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airsmith282
04-17-2009, 03:35 PM
what determens the SCFM in an air compressor id really like to know

JBL37
04-17-2009, 04:36 PM
Not an expert on SCFM for air compressors. It is a compressor rating system that allows a salesman to use a large figure to snow the buyer. The figure one needs to see is the CFM delivered at say 90psi. Most power tools are rated at 90psi, and the manufacturers will give air consumption in cubic feet per minute(CFM). With this knowledge a user can tell if the compressor will operate a tool or tools as the case may be. Which mean one does not have to wait for the compressor to recover. In turn the tool user is more productive.
There may be a conversion factor to switch between these two, but I have never seen or read it. Also, I think Sears was the first company use this rating system. Also they are the ones who had the seven horse power electric motor the size of a shoe box. My day to rant. Jim

Jeffw5555
04-17-2009, 04:48 PM
Jim, you have it totally backward. SCFM means Standard Cubic Feet per Minute, and came into vogue to combat the problem of those using inflated HP ratings.

Comparing SCFM is the only reliable way of comparing compressors output capabilities.

topct
04-17-2009, 04:52 PM
what determens the SCFM in an air compressor id really like to know

Displacement and RPM.

radkins
04-17-2009, 05:14 PM
Jim, you have it totally backward. SCFM means Standard Cubic Feet per Minute, and came into vogue to combat the problem of those using inflated HP ratings.

Comparing SCFM is the only reliable way of comparing compressors output capabilities.


Actually Standard cubic per Minute has been around quite a while and ACFM or Actual Cubic Feet per Minute is becoming more common now. Whether SCFM or ACFM is the best way to rate a compressor is something that will get two engineers to argue so it can be found either way depending on the manufacturer but both figures (if applied honestly) will be close so either is usable as a guideline.

For a short while there (it backfired on them big-time!) a couple of the manufacturers tried to use ACFM for "Assisted Cubic Feet per Minute", as in "Tank Assisted". "Tank Assisted" CFM is total, complete and unabashed BULL$#&^! In that scenario the manufacturer would rate the discharge flow from the tank for a certain length of time which is absolutely meaningless and the flow from the pump into the tank at a given pressure is the ONLY way to get a meaningful rating. This scam did not last long and it has been about 5 years since I have seen one of these darn things so it is not likely anyone will run into a new one but be careful when looking at used equipment. The ones I saw (one was an Ingersoll at Tractor Supply) had a big label on the tank boasting a ridiculous CFM rating, something on the order of 40 CFM from a 5 HP compressor, yeah right!:rolleyes: So be suspicious of compressors claiming as much as twice the CFM of comparably powered models.

oldtiffie
04-17-2009, 05:51 PM
Among other things, the most usual use for the "SCFM" information is to enable a user or buyer to match a pneumatic/air-driven tool and a compressor/air-source with each other.

An air-driven grinder of a plasma cutter may require air at minimum volume per minute (CFM) pressure at a minimum pressure (at the tool - not at the regulator out-put).

Similarly it makes buying a new compressor easier if the required minimum air pressure and SCFM are known.

Having sorted that out, the power requirements can be checked for adequacy to drive the compressor. That is usually labeled or expressed in terms of Volts, phase and amps. So if the compressor and load/tool are matched or adequate and the power is not, the compressor and tool may not be able to achieve their optimum or required - or any - performance/s.

If nothing else, it will show just how much power it needs to drive that air tool - in which case it may really be cheaper and easier to use an optional eclectically-powered tool.

If a compressor shut-off is at 105psi it may be struggling to deliver enough air at say 90 psi - at the tool - for a frequently-used air-grinder even though it does a good job for the occasional "air-blast" or cleaning gun or smaller paint spray-gun use etc.

So the SCFM and power labels allow a pretty good judgment to be made - especially if that terrific "for cheap" air tool from eBay looks like (or does!!) cost a new compressor as well or alternatively that you find that your power requirements are too high for your shop supply.

So - "SCFM" is very useful.

QED.

Evan
04-17-2009, 06:13 PM
what determens the SCFM in an air compressor

Mostly the marketing department. They also would like you to think that you can get 5hp from 1700 watts.

Mark McGrath
04-17-2009, 06:19 PM
Forty years and more ago a compressor was rated by CFM which was at 100 psi.By the seventies that was changing to FAD which is free air delivered which is a lot less.A rule of thumb is a piston compressor,single stage of reasonably decent make will produce 4 cfm@100psi per horsepower.

radkins
04-17-2009, 06:24 PM
If nothing else, it will show just how much power it needs to drive that air tool - in which case it may really be cheaper and easier to use an optional eclectically-powered tool.



A pneumatic tool is probably the most inefficient tool you can use! Air tools waste more energy than just about anything else in a shop, for example a die grinder or cut-off tool may require a 5 HP compressor to keep up with it and some may not even be able to. What you will have then is a situation where you will be using 5 HP to run a fractional HP tool. In fact in the case of a die grinder it would take the flow produced from at least a 5HP compressor to equal the power of most quality electric die grinders. The bottom line is you will be paying for the power to run a 5 HP motor that in turn runs a tool with usually less than 1 HP, not very efficient at all. Except for spraying operations it makes more economic sense to use electric tools when possible but of course the air tool is almost always smaller, lighter and more durable (heavy loading and stalling will not hurt them like an electric tool) so they may be more practical in that sense. As for matching the compressor to the tool using the SCFM ratings it is a good idea to consider that compressors tend to be over-rated as to how much CFM they will produce while tools tend to be under-estimated (by a LOT!) in how much air they will require, seems everyone is willing to stretch the ratings to the limits to make their product look good!

