This page last updated: 11 April 2021.

In March 2021, during the second year of all things Nikola, my little Jun-Air 6-25 compressor sprung a pin-hole leak from its tank.  While this might have been repairable it gave me the opportunity to increase my air supply; the little compressor had never been quite sufficient.  However, as is often the case, my specification was somewhat narrow, I could not find a compressor that met it.  Hence I decided to put together my own.


For reference, the sizes of pipe threads used below are as follows:

Major diameter (i.e. outside diameter of male thread)
Threads Per Inch
1/8" BSP, G 1/8, R 1/8
~9.7 mm
1/4" BSP, G 1/4, G 1/4
~13.2 mm
3/8" BSP, G 3/8, R 3/8
~16.7 mm
1/2" BSP, G 1/2, R 1/2
~21.0 mm
3/4" BSP, G 3/4, R 3/4
~26.5 mm

BSPP means BSP parallel (P for parallel), which is the default for BSP (AKA G).  BSPT (AKA R) means that the male version of the thread has an outside diameter which is tapered (T for taper), increasing from the size given above slowly along the length of the thread; this means that a seal is formed purely by the mating of the threads themselves (with some PTFE tape), no shoulder on the end of the pipe with a gasket/washer etc. is required to form an air-tight joint.  The female version of this thread is just BSPP (AKA G) but can be denoted Rp, presumably to indicate that there is no reliable shoulder to butt against; the mating of the threads must make the seal.

Some Numbers

My requirements were:
  1. be as silent as the Jun-Air 6-25 machine (i.e. 45 dBa), since it is run in a loft workshop,
  2. be able to deliver more air than my existing Jun-Air 6-25 machine and for roughly two minutes; the motor on the 8 bar, 25 litre, Jun-Air 6-25 machine would normally kick-in within 30 seconds in my application and is not meant to run more than 50% of the time,
  3. be of a vertical rather than horizontal construction; I have very little space.
To put better numbers on the air delivery, since that is pretty crucial, the spec for the Jun-Air 6-25 says it has a "displacement" of 50 litres/minute and a "FAD" of 32 litres/minute at 8 bar.  The displacement number is apparently an internal thing, the number to conjure with is FAD or Free Air Displacement.  So 32 litres/minute of air ends up in the 25 litre reservoir built into the Jun-Air machine, hence it should take less than a minute to fill up.

I wanted roughly four times my existing capacity, so that implied a reservoir of 100 litres, and I'd like a slightly higher pressure, let's say 10 bar.

There are lots of videos out there on the internet of people taking any old reservoir tank, e.g. one from the air braking system on a lorry, scraping off the rust and bolting a refrigerator motor/pump to it.  The very useful MIG welding forum has an FAQ which links to a very useful article in the web archive where it says:

Remember, when you're sizing your compressor, that 1 HP [746 Watt] of compressor output generates about 4 CFM [113 litres/minute] of compressed air at about 90 PSI [6.2 bar]. This is the accepted wisdom for air compressors over 10 HP in size. For smaller compressors, those under 10 HP, you must rely on the figures provided by the manufacturer or guestimate that you're getting around 2-3 CFM [57-85 litres/minute] of compressed air per HP @ 90 PSI.

Refrigerator or freezer motor/pumps don't exceed 10 Watts, so they would take a little while to fill a 100 litre tank.  To fill a 100 litre tank in a couple of minutes (hence allowing a continuous flow in a pinch), applying the above rule of thumb, my motor/pump would need to deliver around 50 litres/minute which suggested at least a 1 HP or ~750 Watt motor/pump.  Looking at the fixed-spec Jun-Air machines the motor/pump is 400 Watt and, for larger reservoirs all they do is add more motor/pumps, the 150 litre machine having six of them.  The problem for me with these machines was that the vertically-oriented ones maxed-out at a 25 litre reservoir, going horizontal for the 40 litre and 150 litre reservoir versions, so they didn't meet my physical spec.

Having checked around the internet, the only quiet motors available were those built into the Jun-Air or Bambi hobbyist/dental compressors; these have the additional advantage that the unit is a motor and a pump, two problems solved in one. I already had one Jun-Air compressor motor/pump.  And I found someone selling another Jun-Air machine with a leaky reservoir for parts on E-bay and bought it for 26.  That was enough to get me started.

Sketch Design

A compressor consists of a motor driving an air pump to deliver air into a reservoir tank (so that the motor doesn't have to run continuously).  The reservoir tank is fitted with an adjustable pressure switch that turns the motor on when it is at less than the desired pressure (with a little hysteresis).  There is a non-return valve in the pipe going from the pump to the reservoir tank, so that the pump can be switched off without air escaping, and the reservoir tank also has a pressure relief valve to prevent accidents.  Those are all the essential components.

