New compressor guidance...

[Davek0974]

When i finally get to take my cnc plasma table home from the day job, I am pretty certain I will be needing a new compressor. The one I have has been tested by using a known orifice size, a pressure gauge and some maths and it can **just** about maintain 80psi at around 3.5cfm usage, its a 2Hp 50ltr very noisy single stage unit.

The 30XP plasma cutter needs 3.5cfm of stable 80psi air pressure so this unit is going to be running 100% while i am cutting, I expect this is not good. I might need a little more if I move up to the PM45 plasma unit later.

Are there any suggestions or things to look for, I reckon 3Hp is my limit, what about tank size? Single or twin stage?

This will be a budget buy but there seem to be 1000's of them out there and the prices all seem in line and pretty good.

It's only a home shop so will only be running a couple of times a week.

The 3Hp limit is due to having limited electricity capacity.

[justanengineer]

JMO, but I'd start off looking at your available space to determine what you can fit for a tank. Then I'd look at your available power and determine your max motor hp. I'd then buy bigger in both areas and find a way to make it work.......jk......kinda. :P

Seriously tho, not sure whats available overseas. The standard for compressors here is Quincy and the higher end of several other brands that have full pressure lubricated compressor heads (pumps). If you arent aware, that means instead of the pump being simply splash lubricated like a lawnmower engine, it has an oil pump which makes the head last virtually forever. My current Quincy is 60+ years old but still going strong, and is easily rebuildable when the time finally comes. I would suggest finding an industrial quality pump that is rated on the low end for the hp you want (can always upgrade the motor if you get more power later), the biggest tank you can fit, and designing your own system. I'm not a fan of any of the box-store homeowner grade crap compressors, theyve got ridiculously thin tanks and crap heads, not to mention theyre deafening. Industrial compressors measured in real horsepower (not the Chinese marketing variety) are normally VERY quiet comparatively.

[ironmonger]

When i finally get to take my cnc plasma table home from the day job, I am pretty certain I will be needing a new compressor. The one I have has been tested by using a known orifice size, a pressure gauge and some maths and it can **just** about maintain 80psi at around 3.5cfm usage, its a 2Hp 50ltr very noisy single stage unit.

The 30XP plasma cutter needs 3.5cfm of stable 80psi air pressure so this unit is going to be running 100% while i am cutting, I expect this is not good. I might need a little more if I move up to the PM45 plasma unit later.

Are there any suggestions or things to look for, I reckon 3Hp is my limit, what about tank size? Single or twin stage?

This will be a budget buy but there seem to be 1000's of them out there and the prices all seem in line and pretty good.

It's only a home shop so will only be running a couple of times a week.

The 3Hp limit is due to having limited electricity capacity.

Even if you are in a home shop, running out of air in the middle of a project would be no fun...

When you are sizing pumps or compressors for continuous flow, the tank size is irrelevant. The compressor needs to be able to supply the CFM at pressure without the tank as a consideration. An additional consideration is the filter, dryer system that you need. Both of them will have some pressure drop, and that needs to be factored in. If you use the compressor to raise and lower the tank water level the air tank should be sized appropriately for that massive air volume as compared to the cutting flow. If your expected cutting operation can be carried out in a short enough period so as to not exhaust the tank, the storage can help, but it sure doesn’t take long to knock a tank down to 85 psi or so, and any storage below the required delivered air pressure is not helpful.

A belt driven, lower RPM head will last longer, and feeding it clean dry air in the first place helps as well. I don't know your your physical layout, but I have located my compressor in my basement, and piped the air out to my attached garage where most of the air is used. My compressor is 30 years old (I hope it's not listening... I don't want to buy a replacement just yet...) and gets an annual oil change, and while it's not flogged as heavily as yours might be, it breaths clean indoor air that is air conditioned in the summer. Feeding it clean dry air does not mean that the air compressor need to be located in the house, only the air intake.

paul

[Davek0974]

Ok thanks, my situation is a concrete slab double garage at the side of the garden, it's not damp and kept above dew point in winter, machines never rust so I must be doing something right

Just found the manual and the little noisemaker I have is a 2Hp 1500W lump of crap that is rated 7.5cfm but has no FAD figure and I think that's the important one?

