Given Find Water Properties

Transcription

1 Venturi Example Given: A venturi is to be used to measure a 50 gpm flow of 70 F water in a 4-in ID pipe. Find: Select a throat diameter that provides Re d > 00,000 in the throat, and determine what differential pressure must be measured. Water Properties: µ = lbm/ft-s =.37 lbm/ft-hr ρ = 6.3 lbm/ft 3

2 Venturi Example (Cont d) Sol n: Re d = ρvd/µ and, or: m& = m& = ρ Q 3 lbm gal min ft ft min hr gal m & =15,000 lbm Next, want Re in terms of mass flow rate: hr V d 4 m& ρ π m& = V = 4 ρ π d

3 Venturi Example (Cont d) Putting Re d in terms of mass flow rate... Re d = ρ ρ 4 m& π µ d Now can find d for various Re d (at highest mass flow rate): d = π d = π 4 m& Re d 4 m& d µ µ

4 Venturi Example (Cont d) For Re d = 00,000: lbm 4 15,000 d = hr = 0.336ft = lbm 00,000π.37 ft hr Since D = 4.00 in, we need a smaller d and should have no problem with Re d > 00,000. Choose d = inches for convenience. 4.03in

5 Venturi Example (Cont d) lbm 4 m& 4 15,000 Re hr d = = = π d µ lbm π ft.37 1 ft hr 403,000 Next we rearrange the YMCA Equation: Q = Y M C A ( P P ) 1 ρ

6 Venturi Example (Cont d) Solving for P = P 1 -P : P = Y ρ M Q C A For incompressible fluid (water), Y = 1. π ft 1 A = = ft 4

7 Venturi Example (Cont d) 4 π ft 1 A 1 = = ft 4 M = 1 1 A A 1 = 1 1 ( ) = 1.033

8 Venturi Example (Cont d) Correction factor C can be obtained from the obstruction meter handout or from text Table 10.1 (p. 80). Use Table 10.1 for a machined entrance, note we have β = d/d = in/4 in = 0.5, and Re D = 01,000 (calculation not shown). β, D and Re D > 10 5 fall within the given ranges, so the value C = applies.

9 Venturi Example (Cont d) Inserting these values into the equation for P: P = lbm ft gal min lbf P = 597 ft = gal ft ( ) ft psi 1min 60s ft lbm lbf s

10 Rotameter Simple device for liquid and gas flow measurement This is a variable area meterthe rotameter uses a tapered tube with ball or float. As flow increases, float moves upward until fluid lift balances weight of float. Rotameter scale indicates fluid for which it is calibrated. Other fluids need correction.

11 Typical Rotameters

12 Rotameter Correction Rotameters are calibrated for only one fluid at given T and P (P not important for liquids). When the fluid is different or the conditions are different, the rotameter reading must be corrected to obtain the correct flow rate. Significant changes in use of rotameter yield loss in accuracy.

13 Rotameter Correction- Liquids For liquids, the correction equation is: Q Q act gage = ρ ρ b b ρ ρ act gage gage where Q gage is gage flow rate reading, Q act is corrected (actual) flow rate, ρ is density, subscript gage indicates fluid and conditions for which gage was calibrated, subscript act indicates actual fluid and conditions measured, and ρ b is the float density. 1 ρ ρ act 1

14 Rotameter Correction- Gases The correction equation for gases is: Q Q act gage = P gage P act T T gage gage where Q gage is gage flow rate reading, Q act is corrected flow rate, P is absolute pressure, T is absolute temperature, M is molecular weight, subscript gage refers to conditions for which rotameter was calibrated, and subscript act refers to actual measured conditions and fluid. act M M act 1

15 Rotameter Correction Example Given: A rotameter stamped scfh 70 F, 10 psig reads 36.3 scfh for a flow of O at 7. psig and 9 F. Find: The actual oxygen mass flow rate assuming that standard T and P are 60 F and 14.7 psia. Sol n: Use the rotameter correction equation to obtain the volumetric flow rate of O.

