Machinery's Handbook, 31st Edition
2760 Flow Measurement Thermal Mass Flow Meters: These devices are used almost exclusively for gas flow ap - plications. Resistance temperature detectors (RTD) exhibit a linear change in resistance with respect to temperature. To measure mass flow rate, two RTD probes are used. One probe measures the fluid temperature, while the second probe is maintained at a higher temperature. As fluid flows by the heated probe, it dissipates the heat. The energy required to maintain the heated probe temperature is proportional to mass flow rate of the fluid. Some fluid energy is lost due to the obstruction of the probes in the fluid stream. Flow Control.— Flow control is accomplished with manual or automatic valves, for both liquids and gases. Monitoring and control are often combined into a single device, in which flow is measured and the results are used to control a proportional valve to regulate flow. There are many types of flow controllers available, including manual needle valves and fully automated flow controllers. When density and viscosity is known and constant, and temperature is known, mass flow control can be performed. Mass flow controllers are usually used with gases, but some are available for liquids. Commercial flow control de - vices range in accuracy and flow capacity, and often contain proprietary control software and flow measurement technology. Coefficient of Velocity, C v .— C v is defined as the flow rate in US gallons per minute (gpm) of water at 60°F with a pressure drop across the device of 1 psi. This is directly applicable to hydraulic components, but C v is also applied to pneumatic devices as an indication of their flow capacity. This value is normally used when matching valves to actuators, or sizing other fluid power devices. Flow capacity of valves in a circuit must be equal to or greater than flow required by the devices they supply. For actuators, higher C v values can also indicate faster movement capability. C v is typically used in the United States and calculated using English units. The SI sys- tem has its own flow coefficient, K v , which is calculated using metric units. These two values are not equivalent. K v is defined as the flow rate in cubic meters per hour (m 3 /h) of water at a temperature of 16°C, with a pressure drop across the valve of 1 bar. C v is commonly used by manufacturers for both English and metric products, and is normally provided in catalogs. The equations used to calculate C v and K v are listed below. The equations for liquids are simpler than those for gases, because gases are compressible. Conversion: C v = 1.157 K v K v = 0.864 C v C v and K v Equations for Liquids for Non-choked Turbulent Flow when D p < F L 2 ( p i - F F P v ) C v Q = K v Q =
SG ∆ p ----
SG ∆ p ----
for Choked Turbulent Flow when D p ≥ F L 2 ( p
i - F F P v )
Q F L ---
Q F L ---
SG p i F F P v – ------------
SG p i F F P v – ------------
=
=
C v
K v
for Laminar or Transitional Flow
Q F R --- SG ∆ p ----
Q F R --- SG ∆ p ----
=
=
C v
K v
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