3HP and 2.5HP have same CFM

[v8burbling]

Hi there,
My first post on Mig (been reading for years though !)
Buying a compressor to run a plasma cutter and hopefully air tools.

Have found two 50litre models, one is 2.5HP motor, the other is 3HP but they both say around 7.3CFM.
Is it worth paying the extra for the 3HP (about an extra £15) if it still gives out the same CFM ?
Am I missing something ?

Second question is do I need to be careful with anything else like hose fittings or any other details ?

Thanks.

 

[rtbcomp]

It's all in the definition of CFM (Cubic Feet per Minute). A given mass of air will occupy less volume the higher its pressure, so a CFM figure on its own doesn't tell us much.

Personally I'd pay the extra £15, even if they pump the same amount of air the bigger motor will be less stressed and will start more easily on cold days.

 

[v8burbling]

Good point. You need the PSI too, right ? They both seem to be around 115psi. Good point on the motor stress though.
Perhaps they just rounded the motor HP up a bit ha ha !
Thanks.

 

[Paul G]

7.3 isn't a lot.

I have one about that size and it will just about tackle 4 wheel nuts before I have to stop and let it catch up.

I also have an air nibbler (the toenail clippings type). It takes three attempts to cut a metre wide ally sheet in half.


Paul G

 

[v8burbling]

Hi Paul,
What size tank do you have with your 7.3CFM unit? The one I am looking at is 50litre, which is says is just about enough for most air tools. Also found a handy table on Machine Marts webpages:
https://www.machinemart.co.uk/pages/bgaircompressors

They assume you will only be using a tool for 25% of the time though.

My understanding was that pump usually can't suppy all the air demand so the tank is like a reservoir to help with the demand. What I'm not sure about is, does the pump run when you demand air and the tank supply is diminishing or does it wait until the tank is nearly out before it kicks in ?

 

[Fintray]

It will kick in when the air pressure in the tank drops to say 80psi.

 

[Kent]

I am no expert on air stuff by any means but take care you need CFM actually delivered at the using end (I fell into that trap myself)

 

[MoreWellie]

Another consideration might be that 3Hp are usually recommended to be on a 16amp circuit whereas I think 2.5HP is usually ok on a 13amp plug

that said my 3Hp is on a 13amp plug but I did go through a few fuses before I found one that lasted and no, it is not a nail!

 

[rtbcomp]

Have you got links to the compressors in your original post?

There may be other considerations apart from cfm.

 

[v8burbling]

Ha ha. It shouldn't be drawing anything near 13A though, that's around 3kW. The 3HP I am looking at is around 1.5kW.
However, motors can take up to 6 times their current on start up so there may be a peak of well over 13A.
It is a common misconception that a fuse rating is the current that it won't allow through, however it is more like the continous rating. So a 13A fuse will blow at 13A but only after a certain time and it could well flow 20A or more for a very short time without blowing. The actual thing that blows a fuse is an amount of energy (I2t) where I is the current and t is the time. So what you need is a fuse that is designed to take short peaks without blowing. These are usually referred to a slow-blow fuses. The alternative is fit a 16A plug and distribution socket.

 

[v8burbling]

These are the two I was looking at:
3HP unit
http://www.ebay.co.uk/itm/131406495613?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

and
2.5HP unit
http://www.ebay.co.uk/itm/111576570869?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

 

[Paul G]

Hi Paul,
What size tank do you have with your 7.3CFM unit? The one I am looking at is 50litre, which is says is just about enough for most air tools. Also found a handy table on Machine Marts webpages:
https://www.machinemart.co.uk/pages/bgaircompressors

They assume you will only be using a tool for 25% of the time though.

My understanding was that pump usually can't suppy all the air demand so the tank is like a reservoir to help with the demand. What I'm not sure about is, does the pump run when you demand air and the tank supply is diminishing or does it wait until the tank is nearly out before it kicks in ?

The compressor is mounted on a 25L tank, but I have an additional 50L tank that I can utilise at the flick of a ball valve.

Thing is though, I don't gain anything. It takes about a minute to fill the little tank ( 125psi) and it takes about 3 minutes
to fill both, so to put it another way, I undo 4 wheel nuts per minute, or wait 3 minutes then undo 12.

It's not the size of the tank that matters IMHO, it's that all important CFM.

Cheers
Paul G

 

[skotl]

Hi there,
My first post on Mig (been reading for years though !)
Buying a compressor to run a plasma cutter and hopefully air tools.

Have found two 50litre models, one is 2.5HP motor, the other is 3HP but they both say around 7.3CFM.
Is it worth paying the extra for the 3HP (about an extra £15) if it still gives out the same CFM ?
Am I missing something ?

Second question is do I need to be careful with anything else like hose fittings or any other details ?

Thanks.

See this thread for a similar question re the small compressors vs slightly larger (11cfm / 100L) units (disregard the first 1/2 page which is gibberish about me upgrading a wee compressor).

I had bought one similar to those that you posted links to, for the same requirements; plasma cutting and some air tools.

To be brutally frank, I wasted my money. For plasma cutting, it was kicking in every 30 seconds and was very, very noisy. And it was pretty crap with the air tools - wouldn't even turn the wheel nuts with my impact driver.
You asked about hose fittings and the single most important accessory to buy is 10mm hose, not the 8mm that a lot of these things come with.
Even with a 10mm hose, though, I should have saved my money and bought a better / bigger belt-driven compressor (which I ended up doing).

It's quieter, comes on far less often (due to the 100L tank) and has a lot more oomph than the little one.

I'd save up a bit more...

 

[v8burbling]

Hi Skotl,
That was a really useful link thanks. I think I'm convinced that the 100litre is probably what I should be buying but it's an extra £150 plus I'm going to be working quite a way from the nearest plug scocket and it sounds like the higher power motors with the 100litre units won't like being fed through extension leads. So I'd probably have to get a longer (8-10m) air hose, which would probably just kill a lot of the CFM anyhow wouldn't it.
Wondering whether to put up with a 50litre for now and then get a 100litre when I get a proper workshop/garage built.

 

[rtbcomp]

............
Wondering whether to put up with a 50litre for now and then get a 100litre when I get a proper workshop/garage built.

Sounds like a good plan - you can always use them both together for even more air.

 

[v8burbling]

Two last questions;
1. Someone mentioned an air-driven nibbler. How do they compare to a plasma cutter ? Would it be worth spending more on the compressor and getting a decent nibbler and not buy a plasma cutter ?
I am doing car body restoration and need to cut out rusted metal and cut patches to repair the panels. Metal will be up to around 3mm thickness.

2. What is "oil-free" and is it important / useful. Presume it is for paint spraying ? The compressors I've been looking at don't state which they are so presumably they are not oil free ?

 

[the snooper]

nibblers cut out strips of steel, electric metal shears I think are better, 3mm on car panels that's quite thick

 

[Maker]

The link for the 3hp one says "input power: 1500W", 1.5 kW is 2 hp, not 3. I don't know how the output power has been calculated but that's closer to 1 1/4HP. I don't think I'd bother with one of these.

 

[v8burbling]

nibblers cut out strips of steel, electric metal shears I think are better, 3mm on car panels that's quite thick

Yes, it means chassis work unfortunately !

 

[v8burbling]

The link for the 3hp one says "input power: 1500W", 1.5 kW is 2 hp, not 3. I don't know how the output power has been calculated but that's closer to 1 1/4HP. I don't think I'd bother with one of these.

That's interesting. Funnily enough I suspected the same so emailed the seller asking for a pic of the spec plate on the machine. Good it's not just me thinking that then. Thanks.