File Name: fundamentals of temperature pressure and flow measurements .zip
Flow is classified into open channel flow and closed conduit flow. Open channel flow occur when the flowing stream has a free or unconstrained surface open to the atmosphere. Flows in canals or vented pipelines - like drain and sewers - which are not flowing full, are typical examples.
In open channel flow the force causing the flow the force of gravity on the fluid. A progressive fall or decrease in the water surface elevation occurs as the flow moves downstream. Closed conduit flow occurs when the flow is caused by a pressure difference in the conduit. Flow in water supply pipes or district heating pipes are typical examples. The flow rate depends mainly on the pressure difference between the ends, the distance between the ends, the area of the conduit and the hydraulic properties of the conduit - like the shape, roughness and restrictions like bends.
In a differential pressure drop device the flow is calculated by measuring the pressure drop over an obstructions inserted in the flow. The differential pressure flow meter is based on the Bernoulli Equation where the pressure drop and the further measured signal is a function of the square flow speed. Note that it is common to use " head " instead of "pressure". With an orifice plate, the fluid flow is measured through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe.
The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict. The orifice plates are simple, cheap and can be delivered for almost any application and in any material.
The Turn Down Ratio for orifice plates are less than Their accuracy are poor at low flow rates. A high accuracy depend on an orifice plate in good shape, with a sharp edge to the upstream side.
Wear will reduce the accuracy. Due to simplicity and dependability, the Venturi tube flowmeter is often used in applications where it's necessary with higher Turn Down Ratios , or lower pressure drops, than the orifice plate can provide. In the Venturi Tube the fluid flowrate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference.
This provides a Turn Down Rate Note that the manometry for a venturi tube or orifice should be installed below the hydraulic grade line or pipe. Flow nozzles are often used as measuring elements for air and gas flow in industrial applications. The flow nozzle is relative simple and cheap, and available for many applications in many materials.
The Turn Down Ratio and accuracy can be compared with the orifice plate. When a gas accelerates through a nozzle, the velocity increase and the pressure and the gas density decrease. The maximum velocity is achieved at the throat, the minimum area, where it breaks Mach 1 or sonic.
At this point it's not possible to increase the flow by lowering the downstream pressure. The flow is choked. This situation is used in many control systems to maintain fixed, accurate, repeatable gas flow rates unaffected by the downstream pressure. After the pressure difference has been generated in the differential pressure flow meter, the fluid pass through the pressure recovery exit section, where the differential pressure generated at the constricted area is partly recovered.
As we can see, the pressure drop in orifice plates are significant higher than in the venturi tubes. The rotameter consists of a vertically oriented glass or plastic tube with a larger end at the top, and a metering float which is free to move within the tube.
Fluid flow causes the float to rise in the tube as the upward pressure differential and buoyancy of the fluid overcome the effect of gravity. The float rises until the annular area between the float and tube increases sufficiently to allow a state of dynamic equilibrium between the upward differential pressure and buoyancy factors, and downward gravity factors. The height of the float is an indication of the flow rate. The tube can be calibrated and graduated in appropriate flow units.
The rotameter meter typically have a TurnDown Ratio up to In a velocity flowmeter the flow is calculated by measuring the speed in one or more points in the flow, and integrating the flow speed over the flow area. The pitot tube are one the most used and cheapest ways to measure fluid flow, especially in air applications like ventilation and HVAC systems, even used in airplanes for speed measurent.
The pitot tube measures the fluid flow velocity by converting the kinetic energy of the flow into potential energy. The use of the pitot tube is restricted to point measuring. With the "annubar", or multi-orifice pitot probe, the dynamic pressure can be measured across the velocity profile, and the annubar obtains an averaging effect. The calorimetric principle for fluid flow measurement is based on two temperature sensors in close contact with the fluid but thermal insulated from each other.
One of the two sensors is constantly heated and the cooling effect of the flowing fluid is used to monitor the flowrate. In a stationary no flow fluid condition there is a constant temperature difference between the two temperature sensors. When the fluid flow increases, heat energy is drawn from the heated sensor and the temperature difference between the sensors are reduced. The reduction is proportional to the flow rate of the fluid. Response times will vary due the thermal conductivity of the fluid.