PTSideshow
04-17-2009, 06:50 PM
Another thing to look for is their definition of SCFM .As some are now saying Sustained Cubic Feet Minute which as been stated the market department
:D

airsmith282
04-17-2009, 06:52 PM
o ill be a bit more specific then i gota 8 gallon tank my rating is 4.5 to 5 scfm if i add on a second air tank say 7 gallon with it total now is 15 gallons is this going to increase my SCFM or CFM

Evan
04-17-2009, 07:42 PM
Short answer is No. The compressor pump and the motor hp determine the Standard Cubic Feet per Minute that can be delivered. That spec is an average over time and changing the receiver volume won't make a difference.

oldtiffie
04-17-2009, 09:10 PM
Not so.

The SCFM rating is at the compressor.

If multiple receivers/"tanks" are used together in manifold, and the air demand at the tool is short enough or low enough, there is a chance that the compressor will keep running sufficiently long between demands such that all of the tanks are charged to the pressure regulator cut-off pressure at the compressor. The available supply to meet the demand is determined by the manifold. If the demand exceeds the supply available on a "make up" basis by the compressor, the manifold pressure will progressively reduce and because of the additional volume will take longer for the compressor to re-charge it.

So additional tanks/receivers in manifold may well be a viable option - depending upon available circumstances.

Its a "judgment call" and "trade-off" option.

Here is my 230v single phase compressor - which works very well for me. Just keeps ahead of my plasma cutter. Note the "Free Air Delivery" (rate): 117 litres/min (4.15 CFM) which is the compressor outlet compared to the inlet volume air (at atmospheric/ambient air pressure): 245 litres/min = 8.6 CFM which is an indication of compressor inefficiency. (The compressor is Italian and is a "Trade" unit with a maximum/cut-off pressure of 10 bar = 10 atmospheres ~ 145 psi)

http://i200.photobucket.com/albums/aa294/oldtiffie/Compressor/Compressor_1.jpg

And its "plate":
http://i200.photobucket.com/albums/aa294/oldtiffie/Compressor/Compressor_2.jpg

That "too big" compressor might just eat up that power reserve you had for your newer/better/bigger what-ever tool/s that you just bought!!!

There are no "free lunches" or "magic puddings" or "free rides" - or a re-run of the "Loaves and Fishes" effort either!!
http://en.wikipedia.org/wiki/Magic_Pudding

http://en.wikipedia.org/wiki/Free_riding

Evan
04-17-2009, 09:22 PM
The question was "does it increase the SCFM" rating. The answer is no.

john hobdeclipe
04-17-2009, 11:07 PM
http://www.engineeringtoolbox.com/scfm-acfm-icfm-d_1012.html

radkins
04-18-2009, 12:10 AM
No, adding tank volume will do nothing for the CFM rating because that is determined by how much air is going into the tank not how much is coming out or stored in the tank. It is an all too common mistake to go to the trouble and expense of adding storage capacity in an effort to increase a compressor's performance but in almost all cases it does little or nothing to help the problem. Think of it this way, the tank obviously can not supply more air than is pumped into it (although you may not get the guys who support those goofy perpetual motion machines to believe that! :D ) so if, for example, you double the size of the tank you would double the time you could use it before having to stop and let it catch up. The problem here is that you would also double the time it takes to catch up so after the first start-up cycle you gain exactly NOTHING so after, again as an example, 15 or 20 minutes or so you will have spent exactly the same time working vs waiting with either tank. The smaller tank would be more and shorter cycles while the larger tank would be longer and fewer cycles but the total either way is theoretically the same. Actually with the larger tank you would in all probability lose running time (that's why I said theoretically) because the extended run time of the small pump trying to fill the larger tank would lead to over-heating which in turn leads to reduced compressing efficiency, adding extra or larger tanks is almost always a losing proposition.


Start out with a full tank and time how long it takes to reach start-up pressure with the tools you are using, usually with a compressor as small as the one we are discussing here that will be only a short time probably measured in seconds. That is all the extra run time you would gain if you doubled the tank size and then that would only be until it reaches the first start-up cycle, anything after that will just be lost to the extra recharge time and you gain nothing.

oldtiffie
04-18-2009, 12:30 AM
Thanks Radkins.

I have no issue with what you say.

Your post also indirectly addresses the issues of large vs small "tanks"/receivers on a compressor.

My mention of manifolding does not detract from what you say (or it wasn't intended to).

The principles of manifolding and sizing of receivers is not dissimilar, so in effect the manifold extends the effective available air storage volume.

If a large volume of air is required occasionally (as say for a hydro-pneumatic car hoist in a garage - or pumping up/inflating a large balloon) the manifold would boost or assist the compressor at a combined rate that the piping could accommodate until such time as the reserve capacity was depleted to below the minimum air volume and pressure required. The compressor will use what-ever chance it has to replenish the recivers during the no draw-down time. It may "make it" or it may not. It is an option where compressor capacity is limited. It may well be that if this "works" that a larger capacity compressor is not needed - and it may not.

I referred to the "pipe-line" and its adequacy, but that is a related matter that is not the subject of this discussion - as is the case with air cooling-off in the receivers.

I hope we agree even if my posts may have inadvertently seemed that we don't.

airsmith282
04-18-2009, 06:11 AM
ok here is what iam after my CH 8 gallon compressor is such that i can use my sand blaster intermedit according ot the lable on the thank i want more run time before the motor kicks back in, my plan is to add on a second tank about 7 gallon unless i can finda cheap bigger one then 7 gallon, its to my under standig once both tanks are up to full pressor that this will give me more run time , my sand blaster min requiremnet is 5 cfm or scfm,, max is 9 so by incresing tank volum in this case its not going to cut the motor back in as soon am i right or wronge in this..

see my 3 gallon CH max psi is 100 and at 80 it kicks the motor in takes for ever to charge and wont run my 1/2 inpact gun

my 8 gallon ch will run anything i slap on the hose and it s rated at 125 psi max presser

so how much air a tank holds seems not to be an issue its how much air of that 100 or 125 psi i can relase at a time that seems to be and issue , if i was to increase the air flow out put allowece in the 3 gallon it would then run all my air tools, thus in the case of out put CFM it would be increased correct?????

iam loooking at a 4 gallon stack form ch today iam gongto checks its cfm ratings and see the difference it also has a oil motor in it where mt 3 gallon one is oilless,, anyhow my 8 gallon system either way is my main issue here either way ,,

Evan
04-18-2009, 07:05 AM
If you increase the storage capacity this is what happens:

It takes longer to draw it down to the pressure it turns the motor on. You will have more time that the air is close to the high end of the pressure range.

