Leaks, molecular flow

When calculating pressure relationships and types of gas (viscosity) it is necessary to keep in mind that different equations are applicable to laminar and molecular flow the boundary between these areas is very difficult to ascertain. As a guideline one may assume that laminar flow is present at leak rates where Q > 10 mbar l/s and molecular flow at leak rates where Q < 10 mbar l/s. In the intermediate range the manufacturer (who is liable under the guarantee terms) must assume values on the safe side. The equations are listed in Table 5.2. 

If it is assumed that molecular flow occurs in the leak then, according to Equation (2.37) ... 

Again, as shown in Example 4.1, molecular flow would predominate at the leak outlet. 

For a barrier consisting of two sizes of straight circular holes, with Cmoi fraction occupied by small holes of radius through which pure molecular flow takes place and eyjs fraction occupied by larger holes of radius Zyis througjt which viscous flow takes place, the molar velocity for this viscous leak model would be... 

Here F is known as the permeability, Fo is interpreted as the permeability for molecular flow, and S is sometimes called the slope factor. Comparison of Eq. (14.10) with (14.8) and (14.9) shows that a physical interpretation can be given to the parameters Fo and S in terms of pore radius and void fraction for the mixed flow and viscous leak models ... 

Barrier separation efficiency, viscous leak model. In the viscous leak model, flow through the small holes of radius is of the separating, molecular type dealt with in deriving Eqs. 

On the other hand, viscous flow will dominate in the leak when conditions are such that the mean free path is small compared to the leak dimensions. In this instance, the flow rate is independent of the isotopic mass and a correction which is proportional to 1 y/M is necessary to compensate for molecular flow from the ionization chamber. 

The problem of determining the pressure of neutral gas in the source, especially at relatively low pressures, was for some time considerably more difficult. Before satisfactory low-pressure gauges were available, the source pressure was determined as follows The repeller was biased negatively to the source in order to collect all of the ions formed. Argon or neon, of known ionization cross section, was admitted to the source under conditions of molecular flow through the leak and the total positive ion current collected on the repeller was recorded as a function of reservoir pressure while maintaining a constant, known current of electrons. Under these conditions. 

A basic batch inlet is shown in Figure 2. A small quantity of gas is expanded into an evacuated volume, so that the pressure is low enough for molecular flow to occur in the leak to the mass spectrometer. No mixing of flow regimes occurs, so fractionation is absent. However, it involves more hardware than other inlets, and cannot give truly... 

Methane leaks from a tank in a 50 m3 sealed room. Its concentration is found to be 30 % by volume, as recorded by a combustible gas detector. The watchman runs to open the door of the room. The lighter mixture of the room gases flows out to the door at a steady rate of 50 g/s. The flammable limits are 5 and 15 % by volume for the methane in air. Assume a constant temperature at 25 °C and well-mixed conditions in the room. The mixture of the room gases can be approximated at a constant molecular weight and density of 25 g/mol and 1.05 kg/m3 respectively. After the door is opened, when will the mixture in the room become flammable ... 

In the late sixties, Blatt and co-workers at Amicon developed a thin-channel cross-flow device for plasmapheresis.30 In this device, red cells and plasma could be readily separated with a 0.6 ju MF membrane at an acceptable flux. As shown in Figure 2.60, the flux increases with the cross flow velocity. However, there is a limiting velocity above which the degree of hemolysis is unacceptable. We discovered that pore sizes above 0.8 ju occasionally leaked nonhemolyzed red cells while pore sizes below 0.2 ju retained some of the higher molecular weight plasma proteins (notably albumin and IgG). Therefore, pore sizes between 0.4 and 0.6 ju were selected with 0.6 ju preferred because of higher plasma fluxes. 

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