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Bryce.R
05-17-2012, 08:18 PM
Hey everyone :) my old man is buying an air compressor for our shop. He wants to get one on wheels with a 50 litre tank, i want to get one with a 110 litre tanks with pallet feel. The compressor needs a 15 amp plug so its not as though we are going to be able to move it anyway. He is not convinced though :mad:Can you help me out with some good arguments to get the compressor with the larger tank? :)

oldtiffie
05-17-2012, 09:05 PM
This is my current one which is very good.

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

Here are its specs:

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

This will be my next one - gasolene and Honda engine:
https://www.machineryhouse.com.au/C506

No power problems, no "hard-wiring in", no need to install 3-phase and is 100% portable, will run my plasma cutter and a small sand-blaster (if I buy one).

radkins
05-17-2012, 09:30 PM
First it's a serious mistake to think the larger tank makers a larger compressor, it does not! The tank is not sized to increase performance, contrary to popular belief, it is sized ONLY to manage the on/off run cycles based of the pump/motor CFM and projected usage, adding extra tank capacity WILL NOT increase a compressor's performance! Think about it, a tank does not make air it only stores air that's pumped into it and the oft quoted (mis-quoted!) advantage of more "Reserve air" does not make sense. If you double the size of the tank you of course will double the length of time it will operate before running out of air (which usually happens quickly with either size tank on a small compressor) but you must remember you also double the time it takes to recharge the larger tank so the net gain is exactly nothing. The bottom line is that for a given CFM delivery (Cubic feet per minute- converted to however it's rated where you are) the actual run vs wait to catch up time will remain the same during any work period regardless of the size of the tank, small tank=shorter but more frequent cycles and larger tank=longer but fewer cycles, average time remains the same!


Now after all that here's how to choose the best compressor.

Basically you need to determine the amount of air you can get by with by looking at the volume of air required to run each tool (in CFM or whatever) then try to match that as closely as possible to the rated delivery volume of the compressor. Remember, it's the volume of air from the PUMP (the CFM rating) that determines the compressor's performance and not what's stored in the tank! If that CFM (volume) rating is low then nothing else matters much, NOTHING! Because a tank cannot put out more air than the pump is delivering to it, a larger tank does NOT equate to a larger compressor!

gnm109
05-17-2012, 10:14 PM
If you are wise, you will find a compressor with an 80 gallon tank and at least 20 CFM. Preferably with a real 5 hp motor. Only issue would be that your 15 amp line won't be enough for one.

More CFM is better if you want to run a blast cabinet and other large air tools.

flylo
05-17-2012, 10:41 PM
If you are wise, you will find a compressor with an 80 gallon tank and at least 20 CFM. Preferably with a real 5 hp motor. Only issue would be that your 15 amp line won't be enough for one.

More CFM is better if you want to run a blast cabinet and other large air tools.


I agree 20+ CFM @90psi not 40psi. I believe they may run 240V where 15 amps should work.

gnm109
05-17-2012, 11:06 PM
I agree 20+ CFM @90psi not 40psi. I believe they may run 240V where 15 amps should work.

Right. I should have added that the 20 SCFM should be at 90 psi. It would be nice to have a 240 VAC motor, too. For some reason, the company that built mine used 120 VAC. It may have to do with marketing and wider sales since not everyone has 240 VAC available.

Willy
05-17-2012, 11:45 PM
Some very good advice so far.
But the real question is, what is the intended or perceived use of the air compressor?
An impact gun to take off the occasional tight set of nuts, inflating a few beach/camping bits, or maybe blowing off a couple of parts?

Or maybe you intend to sandblast a few pieces of equipment or use an air sander.
Irregardless it's CF that should be your primary goal.

Willy
05-18-2012, 12:04 AM
..................
...................
This will be my next one - gasolene and Honda engine:
https://www.machineryhouse.com.au/C506

No power problems, no "hard-wiring in", no need to install 3-phase and is 100% portable, will run my plasma cutter and a small sand-blaster (if I buy one).

Tiffie, while the concept of an "unchained and independent" source of compressed air is a good one. There is a price.

Think about the luxury of compressed air that is unobtrusive.
No noise in the background while the power source is chugging away doing nothing but eating fuel, wearing out, and making noise.
Not having to deal with the option of pull starting the engine whenever you need a puff of air.
Also keep in mind the extra short cycle increased maintenance you should do, unless of course the unit runs steady and then that will also included more running costs.

