Choked Flow of Gases - okcc.com

1 BOX Q TRUMBULL, CT 06611 CT PHONE (203) 261-6711 TOLL FREE PHONE (800) 533-3285 FAX (203) 261-8331 O'KEEFE CONTROLS CO. 2003 ALL RIGHTS RESERVED e-mail website Flow of GasesWhen the air velocity reaches sonic velocity(P2/P1 .528) further increases in P1 (upstreampressure) do not cause any further increase inthe air velocity through the orifice. Conse-quently it is wrongly concluded that the massflow rate also does not the air pressure (P1) increases, the densityof the air also increases; and since the massflow rate is also a function of density, the massflow rate increases linearly with pressure (P1).Even though the air velocity through the orifice is limitedto the speed of sound, the mass flow rate continues toincrease as the absolute pressure (P1) is Choked ?

2 The parameter that becomes Choked or limited is the velocity of the air. It is moreaccurate to use the term Choked velocity rather than Choked flow when the absolutepressure ratio of air through an orifice is . ConditionsAir at atmospheric pressure enters the orifice and flowsto a downstream vacuum the case of vacuum conditions on the outletof an orifice and where the inlet is at ambientatmospheric pressure, both the air velocityand the mass flow rate become Choked (limited) when sonic velocity is achievedthrough the atmospheric inlet pressure and downstream vacuum,both the air velocity and mass flow rate are Basic ConceptA greatly misunderstood and misappliednotion is that of Choked flow , also referredto as critical flow .In gas flow through an orifice there is an occa-sion where the gas velocity reaches sonicconditions.

3 This occurs for air flow when theabsolute pressure ratio is .528, when thedownstream absolute pressure (P2) is the upstream absolute pressure (P1).Sonic velocity occurs for air flow whenP2 /P1 . air flow velocity is limited once the absolutepressure ratio is . air flow through an orifice with aninlet air temperature of 68oF the Choked (sonic) velocity is 1129 Misconception!Once sonic velocity is achieved in orifice airflow (P2/P1 = .528), it is easy to "assume" thatthe mass flow rate is constant for all pressureratios less than .528; P2/P1 .528. Forexample, when P2 is psia and P1 is , sonic velocity occurs through the P1 further increases there is no furtherincrease in the velocity of the air flowingthrough the for the onset of sonic velocity in orifice air all the Factors!

4 The mass flow rate through an orifice is a func-tion of three basic (flow) is a function of Velocity Density Orifice AreaThe reason for the mass flow rate limitation isthe fixed inlet density combined with the fixedvelocity. The flow charts on pages 20-22 showthe Choked mass flow effect for vacuum con-ditions. At vacuum levels between 15-30" Hgthe mass flow rate is Flow for PositivePressure ConditionsAs in the case of the above vacuum conditionsthere are certain situations in which chokedflow does occur for positive (above atmo-spheric) pressure. By maintaining a fixed in-let pressure to the orifice and allowing theoutlet pressure (back pressure) to vary, thereis a range of outlet pressures over which themass flow rate is example (see chart below). With an inlet pressure of 80 psig, the massflow rate is Choked (limited) for all outletpressures less than psig (includingvacuum conditions).

5 The actual flow rate is constant for theoutlet pressure range of psig to aslow as a complete vacuum. The flow ratecan be obtained from the charts on pages20-22; for an orifice of .010" diam-eter and with 80 psig inlet pressure, thechoked flow rate is scfh (page 20).This flow rate will be constant for alloutlet pressure conditions psig and full VelocityMass FlowRateAir Velocity 0 0 MassFlow Rate AirVelocity Sonic Velocity Conditions Air Flow Outlet PressureInlet PressureFor Sonic .. 68 FP2 = psia = 0 psigP2/P1 Absolute Pressure Ratio0 Velocity1129 ft/sec @68 FChoked FlowChoked VelocityMassFlow Rate 0P2/P1 Absolute Pressure RatioAirVelocity P1 = psiaP2 < P1 VacuumP1 = psia = 0 psigStandardAtmosphericConditionsP1 = psia = psigP2 = psia = 0 psigSonic VelocityThrough OrificeTUTORIAL0 Velocity1129 ft/sec @68 FAir velocity isconstant inthis regionAirVelocity 0P2/P1 Absolute Pressure RatioP1P24820 BOX Q TRUMBULL, CONNECTICUT 06611 CT PHONE (203) 261-6711 TOLL FREE PHONE (800) 533-3285 FAX (203) 261-8331 O'KEEFE CONTROLS CO.

6 2003 ALL RIGHTS RESERVED e-mail website Orifice Air Flow Pressure psigChoked FlowVacuum LevelIn. Pressure psigChoked 5255 216 237250 286 286 160172185 203216242256275305 248 278 292316 347381405464 239265 280314331356392 483532566 648 214233269301320347375400445 473 244 267307343362394428458 509 538 604 638693 763 839 8921021112580931031101191271311371451672 10231 273298343384405443481513570 604 6787167788569431000114612639010611512213 2141146151161185231256303331379424447489 5325686316707507928609471042110612671398 100114126135146156164167177203254282 331 362 415 468496 540587 627697 739 831875 LevelIn.

7 Conditions 70 F, psiaSCFH Standard Cu. Ft. Per HourAbove data obtained with Type B restrictor. Flow rates for other metal restrictors areSLPM Standard Liters Per Minuteessentially the same as for Type B. Above data supercedes previous Orifice Air Flow Pressure psigChoked FlowVacuum LevelIn. Pressure psigChoked 5255 215227247272 223250264 209227 242269285320338367404445472541 LevelIn. Conditions 70 F, psiaSCFH Standard Cu. Ft. Per HourAbove data obtained with Type B restrictor. Flow rates for other metal restrictors areSLPM Standard Liters Per Minuteessentially the same as for Type B. Above data supercedes previous publications.

8 BOX Q TRUMBULL, CONNECTICUT 06611 CT PHONE (203) 261-6711 TOLL FREE PHONE (800) 533-3285 FAX (203) 261-8331 O'KEEFE CONTROLS CO. 2003 ALL RIGHTS RESERVED e-mail website BOX Q TRUMBULL, CONNECTICUT 06611 CT PHONE (203) 261-6711 TOLL FREE PHONE (800) 533-3285 FAX (203) 261-8331 O'KEEFE CONTROLS CO. 2003 ALL RIGHTS RESERVED e-mail website Orifice Air Flow Pressure psigChoked FlowSupply Pressure psigChoked LevelIn.

9 Conditions 70 F, psiaSCFH Standard Cu. Ft. Per HourAbove data obtained with Type S restrictors. Flow rates for other sapphire types areSLPM Standard Liters Per Minuteessentially the same as for Type S. Above data supercedes previous Orifice Air Flow SCFHV acuum LevelIn. Pressure psigMetal Orifice Water Flow

10 Pressure psig BOX Q TRUMBULL, CONNECTICUT 06611 CT PHONE (203) 261-6711 TOLL FREE PHONE (800) 533-3285 FAX (203)