Quality Second hand units are pretty thin over here as most people shy away from them plus I have little transport options to go get a big one.

I have a maximum 32A to use and the plasma needs 13 of those, plus the lights etc so I think 2 to 2-1/2kw is a pretty good limit.

I'll start looking for a twin stage belt drive unit.

[EVguru]

My main compressor is a Fiac 3hp 100 liter unit that I've had for over 20 years now. When I was building my machine shop, Aldi were having their air tools special and I took my friend who helped me build the workshop down to buy a compressor. Actually, I bought the last two 24 litre 2.5Hp units there and kept the second as a smaller portable unit. Mine only gets used occasionally, but my friend has used his a lot. They are supplied to Aldi by Wolf Air and come with a three year warantee.

http://www.ukhs.tv/Workshop/Air-Compressors/Dakota-14-CFM-Air-Compressor

[Davek0974]

Thanks for that,

We have Abac at work and they seem good, there is a small end to their range and my plasma supplier recommends the B312-60 unit as being capable of running the hypertherm 30xp I have or the larger Hypertherm pm45 plasma.

Best price I can find is £474 inc vat delivered slightly more than the unit you linked to so I'm still comparing figures, it seems FAD figures are a trade secret ! The CFM figures quoted mean bugger all, my crap little unit is 7.9cfm but in real life is probably only 3cfm at running pressure.

[radkins]

When you are sizing pumps or compressors for continuous flow, the tank size is irrelevant. The compressor needs to be able to supply the CFM at pressure without the tank as a consideration.l

Exactly right but the opposite is all to often thought to be the case! By far the worst mistake most folks make when selecting a compressor is making a bee-line straight to the biggest tank in the store, after all a huge tank just looks like a huge compressor but unfortunately a bigger tank does NOT make a bigger compressor! CFM is what matters for how well a compressor will keep up and if the tool used requires more CFM than the pump is producing then not a bigger tank, higher HP rating or anything else will make up for it! The manufacturers are well aware of the common "bigger is better" misconception concerning tank selection for a compressor and they play this to the max by mounting anemic pumps with under-powered but over-rated motors onto uselessly over-sized tanks to create the illusion of a big compressor, the unsuspecting buyer gets duped into buying what is a huge LOOKING compressor that will barely make enough air to blow his hat off!

A big tank may or may not be beneficial but either way it won't help keep up with higher demand, whether it's beneficial or not depends largely on the CFM rating and expected demand, basically it has more to do with duty cycle and the larger vs smaller tank mostly means either shorter and more frequent on/off cycles or longer but less frequent cycles with the actual over-all run vs recharge times being exactly the same (with all else being equal). There are some long term advantages for the larger tank such as small power savings and slightly less wear due to the less frequent high torque start-up cycles but over-all demand determines the significance of this. Theoretically during any given work period (past the first start cycle anyway) the available run time vs wait time to recharge will be exactly the same regardless of tank size, the tank obviously can not produce a puff of air more than is put into it. "More reserve air" is a common term used when someone recommends a larger tank but that too is irrelevant, if the tool is exceeding a compressor's abilities and the user is running out of air at an annoying rate then the difference in run time between an 80 gallon tank vs a 60 is going to be mere seconds and then even that will be lost to the proportionally longer recharge time for the bigger tank, nothing is gained.

[radkins]

My current Quincy is 60+ years old but still going strong,

I bought my Quincy used back in '77 as a gasoline powered unit when outfitting my first service truck, that old pump ran dependably outdoors at surface mines for many years despite the adverse conditions it was exposed too, after one new tank and retiring it's second Kholer gasoline engine it was removed and placed on another new tank with a 7 HP GE electric motor where it still resides to this day still running dependably! From 1998 to 2007 it supplied air to my auto body shop running six days a week, it now resides in my home shop and is STILL, after all these years and use, ticking along just fine! Yep I REALLY like Quincys and I have a darn good reason for that!

