Abstract: Oil-free vacuum pumps can offer significant scientific and economic advantages over oil-sealed rotary vane pumps for most lab applications. Forearm Support for Comfort and Safety
Most lab managers appreciate the advantages of oil-free (dry) diaphragm pumps compared with oil-sealed rotary vane pumps. Most important are the scientific advantages; a mismatch between the capabilities of your vacuum supply and the needs of your application can hamper control over scientific processes. For example, rotary vane pumps used for filtration can evaporate the filtrate instead of collecting it. When used for rotary evaporation, rotary vane pumps can cause bumping and foaming that result in sample loss. Why? Oil pumps provide vacuum at 10-3 Torr – three orders of magnitude deeper vacuum than the 1 Torr sufficient for most lab applications except freeze drying, Schlenk lines and molecular distillations. Beyond the scientific benefits, diaphragm pumps offer significant economic advantages when used for routine vacuum applications like aspiration, filtration, drying, evaporation and concentration. Many people overlook these economic advantages because rotary vane pumps are often a bit less expensive to purchase than dry pumps. Such thinking misses the higher lifetime cost of ownership (LTCO) of oil-sealed pumps.
Vapors drawn from vacuum applications like ovens, concentrators and evaporators directly contact the oil in rotary vane pumps (whether belt-drive or direct-drive). To protect the oil from degradation and extend service intervals, these pumps need to be operated with dry ice or liquid nitrogen cold traps. Furthermore, oil mist filters are needed to keep aerosols – created by vapor flowing through pump oil – from polluting the lab air and coating surfaces. Dry pumps don’t need oil mist filters, and cold traps are not usually needed to protect the pump. These accessories add to the purchase cost of oil-sealed pumps, and the coolants add substantially to the operating costs. Meanwhile, cold trap monitoring and refilling are both an inconvenience and a productivity drag on scientists.
Finally, manufacturers of oil-sealed pumps typically recommend routine oil changes a few times a year to protect the pump. Such service incurs the direct costs of oil, waste oil disposal and labor, as well as the indirect cost of lost pump “up-time.” In contrast, quality oil-free diaphragm pumps have recommended service intervals of as many as 10-15,000 hours. For pumps used 20 hours a week, 50 weeks a year, such maintenance intervals suggest several years’ operation without service interruptions.
When accessory and service costs are tallied, the lifetime costs of an oil-sealed pump are often significantly higher than those of an oil-free pump. Coupled with the advantages for the science, the dry pump has both the performance edge and the LTCO edge for most applications. Use the operating cost savings to pay for electronic vacuum control, and you can save precious researcher time, as well. The message is clear: “Never use an oil pump when an oil-free pump can do the job.”
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