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  1. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    Interestingly, after measuring the actual flow from the output of the compressor (using McM #41945K56, I get much lower flow than the rate provided by the factory: Plugging this measured flow rate into the predicted ADIABATIC equation (green line), I get similar results to using the manuf...
  2. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    BTW: To share a little about the project, the air bladder is our partner's proprietary design but it requires a large volume of air to go from full deflation (such as CPR mode) to full inflation (about 8CF @ 1psig). Hence the large high pressure air tanks. My guess is that bedroom air...
  3. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    ...under isothermal conditions? Assuming isothermal, for a FAD of 0.65SCFM, this would result in an actual flow at 50psia of 0.2ACFM: V = Patm*Vatm/P = 14.7psia*0.67SCF/50psia = 0.20CF (assumes STP conditions) This actually matches up well with my observations: Equipment: Werther PTC15...
  4. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    Aborting this thread. Disappointed that I asked a simple question and was barraged with unrelated advice about inflatable beds and accusations of posting homework. Will later document my measurement of tank fill time and predictions based on the manufacturers FAD curve in case it helps another.
  5. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    Sure, I'll take the abuse, as I am a novice when it comes to pneumatics and I highly respect the engineers that lend their expertise here. For this problem, it does not matter what the air is used for as the first stage is filling an air tank. But yes, it will be later used to inflate the bed...
  6. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    What gave you the impression this is homework? Because I tried to lay out the problem clearly? This project is for a phase I prototype of an inflatable hospital bed. Assuming adiabatic expansion, I am off by a factor of 2 in estimating tank fill time. I cannot simply go up in size because...
  7. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    A colleague pointed out that there are two compression stages taking place, that inside the compressor and that inside the air tank. However, it would seem to me that both would fall under adiabatic compression. Unless the air cooled in the tank and heat was absorbed by the tank wall which...
  8. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    A colleague pointed out that there are two compression stages taking place, that inside the compressor and that inside the air tank. However, it would seem to me that both would fall under adiabatic compression. Unless the air cooled in the tank and heat was absorbed by the tank wall which...
  9. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    ...the closest to experimental results (assuming manufacturer-published free air displacement curve). Free Air Discharge (pg-12): Q = (P2 - P1)/Patm * V/t, Basically derived from P1*V1 = P2*V2 thread798-161670 06 Aug 06 The process taking place inside a reciprocating cylinder is so close to...
  10. erk1313

    Sizing Air Tank: Estimating Polytropic Efficiency

    ...manufacturer'shttp://www.eng-tips.com/threadminder.cfm?pid=391 chart), take small iterative steps until the final pressure is reached. P = Patm*Vatm^n/V^n Vatm = Vinitial + Vcompressor; The resulting air volume in the tank, but expressed as (SCF) standard atmosph. pressure and temperature...
  11. erk1313

    Sizing valve for pressurized vessel to relieve under SUBcritical flow

    Duh! Thank you. So finishing up, for the initial condition: Pchoke = 8.5psia Xt = 0.472 Y = 0.942 Flow = 8.56 SCFM (converted from SCFH) Iterating every 0.2 sec... I get the following graph of inlet pressure for a valve of Cv = 2.0 Is this a close approximation I can trust?
  12. erk1313

    Sizing valve for pressurized vessel to relieve under SUBcritical flow

    Would that equation for Xt still apply to non-choked flow? So using the equation to find the critical pressure: Pchoke = 27.83 psia (air exhausting to atmosphere) Then Pressure Drop Ratio: Xt = (16psia - 27psia) / 16psia = -0.688
  13. erk1313

    Sizing valve for pressurized vessel to relieve under SUBcritical flow

    ...7320; conversion constant assumes units SCFH, PSIA, and R temperature Fp = 1; pipe geometry factor without reducers p1: inlet pressure Y = 1 - x/(3*Fk*Xt); expansion factor .....Fk = 1; Ratio of Specific Heats (assume air) .....Xt: Pressure drop ratio factor (huh?) M = 28.97; dry air T1...
  14. erk1313

    Does orifice diameter affect critical pressure when testing for choked flow?

    That is my understanding as well. Increasing inlet pressure will increase mass flow, but not velocity (M). Thanks for the helpful posts... perhaps some time I will be faced with the challenge of a 3ft diameter jet engine and will put this to the test!
  15. erk1313

    Does orifice diameter affect critical pressure when testing for choked flow?

    Thank you for the quick and thoughtful confirmation. Still seems odd that you could achieve sonic velocity through a 4" hole with inlet pressure of 25psia to 14.7 psia.
  16. erk1313

    Does orifice diameter affect critical pressure when testing for choked flow?

    Does orifice diameter affect critical pressure through a converging nozzle, or is it entirely dependent on the ratio of inlet and outlet pressure? (assume Isentropic flow of air). References from other forum posts listed at the bottom. I've seen many references for testing for choked flow...
  17. erk1313

    Tolerances for Bushings on Parallel Shafts

    Also, the shoulder bolt has a 8-32 thread length of only .188", hence the shallow depth.
  18. erk1313

    Tolerances for Bushings on Parallel Shafts

    To designate only the counterbore having positional tolerance, would I simply move the GD&T frame to the right of the counterbore instruction? (I will likely substitute looser tolerance bushings, but this will be helpful in the future.
  19. erk1313

    Tolerances for Bushings on Parallel Shafts

    Good point. Thank you for the helpful advice, Mike.
  20. erk1313

    Tolerances for Bushings on Parallel Shafts

    How about my GD&T, am I going in the right direction here? http://files.engineering.com/getfile.aspx?folder=0019b32f-b3b0-46c2-a103-cba0ce070d9c&file=DRW-016-A-4120-A.PDF

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