Flow constant

The flow constant or velocity constant (k) is a measme of the migration rate of the solvent front. It is an important parameter for TLC users and can be used to calculate, for example, development times with different separation distances, provided that the sorbent, solvent system, chamber type and temperature remain constant. The flow constant is given by the following equation  

Zp = distance between the solvent front and the solvent level [mm] t = development time [s] 

The following example illustrates the usefulness of the flow constant in laboratory work. In a TLC, if the development time for a migration distance of 10 cm was 30 min and the Zq distance is 5 mm, the k value is 6.125 nrm /s. 

Question-. How much time is required to develop a 15-cm migration distance if the sorbent, solvent system, Zq and laboratory temperature remain constant  

This means that more than twice the development time is required for a migration distance which is only 5 cm longer  

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C] = c = C = gas/vapor flow constant depending on ratio of specific heats Cp/(, (see Figure 7-25 sonic)... 

Ch = specific heat of trapped fluid, Btn/lb/°F Cj = subsonic flow constant for gas or vapor, function of k = Cp/Cv, Table 7-11... 

Unlike the orifice or nozzle, the pipeline maintains the area of flow constant and equal to its cross-sectional area. There is no possibility therefore of the gas expanding laterally. Supersonic flow conditions can be reached in pipeline installations in a manner similar to that encountered in flow through a nozzle, but not within the pipe itself unless the gas enters the pipe at a supersonic velocity. If a pipe connects two reservoirs and the upstream reservoir is maintained at constant pressure P, the following pattern will occur as the pressure P2 in the downstream reservoir is reduced. 

Figure 7. Dissolved oxygen for various point-source ammonia loadings. Flow constant at 6000 ft s BOD and NFS ammonia loading held constant.

Hua et al. [71] carried out automated determination of total arsenic in seawater by flow constant-current stripping with gold fibre electrodes in which the sample was acidified and pentavalent arsenic was reduced to the trivalent form with iodide. The arsenic was then deposited potentiostatically for 4 min on a 25 xm gold fibre electrode, and subsequently stripped with constant current in 5 M hydrochloric acid. Cleaning and regeneration of the gold electrode were fully automated. 

Runnels and Eyman [41] report a tribological analysis of CMP in which a fluid-flow-induced stress distribution across the entire wafer surface is examined. Fundamentally, the model seeks to determine if hydroplaning of the wafer occurs by consideration of the fluid film between wafer and pad, in this case on a wafer scale. The thickness of the (slurry) fluid film is a key parameter, and depends on wafer curvature, slurry viscosity, and rotation speed. The traditional Preston equation R = KPV, where R is removal rate, P is pressure, and V is relative velocity, is modified to R = k ar, where a and T are the magnitudes of normal and shear stress, respectively. Fluid mechanic calculations are undertaken to determine contributions to these stresses based on how the slurry flows macroscopically, and how pressure is distributed across the entire wafer. Navier-Stokes equations for incompressible Newtonian flow (constant viscosity) are solved on a three-dimensional mesh ... 

The speed at which the metal moves through the rollers must change in order to keep the volume rate of flow constant through the roll gap. Hence, as the thickness decreases, the velocity increases. However, the surface speed of the roller is constant, so there is relative sliding between the roller and the metal. The direction of the relative velocity changes at a point along the contact area called the neutral or no-slip point (point N in Figure 7.10). At the neutral point, the roller and metal have the same... 

Fig. 4. Minimum requirements for a gas chromatographic system include (I) a column which contains the substrate or stationary phase. 12) a supply nt inert carrier gas (moving phase) which is continually passed through the columns. (3) a means lor maintaining pressure and flow constant. (4) a means of admitting or injecting the sample into die carrier gas stream. (5) a detector which senses the sample components as they elute, and >) a display (recorder). The carrier gas may be any gas that does not react with the sample nr adversely artect the detector. Helium, hydrogen, nitrogen, and argon are often used...

The model includes parameters for relative volatility a, vapor velocity v, tray spacing flow constant kv, flooding factor //, vapor py and liquid pL densities, molecular weight MW, and some known upper bound on column flow rates FmaX. 

In the following we attempt to describe the acetylcholinesterase/choline acetyltransferase enzyme system inside the neural synaptic cleft in a simple fashion see Figure 4.49. The complete neurocycle of the acetylcholine as a neurotransmitter is simulated in our model as a simple two-enzymes/two-compartments model. Each compartment is described as a constant-flow, constant-volume, isothermal, continuous stirred tank reactor (CSTR). The two compartments (I) and (II) are separated by a nonselective permeable membrane as shown in Figure 4.50. 

The Karbate line separator (Fig. 14-110g) is composed of several layers of teardrop-shaped target rods of Karbate. A design flow constant K in Eq. (14-226) of 0.035 m/s (1.0 ft/s) is recommended by the... 

In order to reduce variations in flow due to secondary forces such as capillary forces within reservoirs, the flow resistance at each port is increased by using a long serpentine path as a flow restrictor (see Figure 2.37). This chip design also contains the so-called pair-well design so that two channels from the pair wells (e.g., wells 1 and 8) are connected to a common node. Since the pair channels have the same flow resistance, the change of flow in one channel could be easily compensated by that from another channel and so keeping the main channel flow constant. For instance, a 40% flow in the main channel can be achieved by either -4% from well 1 and +44% from well 8, or 4% from well 1 and 36% from well 8 [295],... 

TABLE 10.8 Drag and Pressure Flow Constants for a 50.8-mm Twin Screw Extruder (78)... 

In the fully developed flow region, the assumptions of steady and isothermal flow, constant fluid density, and independence of the pressure from the radius, resulted in the conclusion... 

Overall system maintenance is key to ensuring adequate and reliable water flows. Constant water levels, open and operable control valves, and fire pumps that are maintained and tested periodically, are some of the requirements that must be addressed as part of a long-term program. 

To obtain TCO layers with sufficient optical transparency, the H2O/DEZ ratio has to be kept higher than 1. Indeed, as mentioned above, a small excess of DEZ compared to H2O leads to ZnO layers having a dark visual appearance. Keeping the DEZ and B2H6 flows constant while increasing the H2O flow rate increases the total gas flow. In this way, the H2O/DEZ ratio... 

Assuming pure reactant feed za = 1, plug flow, constant physical properties and one-reactant first-order reaction, the following dimensionless model can be derived ... 

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