Minimum flow

Since capillary tubing is involved in osmotic experiments, there are several points pertaining to this feature that should be noted. First, tubes that are carefully matched in diameter should be used so that no correction for surface tension effects need be considered. Next it should be appreciated that an equilibrium osmotic pressure can develop in a capillary tube with a minimum flow of solvent, and therefore the measured value of II applies to the solution as prepared. The pressure, of course, is independent of the cross-sectional area of the liquid column, but if too much solvent transfer were involved, then the effects of dilution would also have to be considered. Now let us examine the practical units that are used to express the concentration of solutions in these experiments. 

The advantage of single-pass over cross-flow filtration is that it is an easier system to operate and can be cost effective, particularly if the product to be filtered is expensive, because very tittle of the initial fluid is lost during filtration. However, because the flow pattern of the fluid is directly through the filter, filter life maybe too short for the fluid being filtered. The minimum flow rate needed downstream of the filter must also be considered, especially when there are time constraints to the process. In some situations it may be more advantageous to use a cross-flow system where higher flow rates may be easier to obtain. 

Safety Showers. Safety showers and eyewash fountains or hoses should be installed where corrosive or toxic materials are handled. A large-volume, low velocity discharge from directly overhead should effect continuous drenching, ie, a minimum flow of 20 L/min (50 gal /min). Water to outside showers may be heated to a maximum temperature of 27°C by an electric heating cable. The valves for all safety showers should be at the same height and relative position to the shower head, and they should operate in the same way and direction. The shower station should be identified by paint of a bright, contrasting color. In areas where chemicals harmful to the eyes may be encountered, an eyewash fountain or spray should be available in case of splash accidents. 

Minimum-flow loop left open at normal rates, or byj>ass around control valve is open. 

Close minimum-flow loop or control valve byj)ass valve. 

Type of resin Maximum and minimum flow, m/ h [gal/(min-fd)] Minimum hed depth, m (in) Maximum operating temperatures, (°F) Usahle capacity, g-equivalent/Lf Regenerant, g/L resinf... 

Use minimum flow recirculation lines piped back to feed vessel... 

Pump is operated Match pump capacity to the service at a fraction of, Install minimum flow recirculation line to heat capacity. Possihil- ensure adequate cooling of the pump ities of excessive internal interlock to shutdown pump on mini-recirculation, fre- indication quent seal and Implement procedural controls to avoid operat-bearing failure ing at too low a flow resulting in loss of, Provide deadhead protection containment. CCPS G-23 CCPS G-29... 

What must be done is establish the maximum flow, and the minimum flow, and implement controls. Regarding filters, you ve got to establish the flow and pressure (resistance) that corresponds to the new, clean filter, and determine the flow and resistance that represents the dirty filter and its moment for replacement. These points must be predetermined. The visual graph of the system eurve with its dynamie resistances are seen in this example filtering and recirculating a liquid in a tank. Consider the following graphs (Figures 8-18 and 8-19). 

A preliminary estimate is useful for the linear velocity to be used on the catalyst under study. The linear flow is known for an existing process. For a new process, it can be estimated from flow used in similar processes. An estimate can also be developed for the minimum flow to avoid gradients from calculations (to be presented in Appendix C.)... 

Step 2. System calculations over the entire range of possible operating conditions are required. The range must cover from the air blower minimum flow point to the expander bypass point for all reasonable variations in the applicable parameters of COj/CO ratio, fresh feed rate, flue gas temperature, ambient air temperature, and so forth. 

Step 3. The final selection of a specific air blower and turboexpander must be made after fully considerating the results of Steps 1 and 2. The normal operating point must be located on the system operating map (similar to Figure 4-66) so that reasonable latitude is available in operating variables between normal operating point and air blower minimum flow, and between operating point and expander bypass point. 

Most pumps need minimum flow protection to protect them against shutoff. At shutoff, practically all of a... 

The process engineer must plan for minimum flow provisions when making design calculations. For preliminary work, approximate the required minimum flow by assuming all the horsepower at bloeked-in eonditions turns into heat. Then, provide enough minimum flow to carry away this heat at a 15°F rise in the minimum flow stream s temperature. 

This approach will provide a number aecurate enough for initial planning. For detailed design, the proeess engineer should work elosely with the meehanieal engineer and/or vendor representative involved to set exact requirements, ineluding orifice type and size for the minimum flow line. Also, a eooler may be required in the minimum flow line or it may need to be routed to a vessel. For boiler feedwater pumps, a special stepped type orifice is often used to eontrol flashing. 

The best anti-surge control is the simplest and most basic that will do the job. The most obvious parameter is minimum-flow measurement, or if there is a relatively steep pressure-flow characteristic, the differentia pressure may be used. The latter parameter allows for a much faster response system, as flow measurement response is generally slow however, the speed of response need only be fast enough to accept expected transients. One major problem with the conventional methods of measurement and control is the need to move the set point for initiation of the control signal away from the exact surge point to allow some safety factor for control response time and other parameters not directly included... 

The maximum and minimum flow rate available from the solvent pump may also, under certain circumstances, determine the minimum or maximum column diameter that can be employed. As a consequence, limits will be placed on the mass sensitivity of the chromatographic system as well as the solvent consumption. Almost all commercially available LC solvent pumps, however, have a flow rate range that will include all optimum flow rates that are likely to be required in analytical chromatography... 

Once the set pressure is reached, the valve opens fully and remains open, so long as the set point is exceeded. There is no need for overpressure or minimum flow to keep it open. Thus, it is not subject to chattering at low discharge rates. 

Heat transfer through a pipe wall. A pipeline parr 15m long carries water. Its internal diameter d, is 34 mm and its external diameter is 42 mm. The thermal conductivity of the pipe X is 40 W m K". The pipeline is located outdoors, where the outdoor temperature Oao is -8 C. Determine the minimum flow velocity necessary in the pipe to prevent the pipe from freezing. The heat transfer coefficient inside the pipe is = 1000 W m K and outside the pipe = 5 W m" K aiid = 4 W m -K . The specific heat ca-... 

The minimum flow simply means that this flow must circulate through the pump casing (not recirculate with no cooling) back to at least the initial temperature of the feed, if excessive temperatures are not to dev elop. The best practice is to request the manufacturer to state tliis value for the... 

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