Flowmeters variable area

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Other alternative energy applications such as the concentrating parabolic mirror reflector-type solar collectors, where the temperature of the circulated oil can be high, around 900°F (500°C), require high-temperature flow sensors. For these applications, in addition to the head-type flowmeters, noncontacting (e.g., ultrasonic) and metallic tubes, variable-area flowmeters can also be considered. 

Measurement and control of low-flow rates are a requirement in such applications as fuel cells, purging, bioreactors, leak testing, and controlling the reference gas flow in chromatographs or in plasma-emission spectrometers. The most traditional and least expensive low-flow sensor is the variable-area flowmeter. It has a high rangeability (10 1) and requires little pressure drop. Due to its relatively low accuracy, it is limited to purge and leak-detection applications. 

The variable-area flowmeter is a head-type flow sensor, but it does not measure the pressure drop across a fixed orifice instead, the pressure drop is held relatively constant, and the orifice area is varied to match the flow (Figure 3.98). In gravity-type variable-area flowmeters, increase in flow lifts the float, piston, or vane, and it is the weight of these flow elements that has... 

In a variable-area flowmeter the area open to flow is changed by the flow itself. Either gravity or spring action can be used to return the float or vane as flow drops. 

In some variable-area flowmeters, gravity has been replaced by spring loading. In these units, an increase in flow results in a compression or deflection of a spring, and this motion is used as an indication of flow. These units can be mounted in any position, including horizontally, as flow-through pipeline devices. 

The F and P Tri-flat (Low Flow Rate) Variable Area Flowmeter Handbook Application, Sizing, and Calibration Prediction Data", Catalog 10A9010, Fischer and Porter Co., Hatboro, Pa, Pub. 13317, September, 1959. 

The type of inputs include both frequency and non-pulsing analog inputs such as 0 to 5 VDC, 4 to 20 MA or 10 to 50 MA. Manual inputs are required for the variable area flowmeters. 

The principal classes of flow-measuring instruments used in the process industries are variable-head, variaBle-area, positive-displacement, and turbine instruments, mass flowmeters, vortex-shedding and iiltrasonic flowmeters, magnetic flowmeters, and more recently, Coriohs mass flowmeters. Head meters are covered in more detail in Sec. 5. 

Other sensors which are described in Volume 1 (Sections 6.3.7-6.3.9) are the variable area meter, the notch or weir, the hot wire anemometer, the electromagnetic flowmeter and the positive displacement meter. Some of these flowmeters are relatively less suitable for producing signals which can be transmitted to the control room for display (e.g. weir, rotameter) and others are used in more specialist or limited applications (e.g. magnetic flowmeter, hot wire anemometer). The major characteristics of different types of flow sensor are summarised in Table 6.1. Brief descriptions follow of the principles underlying the more important types of flowmeter not described in Volume 1. In many instances such flow sensors are taking the place of those more traditional meters which rely upon pressure drop measurement. This is for reasons of versatility, energy conservation and convenience. 

Two or more of these conditions can occur at the same time, resulting in asymmetric axial, radial and tangential velocity vectors. Some flowmeters are more sensitive than others to particular types of flow distortion, e.g. orifice meters are affected by pure swirl more than venturi meters are magnetic flowmeters are unaffected by changes in the radial velocity component whereas ultrasonic time-of-flight meters are highly susceptible thereto swirl and asymmetry have the least effect on positive displacement meters and the greatest effect on variable area meters. 

Full-bore meters include variable-head meters such as venturi and orifice meters and variable-area meters such as rotameters. These will be described in some detail. Briefer descriptions are given of other full-bore measuring devices V-element, magnetic, vortex shedding, turbine and positive-displacement meters, ultrasonic meters, and mass flow devices such as Coriolis and thermal flowmeters. 

Variable-area in-line flowmeters (Fig. 18.15), or rotameters, are sometimes referred to as sight gauges because they provide a visible indication of flow rate. These devices, when fitted with proximity sensors (such as capacitive pickups), which sense the presence of the float, can be used in on-off control applications. 

FIGURE 18.15 Variable-area in-line flowmeter (rotameter). 

Flow transmitters. Flow measurements are made in high-pressure lines by sensing the pressure drop across a calibrated orifice or venturi, or by the transmitting variable-area type of flowmeter. The latter meter resembles a Rotameter with float position transmitted electrically. It has the advantage of being an in-line element but is not readily applicable to large flows. 

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