Thursday, January 13, 2011

Tooling up - Vacuum pump part 1

Vacuum Pump

Before I finalised the design of the board itself I thought it would be good to make the tools needed to make the board. In particular the vacuum pump and rocker table.

The vacuum pump is obviously at the heart of vacuum bagging and of the pieces of equipment needed it is the one that for me had the most unknowns about it.

I cruised the net and the general design that kept coming up involves using a compressor out of a fridge with an optional regulator so that its not running continuously. With a well sealed vac bag setup the compressor only needs to turn on about every 15 mins or so so having it run continuously is not necessary and probably shortens the life of the compressor.

In one article I read of someone using a kite pump's deflate side and hand evacuating the bag. He had installed a pressure gauge and one way value and came back every 10 mins or so the check and repump it. Although this seems like a easy solution, having to hang around for the hours that it needs for epoxy resin to fully cure and come back each 10mins sound like a real pain in the arse. On top of that given the sand and crap that ends up in side the pump I wouldn't be that confident of being able to get it down to the sort of pressure needed without the pump itself needing to be in perfect condition. Maybe a brand new one and some quite viscous lubricant inside to seal over imperfections might work ok.

The best resource I found for teh fridge compressor version is on seabreeze and the compressor I made follows this pretty closely. The only material variation on this design that I've seen is to add a reservoir (made from a big diameter piece PVC pipe with end caps). This would be a great addition as it would mean that it could cover compressor failure (they occasional need to be reset if the overload relay is tripped).
and this one as well for a slightly different discussion (and no regulator)

And so here is my finished vacuum pump. I've tested it and holds a vacuum of 28 inHg ( 1 atmosphere is 29.9 inHg - inHg stands for inches of mercury).

To put this in context 1 atmosphere in pounds per square (psi) is 14.7 psi or 10,332 kg/sq m. A 130x39cm is about 0.5 sqm so the pressure that the vacuum pump can apply to the surface area of the board is about 5 tonnes!!!!

PARTS LIST (see the photos below and in subsequent posts for pictures of the parts)

1. Compressor from fridge with connection box and power cable (begged, borrowed or stolen is fine). Size of the compressor doesn't seem to be an issue other than perhaps how long it takes to suck the required volume of gas out. The small one I got from a 75 l bar fridge is 1/15 hp and sucks out all the air in under 30 seconds or so.
2. 8mm and 10 mm pvc tubing (Bunnings)
3. 8mm and 10mm hose connectors (Bunnings has 't' pieces and connectors for joining different size tube )
4. A vacuum advance of a carburetor (local wreckers had one in a throw out bin that they sold me for $20. Most wreckers I spoke to wouldn't sell a vacuum advance on its own, just the whole carby so old ones from a mechanic might be easier/ cheaper.)
5. 240v 10A microswitch (jaycar electronics)
6. A vacuum gauge. These are commonly used on turbo charges and called 'boost' gauges. At repco / supercheap they cost around $80 but usually because they are very bling. I found one on an online retail site for $14 delivered and its perfect. . You need a fair bit of granularity in the pressure measurement so avoid ones where too much of the gauge is taken up with positive pressure measures (as opposed to pressure less than 1 atmosphere ie. vacuum).
7. Electrical wiring and the 240v cable on the compressor.
8. A strong spring for the pressure regulator. One from a metal bed base is perfect. At first I underestimated how much force there is associated with the pressures in the system and used a soft spring and the regulator kept cutting out at just a few inches of mercury (remember 1 atmosphere = 29.9 inHg).
9. A bolt with a wing nut on it for adjusting the regulator setting
10. Strong wire for the regulator linkage
11. A rubber thong ( for mounting the compressor on to minimise )
12. A handful of screws and cable ties
13. Wood for base and for regulator housing


1. The fridge compressor is mounted inside a sealed black metal container and has 3 or 4 copper tubes inserted into it.
I was lucky enough to come across a 75 l bar fridge that had a working compressor but that the gas had leaked out of so wasn't keeping anything cold.

I had tried a few different places to get one without luck: in our area there is a council program from collecting and degassing old (10 year old) fridges, I tried the local recycling centre but they would let me near the dumped stuff and so if not for lucking it on clean up day on the road side the next port of call would have been friends.

If you get one out of an unwanted working fridge remember that it will most likely be full of ozone killing refrigerant which will escape if you cut the lines.

On the compressor one line is the pressure line that's used to compress the refrigerant another is the vacuum side that helps pull it through the expansion valve in the fridge, one is a lubricant line ( the actual compressor is suspended in a lubricating oil in the metal housing) and the other is where the refrigerant is added. On the one I got from the 75 l bar fridge the last of these was permanently sealed off. Here's the inside view of the compressor

The connection box (the black box on the side) is where the start relay for switching on/off is housed along with the overload protection that shuts the compressor down if its working too hard. Be sure to get all of this and the power lead when you salvage it. This compressor had a solid state start relay (PTCR) in it which looks like a simple disc of plastic with 3 connections on it.

This link provides good discussion of how it works.

The PTCR provides increased current to required to get the compressor started and then opens to reduce the current to the lower level needed to maintain operation. The solid state start relays do this by letting the resistance of the device change rapidly as it lets the initial current through. In about a second the PTCR resistance has increased to 30kOhms because the current running through it has heated it up. This causes the relay to effectively be an open circuit. (PTCR is a solid state relay)

The wiring diagrams are usually on a sticker on the compress. You'll also see that the earth wire in the 240v cable will be connected to the chassis of the fridge. This makes a path to earth if there should be a short circuit that send current through the metal chassis. Its a good idea to scrap of paint from the compressor at one of the feet and make sure you connect the earth wire there.

PARTS 2 and 3. These are cheap as chips and the size means they won't collapse under a vacuum and pinch off the pneumatic circuit. The size was largely because that's what I came across at bunnings when I went looking for the brass connectors suggested in the seabreeze article. However, the plastic connector are perfect and even without the cable ties I put around them did not leak. The one thing to keep in mind when choosing the size is how big the tubes on the compressor and also the vacuum advance and the vacuum bag connector if you end up using one. You may end up (as I did) needing to have a couple of different sizes. The connector needed 10mm hose with the compressor was more like 5mm which I sized up to 8 mm using a piece of fuel line that was connected to the carby.

PART 4. Vacuum advance

UFO shaped thing off the carby. It is very basic. Inside the metal housing there is a rubber membrane that moves acording to the pressure in the space above it and so the armature out the bottom similarly moves. That's all there is to it. It converts pressure to displacement which in turn will press or release the button on the microswith to turn the compress on and off.

... to be continued


  1. Minor update. Had to re-build the pump to get it to fit into a cupboard space.... Rebuilt it will all 4 mm PVC tube instead of the larger diameter stuff. The smaller the diameter the less it collapses under vacuum pressures. With 4mm ( which fitted pigtails on the pump easily ) there are lots of garden sprinkler connections and bits and pieces that fit it nicely so it wasn't any problem to get tight fitting connections. Where the connection was just that bit too big in diameter I just put the pvc tube in boiling water and it goes very soft and can be pushed onto large diameter connections plus creates a really tight seal when it cools.

  2. I have an air compressor for using a nail gun, spray system etc. is it possible to use the inlet of this to pump down the vaccum?

    1. An air compressor as you describe is not ideal for a few reasons.
      1. Most pumps do not do well with their inlet restricted more than their outlet. It causes cavitation.
      2. Very noisy
      3. Costly to run

  3. Very nice post.You mention here very important think so clearly.I get here basic idea of various compressor.