Linear flow rates can also be determined independently of Equation (4.26) by measuring the retention time of an unretained component of the injected sample. It then becomes evident that open tubular columns exhibit linear flow rates that are three to five times higher than packed columns, despite the fact that the user would have to replace compressed gas tanks less frequently. The columns are listed from the smallest commercially available diameter to the largest, along with a representative volumetric flow rate passing through the column. Equation (4.26) is used to calculate Рё for the five most commonly used GC column diameters, and these results are shown in Table 4.4. 18) 26ДЇigure 4.8 reveals that with respect to the average linear velocity, the optimum linear velocity for a packed GC column is lower than that for an open tubular or capillary GC column. Jennings has articulated an interesting relationship between linear and volumetric flow rates, F, incorporating the column radius, r c according to: (p. Any comparison of van Deemter curves, such as that shown in Figure 4.8, must use linear velocity because the influence of the column radius is eliminated. It is the linear velocity that conducts analytes of interest through a chromatographicД¬olumn to the detector.

Increasing the pipe length beyond that point will cause the flow rate to fall because the pump is now behaving like a constant-pressure pump.Notice that Equation (4.25) examines column efficiency as a complex function of linear mobile- phase velocity.

In the case of a positive displacement pump, increasing the pipe length causes the fluid pressure at the pump end of the pipe to go up at a constant flow rate, until the pressure limit of the pump is hit. In the limit of an infinitely long pipe, it takes an infinite amount of work to push fluid through it, and with a constant pressure pump the flow rate approaches zero. In practice, the effect of that increased flow resistance depends on whether the pipe flow is being driven by a constant pressure pump or a constant flow (i.e., positive displacement) pump.ДЇor example, if the pipe is driven by a constant pressure pump, increasing the pipe's length decreases the flow rate through the pipe. What happens as a pipe is lengthened is that its flow resistance increases in direct proportion to its length. This constraint is called continuity and is asserted when solving pipe flow problems involving incompressible fluids and pipes that do not flex under pressure. You are correct all the water entering one end of a pipe must leave the other end. So does the flow rate of fluid in a pipe change with its length? I also considered the possibility that fluid flow may be different in laminar and turbulent flows when looking at Poiseuille's Formula and the Darcy-Weisbach Equation. which has a similar relationship between flow rate (Q) and length (L) as seen in Poiseuille's Formula. Just for some context, I'm focusing mainly on the Darcy-Weisbach Equation: This doesn't make sense to me as that would be like saying if I had an almost infinitely long length of pipe, no water would come out of the end of it even if I pumped a lot of water through it. However, some videos that I watched say that as the length of a pipe increases, the flow rate within that pipe would decrease such as in Poiseuille's Formula: If a fluid is incompressible and enters a pipe at a certain rate, shouldn't the rate at which it exits be the same? Otherwise, the pipe would be filled with more water than it can hold and have to either expand or compress the fluid within it.ДЇrom this textbook that I found, it agrees with this idea in that the velocity profile in a pipe changes - the no slip condition causes fluid molecules in contact with the pipe to stop and slow adjacent fluid layers down, causing the flow in the centre of the pipe to increase in velocity so as to maintain flow rate. I have a question about the flow rate in a pipe.

Volumetrix flow rate