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Davek0974
09-22-2014, 06:28 AM
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
09-22-2014, 08:57 AM
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
09-22-2014, 08:57 AM
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
09-22-2014, 10:19 AM
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
09-22-2014, 10:47 AM
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
09-22-2014, 11:32 AM
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
09-22-2014, 03:44 PM
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
09-22-2014, 04:01 PM
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
09-22-2014, 04:12 PM
These seem to have a good following...
http://www.amazon.co.uk/Wolf-Dakota-150psi-Cylinder-Compressor/dp/B007VOLOHG#productDetails

Might be worth a try.

ahidley
09-22-2014, 05:59 PM
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
09-22-2014, 07:29 PM
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
09-22-2014, 09:24 PM
Lubrication has been mentioned. All I can contribute here is that the so called "Oil-less" compressors will eventually self-destruct.

CarlByrns
09-22-2014, 09:59 PM
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
09-22-2014, 11:10 PM
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
09-23-2014, 04:46 AM
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?

Peter N
09-23-2014, 05:26 AM
Dave, you should consider FAD to be around 65%to maybe 70% of the displacement they quote and you won't go far wrong with that.

I have 2 compressors, one is a cheap ALDI unit I've had for 10 years, and next to the bandsaw is the most unloved piece of equipment in the workshop (never even checked the oil....), the other one is an SIP T3/50 belt drive which is a great small compressor - quiet & reliable for the last 6 years, although I wish I had a larger tank than the 50litre one on there. It says displacement is 13cfm but FAD is only 8.5cfm.
I do use it for beadblasting and running an airblast on the CNC. When blasting it runs continously, but with the CNC it only kicks in every 4-5 minutes as I throttle it down to 2-3bar through a very small 1mm orifice.

Davek0974
09-23-2014, 06:30 AM
I think its a toss-up between these two, i can get the Abac a bit cheaper though..

http://www.amazon.co.uk/Wolf-Dakota-150psi-Cylinder-Compressor/dp/B007VOLOHG#productDetails

http://www.spraydirect.co.uk/acatalog/abac_b312_60p_belt_driven_portable_compressor.html

As Peter N says, I have also just been told that FAD is generally 2/3 of quoted displacement so I need a machine with about 12cfm to give a nice 8cfm supply i think.

Which one would you choose?

radkins
09-23-2014, 09:16 AM
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.


While this is correct it unfortunately is rarely the case, for instance a 1/2" impact wrench might benefit when removing one wheel from a vehicle but these times when it would be of any real help would rarely be worth much. The point is that it's an EXTREMELY common misunderstanding that a larger tank will make a compressor do a lot more than a comparable compressor with a smaller tank and so people overlook what really matters in order to get the largest tank they can when it really makes little difference. It is so common that very often people describe compressors as 60 gallon, 80 gallon, etc as if that is what determines the compressor size and they completely ignore the real determining factors! The fact is there simply is no such thing as an 80 gallon, 60 gallon or any other gallon compressor, the darn tank does not make the compressor and actually is one of the least considerations BUT it is the most obvious thing people see when looking at a compressor. I have been around high demand air tools my entire working life using many different types and sizes of compressors, my 23 CFM@90PSI service truck compressor often was not up to the task, and I can assure you the difference in the real world between an 80 gallon tank and a 60 gallon is pretty much zilch with all else being equal! While we can"split hairs" about situations where that extra 20 gallons would save the day it matters little in the real world because at a given rate of demand if the compressor is running out of air at an annoying rate with a 60 gallon tank how long would it take to run out if it had only a 20 gallon tank? Obviously it happen in very short order but that's all that would be gained and even then that is lost to the proportionally longer recharge time, the only time the tank might help is during those periods you pointed out where the recharge would occur during idle time for the tool, running out of air at critical need times is still by FAR the problem and a larger tank will not solve this problem! The fact remains that a 15 CFM compressor with a 60 gallon tank will handily outperform a compressor with an 80 gallon tank but only 13 CFM yet if both are sitting side by side the 80 gallon model will far outsell the 60 gallon version if the price is close due to this "bigger tank is better" myth!

With the exception of rather rare instances where "one tankful" of air is all that's needed to perform the task at hand a larger tank is hardly any help, it can have a few advantages related to duty cycle and the tank size is chosen by the manufacturer based on CFM capabilities of the pump/motor and expected demand in relation to duty cycle not by the amount of air needed to do the job. At least this is true with serious compressors, the type marketed to home shops are based on duping the buyer into thinking he is getting more compressor than he really is! This becomes apparent when seeing uselessly oversized tanks with tiny pumps and over-rated motors bearing "peak HP" numbers.

