Mass flow measurement drops

Shirato, Gotoh, Osasa, and Usami [J. Chem. Eng. Japan, 1, 164— 167 (January 1968)] present a method for determining the mass flow rate of suspended sohds in a liqiiid stream wherein the liquid velocity is measured By an electromagnetic flowmeter and the flow of sohds is calculated from the pressure drops across each of two vertical sections of pipe of different diameter through which the suspension flows in series. 

For example, suppose we want to control the mass flow rate of a gas. Controlling the pressure drop over the orifice plate gives only an approximate mass flow rate because gas density varies with temperature and pressure in the line. By measuring temperature, pressure, and orifice-plate pressure drop, and feeding these signals into a mass-flow-rate computer, the mass flow rate can be controlled as sketched in Fig. 8.3a. 

For each thickness, at least 10 different flow rate measurements were obtained in order to cover the range of flow rates that a DL experiences during normal fuel cell operation. To obfain fhe corresponding permeabilify, fhe pressure drop resulfs were ploffed as a function of the mass flow rate. After this, the Forchheimer equation was fitted to the plotted data to determine the viscous and inertial permeabilities. As expected, the in-plane permeabilities of each sample DL maferial decreased when the compression pressure was increased. It is also important to mention that these tests were performed in two perpendicular directions for each sample in order to determine whether any anisotropy existed due to fiber orienfation. 

The typical pressure profile of the standpipe and the valve is shown in Fig. 8.18. As can be seen in the figure, the pressure head over the standpipe Aps, in the absolute sense, is equal to the sum of the overall pressure head, Apt, and the pressure drop over the valve, Ap0. From Eq. (8.92), the particle volume fraction in the standpipe can be estimated from the measured height of the bulk particles. Thus, given the particle mass flow rate, the leakage flow of gas in the standpipe can be estimated from Eq. (8.95). 

Other, less accurate methods (1-3% FS) of solids flow measurement include the impulse and the accelerator flowmeters. In the loss-in-weight-type measurement, the total weight of the supply tank is measured and that signal is differentiated by time. The rate at which the total weight is dropping is the mass flow from the tank. These systems do not provide high precision (1% AF over a 10 1 range), but are suited for the measurement of hard-to-handle process flows because they do not need physical contact with the process stream. 

Experiments to measure pressure drop and flooding limits were performed in a set-up accommodating monoliths with diameters of 43 mm (Fig. 8.16), while the length of the monoliths varied up to total length of 1 meter. The liquid was distributed by a nozzle the gas was introduced in countercurrent mode via mass flow controllers in the system. At the outlet of the monolith, a special device was mounted (Fig. 8.17), which improved draining of the monolith. The pressure drop along the column was measured using differential pressure transmitters. All experiments were performed at room temperature and atmospheric pressure. 

Two thermocouples separated by distance L are imbedded in the test specimen, one directly above the other, whereby the temperature drop T2 — T between them is measured. A differential thermocouple measures the temperature rise ATw of the exit water of the calorimeter as compared to its entrance temperature. The mass flow rate of water F into the calorimeter is monitored, so that over a specific time interval At, the total heat absorbed by the calorimeter may be calculated, knowing the specific heat cp of water. Dividing by the time interval will give the rate of heat flow into the calorimeter under steady state conditions ... 

There are two ways to obtain stjo and st. In the first way, CH4 is measured in the bypass mode before or during the reaction and a comparison with the amount of CH4 after the gas passed the reactor is done. However, this procedure has drawbacks the gas flow through the reactor is never absolutely stable due to uncertainties of the mass flow controllers additionally, during such measurements one cannot measure the outlet gas of the reactor. Therefore, the obtained values have large errors, due to summation, especially at low conversions. In the second way, the sum of the remaining amount of CH4 and all the products is used to obtain si o, and the sum of the reaction products is used to obtain si o - Si. Therefore, the calculation can result in much lower uncertainties. For the detection of the hydrocarbon species, a flame ionization detector (FID), and for CO and CO2 a thermal conductivity detector (TCD), was used. When COT falls below the detection limit of the TCD, it drops... 

In this approach a gas flowmeter is used to determine the amount adsorbed. It can be of a differential type, as in Figure 3.7 (e.g. with a differential catharometer or a differential pressure drop flowmeter) or a simple form with either a sonic nozzle (Figure 3.8) or a thermal detector (Figure 3.9). The last provides a signal which depends on the heat capacity, thermal conductivity and mass flow of the gas it is usually referred to as a mass flowmeter although there is no direct measurement of mass. 

Garimella and Bandhauer [32] conducted heat transfer experiments using the same test sections that were used for the pressure drop experiments of Garimella et al. [24, 25, 27, 28] described above. The high heat transfer coefficients and low mass flow rates in microchannels necessitate modifications to the test facility and test procedures described above. For the small zlx required in the test section, the heat duties at the mass fluxes of interest are relatively small. Calculating this heat duty from the test section inlet and outlet quality measurements would result in considerable... 

The Redwood Mass Flow Controllers (MFCs) were able to control mass flow, but with some limitations. In particular, great care had to be exercised when calibrating the MFCs since the calibration curve sometimes exhibited an inflection point in the flow rate versus set point response. A few MFCs did have linear calibration curves, but many exhibited this nonlinearity to varying extents. This was probably due to the flow measurement and control techniques of these MFCs, which operated by controlling the pressure drop across an orifice. Discussions with Redwood staff indicated that adjusting the internal flow control and measmement parameters for these controllers required a considerable amoimt of work. 

Thus, a large variation in e will result at low air flows depending upon whether it is based on qa or q . Since e-NTU curves are very flat at high e (high NTU), there is a very large error in the resultant NTU, h, and j. The j versus Re curve drops off consistently with decreasing Re, as shown by a dashed line in Fig. 17.38. This phenomenon is referred to as rollover or drop-off in j. Some of the problems causing the rollover in j are the errors in temperature and air mass flow rate measurements as follows ... 

Flow. Nonintrusive sensors that can be maintained at the process temperature are ideally suited to measure the flow rate of feed and product streams. Magnetic flow meters are suitable and inexpensive choice for aqueous streams. Organic streams with low dielectric constants require a vibrating tube mass flow meter to satisfy these criteria. Although commonly installed, flow meters that operate by inducing a pressure drop proportional to the flow rate present restrictions for solids accumulation that may alter the calibration. An alternative approach is to monitor the rotational speed of a positive displacement pump. Accuracy of this method is subject to wear and tolerances in the pump. 

---