28 April 2003 Retrospective: Semicore Visit

Transkript

1 28 April 2003 Retrospective: Semicore Visit What is highest growth Industry? Rebuild versus remanufacture Importance of Documentation, blueprinting, spares What are barriers to high uptime? Review Homeworks Helium Permeation Example A review of Foreline design Vessel Q Assuming no leaks or traps, mass flow is constant! Roughing line Foreline Q Q Q Q Pump 1

2 Risk Assessment - Employing Factors of Safety in calculations What are the risks of exceeding the critical foreline pressure on a DP? What are the risks of exceeding the maximum sustainable foreline pressure on a Turbopump? Procedure & Lexicon Review 1. Determine the critical foreline pressure required for the DP or turbo pump. Introduction of a safety factor may be prudent. 2. Calculate the maximum credible gas load (throughput) for the system and vessel. 3. Calculate the required speed at the foreline connection of the DP or turbo. 4. Calculate the required conductance for a specified mechanical pump. 5. Calculate the speed required at the foreline pump inlet. 2

3 On the hi-vac side we have Delivered pump speed (Equation 27) Relation between Mass flow & Conductance in Forelines Mass Flow Rate = Total Gas Load = System Throughput 3

4 Estimate total gas load for the DP or turbo total Q system total = Q perm total + Q outgas + Q leaks + Q process A good Ballpark value for total gas load for a small vacuum vessel is 3 x 10-3 Torrl/s DP or turbo Inlet speed curve " ID Inlet Diffusion pump 2000 DP inlet speed [l/s] DP inlet pressure [Torr] 4

5 Plot mass flow for the pump as a function of pressure Mass flow [T-l/s] DP inlet pressure [Torr] For the specified mass flow, find the inlet pressure Mass flow [T-l/s] DP inlet pressure [Torr] 5

6 DP inlet pressure with gas load " ID Inlet Diffusion pump 2000 DP inlet speed [l/s] DP inlet pressure [Torr] Vessel Critical foreline pressure for many small DP s is 300 mt Roughing line P crit = 300 mt Pump 6

7 Speed required at the DP or turbo foreline connection Q = S System Fore P Crit total Q system Calculate speed required at the foreline connection = S fore P crit Tl / s = S fore 0.3T S fore = Tl / s 0.3T = l / s 7

8 Speed required at foreline Vessel Roughing line S fore = 0.01l/s P crit = 300 mt Pump Design of foreline pumping system Vessel CRoughing fore =? line l/s S fore = 0.01l/s S p=? l/s P crit = 300 mt Pump 8

9 Design of a foreline pumping system Select the mechanical pump to rough the vessel (know S p at P crit ) Knowing S p and S fore calculate C fore at P crit (Equation 24). 1 S fore = 1 S p + 1 C fore Pump inlet speed at P crit 10.0 Pump Inlet Speed [l/s] Pressure [Torr] 9

10 guidelines for foreline design: In general, the inlet pump speed (specified primarily to expediently rough a vessel) will be more than adequate to back the DP or turbo. At 300mT the speed of most mechanical pumps is around 70% of the free-air displacement. Knowing C fore, the length of the foreline, calculate the foreline minimum diameter D. C = 3000P D4 + 80D 3 L 10

11 Assumptions: Permeation Example Helium Party Balloon 2 Diameter in Uninflated condition, with Wall 14 Diameter in inflated condition Creates a Dia. 5 Impression with a 5 lb. book Assumptions 11

12 Permeation Rates Natural 337 Rubber Helium : K p =2.2x10-11 m 2 /sec Nitrogen: K p =5.3x10-12 m 2 /sec Oxygen: K p =2.0x10-11 m 2 /sec Permeation Equation Q = 10 k p A p d Kp permeation rate, Torr-Liters/s Q leak rate m^2/sec A area, cm^2 Delta p, pressure differnce, Torr 12

13 Differential Pressure P= F/A = PSI = 13.1 Torr, where F= 5 lbs A=.785 *25 = 19.6 sq-in Aside: Know the following: 760, 14.7, π/4 How else could you measure this pressure? Permeation Thickness Assume Conservation of Rubber Volume V inflated =V deflated 4πr 2 t (t<<r) t inflated = t deflated (r deflated /r inflated ) 2 t= 0.002*2.54/49=1.04E-04 cm 13

14 Permeation Area A sphere =4 πr 2 =3971 sq-cm (Eq 28) Permeation Rate Q=0.011 Torr-liters/sec 14

15 Other Calculations Volume V sphere = 4/3 πr 3 =23.5 liters Internal Absolute Pressure = 773 Torr Molar Volume (approx) 23.5 Torr 777 Liters = 18,100 Torr-Liters Time To Deflate 10% Need to Assume constant Pressure Condition For Helium 1816 Torr-Liters/(.011 Torr-Liters/Sec) 46 h For Nitrogen Ratio of rates (Nitrogen and Helium) 4.15x, so about 8.0 days What about oxygen? Is natural rubber the best material for party balloons? 15