What air compressor should I buy?

Actually, I don't want a specific recommendation (though I would be glad of some), but I wanted to check out what I might need.

My main confusion is CFM. Despite having read various posts and watch some You Tube videos, I am still not entirely clear if CFM is the cubic feet that go INTO the compressor (at one bar) or the cubic feet that go out of the compressor at (say) 10 bar. I am imagining that one is of the order of 10 times more than the other (9cfm out at 10 bar presumably means 90 in).

You will ask what I want it for? Well, apart from rounding of my collection of tools with which I lack any obvious skill, I have a number of ideas.

Most immediately I want to be able to see if my steam engine (2 x 2.5 inch cylinder) will run. I don't desire to run it at full pelt for half an hour, but it would be nice to know if it could turn over.

I'm also interested in small scale sandblasting at some point, a little bit of air for cleaning out holes and swarf in hard to get to places (suitable PPE will be employed) , perhaps some painting at some point in the future.

But as much as anything it's to provide a generally useful facility for the things I don't know what I want to do yet.

It MUST be quiet (ish) 60 - 70db seems a reasonable sweet spot. I'm thinking of something around 9 - 12 cfm, 24 or 50 litre tank and 9 / 10 bar. Of which there seems to be a wide range.

I did note the video which Jason posted recently, but was unable to read the brand on it. it sounded good (which is to say fairly quiet).

As usual and comments will be appreciated

Iain

Most cheap compressors make them sound better by quoting the displacement in CFM (cubic feet per minute) but what you get out is FAD ( Free Air Delivery) which is usually less by about 30-40%

There are many of teh "quiet" compressors that are basically the same just different colour and logo. KIEI from ManoMano looks the best buy

Thanks, Jason.

I can find ManoMano which looks like a good site and I can find a KIEI compressor on eBay (though the one I've found doesn't look that great), but I can't find a KIEI on ManoMano.

Any ideas?

Iain

Many folks say you need a large compressor for sand blasting but provided you don’t intend to do large parts or lots of them in one go you can get away with something more modest. I don’t think any of the silent compressors are man enough but haven’t tried it. I use a 2.5 HP 25 litre with my Clarke blasting cabinet. If you do a search you can download the manual for the Clarke sandblasting cabinet and I think they recommend a minimum size compressor.

Having built a few compressors in the past & doing a lot of car body work & painting, i know a little bit. My present one is a purchased Clarke 14CFM which has a 3HP single phase motor & a 150 litre tank charged to 150psi. This does most jobs.

So i have many spray guns, some of which are classed as hvlp. High velocity, low pressure. They are ok but my main goto is a Devilbis gti pro. These require around 9cfm to operate correctly at anything from 45psi up to around 90. I spray at 50 to 65psi. At this rate i could spray a whole car with my setup. OK so far.

Now look at a sand blaster cabinet & you need a huge volume of air if you are doing large jobs. The same with all my orbital sanders. The large sander is rated at 14cfm requirements. If i use this in anger the compressor never gets a rest & struggles to keep up.

My brother in law bought the wee Aldi compressor a few yrs ago with the intention of doing some sand blasting. That was a definate no no.. Not sure what Aldi said it was rated at but definately was no where near there claims.

It all depends on what you expect from it. If you are going to do short bursts of blasting with the DIY kit or paint small items. Or running your engine then i think a small one would suffice.

Obviously the bigger the better with up to what a single phase unit can give , approx 3hp. Then that would be your ideal. Mine is brilliant for what i need but is pretty noisy in operation.

I have heard say that the hydro vane types can be quieter than the piston versions.

Unfortunately they are not cheap item on the shopping list.

I think the Clarke i have is now around £450. I think when i bought it some years ago it was £375.

Steve.

Edited By Steviegtr on 03/11/2020 23:21:06

Iain

You need to be very careful when considering any of the inexpensive import compressors as many of the specifications are contradictory. Often flat out impossible.

The smallest quiet compressor that I know to be capable of running a proper sandblaster is the Hydrovane 502. I have one on my Guyson blast cabinet. It is capable of keeping up with the blast gun demand at 110 -120 psi.using the smallest size nozzle. I'd prefer a bigger nozzle to work faster but it gets the job done.

