Most composite propellants either need or benefit from degassing before the uncured propellant is cast into grains. I have heard from others in the EX community that an old refrigerator compressor can be used as a decent vacuum pump. I'm kind of a pack rat, and rarely throw out anything that may have useable parts in it. Over the years I have acquired a number of refrigerator or freezer compressors, mostly when a coolant line leaked and unit no longer functioned. They also make a nice small air compressor, great for a back up compressor or to keep in that odd place you wouldn't keep a regular air compressor. So I have a couple of them around anyway, but I also had this great big 8' ice cream freezer sitting in my back yard. It was only three years old when it developed an internal cabinet leak that was unrepairable. I had been eyeing it for some time, with a vacuum pump in mind for it. I figured the commercial compressor should be heavier duty than a household unit. Boy was I right, was it ever heavy! I managed to remove the compressor/condenser unit and drag it into my shop with a hand truck.
After cleaning it up a little, I made some clean cuts to the copper intake and exhaust lines from the compressor. I plugged it in and it ran perfectly. It still had all the thermostat and all the associated wiring, so I decided to clean all that off and just rewire it with the plug in cord to turn it on. I then removed the fan and the condenser unit. All that was left was the compressor, the box with the large start capacitors and the metal base it was mounted to. It still weighs about 175 pounds. I think I'll mount some wheels to the base so I can pull it around. It may be overkill for a vacuum pump, but I'll go ahead and hook up some fittings at the output side too, that way I can use it as a regular air compressor too.
Here it is after the fan and condenser housing were removed.
The condenser is a nice find too. I've been into solar and wind energy for some years now. The condenser has over 120' of copper winding through it, what a great compact heat exchanger it will make! But my solar water heater is another story.
What I need is a few fitting to finish off the vacuum part of things, and a vacuum gauge to see what kind of vacuum it will pull. Just a finger over the intake seems promising. Try as I may, I couldn't find my old refrigeration gauges, so I'll have to buy a new one. I have a PVC pipe case I built to house an underwater video camera, it hasn't been used in years and should make for a fine vacuum chamber as it has see through ports on either end.
To the left is the vacuum chamber lid, it is made from 1" thick plexi glass of some sort, I don't even remember the exact material but it was used as windows on my home built submarine some 20 years ago. I drilled two holes in the top and taped them for 1 /4" pipe thread. The port on the right has a 1/4 turn ball valve that leads to the vacuum pump. The port on the left has a needle valve for venting the chamber to atmospheric pressure. The lid has two o-ring seals, one on a lip that extends into the chamber, the other rests on top of the chamber. I made a new o-ring for the top seal as the original o-ring was becoming checked due to age. The yellow box on the right is my o-ring making kit. It looks like I need to order some more o-ring rope as I'm getting low. It's really very easy and inexpensive to make your own o-rings, no more searching for the correct size. I've had this kit for about 20 years.
Here is a picture of the vacuum chamber with the lid on. Like I mentioned earlier, this was a case I made for an underwater video camera a long time ago. The hasps that lock the lid on aren't needed, as the vacuum holds the lid in place. I bought a vacuum gauge, it was designed for automotive use, and they are available at about any car parts store.
At the moment, I'm pulling about 23 inches of mercury with the pump I'm using. That's not very good. I do need a check valve yet, so that may help some. If the check valve doesn't help, I'll end up buying a real vacuum pump.