A bigger tank simply does not make a bigger compressor!!!!!

EVguru
09-23-2014, 11:48 AM
I think its a toss-up between these two, i can get the Abac a bit cheaper though..

A bit cheaper than the Wolf, or a bit cheaper than the listed price (which is a LOT more than the Wolf)?

The Wolf claims a bit more displacement too.


I have also just been told that FAD is generally 2/3 of quoted displacement so I need a machine with about 12cfm to give a nice 8cfm supply i think.

You'd get a bit more air for the same power from a screw or vane compressor, but you're unlikely to get one in your budge range.

EVguru
09-23-2014, 11:51 AM
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.

Doesn't happen here. We have standards.

danlb
09-23-2014, 12:03 PM
It is so common that very often people describe compressors as 60 gallon, 80 gallon, etc as if that is what determines the compressor size and they completely ignore the real determining factors! The fact is there simply is no such thing as an 80 gallon, 60 gallon or any other gallon compressor, the darn tank does not make the compressor and actually is one of the least considerations BUT it is the most obvious thing people see when looking at a compressor.

Sometimes the tank is a consideration. My neighbor uses his all the time for air powered nailers and such. He has a packed garage that must be cleared at the end of each day and only a 2x2x3 space for a compressor. He uses a little pancake style since no other style would fit in the space provided. :)

Dan

radkins
09-23-2014, 02:05 PM
Sometimes the tank is a consideration. My neighbor uses his all the time for air powered nailers and such. He has a packed garage that must be cleared at the end of each day and only a 2x2x3 space for a compressor. He uses a little pancake style since no other style would fit in the space provided. :)

Dan


That certainly is a consideration and is the reason my 23 CFM compressor had only a 30 gallon tank, space is at a premium on a service truck! :D

Actually there can be several reasons for choosing a tank size to fit a particular situation but improving a compressor's ability to keep up with an air hungry tool is not one of them, unfortunately that's all too often the first consideration when selecting a compressor. My point in the first post was that it's a big mistake to base the selection of a compressor on the tank size, all too often folks will pass up what really matters such as CFM (the most important!) and durability just to get a more or less useless extra few gallons of storage in the tank, again tank size is more related to duty cycle/cooling periods and not for added performance. For a home shop or small shop with average needs 12 to 15 CFM is a decent air supply and on a compressor in that range 60 to 80 gallons is about right to balance the off/on/cooling cycles but hard as it is to get some to believe it they will rarely if ever see any performance advantage with the bigger tank, assuming identical air delivery. The point is to balance the CFM (the first and most important choice) against things such as quality, after sale service, price, etc and buy based on that with whatever tank the manufacturer chose to use with the particular combo of factors that fits the need and budget. Whatever else is decided DON'T pass up higher CFM which WILL increase performance to get a bigger tank that will NOT, unfortunately due to the "bigger tank is better" myth this very thing happens all too often!

MrSleepy
09-23-2014, 02:56 PM
A friend who worked in a SIP dealership got me a Stratus 2050 at cost in 1995.

It is very similar to this one

http://www.sipuk.co.uk/sip-airmate-tn-3-50-d-oil-lubricated-air-compressor.html

Its a 240vAC 3hp 14cfm displacement with 9.5cfm FAD from a 50 ltr tank.

When I use my plasma at 60psi or spray gun etc..its pretty much running all the time

At some point I may try to get hold of a 100ltr tank ..Fortunately the motor and pump have not been worked hard since I got it.

Rob

lakeside53
09-23-2014, 03:46 PM
If you want your plasma consumables to last.. you'll need an air dryer of some type.

Davek0974
09-23-2014, 04:07 PM
A bit cheaper than the Wolf, or a bit cheaper than the listed price (which is a LOT more than the Wolf)?

The Wolf claims a bit more displacement too.

You'd get a bit more air for the same power from a screw or vane compressor, but you're unlikely to get one in your budge range.

I wish! I can get the abac for about £450, it is about twice the price of the Wolf.

I think I might risk getting the wolf unit, the reviews are favourable and if it does fall apart then the warranty will be tested.