Hydrovane 502 specifications are 3 Hp / 2.2 kW motor, 8 cfm air delivery at 140 psi / 10 bar. Sound level 69 dBA, probably at 1 metre. At lower pressure I get a little more air. Pretty typical numbers for a proper industrial compressor. My experience is that Hydovanes seem to give a little more air in practice than conventional piston compressors of the same rating. The 502 replaced a Vee twin rated at 9 cfm which occasionally struggled a touch on the blast cabinet.

Sanity check figures for evaluating compressor specifications are:-

4.5 cfm per Hp at 100 psi / 7 bar output, input air draw about 6 to 6.5 cfm

3.5 cfm per Hp at 140 psi / 10 bar output, input air draw about 7 to 8 cfm

However specifications referring to input air draw will be unreliable as such are usually used to make an inefficient compressor with poor delivery at the rated output preesure look better than it is.

Sanity check for single phase motor current draw:-

4.3 amps per Hp, 5.5 amps per kw on single phase.

If the numbers on the specification are vastly out of line odds are someone is telling porkies.

Saw one of those twin head diaphragm jobbies rated at 9.6 cfm, maximum pressure 116 psi, 3.5 Hp motor, current draw 5 amps. Does not compute.

Diaphragm compressors tend to be rather inefficient at higher pressures delivering significantly less air than would be expected from the input volume. Especially the inexpensive ones where great internal sophistication cannot be afforded. An inherent characteristic of the breed as the very short stroke combined with diaphragm curvature makes it very difficult to minimise residual volume at the end of the compression cycle.

Screw types are most efficient at converting input air to compressed air at decently high pressures due to the continuous flow. Hydrovanes and the other vane types are almost as good but there is some pulsation in the flow. Then come piston types with diaphragm ones bringing up the rear. No great surprise to discover that efficiency follows price! Piston compressors are a very broad church with considerable variation in efficiency, capbility and price.

Compressor selection is a minefield really. Even the respected names are badging up made down to a price units whose often short lives aren't doing their reputation any good in the lower end of the industrial market.

Clive

Edited By Clive Foster on 03/11/2020 23:24:00

Edited By Clive Foster on 03/11/2020 23:24:49

Edited By Clive Foster on 03/11/2020 23:25:46

Edited By Clive Foster on 03/11/2020 23:26:20

Oddly. Now I've found them...

Iain

I suppose it depends what you call "small scale" sand blasting. This casting is about 9" long

After a couple of mins with a very basic Lidl blaster powered by a 5CFM input compressor with 12lts tank. I'd call that small scale and OK for the odd old model casting or small fabrication.

Taking your model that has a displacement of 0.245lts so ticking over at say 60rpm and a few psi will need 14lts or half a cubic foot. I did find some output figures for these compressors but only the single motor ones and they give an output of 95lts/min @ 1bar and 70lts/min at 4bar so hopefully the 14lts engine will be OK unless under high loadings.

I'm more interested in its use for a fogbuster where I want something quieter than my current 94db one as thet currently would need to run almost non stop. It will also cope with all my engines and no doubt be better for my small blasting needs. My current one is only a cheap basic Machine Mart one but still going good after 30yrs so at £2 a year I'm happy with that.

Edited By JasonB on 04/11/2020 07:48:52

The Binks-Bullows Hydrovane compressors, once used to run continuous air flow sprayguns, are an excellent quiet running source of moderate pressure air. Come in 30 psi and 40 psi varieties. Industrial quality 1/3 rd hp motor and tank level build quality. I imagine they deliver around 2 or 3 cfm. Mine is just the job for running the Bjur Spraymist units on my lathes and mill. One day I'll find 2 more so I don't have to share! Typically £40 to £60 on the used stuff market and, apparently, impossible to wear out in a normal lifetime. Unless you run it dry of oil.

Hydrovane also made a boxed unit for the pub and hospitality industries. Higher pressure output, may be 1 or 1.5 hp motor in a box around 18" cube. I've never seen one in the flesh but it looks to be a potentially useful device.

Sandblasters, like compressors, come in a wide variety of sizes and abilities.

For folk like us the distinction is between the smaller, lighter duty ones most suitable for surface preparation before painting or heavy duty cleaning and those able to shift substantial corrosion, strongly adhered paint et al. Realistically something in the "overlap" range is where most model engineers will be shopping.

This little chap is basically a Badger airbrush on steroids. Air consumption is minimal and it will do a nice job of putting a matt finish on shiny metal ready for painting. Good luck with shifting anything much more than a felt tip pen mark or (very) poorly adhered paint tho'.

