Capillary flowmeters

Most laminar flowmeters (also called capillary flowmeters) measure very low flow rates of gases in applications where other types of meters either give marginal performance or cannot be used at all. Commercial units use either capillary elements or matrix shapes (Figure 3.67). The pressure drop generated by these elements is in linear relationship with the process flow. When higher accuracy and rangeability is desired, thermal instead of d/p detectors are used with the laminar flow element. 

Both pressure drops are small compared with atmospheric pressure (less than about 4000 Pa) so that the compressibility of the gas can be neglected. The Carman-Kozeny equation can in this case be simplified to include the static heads measured directly as shown in Fig. 6. Note that the capillary flowmeter has to be calibrated so that its conductance c (ml/s) is known. 

P205 column, 2. capillary flowmeter, 3. three step cock, 4. thermocouple, 5. sample holder with sample, 6. resistance furnace, 7. electronic microbalance, 8. x-t recorder... 

Fig. 2. Experimental equipment for exposure experiments at very low oxygen partial pressure 1. active Cu catalyst, 2. P2Os column, 3. oxalic acid water saturator, 4. capillary flowmeter, 5. furnace...

Fig. 8-22. Capillary flowmeter, used to meter small gas or liquid streams, shown with high-pressure seal-welded differential-pressure transmitter.

Flowmeter. To monitor flow of carrier gas, a variety of devices are available, such as differential capillary, thermal conductivity, ionization, rotameters, and calibrated soap-film tubes Measurement of the flow may either be continuous or intermittent, and the flowmeter may be placed either in front of the column or at the carrier gas outlet. The soap-film type is most commonly used because of its economy and ease of operation. 

To facilitate measurement and control of larger flow rates, the heat-transfer-type flowmeters can also be placed in a small bypass around a capillary element in the process pipe. The laminar flow element serves to ensure laminar flow and also acts as a restriction forcing a portion of the flow into the sensor tube. The small-size bypass serves to minimize the electric power requirement and to increase the speed of response, but it requires upstream filters to protect it against plugging. 

The required apparatus (Fig. 5) consists of two flowmeters suitable for use with easily hydrolyzed fluorine compounds, one capillary or Rotameter flowmeter for chlorine, two nickel tubes 4 ft. long X 2 in., and two tube furnaces capable of heating the tubes to 400 5°. Metal tubing is... 

Now, we shall present an inexpensive means to meter small flows of gas. Accurately controlling and measuring such flowrates is often difficult. Control valves with small trims coupled to differential-pressure transmitters having small orifices (or long capillaries) are prone to plugging and calibration troubles. On the other hand, most mass flowmeters are expensive for small-scale uses. 

Apparatus is assembled so that dry nitrogen flowing at a rate of about 8 1. per hour through a calibrated flowmeter (made with l-mm. capillary tubing as the orifice) passes through the sulfur trioxide in its flask at about 25° and thence into the reactor. A stream of fluorine also enters... 

The energy flowmeter was designed so that it could shift flow constrictors in the sample line. The ratio of the main flow to the sample flow was called the split of the flow separator. The capillary and valve manifold on the sample line allowed the split to be changed between four different values. This was done automatically in response to changes in the main line flow. The micro-computer of the Flow-Titrator selected the split as to keep the sample flow in the 2-4 scfh range which is the operating range of the combustion system of the Flow-Titrator. 

Flowmeters.— The measurement of gas-flow rate is usually carried out with a soap-bubble meter although other methods involving rotameters and capillary manometers are available. The soap-bubble meter is an inexpensive, easy to operate, direct method which has good accuracy. 

In Figure 25.22, the scheme of experimental setup used for membrane extraction experiments was presented. The setup consisted of membrane contactor with aqueous and organic circuits, two pumps, and the control equipment flowmeters, pressure gauges, and valves. Two phases aqueous and organic solutions circulated countercurrently. Membrane contactor X50 2.5 x 8 Liqui-Cel Extra-Flow, Celgard, was used in the system. The characteristics of the membrane were shown in Table 25.17. The small volume module houses 11,000 capillaries with 1.9 m inner surface area. The module possesses the central baffle, which enables uniform flow inside the shell. 

In temperature indicators or thermometers the temperature is usually transmitted from a sensitive element to the dial by means of a capillary. It is important that this capillary is not kinked or bent or broken frequent flexing will easily cause the latter. Other indicating instruments in common use in chemical factories are ammeters, measuring electric current voltmeters for electric voltage liquid, vapour, and gas meters flowmeters pH meters weighing machines. 

Figure 4.22 Schematic of a pumping system based on a pneumatic gas pressure amplifier with microfluidic flow control via feedback from a sensitive flowmeter. In this way the flow rate is maintained regardless of changes in system back pressure or mobile phase viscosity, and changes in flow rates can be established rapidly and accurately, (a) A gradient system in which the mobile phase composition is controlled via flow rates of both mobile phase solvents, (b) A gradient system in which both back pressures and flow rates are monitored volume flow rates = k. (P(- -P ) and Ug = kg.(Pc - Pg) where k and kg are calibration constants, (c) A demonstration of the precision and accuracy with which controlled flow rates can be changed rapidly at total flow rates in the nL.min range, suitable for packed capillary HPLC. Reproduced from company literature (Eksigent 2005, 2006) with permission from Eksigent LLC.

Finally, Rivkin et al. (1986) also used the capillary flow technique to measure the viscosity of aqueous boron solutions, at temperatures up to 623 K and at pressures up to 30 MPa. In this case they employed a platinum capillary of 500 nun length and 0.3 nun ID, placed in a liquid thermostat in which temperature was controlled with an uncertainty of 0.03 K. A prunp-flowmeter was used to measure the volume of the fluid flowing through the capillary tube at each given temperatme and pressure. The pressure drop across the capillary ends was measmed with a compensation-type differential mercmy pressure gauge with a movable elbow. The details of the experimental apparatus and measurement procedme are given in Rivkin et al. (1979). 

The ratio of the volumetric flowrate out of the purge vent to the volumetric flowrate in the capillary column is termed the split ratio and provides an estimate of and control over the actual volume of sample entering the column. Care should be taken when using the split ratio to estimate actual injected sample volume, or when using it in comparisons between methods on different instruments. There are subtle differences between instruments and measurement techniques that may affect the measured flows. For example, the column volumetric flowrate measured by injecting a nonretained substance is the average column flowrate, not the flowrate at the inlet, while a flowmeter connected to the split purge vent measures the volumetric flowrate at the vent, not in the inlet. With newer, electronically controlled systems, the flows are measured directly at the inlet, or are calculated from the entered inlet conditions and column dimensions. 

Measuring methods which deduce the flow directly from flow characteristics are very common. There are versatile methods being used, which all depend on the velocity of the flowing medium. The most important are measurements with orifice devices, with suspended bodies, and according to a magnetic-inductive method. Beyond that there are flowmeters which use the pressure drop of a flowing liquid in a capillary, and magnetic-inductive, ultrasonic, and Coriolis flowmeters. 

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