAQ complaints in areas of the building that are distant from each other and from the source. 

The calculation of the pressure drop for a chosen exhaust depends on the calculation method (Chapter 9). Pressure drop is usually calculated as the product of a hood entry loss factor, and the dynamic pressure in the connecting duct, p,/. The is expressed a.s p v-/l, where p is the air density and 1/ IS the air velocity in the duct. Some common hood entry loss factors are given in Table 10.4. 

A reversible cycle with turbine expansion split into two steps (high pressure, HP, and low pressure, LP) is illustrated in the T, s diagram of Fig. 4.3. The mass flow through the heater is still unity and the temperature rises from T2 to Tt, = Tq hence the heat supplied (3b is unchanged, as is the overall isentropic temperature ratio (x). But cooling air of mass flow i//H is used at entry to the first HP turbine (of isentropic temperature ratio. xh) and additional cooling of mass flow is introduced subsequently into the LP turbine (of isentropic temperature ratio Xl)- The total cooling flow is then i/( = i/ h + >h.-... 

For two step cooling, now with irreversible compression and expansion, Fig. 4.7 shows that the turbine entry temperature is reduced from Ti. to by mixing with the cooling air i/ H taken from the compressor exit, at state 2, pressure p2, temperature T2 (Fig. 4.7a). After expansion to temperature Tg, the turbine gas flow (1 + lp ) is mixed with compressor air at state 7 (mass flow i/h.) at temperature Tg. This gas is then expanded to temperature T g. 

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