Poor performance of old diff pump.
Posted: Thu Dec 20, 2018 1:26 pm
Hi,
Sorry for my bad english, I'm from Poland. I'm using a research vacuum furnace which is an old used unit transferred to laboratory. Sadly my company decided to refurbish an old furnace instead of making a new one for R&D purposes. It its equipped with old CVC-20 diffusion pump manufactured by Consolidated Vacuum Corporation of Rochester, NY. The company does not exist anymore, as far as I know. The pump is 29 years old. It's a 18 000l/sec diffusion pump connected to furnace via 20" ASA flange. Backing pump (foreline) is Edwards Stokes 412J mechanical pump coupled with Stokes 615 roots booster, capable of going down to 8e-3 mbar. Pump has its original heaters broken long ago, it's now retrofitted with six heating elements from Varian HS-20 (total 12kW vs 11kW original). It is filled with DC-704 oil euqivalent, I've got only photograph of old manual page which says the pump has to be filled with 1 US gal of oil, preferrably DC704. Sadly the pump doesn't have oil themperature T/C. I managed to install thermocouple on pump base near heating elements (outside of pump chamber), and it shows absurdly high values like 510 deg C. I had never operated such small and such old pump. The pump starts to work when heater temperature is about 400 deg C, so it probably corresponds with oil temperature above 200..210 degrees.
I managed to get the pump working, but despite the throughput between 5e-2 mbar and 8e-4 being satisfactory, pump starts to behave poor below this pressure. Pumping speed falls quickly and about 30 minutes are needed to reach 2e-4 in quite small (2,4 cubic meter) chamber. 1e-4 seems to be ultimate vacuum - the pumping speed falls to zero. I managed to get 9,8e-5 after six hours of pumping. The chamber tightness may not be ideal, but I managed to go down to 1e-5 or 1e-6 with bakeout on bigger chambers, having worse leak rates. The pump itself was tested and it's helium-tight. I tried changing the quantity of oil (original level gauge was broken) from 3.8 (1 US gal) to 9 liters, it gives no effect whatsoever. I'm maintaining very big cooling water flow, much bigger than recommended, with 20degC inlet and 33deg outlet, despite this pump runs very hot, you can't touch side walls in half of the pump height. Maybe there is scale in cooling coil preventing heat transfer ?
The pump was visually inspected and flushed with organic solvent and new oil, but the jest assembly has been not disassembled. Can this poor speed be due to some mechanical defect in the jet ?
Sorry for my bad english, I'm from Poland. I'm using a research vacuum furnace which is an old used unit transferred to laboratory. Sadly my company decided to refurbish an old furnace instead of making a new one for R&D purposes. It its equipped with old CVC-20 diffusion pump manufactured by Consolidated Vacuum Corporation of Rochester, NY. The company does not exist anymore, as far as I know. The pump is 29 years old. It's a 18 000l/sec diffusion pump connected to furnace via 20" ASA flange. Backing pump (foreline) is Edwards Stokes 412J mechanical pump coupled with Stokes 615 roots booster, capable of going down to 8e-3 mbar. Pump has its original heaters broken long ago, it's now retrofitted with six heating elements from Varian HS-20 (total 12kW vs 11kW original). It is filled with DC-704 oil euqivalent, I've got only photograph of old manual page which says the pump has to be filled with 1 US gal of oil, preferrably DC704. Sadly the pump doesn't have oil themperature T/C. I managed to install thermocouple on pump base near heating elements (outside of pump chamber), and it shows absurdly high values like 510 deg C. I had never operated such small and such old pump. The pump starts to work when heater temperature is about 400 deg C, so it probably corresponds with oil temperature above 200..210 degrees.
I managed to get the pump working, but despite the throughput between 5e-2 mbar and 8e-4 being satisfactory, pump starts to behave poor below this pressure. Pumping speed falls quickly and about 30 minutes are needed to reach 2e-4 in quite small (2,4 cubic meter) chamber. 1e-4 seems to be ultimate vacuum - the pumping speed falls to zero. I managed to get 9,8e-5 after six hours of pumping. The chamber tightness may not be ideal, but I managed to go down to 1e-5 or 1e-6 with bakeout on bigger chambers, having worse leak rates. The pump itself was tested and it's helium-tight. I tried changing the quantity of oil (original level gauge was broken) from 3.8 (1 US gal) to 9 liters, it gives no effect whatsoever. I'm maintaining very big cooling water flow, much bigger than recommended, with 20degC inlet and 33deg outlet, despite this pump runs very hot, you can't touch side walls in half of the pump height. Maybe there is scale in cooling coil preventing heat transfer ?
The pump was visually inspected and flushed with organic solvent and new oil, but the jest assembly has been not disassembled. Can this poor speed be due to some mechanical defect in the jet ?