Application reports
How to avoid vapour condensation in dry primary vacuum pumps
How to avoid vapour condensation in dry primary vacuum pumps
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 potential applications. However, in applications like freeze drying, vacuum drying, filling of liquid circuit or organic layer coating, dry pumps have not yet found a wide installed base due to their limited capacity of pumping of condensable liquids. The need for high tolerance against water or solvent pumping without condensers has led to development of a dedicated dry pump.
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 the parameters base pressure and pumping speed. Regarding these two parameters, 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 dependant 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 in the thermal behavior of the pumps.
Water Vapour pumping of oil-sealed vacuum pumps is characterized by a partial pressure pWo, the maximum pure water vapour tolerance in mbar and the maximum pure water vapour 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
It is the main target of vapour pumping to avoid condensation within the dry pump. The vapour partial pressure must always stay below the saturated vapour pressure. High flow gas ballast is used to reduce the vapour partial pressure and to increase the pump temperature. In order to avoid condensation the highest pressure within the pump needs to be precisely controlled. The exhaust zone of the pump must not contain any massive flow restriction that might create a local overpressure leading to condensation. Once the condensable liquid has passed the pump in gaseous form it must be achieved that no vapour back-streaming can occur.
These principles have been applied to the adixen ACP 40, a five stage roots pump. To date this pump was available as a standard pump and a version with low flow gas ballast that enabled pumping of humidity. When used for vapour 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. Then jet expansion ensures condensation outside the pump. The liquid is collected in a drainable device that acts also as a silencer. These modifications have increased the condensable pumping capacity of the ACP 40CV to a maximum water vapour flow of 700 g/h at a total pressure of 70 mbar. Measured at 20°C ambient temperature, this flow triples the previous record value that had been achieved with a scroll pump.
Fig. 2: Experimental set-up for vapour flow measurement
The achieved values have been measured by gravimetry. The measurements show strong deviations from Pneurop 6602 standard. Deviations of pumping speed for air and condensable liquid plus different assumptions in gas temperatures in dry pumping are leading to an underestimation of maximum vapour tolerance and maximum vapour pumping capacity.
The positive test results of modified pumps will lead to the release of the new standard product, ACP 40CV, early in 2008.
Authors:
Laurent Saxod
Inopro/Alcatel Vacuum Technology France
98 Av de Brogny, BP 2069
74009 ANNECY Cedex, France
René Sibuet
Alcatel Vacuum Technology France
98 Av de Brogny, BP 2069
74009 ANNECY Cedex, France
Dr. Rudolf Konwitschny
Alcatel Hochvakuum Technik GmbH
Am Kreuzeck 10
D-97877 Wertheim