However:

That means it also spends even more time at lower pressure as well as at higher pressure. Because less air flows at lower pressure it will spend more time at the lower end of the pressure range than at the higher end of the range. You will be waiting longer for it to kick on while using the air at the low end of the pressure range.

Then:

When it turns the motor back on it will take longer to pump back up. If you double the tank size it will take more than twice as long to refill at the high end of the range because the compressor is less efficient the higher the back pressure is. More time will be wasted because of the longer time spent trying to fill an almost full larger tank.


This all means that increasing the receiver size means you spend more time at low pressure than at high pressure than before when discharging and more time waiting at high pressure than at low pressure waiting for it to recharge.

A small receiver means the compressor spends more time near it's actual delivery capacity and less time at the high/low ends of the charge/discharge ends of the pressure range. The bigger the receiver the more time the compressor spends at the ends of the range, especially the ends that are the least useful.

radkins
04-18-2009, 10:01 AM
ok here is what iam after my CH 8 gallon compressor is such that i can use my sand blaster intermedit according ot the lable on the thank i want more run time before the motor kicks back in, my plan is to add on a second tank about 7 gallon unless i can finda cheap bigger one then 7 gallon, its to my under standig once both tanks are up to full pressor that this will give me more run time , my sand blaster min requiremnet is 5 cfm or scfm,, max is 9 so by incresing tank volum in this case its not going to cut the motor back in as soon am i right or wronge in this..

see my 3 gallon CH max psi is 100 and at 80 it kicks the motor in takes for ever to charge and wont run my 1/2 inpact gun

my 8 gallon ch will run anything i slap on the hose and it s rated at 125 psi max presser

so how much air a tank holds seems not to be an issue its how much air of that 100 or 125 psi i can relase at a time that seems to be and issue , if i was to increase the air flow out put allowece in the 3 gallon it would then run all my air tools, thus in the case of out put CFM it would be increased correct?????

iam loooking at a 4 gallon stack form ch today iam gongto checks its cfm ratings and see the difference it also has a oil motor in it where mt 3 gallon one is oilless,, anyhow my 8 gallon system either way is my main issue here either way ,,



In all honestly with what you are wanting to do here you will simply be wasting time and money. That "run time" you are trying to extend, how long will it run now before kicking on? If you double the tank size it will only double that short time you have now THEN you will be in for a proportionally long wait for the compressor to recover, there just ain't no free lunch! If you use the blaster for even just 10 minutes at a time you would still spend the same amount of run time vs waiting time regardless of tank capacity. EXCEPT that with the extra tank you will now exceed the compressor's duty cycle which will lead to over heating and a loss of efficiency, this means that instead of just breaking even you will actually lose run time due to the heat induced extended run recharge times. The bottom line is adding a second tank will not solve your problem.


The tank capacity is probably the most commonly misunderstood function of an air compressor and adding tanks to try to increase performance is an all too common mistake. You can NOT make up for insufficient compressor CFM by adding more tank capacity, it just does not work that way in spite of how it might seem to you. A bigger tank does NOT make a bigger compressor!

airsmith282
04-18-2009, 10:18 AM
adding in the extra tank wil lalow for more run time on say my sand blaster, its not going to increase the cfm unless i mod the output onthe manifold.. iam starting to see that air compressors ar like a co2 pellet gun or a pumper pellet gun if you restric the air allowed to enter the barrel even though the presser and voulm of air is the same comming out of the valve the gun is slowed down this is proven time and time again , if you drill out the trasfer port to the barrel form 1/16 to 3.8 mill the gun will shot from 418 to now over 600 fps , same air pumped in the valve them letting out but the allowed airto travel to the barrel and hit the pellet is now increased .. so if i take a regual kit from an 8 gallon ch compressor and put it on a 3 gallon tank the set it so its at 100 on the cut off then i can use any tool i want on the 3 gallon the only thing i will be lackeing then is the about of stored air in the tank.. so thus i will increse the out put CFM now in a 3 gallon system ..

so in all now the extra tank i want to use on my 8 gallon ch will giveme more tool run time at same cfm which is he goal half way met, so if i want more cfm i need a regulater set forma tank set up thats say out puts to 9 cfm at same presser at 125 psi .,. ori mod existing equipment to do what want..

run time for heating up the motor is an issue that can be controled with the use of a cooling fan to keep the motor from over heating./ the setup i want for the blaster is goingto be ona qick connect system so no mods needed tomy existing ch compresor jus the dummy tank will get a moded set up to take adnvantage of the added air space needed ..

sorry for the bad spelling..

radkins
04-18-2009, 10:46 AM
OK it's your dime but your theory is filled with flaws. No matter how you do it, restrict it, open it up however you do it, the fact remains that adding a tank will only increase the run time at the expense of recharge time, just simple laws of physics. I am going to guess that your blaster will cause the compressor to kick on in only seconds after starting to work as it is now, then by doubling the tank capacity you will only double the number of seconds of run time before you run out of air-you can then go eat lunch while the compressor is recharging. The overheating problem has nothing to do with the motor getting hot (although that will probably happen too) but it has to do with the air itself heating up along with the compressor pump. Hot air expands thus you will actually lose capacity trying to draw hot air from the tank because as it cools upon exit into the system it will draw more from the tank to do he same amount of work as if you were drawing cooler air from the tank. This will be compounded by the fact that the hot pump itself will not compress as well as a cooler pump all of which means you will lose run time instead of gaining and you will lose a lot more than you might think. As I said look at what a short time you have now and the most you would possibly gain would be to double that small amount of time and even that is immediately lost to recharge time. If you want to restrict the airflow coming out and extend your time that way then simply use a smaller nozzle but adding tank capacity will not help you it will cost you. I have seen this tried many times and argued every way possible but the inescapable fact is you can NOT get more air out of a tank than the pump puts in!