16 Rotameter Example (Cont d) Starting with correction equation: Q P act gage T M act gage = Q gage Pact Tgage M act insert values (using absolute T and P!): QO = 36.3 scf Actual O volumetric flow rate: Q = 7.1 scfh 1 1

17 Rotameter Example (Cont d) To get mass flow rate, use: We need to use the ideal gas equation to obtain the density of O at standard conditions of P = 14.7 psia, T = 50 R: ρ = O RT m& = ρ Q and we need to calculate the gas constant for oxygen: P

18 Rotameter Example (Cont d) ft lbf 1545 R R = = lbmol R M lbm 3 lbmol Oxygen density is: ρ O lbf in = in ft = ft lbf R lbm R = ft lbf 48.3 lbm R lbm ft 3

19 Rotameter Example (Cont d) Finally, the mass flow rate of O is: 3 lbm ft m& O = ρ Q = = 3 ft hr.8 lbm hr

20 Spring-Loaded Rotameter The conventional rotameter uses the gravitational force, consequently it must be oriented vertically. Rotameters should be installed using a level to assure correct vertical orientation. Rotameters are available that oppose the flow drag force on the bob with a spring force. These can be oriented in any direction, but typically are less accurate.

21 Spring-Loaded Rotameter

22 Wet Test Meter A positive displacement gas flow meter. Meter is water filled. It counts the number of times a fixed water volume is displaced by gas. To get flow rate, meter is timed for some number of displaced volumes. Gas is exposed to liquid, so corrections must be made for water evaporation into the gas. Normally assume gas is saturated with water vapor before exiting the meter.

23 Wet Test Meter Correction Gas volume is measured at some T and P (T, P also must be measured), but rate is desired at standard T, P (standards vary...) Use ideal gas law: The dry gas flow rate is desired: m& dry gas P = RT std std Q std m& = PV & = RT P dry,m RT m = Q P RT m Q

24 Wet Test Meter Correction Thus, the standard dry gas volumetric flow rate is: Q std = P dry,m P Total measured pressure, P m is the sum of the dry gas partial pressure, P dry,m, and H O partial pressure, which is P sat at T m, so: std T T std m Q m P dry, m = P m P T m

25 Wet Test Meter Correction Putting this all together, the standard dry gas volumetric flow rate is: ( ) P P T Q = m Tm std std P T std m Q m

26 Hot Wire Anemometer A hot wire anemometer is a probe used to measure local velocity (like a pitot tube). It s a small, resistance wire, often platinum or tungsten, held at constant T by electric current. Heat loss from wire is proportional to both ha T and I R, and h is proportional to velocity, so measurement of I can be used to obtain velocity.

27 Hot Wire Construction

28 Hot Wire Anemometer

29 Turbine Meters A turbine meter is a small turbine inserted in a pipe so that all flow passes over it. The blade rotational frequency is sensed as a pulse rate by a magnetic pickup. Meter is calibrated so Q = f /K, where f is frequency and K is the flow coefficient. Typical K units: pulses/gal, pulses/liter, etc. Turbine meters have high accuracy, good transient response, and wide linear range.

30 Turbine Meter Assembly

31 Turbine Meter

32 Paddlewheel Meter Poor man s turbine meter- cheaper but less accurate. Partial insertion in flow.

33 Water Meter Some rate flow meters can integrate volumetric flow over time to get total volume (a totalizing meter). a water meter gives a positive displacement measurement of total volume of liquid passed through (rather than flow rate). Similar meters are used for measuring water usage, gasoline pumped, etc. Following is a nutating vane water meter.

34

35 Mass Flow Meter/Controller A mass flow meter is a gas flow instrument that is relatively insensitive to gas P, ρ and T. Based on relation: q= m& c T p The value of c p is known and relatively constant over a fairly broad T range. The value of q is measured as q = I R. The value of T is measured also, so the only unknown is m&

36 Mass Flow Meter The mass flow meter can be used as feedback to control a modulating valve, thus providing a mass flow controller. Mass flow meter works only on a single gas. Mass flow meters provide about ±1% accuracy at rated conditions and cost $100 to $000. The controller option adds ~$600 to cost.

37 Constant fractional bypass flow Mass flow meter schematic