Application reports

Avoid vapor condensation in dry primary vacuum pumps

High water vapor pumping speed can be achieved with proper gas ballast, temperatures and exhaust

Small, dry vacuum pumps have found numerous applications in vacuum technology. Particle physics, cryo technology, general instrumentation and laboratory use are only a few examples of the applications. However, in applications such as freeze drying, vacuum drying, filling of a liquid circuit or organic layer coating, dry pumps have historically not been chosen due to limited capacity of pumping condensable liquids. The need for high water or solvent pumping capacity has led to development of a dedicated dry pump, the ACP40CV (CV = condensable vapors).

The general advantages of dry pumps compared to oil-sealed pumps are low cost of ownership due to long maintenance intervals, cleaner vacuum without oil back-streaming, and environmentally friendly operation without lubricant disposal. Often vacuum pumps are characterized by base pressure and pumping speed characteristics. Diaphragm pumps are the entry level range of dry pumps. Pumping speeds comparable to compact rotary vane pumps can be achieved with piston, scroll or multistage roots pumps. The performance of these types of dry pumps compared to rotary vane pumps is heavily dependent on the nature of the pumped gas and the pressure gradient over the pump. Additionally, the nature of the gas used has a significant influence on the thermal behavior of the pumps.

Water Vapor pumping using oil-sealed vacuum pumps is characterized by a partial pressure pWo, the maximum pure water vapor tolerance in mbar and the maximum pure water vapor pumping capacity cWo, a weight that can be pumped over time in g/h. This characterization is described in the Pneurop standard 6602.

 

Fig. 1: Adixen ACP40 cross section

Vapor partial pressure must always stay below the saturated vapor pressure. A high flow gas ballast is used to reduce the partial pressure of the vapor and to increase the temperature in the high pressure stages of the pump where condensation can occur. In order to avoid condensation the highest pressure within the pump needs to be controlled precisely. The exhaust area of the pump must be flow restricted. This could create a local overpressure leading to condensation. Once the condensable liquid passes through the pump in gas form, no vapor back-streaming can occur.

These principles have been applied to the Adixen ACP 40, a five stage roots pump. To date this pump has been available as a standard pump and a version with low flow gas ballast which enabled pumping of humidity. When used for vapor pumping precise control of the geometry in the five pumping stages leads to continuous temperature increase within the pump. The maximum stator temperature in the 5th stage may reach 70°C. The gas temperature in this stage is pressure-dependent and can reach up to 105°C.  Jet expansion ensures condensation outside the pump. The liquid is collected in a drainable device which also acts as a silencer. These modifications have increased the condensable pumping capacity of the ACP 40CV to a maximum water vapor flow of 700 g/h at a total pressure of 70 mbar (measured at 20°C ambient temperature). This flow triples the previous recorded value achieved with a scroll pump.