Evan
04-18-2009, 11:22 AM
Read what I wrote again. Adding more storage will mean that the system spends more time at low pressure before it kicks in to pump up. You don't want that. You want the compressor to cycle more often, not less often. That way you get the highest average pressure per unit of time.

airsmith282
04-18-2009, 11:30 AM
if i restict pressure then i lose the ability to run the sand blaster at proper cfm etc.. same as an impact gun wont run as well either.. the second thank is the only option for still using full cfm and pressure ill need for the blaster to run at min cfm.. the longer run time to rechage is not a bad idea form empty my tank is less then 5 min to charge up form the cut off to the restart of the fill my tank tops up in less then 30 seconds so a second tank even thoiugh i now use 2 times more air i can top the 2 tanks in less then a min or a min which is not to bad really so the air wont heat up that much, and i always leave my tank fully chaged now all the time so even with in an hour or so any warm air is now cold again any how ,, i under stand there is give and take no matter which way i go but the extra run time before compressor kick in is going to have to be the one i accept for now as a solution for the time being.. the added cfm thing will have to waite for now but iam sure i can find a soloution there as well with either modding the output or switching to a regulator that can increase the cfm higher ..


now the next question i have is whats the max pressure these thanks will handel before they blow up ,, iam thinking if i can raise the pressure to max at say 160 and then have it restart at say 70 then i might not need the second tank idea , is this possible on and 8 galon tank to saftly use it all the time at 160 psi in and out put or just in put presure.. ???????

radkins
04-18-2009, 11:38 AM
You want the compressor to cycle more often, not less often. That way you get the highest average pressure per unit of time.


The kind of thinking here about adding more tanks in an effort to increase a compressor's performance is exactly why the economy compressors usually are tiny pumps with an underpowered motor mounted on huge uselessly over-sized tanks! The manufacturers know that most people will run straight to the biggest tank in the store because it will LOOK like the biggest compressor, never mind that it may be nothing more than a tire pumper, it LOOKS big! I saw one guy hook up two 60 gallon tanks to a 10.5 CFM Campbell Hausfeld compressor in an attempt to do exactly what this fellow here is trying to do and all he wound up with was a smoking hot compressor that took FOREVER to build up pressure. The fact is a compressor in the 10 to 12 CFM range works OK with a 60 gallon tank but most people would find them more useful with a 40 gallon tank, however just try getting them to believe that. It is a fact that a small compressor mounted on an 80 gallon tank would by far outsell one with a 60 gallon tank even if the 60 gallon had a higher CFM rating, how do I know this-I sold compressors for several years!

radkins
04-18-2009, 11:48 AM
now the next question i have is whats the max pressure these thanks will handel before they blow up ,, iam thinking if i can raise the pressure to max at say 160 and then have it restart at say 70 then i might not need the second tank idea , is this possible on and 8 galon tank to saftly use it all the time at 160 psi in and out put or just in put presure.. ???????


Now you are getting dangerous! Are those tanks ASME certified? Judging by the size and the fact it is an economy type compressor I doubt they are. Look on the tank for a data plate and IF there is one it will give the safe working pressure, the max pressure rating on a data plate is NOT the safe working pressure, DO NOT under any circumstances exceed the working pressure limits. In the absence of any pressure data, either on a data plate or stamped into the tank itself, DO NOT increase pressure above what came from the factory. Even an eight gallon tank rupture can do serious damage and could easily kill you!

Besides that little motor is probably maxed out at the set cut-off pressure as it is and the extra load would almost certainly kill it!

Evan
04-18-2009, 12:10 PM
If it is set to cut out at 145 try setting it to 125. It will pump up a LOT faster and will keep a higher average pressure calculated as a percent of the maximum. The air pump efficiency goes down in direct proportion to the pressure going up. If the tank pressure is 130 for instance it has to squeeze that air to 130 PSI before any air moves into the tank. Then only part of the air goes into the tank because the air pump always has some dead space in the cylinder and connecting line to the check valve. The higher the pressure the less air that actually goes into the tank per stroke.

David Powell
04-18-2009, 12:55 PM
I think we can safely say that we are agreed that, a) air tools are inefficient. b, extra air tanks make more problems than they solve, c, the proper answer is to have a motor with lots of power, a compressor with lots of capacity and a reasonable sized tank relative to the above. Years of messing around have driven me to those conclusions. If the dollars are desperately short then dont waste them on attempts that just wont do the job, save up and buy what you need ONCE. Incidentally I have found that an oversize unit can be considerably quietened by running the compressor about 10 % slower than originally set up by changing pulleys in circumstances when less noise is needed. It seems those last few revs make the most noise, but maybe thats just a hopeful subjective feeling. Regards David Powell.

radkins
04-18-2009, 12:59 PM
Evan, You are probably wasting your time with that one just as I am trying to get across the point about the tanks, just as with a bigger tank higher pressure numbers sells! Just look at those junky little plastic 12 volt tire pumpers sold at Wal-Mart and other places, one boasts that it is a 250 PSI compressor, another claims 275 PSI while still another has 300 PSI printed boldly across the case! :rolleyes: Even if these stupid things really could pump that much pressure what Earthly reason would anyone have for it? They could just as easily make these things FAR more useful by using a slightly higher volume pump that maxed out at maybe 60 PSI or so, this would let someone pump up a flat tire a heck of a lot faster but it would limit the pump to the 60 PSI-so what, why don't they build them this way? They don't do that because a pump that would only pump 60 PSI sitting beside one that claims from 250 to 300 PSI would not sell, never mind it would be far more useful and faster, unless you need 250 PSI in your tires! :D

No matter how many tanks, valves or regulators the air is passed through the fact is there will be absolutely no more air exit the end of that air hose over a given work period than the pump puts out and adding another tank is a total waste of time. If there is one thing that could help, even if only a little, it would be to use this suggestion of dropping the pressure slightly, instead of increasing it, to increase the compressor's efficiency. This would cause an even shorter (although only slightly) run time but over a period of more than one complete on/off cycle it could actually gain some over-all work time.