I searched E-bay for a vertically-oriented 100 litre air tank and found the EXTREMELY useful [if you're in the UK] Context Pneumatic Supplies.  Not only were they able to supply the tank with fittings (for ~300), they were able to supply everything else I needed as well and were very patient with my voyage of discovery concerning compressor construction.  I sent them first the picture on the left.  Then I realised that I might want to amp the system up with more motors/pumps and so I sent them the one on the right, swapping the Y-connector for a manifold and adding a couple more non-return valves:

                  design More

As they only had one of the 100 litre tanks in stock I ordered that immediately while we discussed the above; it was on my doorstep within 24 hours.  It was a beast: definitely a two-person lift.  Holes/fittings as supplied with it were as follows:

100 litre air tank
Air tank fittings

Basically the hole in the top takes the 1/2" BSP, 11 bar, safety valve, the hole in the bottom takes the 1/2" BSP elbow and ball valve, the holes either side take the two 3/4" BSP ball valves and, of the holes in front, one takes the pressure gauge and the other is capped off (this is where the pressure switch will go).  I found that the threads in the tank were a little rough so I bought myself some BSP taps to tidy them up.  For some reason only a single fibre washer suitable for the 3/4" BSP ball valves was supplied so I also bought a set of assorted fibre washers.


In terms of bits that I could re-use I now had these two lovelies:


The one on the right, my old compressor, had spent its working life opening and closing a door somewhere and so had been fitted out for "continuous" operation with an automatic drain (first two pictures below), output filter (with 6 mm push-fit output) and drain bottle:

                  automatic drain, front
                  automatic drain, side
Jun-Air filter with
Jun-Air drain

Basically the mains-driven automatic drain valve opens for 0.3 seconds every up-to-30 minutes, with the intention of letting water that has been compressed out of the wet air leave the tank, while the filter removes any spray from the oil-immersed pump/motor; both drains empty through 4 mm push-fit piping into the bottle.  In my case the reason the tank had sprung a leak was likely because the end of the pipe attached to the automatic drain had clogged and hence the tank had rusted due to the collecting water.  All I needed was appropriate adapters to re-use all of this in the new build.

The thread on the automatic drain was nice normal 1/4" BSPT male but the thread on the inlet connector of the filter was very peculiar, I couldn't find a match for it in any standard (CGA, JIC, NPT, BSP, UNF, etc.); it had far too many threads per inch (> 25) for such a large major diameter:

Jun-Air filter

I asked on the ever-helpful compressor forum and they pointed out that there is a hex inside that bull-nosed connector and, applying some considerable force (a 7 mm hex, a vice and a hammer) I was able to remove it, revealing a 1/4" BSP female thread inside the top of the filter; much better.

Then there was the pressure switch: the one provided with a Jun-Air compressor, though set to 8 bar, was fully adjustable up to 16 bar and was rated at 240 Volts/20 Amps and 2.2 kWatts, easily within what I need for two motors @ 2.9 Amps each and likely OK for four since the effect of the additional load is to reduce the already large > 100,000 cycle operating life (and in any case I have a spare).  The connection to the tank was 1/4" BSPT male again, nice and simple.

Jun-Air pressure

And of course there were the two pumps/motors with their starter capacitors, very slightly different generations of the same model, with nice normal 1/8" BSP male connections:

                pumps/motors, front
                pumps/motors, back

Connecting Up

To connect everything up I ordered the following from a combination of Context Pneumatic Supplies and RS Online, cost altogether about the same as that of the tank:

Purpose Items
G 1/8 female x 4 inputs, G 1/4 female x 2 outputs.
G 1/4 male blanking cap for unused output.
Connect pumps/motors 1/8" BSP male output to manifold 1/8" BSP female inputs.
G 1/8 female to 4 mm push-fit adapter (4 off).
4 mm pneumatic tubing (~2 metres).
non return valves: 4 mm push-fit input, R 1/8 male output (4 off).
4 mm blanking plugs for the above.
Connect manifold 1/4" BSP female output to tank lower right 3/4" BSP female input.
G 1/4 male to 8 mm push-fit adapter.
8 mm pneumatic tubing (~1.5 metres).
8 mm push-fit to G 3/4 male adapter.
Connect tank front 3/8" BSP female output to pressure switch 1/4" BSPT male input.
R 3/8 male to G 1/4 female adapter (in multiple steps).
Connect tank bottom 1/2" BSP female output to automatic drain 1/4" BSPT male input.
G 1/2 male adapter to 4 mm push-fit (in multiple steps).
4 mm tubing (~0.5 metres).
4 mm push-fit adapter to G 1/4 female.
Connect tank upper left 3/4" BSP female output to filter 1/4" BSP female input.
G 3/4 male to R 1/4 male adapter (in multiple steps).
Connect 4 mm push-fit drains from automated drain and filter to collection bottle. 4 mm pneumatic tubing (~3 metres).
Connect filter 6 mm push-fit output to user tap.
6 mm pneumatic tubing (~10 metres).
User tap.
Nice mountable tap with pressure gauge and 6 mm push-fit adapter on either end.
User air connection.
Series 21 female quick-connect coupling.

To be continued...

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