You are working on a Plasma Enhanced Chemical Vapor Deposition (PECVD) solar manufacturing R&D application. Your facility has a high vacuum chamber which requires a turbomolecular pump and you have been asked to research what is the best turbomolecular pump to purchase. *Keep in mind you also require an ISO250 flange and 12 Standard Cubic Centimeters per Minute (sccm) at 1x10-4 mbar. Determine what turbomolecular pump is best suited for the application by answering the following:
What is the pumping speed required in liters/second (assume temperature of 20 degrees C) at 1x10-4 mbar? What is the change in pump speed if you add an ISO250 gate valve that is 10 cm in length?

Solution:

Calculation of standard conditions to actual conditions:
Recall “Standard” conditions are atmospheric pressure (1013.25 mbar) and zero degrees C (or 273 Kelvin). Actual temperature and pressure given is 20 degrees C (293 Kelvin) and 1x10-4 mbar

12 sccm converts to 2200 liters / second as follows:

Selection of a turbomolecular pump:
The process is PECVD which is best served by a robust, magnetically levitated turbomolecular pump. Given the flange size (ISO250) and required pumping speed (2175 liters / seconds) the best fit is a Edwards STPA2203C “Advanatage” turbomolecular pump (Section 2, page 52 of the 2007 Edwards Product Catalog. The STPA2203C “Advantage turbomolecular pump has been approved for use by major etch, ion implant and deposition equipment manufactures.

Determining conductance losses of the valve:
Refer to Section 14, page 6 of the 2007 Edwards Catalog. Use figure 2 to determine pipe conductance of a 250 mm pipe, a meter in length at 10-4 mbar . The conductance is 2000 liters / second. Use the conductance equation given below the figure to determine conductance for the length of 10 cm (0.1 meter)

Cp = C/L
Where Cp in the conductance of the pipe, C is the conductance per meter and L is the length in meters.

Cp =  20,000

Now we can use the value for Cp to correct the pumping speed at the end of the pipe (valve in this case)

Sp = (Cp x S) / (Cp + S) = 1982 liters / second  (S is the pump speed and Sp is the speed with pipe)

10% of the pumping speed is lost to conductance when adding an ISO250 gate valve with a length of 10 cm.