Evan
04-18-2009, 02:38 PM
For a given motor and compressor the point of maximum air delivery is when the pressure is set to just barely cutout while air is being used. This assumes that the compressor is able to keep up with the amount of air demanded.

For regular shop use I have both of mine set to cut out at 110 psi. When I need extra air for sand blasting I gang the two systems together. 110 psi works fine for sand blasting small parts. Unless you are cleaning freighter hull bottoms or similar you don't need more.

airsmith282
04-18-2009, 02:49 PM
ok now my blaster runs at 90 psi so i set the regualator to 90 psi i know iam going to get some good run time there over setting it to 110 psi which seems its the cut off to max psi even though its rated for 125 psi iam talking about my 8 gallon ch compressor here by the way, anyhow if say i get 1 min of total spray time at 90 i would get 2 min at 90 if i had the second tank my motor kick in is set at 70 psi so that gives me 20 perhaps 30 psi of use for..

now whats bugging me here is the CFM really i get 4.5 cfm at 40 and i get 3.7 at 90 not alot of difference here at all really recharge is is under 1 min even if i set the sucker to 110 psi on the out put regulator..

so to recharge 2 tanks that have depleted to the comprssor kick in at 70 psi its not a big deal at all there ,, but like i had said the CFM out put does seem to work like a pellet gun smaller the trasfer port the less FPS increes it to a certian point and you get alot more fps at the same amout of PSI, so one could see a change for the better if you increse the size even by 1/16 more on the out put regulator..

see 100 psi is 100 psi thats fact right, so wont matter on 2 gallon 3 gallon 8 or even 60 gallon its still only 100 psi , now 3 gallon is not alot but its 5 gallon short of 8 gallon and my 3 gallon only put out .83 cfm as opposed to my 8 at 4.5 cfm that has to be regualor output control the same pressor is hitting the release valve so the valve port has to be also smaller on the 3 gallon one in order to account for the lower cfm .. pressor alone does not matter if its 100 its a 100 but the amount of air at a time thats allowed to exit the valve to the regulator now thats the difference in the cfm you get out of the tank at a time thus the higer the cfm the faster the air in the tank deplets and the sooner the compressor kicks in for a recharge, so, if i increase the out put port to a bit larger ill see alot more cfm but ill be recharging sooner unless i turn the psi down on the output valve a bit but at the same time ill still go threw more air but gain more cfm either way ,, ill have to experiment with this on my 3 gallon tank dont want to mess wiht the 8 unless it wokrs

see i saw a ch 12 gallon set up and its got the same CFM ratings as mine so the tank size has noting to do with the cfm out put and has only part to do with the motor but ill bet its the out put regulator valve thats doing it .,....a bigger tank just mease takes longer to fill or drian psi ratings were even the same ,, so i thinks me onto something here ..

David Powell
04-18-2009, 03:27 PM
I have been on a slightly different adventure with air supply. But the results were similar. If you ever went to the Hobby Show in Toronto and saw the Toronto Society of Model Engineers booth, especially in the years 1985 to 2004 or so you would have seen a host of model steam engines , of all sorts of sizes, from 1/4 " bore and stroke up to 3" bore by 3" stroke running on air. WE had 4 115 volt nominally 20 amp outlets supplied by the show, usually they would blow breakers in the wall panels if we ran at more than 12 amps for any length of time, secondly we wanted lots of air at low pressure for most of the models and quite a lot of air at 50 lbs for a beautiful collection of ( compound)engines by one member. Thirdly all the compressors had to run quietly, so we could talk to the public. We, mostly I tried all sorts of things. Our most successful outfit is a large twin compressor( Princess Auto) driven by an ancient repulsion induction motor 3450 rev belted down about 7 to 1, that has a large safety valve set to 60 lbs at which draw is 16 amps. We have a Campbell Hausfeld twin and put an adjustable pulley on that to keep amps down( I suspect the motor is an offshore one) and a couple of other lesser outfits eg an air brake compressor off a truck with ancient 1/2 horse motor , a very low pressure outfit from a hospital ward( NO! the patient didnt take it home under his wheelchair!!!) and others. Years of trial and error proved that Volume of air, at the required pressure, is what is important. All attempts at storage of air proved futile, giving only seconds more of run time if a unit stopped. WE understand the Hobby show will be revived. I hope to meet you there. Regards David Powell.

David Powell
04-18-2009, 03:33 PM
I have been on a slightly different adventure with air supply. But the results were similar. If you ever went to the Hobby Show in Toronto and saw the Toronto Society of Model Engineers booth, especially in the years 1985 to 2004 or so you would have seen a host of model steam engines , of all sorts of sizes, from 1/4 " bore and stroke up to 3" bore by 3" stroke running on air. WE had 4 115 volt nominally 20 amp outlets supplied by the show, usually they would blow breakers in the wall panels if we ran at more than 12 amps for any length of time, secondly we wanted lots of air at low pressure for most of the models and quite a lot of air at 50 lbs for a beautiful collection of ( compound)engines by one member. Thirdly all the compressors had to run quietly, so we could talk to the public. We, mostly I tried all sorts of things. Our most successful outfit is a large twin compressor( Princess Auto) driven by an ancient repulsion induction motor 3450 rev belted down about 7 to 1, that has a large safety valve set to 60 lbs at which draw is 16 amps. We have a Campbell Hausfeld twin and put an adjustable pulley on that to keep amps down( I suspect the motor is an offshore one) and a couple of other lesser outfits eg an air brake compressor off a truck with ancient 1/2 horse motor , a very low pressure outfit from a hospital ward( NO! the patient didnt take it home under his wheelchair!!!) and others. Years of trial and error proved that Volume of air, at the required pressure, is what is important. All attempts at storage of air proved futile, giving only seconds more of run time if a unit stopped. WE understand the Hobby show will be revived. I hope to meet you there. Regards David Powell.

