Setup Cha
ter Two: Installation
14
Setup
System Design
There are several considerations that must be dealt with before start-up:
Valve Selection
The correct valve must be selected to allow a reasonable fast time response.
For example:
Assume a system whose pumping speed is 150 liters/sec. and whose volume is 100 liters. This
means that the pump down time constant (T.C.) will be 0.7 seconds. To control at 100µ, or 0.1
Torr, with an up time constant equal to the down time constant (in the practical case this will
rarely happen), the quantity of gas that the pump is able to draw out at this pressure is 150
liters/sec. x 100µ, or 15000µ L/sec. Converting this to sccm: = 1184.
If the 1000 sccm valve is chosen, it will never get up to 100µ because the pump is slightly faster
than the valve (at 100µ). The valve goes wide open and the pressure drops to the point where the
valve throughput equals that of the pump (somewhat less than 100µ pressure). Two things can
be done to make the system work.
The first, and proper course of action (which maintains the system’s natural fast response), is to
select a bigger valve (2000 sccm) which provides more gas than the pump can extract. If
overshoot occurs, the down T.C. will be a combination of the pump and chamber T.C., modified
by the controller’s T.C. as control point is approached. Basically, if a large valve is selected, the
speed-up control will be a function of the controller’s speed (incoming flow); the down control
will be a function of the pump’s speed.
The second course of action is to reduce the pumping speed. However, this approach (usually
realized by partially closing a valve) slows down the entire system.
Pressure Range
The range of pressures over which the system will work must be selected.
It is recommended that this be limited to 100 to 1; although pressures five decades apart have
been controlled with a single valve.
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