I'd regard Jasons unit as being around the top end of the paint preparation range devices. As Jasons pictures shows it will quickly shift modest surface corrosion and similar. Dealing with deeper corrosion and such like, as in vehicle restoration, is really above its pay grade. Occasional use on small parts maybe but it will be very slow.

If you need to shift deeper corrosion et al with a limited air supply the spot blast guns with a sleeve surrounding the nozzle that pushes against the part to constrain air, and media, distribution are pretty effective. Takes a while to do a reasonable size part in 1/2" diameter spots but they do use less air and are less messy. The nozzles tend not to last well and finding replacements can be difficult. I have one for spot work on big stuff.

My Guyson cabinet is bottom of the pro range. It will cope with most things but would go much better with a larger compressor and nozzle. Tends to be overkill for surface preparation.

As ever its a matter of finding the best balance between what you want to do, what you can accommodate and what you can afford. The latter usually rules.

A Guyson style cabinet is too big for most folks. My "dirty" room where all the grinding and messy prep work happens is a little over 8 ft square, bigger than a fair few folks workshops. Crammed tight. The compressor lives in its own hutch outside.

I have maybe £800 - £900 tied up in blast cabinet, dust collector and hydrovane plus the other essential bits. Generally that sort of money needs serious justification for normal guy or gal in the woikshop.

Clive

Who thinks normal is boring but tends to take things to the extreme!

It's relatively simple in fact.

You want quiet then its either belt drive or Hydrovane. Hydrovane's are a risky 2nd hand buy.

You want 9cfm FAD then it's a 3HP 14 cfm that you need. A £20 suction blast gun will run on 2hp for small jobs if you reduce the size of the pick up with a bit of old hose. BUT 2hp are all direct drive and run a double the speed with treble the noise.

"You want quiet then its either belt drive or Hydrovane. "

My Bambi is most definitely quieter than anything else I have heard in action.

rgds

Bill

A reasonably modern Hydrovane in clearly un-abused condition at a fair price is hardly a risky second hand buy. Reasonably modern tank mounted units in sizes we are likely to use means 501, 502 or HV01, HV02. Maybe a 15 in "one lady owner, only driven to church on Sundays" condition. But the 15 is getting pretty old now. Basically round, painted, air end style. Fair price is maybe £400 and up. Air controlled solenoid valves and motor coupler are the most likely issues. Relatively easy to sort but rarely reflected in the price. Look for a better one.

Three phase 501/HV01 compressors are often rebuilt into single phase 502/HV2 versions of double the output by fitting a 2,800 rpm motor instead of the 1,400 rpm one. Safe enough as its basically what the factory do but if this has been done its clear evidence that someone has been fiddling. At pushing £200 for a motor you have to wonder at the economics of the job and condition of the donor air end. £600 plus from a reliable supplier or walk.

You don't get owt for nowt and bargain shopping around the elderly, sort of oval shape, unpainted aluminium body Hydrovanes is basically asking for serious grief. Parts pretty much can't be got and reassembling one is the sort of jigasw puzzle not to be attempted by the untrained.

Three phase variants run fine on a decent brand inverter as supplied by the likes of Inverter Drive Supermarket. Following their advice I've set 502's up on Schnieder and Eaton DE inverters with very satisfactory results. One in a cold barn needed the high torque setting to cope with sub freezing conditions tho'. Which suggests that a low price no name import might not quite be up to the job mid winter. Compressors in general need the torque boost setting on inverters unless the inverter is oversized.

Back in the day folk often built quite serviceable compressors from industrial refrigerator or or lorry brake units. Run at around 500 rpm by a suitably husky and sat on an old calor gas cylinder. Yup, I did too. Unearthing a lorry brake compressor from a dark corner I ran the numbers to turn it into a compessor unit to sit in the garage for very occasional air supply. Even though I have plenty of bits lying around the economics didn't add up. So I got a cheapy and ear-plugs!

Clive

Edited By Clive Foster on 04/11/2020 13:11:24

Edited By Clive Foster on 04/11/2020 13:25:40

Edited By Clive Foster on 04/11/2020 13:25:59

Agreed, but 9 cfm FAD as well suprises me.

Why are belt drive quieter Dave?

The bigger pump runs at less than motor RPM. They get their 9cfm (Displacement) by running the pump @ 3000rpm which is why the life span is quite short.

Thanks for the input, guys.