radkins
04-18-2009, 03:42 PM
ok now my blaster runs at 90 psi so i set the regualator to 90 psi i know iam going to get some good run time there over setting it to 110 psi which seems its the cut off to max psi even though its rated for 125 psi iam talking about my 8 gallon ch compressor here by the way, anyhow if say i get 1 min of total spray time at 90 i would get 2 min at 90 if i had the second tank my motor kick in is set at 70 psi so that gives me 20 perhaps 30 psi of use for..

now whats bugging me here is the CFM really i get 4.5 cfm at 40 and i get 3.7 at 90 not alot of difference here at all really recharge is is under 1 min even if i set the sucker to 110 psi on the out put regulator..

so to recharge 2 tanks that have depleted to the comprssor kick in at 70 psi its not a big deal at all there ,, but like i had said the CFM out put does seem to work like a pellet gun smaller the trasfer port the less FPS increes it to a certian point and you get alot more fps at the same amout of PSI, so one could see a change for the better if you increse the size even by 1/16 more on the out put regulator..

see 100 psi is 100 psi thats fact right, so wont matter on 2 gallon 3 gallon 8 or even 60 gallon its still only 100 psi , now 3 gallon is not alot but its 5 gallon short of 8 gallon and my 3 gallon only put out .83 cfm as opposed to my 8 at 4.5 cfm that has to be regualor output control the same pressor is hitting the release valve so the valve port has to be also smaller on the 3 gallon one in order to account for the lower cfm .. pressor alone does not matter if its 100 its a 100 but the amount of air at a time thats allowed to exit the valve to the regulator now thats the difference in the cfm you get out of the tank at a time thus the higer the cfm the faster the air in the tank deplets and the sooner the compressor kicks in for a recharge, so, if i increase the out put port to a bit larger ill see alot more cfm but ill be recharging sooner unless i turn the psi down on the output valve a bit but at the same time ill still go threw more air but gain more cfm either way ,, ill have to experiment with this on my 3 gallon tank dont want to mess wiht the 8 unless it wokrs

see i saw a ch 12 gallon set up and its got the same CFM ratings as mine so the tank size has noting to do with the cfm out put and has only part to do with the motor but ill bet its the out put regulator valve thats doing it .,....a bigger tank just mease takes longer to fill or drian psi ratings were even the same ,, so i thinks me onto something here ..


I am not exactly sure what you mean by what you are saying but don't get the discharge rate of the tank confused with CFM they are two different things, CFM is the air coming from the pump INTO the tank and is the ONLY thing that determines how much run time you get over a given work period. If you are having problems with getting enough air flow out of the small tank that is a restriction problem and has nothing to do with tank size. I will try this once more and then you can see for your self what many others before you have found out already, the amount of air that you will have available to work with over any given time period past the first time the compressor kicks on will be exactly the same no matter how many tanks you have hooked up. Think of it this way, the compressor is going to be pumping air while it is running and what air that compressor pumps during the work period is ALL the air you will get no matter how big the tank is! If you have a bigger tank it might at first SEEM like you are gaining something but the pump is not replacing the air you are using. Theoretically you would get exactly the same amount of work time vs wait time over a 5 minute work period, or any work period past the first start-up cycle, but in reality you will get less because the compressor will exceed it's duty cycle and lose efficiency. You probably would not lose much with just 5 minutes or so but the longer you work the worse it would be, in any case what you are trying to do by adding tank capacity is simply impossible because ANY amount of extra run time gained at the first, and succeeding cycles, will most certainly be off-set by the extra waiting time on the recharge-it is absolutely impossible for it to be any other way (except for losing time)!


Now if you have another smaller compressor with that little tank you can hook two complete compressor's together and combine the output from both to extend working time.

ftl
04-18-2009, 04:49 PM
In terms of getting the most high-pressure air from a compressor setup, would it not work to double the tank capacity, but at the same time change the cycle points for the compressor from say 80/100 PSI to 90/100 PSI? That should leave the compressor cycle times similar, while increasing the average pressure from 90 to 95 PSI. I know this does not increase the total volume available at a given pressure, but if you are using the air intermittently it would help to keep the pressure up with a minimum of 90psi available at most times, instead of a minimum of 80psi.

A different issue is the efficiency of the compressor. A friend of mine that has worked in engineering natural gas compressor stations told me that they always attempted to keep the compression ratio at each stage below 6:1. Above 6:1 he says that the compressor efficiency drops significantly because so much of the energy goes into heating the gas. When the heat is lost (in the tank or the pipeline) the pressure drops again.

To put this into perspective ambient pressure is about 15psi which at 6:1 gives you about 90psi. For anything over even 80psi he says a dual stage compressor is much more efficient.

If you use a two stage compressor with only 3:1 at each stage you will get:
15psi x 3 = 45psi x 3 = 140psi.
The 3:1 compression ratio apparently end up with a lot less energy lost to heat. An inter cooler between the stages can also be used to reduce the heat buildup in the second stage.