In general lower thermal conductivity require higher velocity for proper measurement. There is many different manufacturing design of turbine flow meters, but in general they are all based on the same simple principle:. If a fluid moves through a pipe and acts on the vanes of a turbine, the turbine will start to spin and rotate.
The rate of spin is measured to calculate the flow. The turndown ratios may be more than if the turbine meter is calibrated for a single fluid and used at constant conditions. An obstruction in a fluid flow creates vortices in a downstream flow. Every obstruction has a critical fluid flow speed at which vortex shedding occurs. Vortex shedding is the instance where alternating low pressure zones are generated in the downstream.
These alternating low pressure zones cause the obstruction to move towards the low pressure zone. With sensors gauging the vortices the strength of the flow can be measured. An electromagnetic flowmeter operate on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor and the magnetic field is created by energized coils outside the flow tube.
The voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage which is measured by a secondary element. Electromagnetic flow meters can measure difficult and corrosive liquids and slurries, and they can measure flow in both directions with equal accuracy.
Electromagnetic flow meters have a relatively high power consumption and can only be used for electrical conductive fluids as water. The effect of motion of a sound source and its effect on the frequency of the sound was observed and described by Christian Johann Doppler.
The frequency of the reflected signal is modified by the velocity and direction of the fluid flow. If a fluid is moving towards a transducer, the frequency of the returning signal will increase.
As fluid moves away from a transducer, the frequency of the returning signal decrease. The frequency difference is equal to the reflected frequency minus the originating frequency and can be use to calculate the fluid flow speed. The positive displacement flow meter measures process fluid flow by precision-fitted rotors as flow measuring elements. Known and fixed volumes are displaced between the rotors.
The rotation of the rotors are proportional to the volume of the fluid being displaced. The number of rotations of the rotor is counted by an integral electronic pulse transmitter and converted to volume and flow rate. The positive displacement flowmeter may be used for all relatively nonabrasive fluids such as heating oils, lubrication oils, polymer additives, animal and vegetable fat, printing ink, Dichlorodifluoromethane R, and many more.
The thermal mass flowmeter operates independent of density, pressure, and viscosity. Thermal meters use a heated sensing element isolated from the fluid flow path where the flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate and the temperature difference is calculated to mass flow.
The accuracy of the thermal mass flow device depends on the calibrations reliability of the actual process and variations in the temperature, pressure, flow rate, heat capacity and viscosity of the fluid. Direct mass measurement sets Coriolis flowmeters apart from other technologies. Mass measurement is not sensitive to changes in pressure, temperature, viscosity and density.
With the ability to measure liquids, slurries and gases, Coriolis flowmeters are universal meters. Coriolis Mass Flowmeter uses the Coriolis effect to measure the amount of mass moving through the element. The fluid to be measured runs through a U-shaped tube that is caused to vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tubes will deform and an additional vibration component will be added to the oscillation.
This additional component causes a phase shift on some places of the tubes which can be measured with sensors. The Coriolis meter can also be used to measure the fluids density. A common method of measuring flow through an open channel is to measure the height of the liquid as it passes over an obstruction as a flume or weir in the channel.
These factors are more or less related to each other. Example - cost of flow meters increases with accuracy and life time quality. Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro.
Flow is classified into open channel flow and closed conduit flow. Open channel flow occur when the flowing stream has a free or unconstrained surface open to the atmosphere. Flows in canals or vented pipelines - like drain and sewers - which are not flowing full, are typical examples. In open channel flow the force causing the flow the force of gravity on the fluid. A progressive fall or decrease in the water surface elevation occurs as the flow moves downstream.
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Fundamentals of Temperature, Pressure, and Flow Measurements. Robert P. Open navigation menu. Close suggestions Search Search. User Settings. Skip carousel. Carousel Previous.
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Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways. The common types of flowmeters with industrial applications are listed below:. Flow measurement methods other than positive-displacement flowmeters rely on forces produced by the flowing stream as it overcomes a known constriction, to indirectly calculate flow. Flow may be measured by measuring the velocity of fluid over a known area. For very large flows, tracer methods may be used to deduce the flow rate from the change in concentration of a dye or radioisotope.
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