According to Jim Colt of Hypertherm fame, it should only need a 0.1micron (I think it was that size) filter in the line, no drier etc. he has seen issues caused by having too many filters and stuff in the line. The unit has a built in filter reg, the compressor will likely be upgraded to a decnt filter reg as they are always crap on cheap units and I fully expect to fit a decent one. The last one I bought had the reg blocked with swarf! That went straight in the bin and air flow improved massively :)

radkins
09-23-2014, 04:30 PM
At some point I may try to get hold of a 100ltr tank ..Fortunately the motor and pump have not been worked hard since I got it.

Rob

This is exactly the misconception I have been talking about, with the same cfm switching to that larger tank will accomplish nothing except to lighten your wallet, this is a prime example of where a larger tank is useless. During any given work period switching to a larger tank and not increasing the pump CFM would result only changing the rate at which the compressor would switch on/off, the amount of time available for use vs the amount of time waiting on it to recharge will be EXACTLY the same regardless. The number of times it cycles would change but the number of minutes it runs and the number of minutes it is available for use would not.


Why is it that people seem to see only an increase in time before running out of air but ignore the proportionally longer time it takes to recharge? Doubling the size of the tank would double the time before running out of air but it also doubles the time it takes to recharge before it's usable again, it simply swaps one reason for having to wait for another reason. While doubling the run time might seem to make sense in some cases it also creates another problem and that's exceeding the duty cycle by running for excessively long periods with insufficient cooling time in between. This means adding excessive tank capacity can actually be counter-productive since the unbalanced duty cycle situation quite often leads to overheating of the pump which greatly reduces efficiency and actually results in the tank causing the compressor to make less air available during a given work period instead of more.

MrSleepy
09-23-2014, 04:44 PM
This is exactly the misconception I have been talking about


My compressor is never left on permanently... I often charge it up then turn the power off...it may get re-charged if I havent finished spraying etc. Then its bled and drained.


Why is it that people seem to see only an increase in time before running out of air but ignore the proportionally longer time it takes to recharge?

In a pro shop situation it would be an issue if no air is available... but quite often I turn it on then prepare the job..its usually charged well before the time I'm ready.. And if it took longer to recharge..then its obviously a reason to stop for a cup of tea.

Rob

CarlByrns
09-23-2014, 05:03 PM
This is exactly the misconception I have been talking about, with the same cfm switching to that larger tank will accomplish nothing except to lighten your wallet, this is a prime example of where a larger tank is useless.

That's not quite true. Some pneumatic tools (impacts are the biggest offenders) require large volumes of air at pressure to work properly. You cannot run a half inch impact gun under load on a little nail gun compressor (try it).

macona
09-23-2014, 05:49 PM
Lubrication has been mentioned. All I can contribute here is that the so called "Oil-less" compressors will eventually self-destruct.

Unless it is a scroll compressor like mine. They last about 20000 hours before their first service. Of course they do cost about $7500 new. I think I paid $400 for mine with about 100 hours on it.

radkins
09-23-2014, 06:34 PM
That's not quite true. Some pneumatic tools (impacts are the biggest offenders) require large volumes of air at pressure to work properly. You cannot run a half inch impact gun under load on a little nail gun compressor (try it).

I never in any way insinuated that you could, how in the world did you figure that???????

Of course most pneumatic tools require copious amounts of air, after all pneumatic tools are probably one of the most inefficient and wasteful ways to use energy ever devised by man! Until the tank reaches it's first recharge cycle then everything discussed is meaningless, after it kicks on for the first time it all becomes relevant then.

BTW, if that little pancake compressor happened to be charged by a 15 CFM pump and the 1/2" impact required 14 CFM then believe it or not it would indeed run that impact wrench just fine and do it all day long without running out of air even with that tiny tank! The problem with trying to run a high demand tool such as that impact wrench is the fact the little compressor is not putting air into the tank fast enough, mount that same pump on a 60 gallon tank and it would indeed run the impact wrench for a short time (until the tank pressure dropped too low, which would happen rather quickly) but then you could eat lunch and probably take a nap while waiting on it to recharge!


Basically the tank is simply a buffer in the system selected to control the on/off cycles and size is not selected to increase performance, a grossly undersized tank (think that pancake tank with a 15 CFM compressor) for the pump size and expected demand would cause the compressor to cycle on and off at an unacceptably high rate. Conversely if the tank is excessively over-sized it will run too long between cycles, in either case the theoretical mount of air available during any given time period is going to be exactly the same. No matter what the tank size, if the pump can't keep up and the compressor is running out of air at an annoying rate the overall time it will be available for use vs the time waiting for recharge is going to be the same regardless of the size of the tank.

