Economic velocities

R. A. Smith, "Economic Velocity in Heat Exchangers," ASME/AIChE 20th National Heat Transfer Conference, Milwaukee, Wis., 1981. 

Economic velocity The velocity at which gases or fluids are conveyed to ensure that running costs are kept at an economic minimum and that damage is nor caused by erosion. 

Economic velocities for turbulent flow (the most common industrially), taking into account piping capital costs (piping, fittings, pumps, etc.) and running costs (pressure drops), are primarily a function of fluid density [A/ fT oc pu2]. Details are given in the literature.4 Order of magnitude economic velocities are ... 

Rule-of thumb economic velocities for siizing sited pipelines... 

Desired diameter This is determined by gas velocity, which will be in the range between the minimum fluidization velocity and the velocity determined by excessive entrainment. The most economic velocity will be set by balancing gas circulation cost with other costs. 

Because calculations such as these are long and tedious, companies that install many pipelines have solved the problem for a large number of cases and have summarized the results in convenient form. The most popular method is to calculate the economic velocity ... 

This equation says that for a given set of cost data the economic velocity is independent of the mass flow handled and dependent on only the. fluid density and the friction factor. More thorough analyses and far more complicated cost equations lead to substantially the same conclusion. For example, for schedule 40 carbon-steel pipe, Boucher and Alves [16] give the data shown in Table 6.4. 

The table refers to turbulent flow only. For laminar flow, the value of / goes up quite rapidly as the viscosity increases, making the economic velocity go down. Oil companies spend more money pumping viscous liquids (crude oils, asphalt, heating oils, etc.) than do any other companies therefore they have made up the most convenient economic-velocity plots for laminar flow. 

Economic velocity for schedule 40, carbon-steel pipe... 

Why does App. A.4 show the velocity in feet per second for all the water flows given Fromj Table 6.4 and Fig. 6.21 we can see that for water (which is almost always in turbulent flow in industrial equipment) an economic velocity is almost always about 6ft/s. Thus, working engineers often simply select pipe sizes for water or similar fluids by looking at App. A.4 for the pipe size which gives a velocity ofj about 6ft/s (2m/s). 

The economic size for a pipe is the size with the lowest sum of annual charges for thle purchased cost of the pipe and pump and the annual power cost of running the pump or compressor needed to overcome friction. For turbulent flow, this results in an economic velocity which is practically independent jof everything but fluid density it is about 6ft/s for most liquids and about 40ft/s for air under normal conditions. 

If Table 6.4 v ere based on Eq. 6.51 and the friction factor held constant, then the product of thie economic velocity and the cube root of the density would be a constant. How much does it vary from being a constant What is the cause of this variation j... 

As discussed in Sec, 6.13, the economic velocity in a pipeline is primarily dependent on the density of the fluid flowing. For long-distance natural-gas pipelines, the pressures are normally in the range of 500 to 1000 psia, so densities are of the order of 1 to 21bm/ft From Table 6.4 we can estimate the economic velocity at about 20ft/s, which is typical of these pipelines. Thus, this kind of flow does not really correspond to the subject of this section—high-velocity gas flow however, it fits in naturally here, after we have developed the equations for high-velocity gas flow. 

To determine the fluid velocity in a pipe, the rule-of-thumb economic velocity for turbulent flow is used, as given next ... 

The bend radii of pipes should be as large as possible. Normally, a minimum of three times the diameter of the pipe should be enforced for economical velocities. This may he adjusted up for various metals, depending on their fabrication difficulties, e.g. mild steel and copper pipe - three times, 90/10 copper nickel - four times, minimum, and high-tensile steel pipe - five times the diameter of the pipe, minimum. Adjustment for high velocities is, of course, also required - the higher the velocity the larger the radius of the pipe. Elbows of similar radii, i.e., minimum three diameters, are advantageous if these are commercially available. 

Haubenreich, P. N., Estimation of Most Economical Velocity in Reactor Circulating Systems, USAEC Report CF-54-5-26, Oak Ridge National Laboratory, May 5, 1954. 

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