[Davek0974]

These seem to have a good following...
http://www.amazon.co.uk/Wolf-Dakota-...productDetails

Might be worth a try.

[ahidley]

I don't know how it is in England but in the states sears advertises a 4 and I think also a 5 hp compressor that runs on 115 vac. That is impossible. They rate the motors right down to stall speed to get big numbers for sales. I would put more trust in flow rates at a specific pressure.

[radkins]

I don't know how it is in England but in the states sears advertises a 4 and I think also a 5 hp compressor that runs on 115 vac. That is impossible. They rate the motors right down to stall speed to get big numbers for sales. I would put more trust in flow rates at a specific pressure.

Compressors such as those are what I was referring to when I mentioned "underpowered overrated motors", these are usually listed as "Peak HP" ratings which is nothing more than 100% USDA certified Bull$#!*! Another dead give away that a compressor is nothing but an over-rated piece of junk is if it gives a "tank assisted CFM" rating, that term is pure nonsense and means exactly nothing except the manufacturer is trying to con the buyer into thinking the CFM rating is higher than it is.

I don't think the manufacturers can get away with using "peak HP" ratings anymore on 220 volt compressors, they lost a class action lawsuit on that one, but for some odd reason they can (and still do!) use it for the little 110-120 volt units. The lawsuit served a purpose in getting that phony HP rating off the labels but as is usually the case in this type of suit the shys,,,,,,,,,err lawyers collected a bundle and the plaintiffs got a worthless discount coupon towards the purchase of a new compressor, yeah right!!!

[mickeyf]

Lubrication has been mentioned. All I can contribute here is that the so called "Oil-less" compressors will eventually self-destruct.

[CarlByrns]

Lubrication has been mentioned. All I can contribute here is that the so called "Oil-less" compressors will eventually self-destruct.

My Campbell-Hausfeld oilless compressor is 24 years old and still going strong. I've painted and sandblasted with it, running 8 to 10 hours at a stretch with no issues.

The pressure switch has been replaced but the compressor section never been touched.

Quiet, no. Durable, yes. I didn't think it would last this long either.

[J Tiers]

To be fair, a larger tank has some advantages.

If you have a demand for larger amounts of air, intermittently, what the tank does is to average the demand. While a smaller pump may be totally unable to even power the device at all, it can work with a large tank, so long as the average demand is within the pump capability.

Say you need 10 CFM at tank pressure, but you need it only for short times, say 10 seconds out of a minute, because of the way the tool or whatever is used. The demand is for 1.6666 CFM, average. A 2CFM pump can keep up with it, but would be totally unable to power the tool without a tank.

But a larger tank would be able to supply the air during the high demand time, and be "charged back up" over the longer time between the high demands.

of course, if the tank is too small, the short high demand will drain it, and pressure will fall below the need before the short high demand has ended. As soon as the tank is capable of holding sufficient pressure for the duration of the high demand, that's enough. A bigger tank than that doesn't help at all.

And, as soon as the average demand gets past the capability of the pump, you run out of air regardless of tank size.

The only thing a big tank does is to extend the time until the pressure falls below the requirement.

Theoretically, a big enough tank could allow you to work all day at max demand. But you better not want to do that very often, because a small pump will take a long time to recharge that huge tank. But with a pump capable of around 1/3 of the demand, you could work 8 hours, and do it again the next day. Of course, the pump would never stop, at that rate.

[Davek0974]

The issue i have found is that practically no supplier over here lists the actual air delivery capability, I think it's called free air displacement or FAD?

Pump lubed is out of the question on cost i think, splash lube is pretty common here and seems ok.

I only need at most 7CFM at 80psi continuous supply, surely that's not too hard?