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Salvage It! BY NENO STOJADINOVIC A Vacuum Pump from Junk Every young techie needs a vacuum pump. They rate alongside the air compressor, multimeter and soldering iron as one of the most useful tools to take their place in the arsenal. This guide will show you how to make a serious vacuum pump for next-to-nothing. Add a vacuum tank for much the same price and you are loaded for bear. I t used to be that every young scientist needed a vacuum pump. There was metal to sputter, glass envelopes to evacuate when producing X-ray tubes and investment to ‘debubble’ when making castings. Nowadays, there are commercial products available to replace these venerable old staples but hobby scientists have compensated by expanding the scope of their activities. I don’t sputter my own telescope mirrors but I do pot my own coils and ultrasonic transducers and use vacuum to remove all air bubbles from the resin. I don’t make X-ray tubes but I do make robot parts with ‘prepreg’ carbon fibre that uses a process called vacuum bagging to mould the material. And call me funny but I find a strange fascination with the science of refrigeration. In refrigeration systems the flow rate and pressure can be considered analogous to current and voltage in electronic circuits. With the advent of LP gas as a refrigerant and cheap manifold gauges, I find a lot of tech heads like me sitting around watching ice form on their home made evaporator coils. Where do you get it? Vacuum pumps live in the bottom of every refrigerator, except they are cleverly disguised as refrigeration compressors. Countless refrigerators siliconchip.com.au are thrown out due to gas leaks, faulty thermostats or some other minor fault, leaving a perfect compressor that is just ripe for a new purpose in life. Refrigeration compressors of this type are commonly referred to as “sealed units”, and are a simple piston pump running in an oil bath for longevity. Liberating the compressor is simple enough but it needs to be approached with a bit of caution. Most discarded fridges I’ve seen are devoid of gas but the gas lines can possibly be still under pressure and may also be partly full of refrigeration oil. Thus it is important to put on goggles before you begin, and don’t point the pipes at yourself as you’re cutting them. So to begin: peer under old fridges until you find a compressor that doesn’t have capacitors near it. (It’s not difficult to drive a capacitor motor but there are plenty available that don’t STARTING SOLENOID ACTIVE RUN EARTH NEUTRAL START MOTOR CONNECTION (MOTOR CASE) COMMON T THERMAL OVERLOAD SWITCH Fig.1: Wiring diagram for a typical fridge compressor. Note the overtemperature cutout. use them at all, so I avoid the capacitor jobs as a needless complication). Cut all of the gas lines to the compressor, leaving a handy length to work with. Use a pair of side cutters or a tube cutter to sever the lines, as a saw will introduce fragments of metal into the compressor. Cut the electrical connections, unbolt the compressor and the prize is yours! Test drive Firstly, find out if it runs. Most simple compressors use a split phase start winding which is energised by a solenoid that is connected in series with the main, or “run” winding. When the motor is first started, the main winding draws a large current. As the solenoid is connected in series, the heavy current creates a magnetic field that pulls the contacts shut and thus energises the start winding. As soon as the motor is up to speed, the current in the main winding dwindles and the solenoid drops out, thus cutting off power to the start winding. Hopefully, fig.1 should make it all clear. Once you’ve got it wired up, just stand back and plug it in. Quiet fridge-type hum good, flames bad! Once you have a good one up and running, partially block the thick pipe near the top of the compressor. You should feel a bit of suction but don’t March 2011  71 MAINS EQUIPMENT Take extra care when salvaging any old 240VAC mains equipment. Be aware that insulation may have broken down, previous repairs may be “dodgy” ...and mains voltages can kill. Remember that old adage: “If in doubt, don’t!” block the pipe completely, because if you do, the compressor oil will froth up inside the housing and then squirt violently out of the other pipe. If all is well, power it all down and get cracking to change the oil. The oil is drained by undoing the service plug and just letting it pour out. Of course, we don’t have to say that it must be disposed of properly . . . Once it’s all gone, the compressor can be refilled by pumping the new oil into the vacuum (thick) pipe. It is possible to get many different grades of vacuum oil, mostly selected for intended vacuum level and type of pump, but for most jobs I use standard air compressor oil. It is available at most places that sell air compressors and comes in a handy squeeze bottle that allows you to partially fill the compressor before your wrists give out. Then you can use the compressor itself to suck the rest in – just block off the service port and stick the vacuum pipe into the oil bottle. Stop the pump Here’s what you’re looking for. It almost certainly won’t look as pristine as this (in fact, it will probably be at least a bit rusty and/or covered with grease and dust). But most fridges these days have a compressor looking something like this one. every now and again to check if the oil level has reached the level of the service plug and once it’s oozing out you are done. Pull Vacuum Vacuum pumps require you to learn a new vocabulary and some new physical principles. First of all, if you convince all of the air molecules in a container to vacate the premises, you will have yourself an absolute vacuum. Fig.2: a compressor with the lid cut off. Gas enters the stub of pipe near the bottom of the figure and exits through the long thin pipe that loops across the top. This pipe always exits into an equally thin pipe on the outside of the housing. The compressor is normally suspended from three springs but these are disconnected in the photo. 