The KIEI one mentioned by Jason looks like a sweet spot. The noise is tolerable (I have neighbours and a wife who stresses).

The sandblasting type requirements I can imagine would be covered by this and the cheapy type blaster from what I can see. Cleaning off some rust or paint sounds about the limit for me anyway - and the jump up to more industrial would be significant. Still I think some kind of home made cabinet would be in order.

I take it that the connectors are reasonably standard so if I buy this compressor and then some tools from Aldi or Lidl, they will 'just work'. I know it sounds unlikely, but one can hope!

Iain

The common small push in quick detachable connectors come in two styles.

PCL and PCS / Euro.

PCL has always been the usual "over the counter at the shop" style in Britain but PCS / Euro has become steadily more common in recent years on inexpensive import compressors. Anything from Aldi / LiDL will be PCS / Euro of the lower end import breed. Prices are remarkably good but quality does vary. You may get leaks when mixing and matching from different batches. Usually OK but not always.

Historically PCS / Euro was always more expensive for good quality units as being considered harder to make leak-proof and less resistant to abuse. Was generally reserved for applications needing its bigger bore for more air throughput.

PCS / Euro is harder to mate than PCL under pressure.

Both styles can be got with a range of threaded connectors. You will almost certainly be requiring 1/4 BSP. Male and female in both taper and parallel can be found. Although the LiDL / Alidi connectors and bits kits are good starter sets expect to visit the local plumbers merchant, Screwfix et al for "just one more" if you get ambitious.

Screw joints will almost invariably be 1/4 BSP at the sizes you are considering. Taper or parallel at the makers whim.

At the affordable import end of the market threads that screw into cast alloy handles and the like where metal volume is restricted may well be oddball. Generally if you have a male / male adapter screwed into handle the visible thread will be 1/4" BSP parallel and the hidden one going into the handle quite likely something odd. So don't unscrew it. Same can apply to a basic hose tail. Try not to break them. I had major grief on a fix-it job where the tail had been broken, the remains thrown away after extraction and Mr Optimist appeared at my workshop door expecting me to have one in stock! "its not BSP" quoth he.

The inexpensive extending hose reels use a simple plastic tube style hose rather than the proper stuff. OK but its vulnerable and not super durable. Save up for the proper re-enforced stuff. Usually the manual wind up drums from Aldi / LiDL have proper but not always.

If blasting a cabinet is pretty much essential if sand isn't to go everywhere. Make sure it seals well and, if you haven't got an extractor expect to spend half your time waiting for the dust to settle.

Clive

The compressors from the fae east are Ok mine must be 30 years old,make sure you get a big tank,also make sure the compressor stands level as the lube oii capacity is limited,dont use on a driveway with a steep slope to inflate car tyres,first time my neigbour did it,I managed to to free off the big end which just nipped up,second time he did it the bearing seized solid and scrapped. A friend who started reconditioning motor parts as a business started with A hydrovane and Guyson shotblast cabinet cabinet and expanded to a large walk in unit,and he forever suffered from air suply problems ,he even linked two really serious compressors together,if a shot blast set specifys that it uses x amount of air then double or treble the compressor size.specifiedI think the Hydrovane was popular at one time as it was supposed to not require an air tank,so getting rid of insurance and inspection problems, but my friends Hydrovane could not really keep up with the demand of the Guyson cabinet,increasing the suply pipe diameter using large dia steel water pipe did give the effect of a resevoir which helped a bit.

Iain, I bought a New 3hp 14cfm ABAC (a better air compressor) 20 years ago as a teenager, it's been a fantastic compressor in general, I think they're made in Italy, it was quite reasonable at the time, 4 hundred and something, definitely worth a look at them, not sure what their prices are like now.

For airline couplers, try screwfix, the euro type quick release ones they sell are made here in the UK, they are very reasonable priced, and are the best I've used, they don't hiss when not in use but still connected.

60 to 70 db is very quiet! A supposedly quiet room is at about those levels.

(The decibel scale is logarithmic, not linear. So an increase of 3 db means that the noise level has doubled )

You may have problems finding any compressor that quiet, let alone one delivering 9 CFM No doubt such machines are available, but at a price that precludes hobby or even light industrial use.

A belt driven compressor will be quieter than a direct drive one, not that I can think of a direct drive reciprocating unit that delivers 9 CFM.

If you must have noise levels of 60 -70 db, include building an anechoic chamber for it in the budget.

Howard