This is a 3 cubic foot per minute (free flow) Edwards pump. This is a professional vacuum pump, not a "service" pump like is used in air conditioning. A professional vacuum pump will have better performance at the low end of the pumping range, in the range below 100 microns. This pump will pump to below 5 microns. The motor on the pump has been replaced with a variable speed DC motor so the pump can be run at full volume when doing initial roughdown, and then slowed down to an idle when maintaininig the vacuum. This cuts wear on the pump drastically, reduces power consumption, and reduces noise.
AMATEUR VACUUM TUBE MAKING
MECHANICAL PUMPS USED IN TUBE MAKING
This is a 15 cubic foot per minute Welch 2 stage rotary pump. It is a professional vacuum pump also. This is a fairly large pump and is suitable for pumping a bell jar 12 inches diameter and 18 inches tall in a reasonable time. This pump also has had the motor replaced with a variable speed DC motor. The power consumption is only 180 watts on full speed.
This is a premium Welch 5 cubic foot per minute THREE STAGE rotary pump. Three stage pumps are not available. These pumps were only produced for a very limited time and are now only available on a chance find basis. They have excellent performance in the below 50 micron range, having a practical low end of around 5 microns. This pump has had its motor replaced with a variable speed DC motor.
This is the classic Welch belt driven vacuum pump. This is a single stage unit but two stage pumps are common and are the ones to use. These pumps are reliable and will run virtually forever if you keep oil in them. The rebuild kits are still available for these pumps. The main disadvantage of these pumps is that they are big and heavy for a given displacement.
This is one of the small $100 refrigeration service pumps. Note the small size of the pump in comprison to my hand! The square part of the pump is right at 4 inches across. The pump has a 1.2 cubic foot per minute displacement, which puts it in the smallest of the pump categories. The pump is specified at 20 microns final pressure and surprisingly, it did achieve that. The main factor was the very long time it took to get there. It was tested with only a baratron connected, which makes the pumped volume very small for the test. It took right at one minute to get to 100 microns. Then, it took 8 minutes to get to 50 microns. It took a total of one hour and ten minutes to get to the ultimate pressure of 20 microns. Compare that to the 4 minutes it takes the 3 stage unit above to get to below 20 microns, and that on a chamber a hundred times larger than the mini pump test! So, while the ultimate pressure of the pump is completely satisfactory, the pumping time will really irritate you. Go for a 5 cfm pump for your setup.
This is a homebuilt pump that was made from an old refrigerator compressor. The compressor was a single stage rotary vane type in a hermetic steel case. These compressors are not useable for vacuum work as they are for two reasons. First and most serious, is they have a built-in check valve. This check valve takes about 10 torr of pressure to open. Thus, you cannot get the compressor to pump lower than 10 torr as it comes from the refrigerator. The second problem is that the refrigeration oil is not suitable for vacuum work. It has a vapor pressure of about 100 microns so you would not be able to get below that even if you removed the check valve.
To get around these probelms, the compressor was cut out of the hermetic steel case, a daunting job, and completey dismantled and the oil cleaned out of it. The check valve was removed and the motor was removed. Then, the pump reassembled and mounted to the aluminum plate in the photo, and a steel case for it made from an old kitchen pan. A shaft seal was mounted to keep the oil from leaking. Good grade vacuum oil was used. The pump worked fine but being a very small single stage unit it only would pump to around 100 microns ultimate pressure. This is not a practical way to go. This was done back before ebay and the flood of cheap vacuum pumps that are now available.
The pump you want for your system is a 3 to 5 cubic foot per minute two stage rotary vane pump. Preferrably a professional vacuum pump, but a refrigeration service pump is completely satisfactory.