The only time the independence pays for itself is when you don't have the option of a mains powered source.
Not sure where you may need air. Is it remote?

flylo
05-18-2012, 12:42 AM
Picked up 2 last week I;ve never seen. These are 20-25 gallon huge cast iron finned 120V motors true 3HP, 2 stage V twin pumps, twin belt drives. They are basically made for 60 gallon verticle tanks but on smaller roll around tanks. I love them as they go to 135PSI in no time, run slow, quiet, could run all day & nothing gets hot (except the finned copper tube between the heads. I'm sure they're older imports with oil sight glasses) & 2 outlets & 2 handles each as they weigh about 300# each. Tagged Astra on the blue one & Cummins/IHC on the other. Jst odd I've never saw one & buy 3 in a week. These are imports but very good quality & performance. I'll try to get a picture. One has 4 wheels which was a great idea someone had.

flylo
05-18-2012, 01:04 AM
Look up Harbor Freight - item#56101. Just bought a slightly used one still bolted to the crate bottom for $300. Except mine has the Honda clone engine & it's 2 years old. Divorce sale.Too bad your 1/2 a world away. This would put a smile on your face. Turn the key & 0 to 180 in nothing flat!



This is my current one which is very good.

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

Here are its specs:

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

This will be my next one - gasolene and Honda engine:
https://www.machineryhouse.com.au/C506

No power problems, no "hard-wiring in", no need to install 3-phase and is 100% portable, will run my plasma cutter and a small sand-blaster (if I buy one).

BobL
05-18-2012, 01:05 AM
Hey everyone :) my old man is buying an air compressor for our shop. He wants to get one on wheels with a 50 litre tank, i want to get one with a 110 litre tanks with pallet feel. The compressor needs a 15 amp plug so its not as though we are going to be able to move it anyway. He is not convinced though :mad:Can you help me out with some good arguments to get the compressor with the larger tank? :)

As others have said, the larger tanks is not going to help much unless it is matched by a larger compression unit.

Even though my compressor has wheels, compressors on wheels that are wheeled up to the job are a PITA. They are noisy and they and the lines always get in the way. One of the best things I did in my new shop was to put the compressor outside in a noise suppression chamber and retic the air inside my shop. A full retic is not essential just run some line to a couple of points inside and then use a retractable air hose from there in. After using my setup for 12 months I'd never go back to a compressor inside my shop.

J Tiers
05-18-2012, 01:52 AM
As others have said, the larger tanks is not going to help much unless it is matched by a larger compression unit.


Sounds good, isn't necessarily so.

If the prime need is relatively infrequent use of a fair amount of "surge" CFM, a large tank and small pump MAY be just right.

if you had exactly the same compressor pump, and a little "hot dog" tank, you couldn't do that same job...... Not unless you got a much larger pump that could supply the maximum surge CFM.

it's all about the "average"...... you can't escape getting enough pump to take care of your average CFM...(or the total daily CFM). But if that use is infrequent use of a lot for a short time, you may well be able to do better with a smaller pump if you have a large tank.

The only things are that you need enough tank volume to supply your total air volume for that short time, since the pump isn't going to help much during the usage time, and you need to not require much more air until the tank is full again..

lakeside53
05-18-2012, 02:45 AM
I agree - a larger tank is very useful if you needs are "bursty". Same with running at tank at 175psi (and rated for such use!) and having say a 100psi regulator.

BobL
05-18-2012, 02:50 AM
Sounds good, isn't necessarily so.

If the prime need is relatively infrequent use of a fair amount of "surge" CFM, a large tank and small pump MAY be just right.


Sure I agree in terms of air delivery, but unless the pump is designed to cope with longer pump up times a small pump and large tank is also more likely to wear out the pump and is why it is not wise to just keep adding tanks to a small compressor.

On something like a compressor most of the pump wear happens when the machine is at its hottest. So in the case of a small compressor it's probably better if it runs more frequently, for short periods at lower temps, than for long periods infrequently where it has to operate at higher temps for much longer at the end of the pumping cycle.

radkins
05-18-2012, 11:01 AM
I agree with Bob, while it can certainly be argued that "burst" air supply would indeed be helped by a larger tank this is in most cases going to comprise only a small percentage of the compressor's use, basically only during the odd times where only one tank of air would do the job and the little extra might make the difference. The question is asked all the time "should I get an 80 gallon tank compressor or settle for a 60 gallon" as if the 60 gallon is somehow inferior when the fact is they could not tell the difference. Fellows you can argue all day about tank sizes and the manufacturers of the home shop type compressors love it! They have for years been sticking tiny pumps on top of cheap huge uselessly over-sized tanks knowing full well that a lot of guys, not knowing any better, will run straight to the biggest tank in the store because a big tank just looks, well it looks like a big compressor! This "reserve air" only helps in the odd instance where the compressor is needed for only one run cycle then any time that is gained by the extra tank capacity is lost to the proportionally longer recharge time. I am not saying a big tank is a bad idea just that on a properly designed compressor the tank will be chosen by the designers to balance the run/recharge cycles and adding more tank capacity is in most cases not only useless but in some cases can be counterproductive.