My friend claims that this would get you significantly more energy in the compressed air for the electrical energy you put in by wasting far less in heat.

airsmith282
04-18-2009, 05:37 PM
ok how about this my small 3 gallon ch compressor can not run my 1/2 inch impact gun , my 8 gallon one will run the hell out of it both comprssors set at 100 psi output

why wont the smaller machine run the impact gun..

this was done with neither compressor motor running

so fully chaged tanks on each

unpluged the power to each machine both set up for 100 pis output

same results

so dont tell me its the compressor motor that determens what tool it can run when both machines are fully charged with air and psi output it set the same on each one ..cause that dont make any sence

you do have and input cfm and and out put cfm that i get

so far all i have heard is its all upto the motor when i cant see that being the factor the main factor cause its not realeasing any air into the tank when the sucker is not pluged in,, and its not running when it is pluged in and switched on an yet psi is the same set up so the relaing cfm has got to be dictated my the size of the output valve hole , restriced out put air flow at same psi means less cfm tank size means how much air you got in there to use part of the CFM if you put in 9 cfm and you get output of 4.5 then thats half output at 100 psi in this case

so if oyu can only put in 2 cfm you get .83 cfm out and thats part if the output of the regulators hole size, you inlarge the size of the hole you empty the tank faster and you increase CFM thats what i was getting at.. psi is psi reducing it to say 40 form 90 means longer run time for air before the compressor kicks back in but if the tool wont work its useless and also if there is not enough cfm discharge at 100 psi the tool also wont run ,, so ok what if you where to increase the input hole from the pump to allow more into the tank faster does that make sence at all ..???????

there has to be a way to get more cfm out of a 3 or 8 gallon set up with out using a bigger motor on either one..

i looked at wall mart today at 5 gallon twin stack compressor so 2.5 per bottle , any how and its an off shore brand any how the motor is much smaller then my 8 gallon one but it runs at 5.2 and 4.5 oiut put cfm so much higher then my ch 8 gallon one the size of the motor was physicly smaller but the HP was like 3 peak and 2 continuis where my ch is 1.3 continuis .. so not sure on that one there

David Powell
04-18-2009, 06:53 PM
Over the years we tried all sorts of the cheaper compressor outfits. The direct drive ones are noisy, output tends to fall as they get hot, motors have cooked( Only offshore ones). One direct drive job broke its crank in the middle of the show. I have one in the shed now, sure it pumps air but once the pressure gets up to about 75 lbs it pumps about half and half crankcase oil. What you need is a belt driven two cylinder compressor of a size that will keep a 2 to 5 hp motor busy but not overloaded and about a 20 to 40 gallon tank and the controls and safetys to match. It wont plug into a 15 amp socket and you wont be able to carry it. Yesterday.s Princess Auto flyer shows item no 8059404 which they call a 4.5 hp twin cylinder pump for $ 149. They dont give cfm, but do say 155psi and 1400 rpm max. Likely you COULD do better buying secondhand real industrial stuff secondhand, theres lots of businesses closing down these days, but mind it isnt worn out when you buy it. A bit of help is worth a lot of advice, come down here with$ 500 t0 spend, stay with me a day or two ( NOT this week granddaughter is here) and we will find or build you a damn good one, and you can drive my steam models. Regards David Powell.

radkins
04-18-2009, 08:20 PM
ok how about this my small 3 gallon ch compressor can not run my 1/2 inch impact gun , my 8 gallon one will run the hell out of it both comprssors set at 100 psi output

why wont the smaller machine run the impact gun..

this was done with neither compressor motor running

so fully chaged tanks on each

unpluged the power to each machine both set up for 100 pis output

same results

so dont tell me its the compressor motor that determens what tool it can run when both machines are fully charged with air and psi output it set the same on each one ..cause that dont make any sence

you do have and input cfm and and out put cfm that i get

so far all i have heard is its all upto the motor when i cant see that being the factor the main factor cause its not realeasing any air into the tank when the sucker is not pluged in,, and its not running when it is pluged in and switched on an yet psi is the same set up so the relaing cfm has got to be dictated my the size of the output valve hole , restriced out put air flow at same psi means less cfm tank size means how much air you got in there to use part of the CFM if you put in 9 cfm and you get output of 4.5 then thats half output at 100 psi in this case

so if oyu can only put in 2 cfm you get .83 cfm out and thats part if the output of the regulators hole size, you inlarge the size of the hole you empty the tank faster and you increase CFM thats what i was getting at.. psi is psi reducing it to say 40 form 90 means longer run time for air before the compressor kicks back in but if the tool wont work its useless and also if there is not enough cfm discharge at 100 psi the tool also wont run ,, so ok what if you where to increase the input hole from the pump to allow more into the tank faster does that make sence at all ..???????

there has to be a way to get more cfm out of a 3 or 8 gallon set up with out using a bigger motor on either one..

i looked at wall mart today at 5 gallon twin stack compressor so 2.5 per bottle , any how and its an off shore brand any how the motor is much smaller then my 8 gallon one but it runs at 5.2 and 4.5 oiut put cfm so much higher then my ch 8 gallon one the size of the motor was physicly smaller but the HP was like 3 peak and 2 continuis where my ch is 1.3 continuis .. so not sure on that one there



Again I am not exactly sure of what's happening here but I take it that the small tank will not run the impact right from the start? If so that is, as I said earlier, a restriction from the tank and has nothing to do with tank size. If you open up the discharge fitting from this small tank that would solve that problem but three gallons is not going to run an impact wrench very long, not even a small one. If you increased the size of the three gallon tank to 30 gallons it still would not run the impact any differently if you did not open up the discharge fitting. The CFM rating has nothing to do with how fast a compressor can discharge air (or in other words how much air from the tank to the tool), the CFM rating refers to how fast the pump can replace the air that is being used, or another way to look at it would be how much air (in CFM) the compressor produces over a one minute period.


so dont tell me its the compressor motor that determens what tool it can run when both machines are fully charged with air and psi output it set the same on each one ..cause that dont make any sence

Past the first on/off cycle that is exactly the way it is! When you first start out you can run anything from the small compressor that you could on one that is ten times as big, BUT ONLY UNTIL THE PRESSURE DROPS TO THE CUT-IN POINT! After the first time the compressor kicks back in the pump/motor is the ONLY thing that determines how much run time you will have because no matter how big the tank is the pump has to replace the air you are using. That's why no matter how big your tank or how high the cut-off pressure the run time vs the wait time will remain the same, there is just no way around it.



