New compressor guidance...

[Peter N]

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]

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-...productDetails

http://www.spraydirect.co.uk/acatalo...ompressor.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]

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 [email protected] 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]

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]

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]

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]

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!

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]

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]

If you want your plasma consumables to last.. you'll need an air dryer of some type.

[Davek0974]

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]

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]

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]

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]

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]

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?