72  Silicon Chip The gauge pressure, relative to atmosphere, will be -101kPa or -14.7psi if you’re over 45 years old. Americans will say -30in Hg, while Europeans will say minus one bar or possibly -760mm Hg. I buy little vacuum gauges on eBay (see Fig.5) and they can tell you that the typical home-made vacuum pump will pull better than -25in Hg if allowed to run for a while. Remember that below certain vacu- Fig.3: the casing itself. The loop at the bottom is a heat exchanger and leads out to two thin pipes near the bottom of the casing. Gas is drawn directly into the casing and the inlet port can just be seen at the top of the figure. Inlet gas is hot and not very dense so it always flows through large diameter pipe. The service port can be seen at right. siliconchip.com.au You will inevitably draw impurities into the pump while it’s on evacuation duty and some of them can be explosive when subsequently in contact with compressed air. And following on from that, vacuum pumps will be damaged if you allow them to suck up crud. The cure is to use a separator. They are commonly used in distillation (especially the moonlight kind) so look them up or else drop me a line and I’ll show you how to build one out of an old Primus bottle. Fig.4: a reservoir tank made from a 9kg gas If you want to suck up large bottle, adapted to a standard air line fitting. quantities of liquid, dust etc, It is worth knowing that most pipe fittings in it is best to use a large vacuum Australia have a BSP (British Standard Pipe) tank as a reservoir. Old gas botthread. This includes air lines, refrigeration, tles can be found at most dumps, plumbing, irrigation, etc so it is fairly well universal. BSP fittings will even screw into most and it is a simple matter of buying a matching fitting to convert American (NPT) fittings. Note though that gas it to a vacuum tank – see Fig.4. lines use a tapering thread (BSPT) while most other pipes use a parallel thread (BSPP). Places that sell barbecues are a gold mine of fittings, valves um levels you are not so much pump- and hoses that can be re-purposed ing air as convincing air molecules to for our warped needs. Once you’ve float into your pump. evacuated the tank, you’ll be amazed Once it’s all up and running, I like at the sheer power of a measly one bar to solder a hose barb onto the vacuum of (absent) air. line to suit the job at hand, perhaps build it into a handy tool box so I can Politically correct message about refrigerant carry it around, all nice and neat. Note that standard compressed air Most of the fridges I’ve seen at the hoses and fittings work just as well un- local dump have been very obviously der vacuum as they do under pressure. empty of gas; it seems nobody treats You will find that the pump ejects malfunctioning whitegoods very well an oil mist while it’s running. I catch and broken pipes are common. I even the oil in a can and run a hose to the found one that was riddled with bulvacuum line so I can draw the oil back let holes! in once a bit has collected. It is environmentally unfriendly and illegal to release refrigeration gas to Care and feeding the atmosphere, so if you can’t find a First and foremost, don’t ever use ‘pre mauled’ unit, the next best thing your vacuum pump as a compressor. is to go and visit your local refrigera- Fig.5: you’ll find many vacuum gauges on eBay, new and used, ranging from next-to-nothing to next-to-ridiculous. This one, for example, was brand new and about $25 plus postage. Most common are the combined vacuum/ boost gauges intended for automotive use – they’re fine for our purposes and often the cheapest (sometimes <$10). tion mechanic. Refrigeration repair is yet another industry that has been hit hard by cheap imported goods and many people lug in a fridge only to be told that a repair would cost more than a new one. Suddenly you have a fridge waiting for you and the mechanic will suck all of the gas out for a small fee. Even better is that mechanics are generally friendly folks and have all sorts of goodies for the likes of us. How about a 12V fridge compressor? (They are very common in caravan units). Or else maybe a complete condensor unit? They are those boxes that you see on the sides of buildings used for coolrooms and split system air conditioners, and they are full of goodies. I got a working unit for $10. SC WARNING: Use a full face shield when evacuating glassware. Glass will eventually weaken and implode when subjected to vacuum, causing shards to explode outwards. Issues Getting Dog-Eared? REAL VALUE AT $14.95 PLUS P & P Protect your investment with these handy binders. Each one holds one year (12 issues) safely and securely with “easi-wire” insertion. Price just $14.95 plus $10.00 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or ring (02) 9939 3295 [9AM-5PM Mon-Fri] and quote your credit card number. Available in Australia only (ie, not available for export). siliconchip.com.au March 2011  73