Air compressor

[Fen]

Firstly, this is off topic. I realise it's the 944 section and there is probably a more suitable area, but I do't go out of the 944 area much... Feel free to ignore the rest.

I want to buy an air compressor for the garage. I've found some info on the interweb that is intended to help choose the right type but most of it is just common sense stuff to do with SCFM and where they try to advise on things like tank size or operating pressure they are either very non-committal or even contradictory.

I'd appreciate the advice of anyone who knows much about them then. My wish list is that it will be electric (single phase) and that I can use it for general tyre inflation plus some air tools. I would like to be able to use it to spray paint, or run a blasting cabinet, but only at an occasional and "amature" level. Of course I don't want to splurge a fortune on it, nor even a reasonable amount one one that is more than I need.

I'm not clear what volume it needs to shift, what pressure it needs to reach, whether I want a big or small tank (nor even what constitutes a big or small tank for that matter), if it should be belt or direct drive, duty cycle, HP etc. Anyone able to offer advice?

 

[Guest]

A friend runs his garage with one of these ... http://www.abac-support.co.uk/individualmodels/Beltdrive/0036HS526,%20B3914_150S.htm More than capable of handling all the air tools and gear. He has it mounted solid and has the air supply mounted on the wall evry 6ft or so ... Really handy...! Dont know if this is much help to you....

 

[zcacogp]

Fen,

I can offer no words of wisdom derived from experience, but the one thing that sticks in my mind from hearing people discussing them is that you cannot over-spec an air compressor. Work out what spec will meet your needs and double it. Then buy that.

I have been offered compressors by two friends who have equipped their workshops with them, and found that they simply aren't man enough for the job. (I declined both, because I have never had a workshop big enough to use compressed air.)


Oli.

 

[Dave Wilkinson]

Fen,

As has already been mentioned, much has to do with your intended use. The size of the tank determines the capacity of the compressor to sustain the air flow. The smaller the tank the more often the compressor needs to run in order to top up the reserve. If you are just running a single outlet (as opposed to a workshop with multiple outlets) a 25 litre tank will probably be sufficient but you need to be careful with the flow capability.

Most of the small 25 litre compressors (look here for some examples from SIP) will be OK for most things - with the exception of the impact wrench used for wheel nuts. I've got a 24 litre SIP Airmate and it's fine for tyre inflation, small air wrenches and spraying - but it won't handle an impact wrench. Most air tools will have in their spec the minimum air flow required to operate them - usually in cf/m - and you'll find that the minimum needed for an impact wrench is usually marginally below that supported by the small 24/25 litre compressors.

The real problem is that the smaller compressors are available quite cheaply (I bought mine from Costco for less than £90) but as soon as you go up the the 50 litre versions, the price starts to escalate massively. It just depends on how much you can justify for what you can achieve.

You'll find that most of the smaller units are electrically driven, direct drive - it's only the more powerful units that introduce belt drive or petrol driven motors.

One other small point - if you plan on using the compressor for spraying anything where you want a decent finish plan on getting a switchable oiler/oil filter so you can use the tools than need the oil in the air flow and have the facility to switch off the oil when you spray.

Good luck with the search.

Regards

Dave

 

[Fen]

Thanks everyone. All that is pretty much what I expected but I found something on line that suggested a big tank is a disadvantage with too small a pump; the logic being that once you draw the pressure in the big tank down and the motor starts it will take it a long time to bring the large volume back up to a suitable pressure, and that for short burst type usage a small tank is better. I follow the logic, but it's contrary to what I had thought was good sense.

I also found some info suggesting that electric compressors running on single phase don't always make the theoretical HP figure quoted as they ssimply can't get enough power. Admittedly that was a US article and they are hamstrung with a 110V 15A supply there where here we have 240V 10A so a little more oomph available (about 3.2 HP I think).

My intention is that maybe I'd have a line running around the wall with multiple outlets, and possibly I'd have a cabinet for blasting as well, but I'd only ever run one tool at a time as I'd be working alone almost all the time. I'd be looking for as good a finish as I could get when spraying, but realisitcally I'd give it to the pros is I wanted a top job and I'd probably only take on non-critical stuff like wheels or perhaps the spare wheel carried of the Jeep which needs doing now. I'd like to be able to run an impact wrench though.

I'd rather spend a reasonable amount on one that does the job than a cheapie that isn't going to do everything for me, but as noted they do start to ramp up when you get out of the real cheapo end of the market so paying for more than I need could get expensive quite quickly. I've found a 5.5HP petrol one for about £350 which might be a good option as, although I have power in the garage I can't get 5.5HP out of it and there's always petrol around for the mower / chainsaw / strimmer etc., though I'd have to run something to outside for the exhaust. It's 50 litre and delivers 8.1 CFM at 90 psi (15CFM free air). Is that enough for an impact driver?

 

[Dave Wilkinson]

ORIGINAL: Fen
It's 50 litre and delivers 8.1 CFM at 90 psi (15CFM free air). Is that enough for an impact driver?

I think this is where it gets difficult. I had a look at the SIP site (http://www.sip-group.com) and their impact drivers quote a required airflow of 4 - 4.5cfm. Their small 25 litre compressor supposedly delivers 9cfm but I just have my doubts, based on my own experience, that it would work. I think the only way you are going to get any comfort here is to buy both the compressor AND the impact wrench from the same source on the understanding that one will drive the other - and complain if it doesn't!!

Regards

Dave

 

[Fen]

I had a look at an impact driver today, which claimed to need 6.8CFM (I think it was) at 90psi. The petrol compressor I saw should be able to drive that, but there isn't much margin for error. Certainly the smaller electric ones they had in the shop were only capable of 6 or 7 CFM free air so they wouldn't manage to keep up. Tricky this...

 

[Hilux]

Fen, go for at least a 50 litre tank and look to need 8CFPM for most tools.

I`m going over to air tools with my next build and running 2" pipe from the compressor to above where I work with 1/4" take offs as the pipe itself also acts as a reservoir. If I run out of puff I`ll plumb in a couple of pub Co2 bottles in line to increase the capacity.

A small tank empties quicker so will give the CFPM but not for long hence the recommendation above.

If you do start to use air tools more get an in line oiler too.

 

[Fen]

Thanks Paul, definitive numbers is good. However...

8CFM - is that free air, or at a working pressure? 90psi seems like a good point for working pressure (is that the pressure component of SCFM with the temp being 65 Fahrenheit?). Any idea how much HP is required to produce 8CFM at 90psi? Surely that's a calculable value if someone knows the formula (not including losses of course). Armed with that info in addition to your recommendations above I could work out if a single phase electric type is capable of producing the numbers I need, then convert to l/min as some sold here are metric, and start looking in earnest for a compressor.