Attainable pressure

The required vacuum pressure in a system can only be achieved if the vacuum pump can deal with the gas load. Low gas pressures can be obtained if  

A cylindrical vacuum chamber has an internal diameter of 70 cm and a length of 2.5 m. It is made of cleaned stainless steel and evacuated with a combination of diffusion and rotary vacuum pumps. Measurements of the outgassing rate of the chamber material, cleaned according to the same method, give the following data  

Assuming that the chamber ends are hemispherical, the area of the walls and chamber ends (neglecting the entrance area of the diffusion pump, gauges, etc.) is  

A vacuum vessel (V = 50 L) is evacuated by a diffusion pump backed with a two-stage rotary vacuum pump. There is a leakage of air into the vessel and a desorption gas load (water vapour) of 1 x 10 4 mbar L s 1. The pumping speeds of the diffusion pump for air and water vapour are 100Ls1 and 120Ls1, respectively. Between the diffusion pump and the vessel is a valve having a conductance for air of 100 Ls 1. 

The surface area of a vacuum chamber is 2 m2 and the outgassing constant is 1 x 10-4 Pam3 s mf2 (not varying with time). N2 is admitted at a rate of 2 x 10 6 Pa m3 s 1. 

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The shell and tube sides of heat exchangers can be designed to contain the maximum attainable pressure on either side, eliminating reliance on pressure relief to protect the exchanger shell in case of tube rupture. 

As well as the normal addition reaction, an extremely exothermic decomposition reaction may occur, particularly at high vessel loadings. At loadings of 0.8 ml of 1 1 mixture per ml, the violent reaction, catalysed by iron(III) chloride, initiates at —40°C and will attain pressures above 0.7 kbar at the rate of 14 kbar/s. At 0.5 ml loading density, a maximum pressure of 68 bar, attained at 114 bar/s, was observed. 

A volume of gas is enclosed in the space bounded by the rotor, the stator and the two vanes (see Fig. 1.6). The pump removes the gas by compressing it to a pressure slightly higher than the atmospheric pressure. This overpressure opens the spring-loaded outlet valve, and the gas escapes to the atmosphere. A thin film of oil makes the final seal therefore the ultimate pressure depends also on the oil vapour pressure. With one stage, the lowest attainable pressure is about 10 2torr and with two stages in series 10-3 torr. 

Several authors measured attainable pressure drops as a function of the external resistance for different groups of volunteers. Healthy male subjects (during maximal inspiration) are able to create a pressure drop (on average) of 6.7 kPa through an airflow resistance of 0.038 kPa° min 1, (which is in the range of that of marketed DPIs, see Table 3.7), whereas females are able to create a pressure drop of 3.8 kPa under the same resistance [22]. Differences between different groups of patients depend on the degree to which pulmonary func-... 

In many applications it is expedient to relate the attainable pressures at any given time to the pump-down time. This is easily possible with reference to the nomogram 9.7 in Section 9. 

Theoretical analysis of gas detonation leads to the conclusion that a shock wave propagates at the detonation front, compressing and heating the gas mixture. The chemical reaction runs in the already compressed gas, and it is only after completion of the reaction that the state of the explosion products calculated in the classical theory is attained (pressure pc, velocity wc, temperature Tc). In particular, in the wave front the velocity w1 and the pressure p1 of the compressed gas are approximately twice as large as in the reaction products w1 2wc, p1 2pc. The amount of the compressed gas at the pressure px and the thickness of this layer are proportional to the chemical reaction time, r. 

To attain pressures lower than that produced by the oil immersion pump, a vapour diffusion pump is employed which gives pressures down to 5 x... 

The density of the polymer at 25°C was obtained by using a molded disk, 0.125 in thick and 2 in in diameter. The melt density at processing temperature was obtained with an Instron Capillary Rheometer with plugged exit. The isothermal compaction at meltprocessing temperature was conducted at a plunger speed of 0.05 in/min with attainable pressures up to 25,000 psi. 

A possibility to reduce the number of cold heads/compressors would be to shield the 50 K panel with a LN2-cooled baffle. This, however, would reduce Xe owing to transmission restrictions, thus requiring an equivalent increase in Ac. Further, the cost of a LN2-cooled baffle and LN2 have to be considered. An unexpected problem is that, at 80K, ps Xe is 10 2 mbar. If the sticking coefficient of Xe is significant then this may limit the minimum attainable pressure in the system. 

Gas Sources and Attainable Pressure in Vacuum Systems For 95% of the steady-state signal ... 

The concluding part of the chapter considered the influence of various gas sources on the attainable pressure in vacuum systems (Examples 4.24-4.26). 

In Chapter 4, the gas sources and loads which are present in every vacuum system are considered. Since the gas loads from different sources have to be known or estimated to quantify the size of the pump set, their influence on attainable pressure is considered. 

With many systems, especially those containing microporcs, low pressure hysteresis (indicated by the dashed lines in Figure 3), may be observed extending to the lowest attainable pressures. Removal of the residual adsorbed material is then possible only if the adsorbent is outgassed at higher temperatures. This... 

The experiment began by charging the equilibrium cell with about 30 cm3 of either phenoPp-cresol or phenol-water solution mixture. The cell was then pressurized with either methane or carbon dioxide until the phenol clathrate formed under sufficient pressure. The systems were cooled to about 5 K below the anticipated clathrate-forming temperature. Clathrate nucleation was then induced by agitating the magnetic spin bar. After the clathrates formed, the cell temperature was slowly increased until the clathrate phase coexisted with the liquid and vapor phases. The nucleation and dissociation steps were repeated at least twice in order to diminish hysteresis phenomenon. The clathrates, however, exhibited minimal hysteresis and the excellent reproducibility of dissociation pressures was attained for all the temperatures and found to be within 0.1 K and 1.0 bar at each time. When a minute amount of phenol or p-cresol clathrate crystals remains and the system temperature was kept constant for at least 8 hours after attaining pressure stabilization, the pressure was considered as an equilibrium dissociation pressure at that specified temperature. 

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