Let's use a common example of a bead blast cabinet, this is usually a high demand situation where the home shop owner often finds himself running out of air and waiting on a recharge. If for instance the compressor discharges to an unusable point in 2 minutes then takes 4 minutes to recharge then in a 1/2 hour work period it would be available for use for a total of 10 minutes out of that 1/2 hour but would require 20 minutes of waiting, increasing the size of the tank to double it's capacity (usually impractical on most compressors due to the duty cycle) would double the use time but it would also double the recharge time resulting in EXACTLY the same 10 minutes of use vs 20 minutes of recharge during that same work period. While at first thought it might seem that 4 minutes vs 2 minutes of run time might be a big advantage, and in some intermittent use situations it might very well be, the proportional wait increase to 8 minutes from 4 offsets any gains and all this does not even take into account potential efficiency losses due to exceeding the design duty cycle and resulting heat. We can bat back and forth all day with hypothetical, and even some real, situations where one tankful of air will do the job but in the real world it rarely ever works that way. The whole point, again, is that the tank is all to often the deciding factor when buying an air compressor and that can be serious mistake! No matter how big and impressive that tank looks it simply can not put out more air than is put into it and dwelling on what it can do with just one tankful of air is quite meaningless, a bigger tank can not and never will be able to make a compressor bigger, that's simply not the purpose of the tank! Adding an extra tank or increasing the size of the original compressor tank is almost always, except in maybe a few rare situations of intermittent use, going to be an exercise in futility and waste of money, it simply trades one reason for waiting for another!

Did you know that some large high CFM rotary compressors don't even use a tank?

jlevie
09-23-2014, 09:33 PM
I have no idea what is available on the other side of the pond. But since you say that your 2hp compressor could almost keep up, and that wiring limits you to a 3hp, I'd suggest only looking at commercial units that are rated for a 100% duty cycle. They'll be more expensive, but they can run flat out all day long. I like twin cylinder V-type compressors as the run cooler and last longer.

J Tiers
09-23-2014, 11:34 PM
While this is correct it unfortunately is rarely the case, for instance a 1/2" impact wrench might benefit when removing one wheel from a vehicle but these times when it would be of any real help would rarely be worth much. .........

It can be the case for home users..... so long as the tank doesn't leak down (like mine does). Trying to quantify it as "removing one wheel" or whatever, is not helpful to understanding, because of course you and I have NO IDEA what a particular user will do, and can't possibly make a statement as to what particular job is going to be typical, how much air it takes, and/or whether "X" CFM is enough..

If someone wants a particular job done, and has info on CFM requirements, peak, average, or duty cycle, then the thing can be calculated. A yea or nay statement can be made.

You can list a slew of jobs where it would have no chance. I can list an equal number of jobs which would be fine. There's no point in even starting up with that noise.

It's the exact same duty cycle deal as a cheap little Lincoln welder, with the exception that if the tank isn't full when you get there, it has to fill first..... at least the welder works best when you first turn it on.

Fact of the matter is that the tank will hold up above some pressure for a certain time at a given airflow, and after that, it has to be pumped back up. A big pump does that fast. A little pump does it slower. If that's good enough, then it works, if not, it won't.

Davek0974
09-24-2014, 03:41 AM
I have no idea what is available on the other side of the pond. But since you say that your 2hp compressor could almost keep up, and that wiring limits you to a 3hp, I'd suggest only looking at commercial units that are rated for a 100% duty cycle. They'll be more expensive, but they can run flat out all day long. I like twin cylinder V-type compressors as the run cooler and last longer.

V-twins are very rare over here, most 2-stage are now combined units in one block, no V any more.

I'm pretty confident that for my intermittent usage, the Wolf unit will work out.

radkins
09-24-2014, 09:11 AM
It can be the case for home users..... so long as the tank doesn't leak down (like mine does). Trying to quantify it as "removing one wheel" or whatever, is not helpful to understanding, because of course you and I have NO IDEA what a particular user will do, and can't possibly make a statement as to what particular job is going to be typical, how much air it takes, and/or whether "X" CFM is enough..

If someone wants a particular job done, and has info on CFM requirements, peak, average, or duty cycle, then the thing can be calculated. A yea or nay statement can be made.

You can list a slew of jobs where it would have no chance. I can list an equal number of jobs which would be fine. There's no point in even starting up with that noise.

