Basics of Flow measurements using Hotfilm - anemometer

Transcription

1 Basics of Flow easureents using Hotfil - aneoeter 1/9

2 Inhaltsverzeichnis: 1. Definitions 1.1. Air Velocity 1.2. Aount (of Gas) 1.. Flow Mass flow rate Voluetric flow rate 1... Standard voluetric flow rate 2. Flow forulas. Flow easureents using E+E Hotfil aneoeter 2/9

3 1. Definitions: 1.1. Air Velocity: Definition: Air Velocity describes the distance an air olecule is oving during a certain tie period Units: Nae of unit shift Converted to SI - Unit /s Meter per second /s 1 /s Kiloeter per hour k/h 0, /s Centieter per inute c/in 1, /s Centieter per second c/s /s Meter per inute /in 1, /s Millieter per inute /in 1, /s Millieter per second /s /s Foot per hour ft/h, fph 8, /s Foot per inute ft/in, fp 5, /s Foot per second ft/s, fps 0,048 /s Furlong per fortnight furlong/fortnight 1, /s Inch per second in/s, ips 2, /s Knot kn, knot 0, /s Kyne c/s /s Mile per year py 8, /s Mile (stat) per hour ph, i/h 0,44704 /s Mile (stat) per hour i/in 26,8224 /s /9

4 1.2. Aount (of Gas): 2 The aount of Gas n is the nuber of oles of gas. 1ol olecules. For better understanding and its copact atheatical forulation, the properties of gases are described using an idealised odel called ideal gas. The aount of an ideal gas is defined using the ideal gas law: p V n R T J using the universal gas constant R ol K Volues V, pressure p und teperature T are state variables. They are defining the state of the gas aount n. For known coposition of the gas, its aount n can be substituted by its ass p V R i T using R i als individual gas constant. It is dependent on the coposition of the gas: Gas Air O 2 N 2 H 2 CO 2 indiv. Gas constant R i ,8 296, ,9 Finally the aount of gas can be detected in two different ways: 1. Weighing of the aount easuring the ass of the aount. With known weight of one gas olecule, the aount of gas can be calculated. Mass, easured in kg or g 2. Deterination of the state of the gas: easuring of the volue V. With known teperature T and pressure p, the aount can be caluculated using the ideal gas law. Volue, easured in, d or c The ratio between those two quantities is defining the density ρ. ass kg kg g density ρ,, volue V d c The density is a paraeter of the state of the ediu and is defined by the individual gas constant R i, teperature and pressure of the ediu. ρ p R T i 4/9

5 The density ρ of the gas is linearly dependent on its pressure p und its teperature T. Deonstrative, that can be shown using a cylinder whose volue can be variated by a oving piston p 1 T 1 ρ 1 p 2 >p 1 T 2 T 1 ρ 2 > ρ 1 p p 1 T >T 1 ρ < ρ 1 The syste is absolutely leak proof, aount and ass of the filled ediu is constant. To increase the pressure inside the cylinder the piston is pressed downwards (p 2 >p 1 ), the gas is copressed, the density of the gas is increasing, the volue is decreasing. On the other hand, by heating the gas inside the cylinder (T >T 1 ), the density of the gas is decreasing and the volue is increasing! Suary: The ass of a gas with known coposition is a direct easure for its aount. The volue is just defining the aount of a gas in cobination with its teperature and pressure, but is independent fro its coposition! To get a teperature- and pressure independent voluetric-quantity, the easured volue is recalculated to the volue which would be easured at standardized conditions. This standardized volue V Standard is a voluetric quantity independent fro coposition, pressure and teperature of the gas. Standard conditions: standard teperature 0 C 27 K standard pressure 1,01 bar Standard volue: V 1 V 2 <V 1 V >V 1 absolut pressure[ bar] 27 K V s tan dard 01 ( 27 + teperature[ C] ) 1, bar 5/9

6 1.. Flow A v Flow defines the nuber of Gas-olecules streaing through the surface A during a certain tie- period. The quantities for flow are based on the different quantities for an aount of gas: ass, volue and standard volue Mass flow rate The Mass flow rate is defining the aount of a fluid with known coposition flowing through a surface A in a certain tie-periode. Mass & tie [ kg, g] t[ h, s, in] Nae of unit shift Converted to SI - Unit kg/s Kilogra per second kg/s 1 kg/s Kilogra per inute kg/in 0, /60 kg/s Kilogra per hour kg/h 2,7778*10-4 1/600 kg/s Gra per second GRPS (g/s) 10- kg/s Gra per inute GRPM 1,666667*10-5 kg/s (g/in) Gra per hour GRPH (g/h) 2,7778*10-7 kg/s Pound per second LBPS 0, kg/s Pound per inute LBPM 7,55987*10- kg/s Pound per hour LBPH 1,26*10-4 kg/s Voluetric flow rate The voluetric flow rate is defining the volue of a fluid for the present state, represented by teperature and pressure, streaing through the surface A in a certain tie-periode. 6/9