Increasing the size of the supply line fitting from the pump to the tank will not help because it is such a small amount of flow that line would have to be really small to cause a restriction.

airsmith282
04-18-2009, 08:36 PM
i talk to a buddy tonight and he explained to me where the problem is and its not t do wiht tank size its the compressor it self the piston size has alot to do wiht CFM so if i gota tiny piston like in the 3 gallon then i get less then 1 cfm in this case so now way in hell its going to do more hten run a small nail gun or inflate tires, were has my 8 gallon compressor has a very large piston so its able to produce more CFM aka input is about 9 out put is 4.5 any how thats the facts right form him,, better explaination to me and now i under stand as wel that unless i was to put on a larger comperssor on the tank i will not run more hten the brad nail gun or inflate tires wiht the 3 gallon unit , i was able to run my die grinder on it for a few seconds and run my cut off tool for a few seconds but no way for impact gun and the air ratachet well its didnt do well either but anyhow i now know a tin more then i did before and perhaps i might sit down and see if i can design up a better system someday , iam going to buy the off shore 5 gallon twin stack i saw only cause it will put ot alot more cfm then my 8 gallon setup which will be handy on my sand blaster.. anyhow thanks everyone for the info and no iam not goingt o jack up the max PSI my tank wont handel it and iam not stupied but it was and idea just not a good one...

Evan
04-18-2009, 09:07 PM
That is what we have been telling you all day now.

Would you please download and use a spell checker. It's really hard to decipher what you write. It won't fix everything but using one will help you to improve your spelling. I make many typos when I type and I spend a lot of time proof reading before I post anything. Even then I don't catch them all but I will go back and edit my posts to fix them.

I almost never get after anybody for spelling, it's too easy to have it boomerang back at you but your spelling is terrible. It's about making yourself understood so that when you ask a question people will be able to give proper advice.

Sorry to be so blunt but in the end you will be much better off if you take my advice.

oldtiffie
04-18-2009, 09:19 PM
Let's cool it down a bit.

I agree that the "last little bit" as regards pressure is the hardest. My 145 psi compressor is really working from about 130psi onward - but it does what I need it to do.

Next, that air is pretty hot when it gets to the tank and it cools and contracts and tank pressure drops and the compressor kicks in again - Boyle's Law - as expected. It then pretty well stays at 145 psi then.

Next, I have an unloading valve on my compressor which unloads the compressor>tank line so that the motor starts under no load conditions. Not having an unloader is a sure path to a cooked motor.

Motor cooling is next - mine has a very good axial flow shrouded system - but it only runs when the motor does.

I agree -as I've said previously - as regards extra tank size and manifolding - there is no free lunch. But if extra tanks are installed on a manifold and each has its own isolation/on-off valve you can choose which ever combination of arrangements you like to suit the occasion.

A large/better compressor will require more power and may even require considering installing 3-phase power if you only have single phase.

The amount of power (and number of phases) you can have at the point of supply will be dictated by your supply utility and not necessarily by the current supply to the main board (usually where the meter is).

Using a lot of power may require some "load-shedding" depending on what power you are drawing/using on other equipment at the time.

But back to the compressor.

Realistically the sizing of the compressor is or should be determined by the air supply requirements at the tools - not at the compressor.

There are usually two pressure guages on the compressor - "tank" pressure (in the tank) and "line pressure" (down-stream from the line-pressure regulator.

If you set your line pressure to say 90psi in static state - that is where it will stay until there is an air demand from a tool. The line pressure will, depending on the demand an the efficiency of the regulator, may drop to say 70psi due to the pressure-drop across the regulator. Now if you say set a guage at the tool (static - no demand) of say 80psi you may well find that as soon as you start the air tool that the line pressure a the tool is only 50 psi - due to pressure loss/drop in the air line line between the regulator and the tool. Getting and maintaining say 90psi at the tool may be well near impossible so you have to "back-track" along the line until you guessed it - the compressor.

So the line is quite often just as much a problem as the compressor is.

That is why I went for a 15 bar (145psi) compressor that "switches in" at 7 bar (100.5psi) and "off" at 10 bar (145psi). The motor on my compressor is 2 HP and draws 1.5Kw (say 6 Amps) which is well within my power capacity.

http://i200.photobucket.com/albums/aa294/oldtiffie/Compressor/Compressor_2.jpg

I also bought it for use around the house and property so it needs to be within the capacity of the house outlets and power.

I refused to put in an air line in the shop as it was of no benefit at all as there are too many sources of line loss and condensation. I just use a short 10mm (3/8") hose and take the compressor to the job if needs be. Otherwise I use a 50 foot close-coiled 6mm (1/4") line for general air work and leave the compressor where it is.

For normal "air gun" work I use 40>60 psi as it does as well as 90psi and it is less demand on the compressor.

I gave away all my (several) air-powered "die" and other grinders and tools and just use electric ones now. The big problem with air grinders is that they really do compound the problems of grinding grit even on electric tools as they blow the grit everywhere. It is made worse if I am limited to air-line connections in the shop. I don't have that problem with the portable compressor and I can and do take all my grinding (other than tool and cutter and surface grinding) out of the shop into the concrete car-port apron or onto the gravel drive. (Same with welding!!).

If I had a need for a larger capacity compressor I would buy an engine-powered (gasoline) twin/triple cylinder 2 or 3-stage compressor with an inter-cooler - but a portable one for the previous reasons.

So "sizing" a compressor as regards capacity, tank sizes and numbers as well as power and air capacity at the tool is not easy - nor is it cheap.