This my presentation for vacuum class 2014

1. Gas Flow
1
Gigin Ginanjar
01312003

2. Content
2
• 3.Gas Flow
– 3.1 Flow Regimes
– 3.2 Definition of Throughput and Pumping Speed
– 3.3 Conductance
• 3.3.1 Definition
• 3.3.2 Combination
• 3.3.3 Long Tubes
• 3.3.4 Orifice
– 3.4 Calculating Pumping Speed at Different Places

3. 3.1 Flow Regime (1)
3
• Flow Regime: Flow that through all different type of flow (because
vacuum system)
• Vacuum system start form turbulent flow ,when pressure fall flow
change to laminar flow. Both flow are viscous ; molecules striking each
other and pushing each other
• Transition from turbulent to viscous flow depend on Reynold number;
which function of flow velocity, mass density, tube diameter

4. 3.1 Flow Regime (2)
4
• Knudsen number
퐾푛 =
퐿
푑
L=mean free path ;mfp [L1]
d=characteristic dimension of the system [L1]
• In viscous flow mfp< character dimension ; gas
to gas collision ; Kn <0.01
• If pressure reduce then mpf = characteristic
dimension; gas to wall interaction
• In region 1>Kn>0.01 transition region;
• Kn>1 Flow considered molecular flow; In
molecular flow gas –wall collision predominate,
The wall interaction is diffuse reflection

5. Molecular interaction with surface
Diffuse reflectance . The length of the arrow for desorption
is proportional to the probability of desorption in
that direction
5
Specular reflectance: The angle of reflection equals the
angle of incidence
Example Flashlight to mirror
Cosine distribution
Example Flashlight to wall with mate finish
Wrong: ping pong ball bouncing in the table
Right : Glue many ping pong ball on a table
And attempt to bounce to another surface
Wrong: Incoming ball is very large compare
to roughness of surface
Right : Consider that molecule do not bounce.
Consider absorption, there resident time and desorbs
Molecular interaction with surface

6. Molecular flow
Molecular Flow; random traversing back & forth
of the molecules from wall to wall with progress
of molecules through the vacuum lines a matter
of statistics
Molecule arriving from region 1 move region 2
To gases not collide effected one and another
Higher gas density region 1
More molecule move right than to left
6
Three different regions with P1>P2>P3
Section A

7. 3.2 Definition of Throughput and Pumping Speed
7
The rate flow trough the passage is depends on
-Capacity of the pump
-Geometrical shape
-Type of flow
-Gas Characteristic
Gas Flow rate – Mass Flow Rate Q
- Volumetric Flow Rate S
Mass Flow Rate Q or Throughput
The net number of molecules passing a given plane per unit time
Unit : pressure volume per unit time, example torr-liter per second

8. 8
Steady state condition : the pressure at given location not changing
as function of time
Pseudo-Steady state condition: no leak valve but gas evolving from the walls
act similar to leak valve

9. Volumetric flow Rate
Volumetric flow Rate S or pumping Speed: the actual amount of substance
which moved a distance d is not specify size that depends on Pressure
9
Unit volumetric flow is volume per unit time example liter/sec
Volumetric flow rate and mass flow rate are related to pressure by equation
푄 = 푆 푥푃

10. 3.3 Conductance
10
• 3.3.1Definition
Conductance property of component which usually fills a certain amount
of three dimensional space whereas volumetric flow rate is property of a
position in space ( plane)
퐶 =
푄
푃1 − 푃2
Unit for Conductance same as volumetric flow is volume per unit time
example liter/sec

11. Combination
11
• 3.3.2 Combination Q = constant
푃1 − 푃2 =
푄
퐶1
푃3 − 푃4 =
푄
퐶3
푃2 − 푃3 =
푄
퐶2
푃1 − 푃4 = 푄
1
퐶1
+
1
퐶2
+
1
퐶3
푃1 − 푃4 = 푄
1
퐶푡
1
퐶푡
=
1
퐶1
+
1
퐶2
+
1
퐶3
퐶푡 = 퐶1 + 퐶2 + 퐶3
Conductance in series
Conductance in parallel
Remember ,Q=Mass Flow Rate/throughput

12. Long Tubes
12
• 3.3.3 Long Tubes
Long tubes has length significantly greater than its diameter
Viscous flow od dry at 20oC
퐶푣 =
3000 < 푃 > 퐷4
퐿
Molecular flow od dry at 20oC
퐶푚 =
80퐷3
퐿
<P>= Average Pressure , Torr
D= tube diameter, inches
L=Tube Length ,inches

13. Orifice
13
• 3.3.4 Orifice
Conductance of orifice is not infinity
퐶0 = 11.6 퐴
1
퐶푡
=
1
퐶표
+
1
퐶푙
A =Area in cm2

14. Condutance ?
14
• example
A Tube 10 inches long and
0.5 inches diameter
80(0.5)3
퐶푙 =
10
=1.0 liter/sec
퐶0 = 11.6 휋 0.25 푥 2.54 2 =14.7 liter/ sec
1
퐶푡
=
1
1.0
+
1
14.7
=1.07
퐶푡 =0.94 liter/sec
A Tube 4 inches long
and 6 inches diameter
퐶푙 =
80(4)3
6
=853.33 liter/sec
퐶0 = 11.6 휋 2 푥 2.54 2 = 940.45 liter/ sec
1
퐶푡
=
1
853
+
1
940
=0.00223
퐶푡 = 447.39 liter/sec

15. 3.4 Calculating Pumping Speed at Different Places
15
• f 푃1 =
푄
푆푙
푃표 =
푄
푆푝
퐶 =
푄
푃1 − 푃표
퐶 =
1
1
푆1
−
1
푆푝
1
푆1
=
1
푆푝
+
1
퐶푡
1
푆푎
=
1
푆푏
+
1
퐶푎푏
From equation previously
Conductance equation
Pumping speed
Generalized equation

16. Example
16
• Example
Pump with pumping speed 200 liters/sec
Attached to chamber via 6 inch tube with 10 inches diameter
Conductance = 951 liters /sec
Pumping speed enter the chamber?
1
푆푐
=
1
200
+
1
951
푆푐=0.936 liters/ sec
6 inches long and 10 inches diameter
퐶푙 =
80(6)3
10
=1728 liter/sec
퐶0 = 11.6 휋 2 푥 2.54 2 = 2116.01 liter/ sec
1
퐶푡
=
1
1728
+
1
2116.01
=
퐶푡 = 951.21 liter/sec