It's the exact same duty cycle deal as a cheap little Lincoln welder, with the exception that if the tank isn't full when you get there, it has to fill first..... at least the welder works best when you first turn it on.

Fact of the matter is that the tank will hold up above some pressure for a certain time at a given airflow, and after that, it has to be pumped back up. A big pump does that fast. A little pump does it slower. If that's good enough, then it works, if not, it won't.



I gave the "one wheel" as a "just for instance" and of course there are many situations where the same would apply, such as needing to use an air blow gun for a short period, maybe to run a pneumatic jack, or any number of other situations and I never disputed that. In these instances you are correct and I never disagreed but I think you may be missing my point, it's the all too common mis-understanding that leads some folks to think a bigger tank is going to help their compressor keep up with an air hungry tool. The situations where one tankful of air may do the job or intermittent use will allow for the tank to recharge while it would not be used anyway is not usually the problem, it's where people are having to wait for the compressor to catch up while trying to run high demand tools such as pneumatic grinders, cut-off tools, bead blasters etc where the problem really lies. It's not at all uncommon for folks to waste time and money trying to install extra tank capacity which does not help in THESE situations or go out and buy another compressor and make the wrong selection by passing up the factors that really matter because they are looking at a big tank when what they really need is CFM! Over on the auto paint and body forums I visit it's common to have newbies (the old timers already know better!) ask about what compressor do they need and the usual question is "do I need a 60 gallon compressor or will I need an 80 gallon model?" as if that's what matters most when in reality it hardly matters at all! I know that's paint and body work but the principle remains the same and it is still a fact that a (for instance) 15 CFM compressor with a 60 gallon tank, or even a 40 for that matter, will easily outperform an 80 gallon version with only 14 CFM! The bottom line is if the CFM is not there when selecting that new compressor then a bigger tank is simply not going to make up for it in spite of what some folks want to think except in situations maybe like you mention where one tankful of air might get the job done, honestly how many of us buy a compressor thinking they are only going to need one tankful of air at a time? At other times where most people are running out of air that bigger tank only trades one wait for another!

Davek0974
09-24-2014, 09:25 AM
The tank volume means practically bugger all in my case :)

It's running a plasma cutter mostly so if the pump can't handle it, the plasma will cut out and the part is wasted (usually).

I've purchased the cheapo one on the strengths of the customer reviews and that they have a good warranty, if it falls to bits then its their problem.

I need (at present) a modest 4cfm @ 80 psi constant, in the future i might get the next plasma cutter up which needs 7cfm @ 90psi , the compressor listed shows the useless displacement figure of 14cfm and general advice seems to be to take 2/3 of that as usable delivery which gives me a possible 9.3cfm of usable flow rate.

Having a reasonable tank at 90ltr will at least allow the plasma to get running before the compressor kicks in, what I don't want is for both to start together as that would certainly blow a breaker or two ;)

Rich Carlstedt
09-24-2014, 02:30 PM
Dave, As Paul mentioned somewhat in the second post on this thread, air temperature is important.
Giving the compressor cylinders the coolest air possible will improve compressor efficiency dramatically.
In an Industrial application,I increased compressor (2x 90 HP) performance 25 % when I mounted the air intakes
outside, including the intake Filter units .
This also takes the compressor noise out side .

If the receiver tank is cool to the touch, you are doing good.
If it is warm, watch your drier !

Rich

Davek0974
09-24-2014, 03:15 PM
Thanks Rich,

I will check on the temperature, I did read that hot air is not good. I'm not sure the neighbours would agree with my putting the air intake outside though ;). It's fairly cool in my shop all year though as the roof is well insulated and walls are concrete slab.

I have read that 10-20' of iron pipe can be used as a cooler to good effect if needed.

