Pumping of process gases
The design of suitable pump systems for the continuous pumping of process gases is generally based on the assumption of steady state conditions. In contrast to evacuation processes, time dependent effects such as acceleration and deceleration of pumps, heating, desorption etc. are neglected. The two essential factors for the selection of a suitable pump system for steady state operation are the gas flow to be pumped and the pressure to be maintained.
In vacuum technology, a general distinction is made between the pressure ranges rough vacuum, fine vacuum, high vacuum and ultra-high vacuum. The lower the desired pressure level, the more difficult it is to achieve. For each pressure range, there are pump types that are better or worse suited for a particular application, depending on their compression principle and internal design. The following overview shows the main working ranges of the individual pump classes:
In steady state vacuum processes, the target pressure largely determines the type of the pump needed that is closest to the chamber. Its ultimate pressure should be well below the target pressure in order to provide a good pumping speed under process conditions. Since the ultimate pressure of a vacuum pump is determined by that pressure at which the pumping speed becomes zero, pumping of process gases at a pressure close to the ultimate pressure is not practical. In that case the vacuum pump would have to be unreasonably large. The target pressure is first of all the pressure to be maintained for the actual process and is also referred to as the process pressure.
A second pressure value, the so-called "base pressure", is often specified for the design of the pump system. This value is more or less well below the process pressure and about a decade above the theoretical ultimate pressure of the system. E.g. it is important for conditioning the vacuum system before starting process operation. The base pressure represents the equilibrium between the installed suction capacity and the gas load resulting from leaks, permeation and desorption. As the desorption effects fade, the equilibrium pressure develops asymptotically over time.Thus, the desorption is not taken into account for design calculations.
The gas flow
In vacuum technology, the use of the of the property gas flow as the product of pressure and volume flow has proven to be helpful. The gas flow to be maintained during a vacuum process determines the size or the nominal pumping speed of the pumps required. In practice, there are several terms used synonymously: gas flow, gas load, flow rate, throughput, p-V-flow. In the following, we will solely use the term gas flow (Q). It can be determined anywhere in a vacuum system from the local values of pumping speed S and pressure p:
Q = p × S
That way, for a given target pressure there is a direct relation between the provided pumping speed by the vacuum pump and the pumped gas flow. The gas flow to be effectively pumped by the pump or the pump system comprises of:
- the amount of gas used for or emanating from the process itself
- the amount of gas entering the system through leaks, by permeation etc.
- any purge gas quantities possibly required to protect the pumps
The pumping speed of vacuum pumps is usually constant over a certain pressure range only and decreases towards low pressures. Therefore, the actually available pumping speed depends on the pump's inlet pressure. To be able to maintain the target pressure at the total gas flow coming from the installation and the process itself, the selected vacuum pump has to provide a pumping speed that corresponds to this pressure and flow.
To pump at a pressure of 10 mbar a gas flow of 100 mbar × l/s one needs a vacuum pump providing at 10 mbar at least a pumping speed of 10 l/s or 36 m³/h. Suitable pumps are, for example, SOGEVAC SV 40 B or ECODRY 40 plus. Reducing the target pressure to 1 mbar the necessary pumping speed increases to 360 m³/h. That means, one would need a SOGEVAC SV 470 B or a DRYVAC DV 450 then.