These applications will - due to browser restrictions - send data between your browser and our server. The gas, in the vacuum chamber at 1 x 10-6 Torr, (position 1) is in molecular flow, where the gas molecules move in a random direction and collide more frequently with surfaces inside the chamber than they do with other molecules. Read the description of the equation, I believe it addresses your discussion. �Ul��.sz�O$� �p�נudu���2�R���bb�&�R��E����W/h���N.�]�!�������D�=3]��c5�0��f��#�jK�Z2AR�0�$sb��Y]c�\QS�)��VF+ՅMt�4�8�ٝ D�’��9 D_�PyY�G�C��O� z��l2�B�G攉J&}� Conductance is the reciprocal of Resistance     C = 1 / R. An informal definition of conductance is the capacity of a pipe to allow a volume of gas (at any pressure) flow from one end to the other in unit time. The units used must be consistent. Photo courtesy of “Modern Vacuum Practice” p. 320. as above. Make sure that the units used all match, you can’t mix cfm and l/sec. (At pressures above 1 Torr the higher values of conductance allow lots of gas flow and the pump speed loss is much lower.). In a typical vacuum furnace the chamber would also have an oil diffusion pump mounted on it and other pipelines and valves are used to allow the furnace to be evacuated by the pumps in the correct sequence. At 6 x 10-2 torr, conductance C = 1000 l/s (per meter). ���9f*�9x�Ф°u&��d�����Xy^�L�.��C�͗J�~���ȡmgF D�����2����(��f��|�-)=? For throughput: the pressures at different points in the system show the very high compression ratios developed by the oil diffusion pump and mechanical pump. That would indicate that the 100 mm (4”) pipe is a much better pipeline size for this example than a 70 mm (3”) pipe. Putting the selected pump size into the pump down time version of the equation gives us: T  =  FV / S   and   T  =  11 x 120 / 150          which becomes 1302 / 150    and    T =  8.8 minutes. The conductance between two points in a vacuum system is expressed as the quantity flow rate of gas flowing through a device divided by the resulting pressure drop. Vacuum in %, torr, mm Mercury, psi and kPa. *Eng-Tips's functionality depends on members receiving e-mail.

Now, looking at roughing lines of 2 m and 3 m length, we will see quite a difference. It occurs in the roughing line, the foreline and the high vacuum piping and affects the amount of time taken to evacuate a vacuum chamber to its required base or process pressure. That can be denoted in several ways; cfm, cu ft /min or ft3 min-1. The last conversion needed is to convert the selected pump speed of 150 cfm to l/s units. ��!0�%3�7��3v X[�� Tutorial on Gas Flow, Conductance, Pressure Profiles Definition of “vacuum ranges” •Linked to the Knudsen number and the flow regimes, historically defined as in table below [1]; •With the advent of very low-outgassing materials and treatments (e.g. Note on the chart, or off the top of the chart, that at pressures approaching atmospheric pressure conductance is high and little or no restriction on pump speed is seen. We will look at both options to see the difference. b��)�б�Z�Q

For this simple example we will ignore the valve conductance and only consider the pipeline which may have a total length of 2 meters or 3 meters depending on the system design.

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.The Eng-Tips staff will check this out and take appropriate action. If the mechanical roughing and backing pump is similar to a Stokes 212, having a rated pumping speed (Sp) of 150 cfm, it will have an inlet size of 3 inches. Fig.

5 that shows conductance “per meter” of various diameter pipes at different pressures. For a roughing line of 2 m length, Se is 146 / 2 = 73 cfm. I would suggest that most users don’t really give it any thought. October 10, 2013 by VAC AERO International. Photo courtesy of “Modern Vacuum Practice” p. 320. as above. To complete the calculation we have to express the pump speed (Sp) in l/sec. If the roughing line was 3 meters long, for the same pump the effective speed would be 124 cfm at that pressure. This is due to the fact that the Roots blower reduces the pressure by about one decade from the one-stage pump ultimate vacuum as well as adding a much higher pumping speed to the system once it is at full pumping speed. Now let’s compare that result for the same chamber volume with a more normal pipeline arrangement for that large volume. If you want to promote your products or services in the Engineering ToolBox - please use Google Adwords. The gas molecules in transitional flow now move down the pipeline to the lower pressure generated at the inlet of the mechanical backing pump where they are compressed again, up to atmospheric pressure, and exhausted from the system (position 3). The example shown starts at an already low pressure in the vacuum chamber. The vacuum chamber pressure “P” is shown as 1 x 10-6 Torr which is a typical pressure for many vacuum applications. Click Here to join Eng-Tips and talk with other members! The calculation above is available as an excel file -. If the roughing line is 3 m long, Se is 146 / 3 = 49 cfm. It’s a goal shared by embedded computing manufacturers like Abaco: we never forget who we serve.This case study describes how a major international contractor integrated an Abaco single board computer at the heart of its CAS/CSAR solution. Promoting, selling, recruiting, coursework and thesis posting is forbidden. Velocity in a NPS 3" steel pipe with standard air volume flow 100 scfm at 30% vacuum can be estimated to approximately 3000 fpm as indicated in the diagram below: Standard and Actual Air Volume - l/s This chart shows mbar on the pressure scale and the conversion is 8 x 10-2 mbar (a 3/4 ratio).

This knowledge is used to select the correct size of mechanical vacuum pump to rough out the vacuum chamber in the required time. This gives a small safety factor to allow for process outgassing, but some processes may need a larger pump if the outgassing from the furnace and product surfaces is large, i.e. That gas is then compressed about another 15,000 times in the mechanical pump allowing it to be exhausted into the air at 760 Torr. Looking to make the design and manufacturing of your products more agile? Copyright © 1998-2020 engineering.com, Inc. All rights reserved.Unauthorized reproduction or linking forbidden without expressed written permission. Pipe load calculator solving for vacuum load per linear length of pipe given internal vacuum pressure and inside pipe diameter Transitional flow will be a changing mix of laminar and molecular flows. [Fig.2 black arrows]. At some point the time needed to evacuate the added volume cancels out the increased effective pump speed at the chamber and no improvement is seen. These applications will - due to browser restrictions - send data between your browser and our server. it offers resistance to the flow. Figure 8 | Flow through a pipe in the continuum range Figure 9 | Flow through a pipe in the molecular range. Even though the port on your tank may be small, the air is only traveling through a short distance, so the restriction is minimal.

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