Seastar
09-24-2014, 04:46 PM
The best air compressors built in the USA are C-Aire in Wyoming, MN
They use top notch components and make great tanks.
Look them up.
I don't have any interest in them but do have one of their two stage units in my shop in Minnesota.
It just works.
Bill

radkins
09-24-2014, 07:11 PM
For those folks here in the USA it might come as a surprise that, believe it or not, Harbor Freight has a couple of real bargains on compressors! They have two different "brands" with Central Pneumatic being the brand name of the compressor shaped lumps of scrap iron and aluminum they sell and the "US General" brand which happens to be a re-badged American built Belaire at about 2/3 the cost of a Belaire brand! These are NOT Belaire clones, they actually are the genuine article made right here in the USA (South Carolina) with most everything including the tank being manufactured here except the pump itself which is a very good quality pump built in Italy. They come with Square D electrics and either a Century or a Baldor motor depending on the model and are very well built units for a home or small shop type application. I think they only have a couple of different models now having dropped the top-of-the-line model that had a 5 HP Baldor motor and two stage pump mounted on an 80 gallon tank but I have read that there will be another version of that one available sometime next year. This is ONLY the Black painted US General brands they have and they are not to be confused with their Chinese junk compressors, it's somewhat of a mystery why they don't advertise these being American built but for whatever their reasons they don't mention it anywhere that I have seen.

Maybe they just don't want to spoil their reputation? :rolleyes:

Davek0974
09-25-2014, 02:10 AM
The best air compressors built in the USA are C-Aire in Wyoming, MN
They use top notch components and make great tanks.
Look them up.
I don't have any interest in them but do have one of their two stage units in my shop in Minnesota.
It just works.
Bill

Well i would, if I wasn't in the UK :)

Seastar
09-25-2014, 08:38 AM
Dave
Sorry about that.
I need to read more carefully.
C-Aire does build great compressors though.
Bill

larry_g
09-26-2014, 01:10 AM
I'll have to say I agree with Radkins on the tank size issue. If the compressor cannot deliver the air required for the job then the tank size becomes irrelevant and you will be playing a waiting game. Or as said by one above , scrapping parts.

One other issue not brought up here is the duty cycle of the compressor your going to use. If you plan on running long cut jobs the compressor may not have the duty cycle to withstand use. A lot of homeowner compressors are rated as low as 4 starts per hour.

lg
no neat sig line

Davek0974
09-26-2014, 03:33 AM
I'll have to say I agree with Radkins on the tank size issue. If the compressor cannot deliver the air required for the job then the tank size becomes irrelevant and you will be playing a waiting game. Or as said by one above , scrapping parts.

One other issue not brought up here is the duty cycle of the compressor your going to use. If you plan on running long cut jobs the compressor may not have the duty cycle to withstand use. A lot of homeowner compressors are rated as low as 4 starts per hour.

lg
no neat sig line

Thats a good point, it's not listed anywhere in any of the adverts so I am just going to run it how i want and see how it stands up. If they do not state clearly a recommended duty cycle then they cannot complain if i kill it.

lakeside53
09-26-2014, 11:36 AM
On my 3hp (real) Quincys I have a couple of options for running. If the starts excessed 6-8 per hour they recommend just running them continuously and using the free-run unloader (it just closes the cylinder valves when it needs to pump). Of course, the compressor is good for 50,000 hours... and running unloaded it's very quiet and uses little power.

The starting duty cycle is is mainly to save power (particularly for those with peak demand metering which I don't have) and to protect the motor.

Davek0974
09-26-2014, 02:13 PM
Somehow I doubt my cheapy unit will have the option running free :)

It probably won't need to start more than 4-5 times an hour, it's not going to be working flat out as I don't do production runs, mostly one-offs so plenty of thinking time between jobs.

Davek0974
09-29-2014, 06:02 AM
Compressor arrived this morning:)

I can confirm from the depths of the manual that there is a duty cycle:-
50% duty cycle,
15 minutes continuous running,
12 re-starts in 1 hour,

These seem a bit tight for a heavy-duty compressor recommended for "a professional DIY hobbyist" or "enthusiast" although I'm not sure what a compressor enthusiast is?? :)

Anyway, one really good thing is that it's practically silent, compared to the bone-shaking ear-splitting direct drive units it's replacing anyway, might even be able to work in the shop while its running now!

I'll have the plasma running later so will see how it keeps up.

MrSleepy
09-29-2014, 07:05 AM
These seem a bit tight for a heavy-duty compressor recommended for "a professional DIY hobbyist" or "enthusiast" although I'm not sure what a compressor enthusiast is?? :)


The heavy duty workshop continuous use types usually are either heavy cast iron bore or screw compressor.

http://i897.photobucket.com/albums/ac180/MrSleepy123/_57_zps5f83d294.jpg


http://i897.photobucket.com/albums/ac180/MrSleepy123/_57_zpsd48c7300.jpg


Rob

Davek0974
09-29-2014, 08:10 AM
Hydrovanes are nice units.

In its defence, this unit does at least have a cast iron block :)