7 volue V & tie [, l] t[ h, s, in] Nae of unit shift Converted to SI - Unit /s Cubiceter per second /s 1 /s Cubiceter per inute /in 0, /60 /s Cubiceter per hour /h 2,7778*10-4 1/600 /s Cubicdezieter per second d /s 10- /s Cubiccentieter per second CCS (c /s) 10-6 /s Cubiccentieter per inute CCM (c /in) 1,666667*10-8/s Litre per second LPS (l/s) 10- /s Litre per inute LPM (l/in) 1,666667*10-5 /s Litre per hour LPH (l/h) 2,77778*10-7 /s Millilitre per second l/s 10-6 /s Millilitre per inute l/in 1, *10-8 /s Cubicfeet per second CFH (ft /s) 2,8168*10-2 /s Cubicfeet per inute CFM (ft /in) 4,71947*10-4 /s Gallons per inute (GB) GPM 7,57682*10-5 /s Gallons per inute (US) GPM 7,4147*10-5 /s 1... Standard voluetric flow rate Standard voluetric flow rate is independent fro state and coposition. standard volue tie s tan dard V s tan dard [ N, Nc t[ h, s, in] ] Nae of unit shift Converted to SI - Unit N /s Nor-Kubiketer pro Sekunde N /s 1 N /s (Geran) Standard Cubiceter per second SCMS (S /s) 1 N /s Standard Cubiccentieter per second SCCS (Sc /s) 10-6 N /s Standard cubiccentieter per inute SCCM (Sc /s) 1,666667*10-8 N /s Standard Litre per second SLPS (Sl/s) 10 - N /s Standard Litre per inute SLPM (Sl/in) 1,666667*10-5 N /s Standard Litre per hour SLPH (Sl/h) 2,77778*10-7 N /s Standard Cubicfeet pro second SCFH (ft /s) 2,8168*10-2 N /s Standard Cubicfeet pro Minute SCFM (ft /in) 4,71947*10-4 N /s 7/9

8 2. Flow forulas Volue flow is the direct cobination of the surface A, the fluid is streaing through with its average velocity. Voluetric flow rate average velocity flow surface [ / s] v[ / s] A[ 2 ] To calculate the standard volue flow it is necessary to easure actual pressure and teperature. Standard voluetric flow rate voluetric flow rate at standard conditions p[ bar ] 27 K [ N / s] V & [ / s] standard 01 ( 27 + T [ C ]) 1, bar With known density of the fluid (Air) at standard conditions it is possible to calculate the ass flow. Mass flow rate standard voluetric flow rate Air density at 0 C and 1,01 bar & [ kg / s] V & s [ N / s] 1,275 [ kg tan dard / N ] 8/9

9 . Flow easureents using E+E Hotfil aneoeter: The easureent principle of a hot-fil aneoeter is to easure the nuber of olecules which are streaing over the heater sensor, and so conduct heat. With rising air pressure, the density of the gas is increasing and with the sae velocity ore olecules are streaing over the sensor. So, to easure the air velocity of the olecules at any pressure, this pressure influence has to be corrected. At E+E all Air Velocity Transitter are calibrated at Standard Pressure of 1,01 bar. For diverging air pressures fro 1,01 bar, that influence has to be corrected. Air velocity: v[ / s] v Transitter 1,01bar [ / s] p[ bar] The teperature also has a sall influence on the heat transfer fro the sensor. E+E Transitters have ipleented internal teperature copensations to correct that influence. By easuring the average air velocity in a Tube and with known cross section surface A, the volue flow can be calculated. Voluetric flow rate: [ / s] A[ 2 ] v Transitter 1,01bar [ / s] p[ bar] In ost industrial applications the voluetric flow rate at standard conditions has to be easured. In that applications the E+E Hotfilsensor and its calibration at standard pressure shows its ain advantages. Because of the opposed pressure influence of sensor and conversion of voluetricflow to standard-voluetric flow, the E+E Hotfil-Sensors easure standard voluetric flow and ass flow independent to pressure. Standard voluetric flow rate: standard [ N / s] A[ 2 ] v Transitte r [ / s] 27 K ( 27 + T [ C ]) Mass flow rate: & [ kg / s] A[ 2 ] v Transitte r [ / s] 27 K ( 27 + T [ C ]) 1,275 [ kg / N ] 9/9