In the case of the usual 80 vs 60 argument think of it like this, if the compressor is already running out of air at an annoying rate with a 60 gallon tank than just how long would it take to run out of air if it had only a 20 gallon tank? Obviously it would out in an extremely short time (but would also recharge in a very short time!) but those few extra seconds is all the extra time the 80 gallon tank would add to the 60 gallon's run time but the kicker is that THEN even that is lost to the longer recharge time! Of course if it's not running out of air with the 60 gallon tank then there's no problem in the first place.


The bottom line is waaaaay too much emphasis is placed on the size of the storage tank and far too often people look at these tanks and overlook what really matters-THE CFM RATING!!!! An 18 CFM rated compressor with a 60 gallon tank (or even a 40 for that matter) will easily outwork a 16 CFM compressor with an 80 gallon tank but that bigger tank will outsell the smaller one two to one because of the myths and misunderstandings. The fact is the size of the tank (within reason of course) simply does not matter much and matters little at all from a performance standpoint so choose that compressor based on CFM and quality in the price range you can afford and don't worry about the tank!

A BIG TANK DOES NOT MAKE A BIG COMPRESSOR!!!

gnm109
05-18-2012, 11:50 AM
If you like small compressor tanks, that's fine. Discussion has been had here about large compressor tanks with small pumps. Who builds those? The compressors I've seen with smaller tanks, usually at Lowes or Home Depot, generally have one of those smaller compressors with alloy crankcases. They usually say that they have a cast iron liner, but who knows? They're all made in China, too with those nice Chinese motors. (Google Ingersoll-Rand sometime using keywords "compressor fires".)

How about getting a compressor with a big tank, a large compressor, enough CFM and a motor big enough to drive it?

Who wants a compressor with less than 20 CFM anyway? I sure don't. A compressor needs to be sized to match the largest air consumer in your shop. In my case, that would be three items: A large bead blasting cabinet, a 3/4" Impact wrench, and a large sand blasting unit. All of them are air hogs. Using those toos, with a small compressor, it would be running all of the time and I'd have to listen to it, too.

When I bought my present compressor I wanted air, lots of it. I paid big bucks and got an excellent unit. That was 15 years ago and I've never been sorry. I've had several small compressors and worn them all out. If you need air, don't buy a small compressor with a small tank and a small motor. That's one place where you don't want to economize.

My opinion only, of course. LOL.

lakeside53
05-18-2012, 12:28 PM
The argument that 80 only lasts a few seconds longer than a 60 can be extended - get a 40 or 20 then - you're only getting a few seconds! I don't buy it... a great deal of what a compressor gets used for is " bursty" and a larger tank works fine for that. If you need a certain continous cfm, then sure, get a compressor suited for that. If it's so important to match the compressor to the tool, then why have a tank at all?;)

I have a 60 with a 3hp Quincy pump, because that's how I bought it. It's not for sandblasting, but it does all I need of it.

radkins
05-18-2012, 12:31 PM
A compressor needs to be sized to match the largest air consumer in your shop.



That's it in a "nut shell" as the old saying goes (wonder where that saying comes from? :confused: )


The whole point about the large vs small tank size was not that one is better or worse than the other, that depends on the size of the pump/motor, but rather the point is that choosing a compressor based on the size of the tank can be a serious mistake. It's easy to find examples of the 80 gallon tank compressor with less CFM than a 60 gallon model (a particular model of Campbell Housfeld vs some Quincys for example) and buying lower CFM to get a bigger tank is most certainly going to result in LESS performance not more, comparing a CH to a Quincy is a bad comparison for other reasons but the point remains the same. A quality compressor is going to have the tank sized to match the pump and available power so the point is buy based on CFM rating matched to the projected shop demand and don't worry about the tank, CFM is what makes the difference not the tank capacity!

radkins
05-18-2012, 12:43 PM
The arguement that 80 only lasts a few seconds longer than a 60 can be extended - get a 40 or 20 then - you're only getting a few seconds! I don't buy it... a great deal of what a compressor gets used for is " bursty" and a larger tank works fine for that. If you need a certain continous cfm, then sure, get a compressor suited for that.

If it's so important to match the compressor to the tool, then why have a tank at all?;)



I am NOT saying a small tank is better, just that a larger tank is not necessarily better either!

You might be surprised to know that some high CFM screw and rotary compressors DON'T have a tank or even if they do it can be very small. As far as your "get a 40 or 20 gallon then" example you are missing the point, I said within reason but even those small tanks would still provide the same average run vs recharge times with all else being equal! Of course if the tank is too small it creates other problems such as too frequent cycling even if available use times remain the same but in the case of the 40 vs 60 it may not be as bad as you seem to think! One popular tank option we had was to use a truck bumper, because of available space, as the tank on some service trucks (6" diameter x 3/4" wall) which was about 40 gallons capacity with a 24 CFM compressor, there was no complaint about tank capacity. Believe what you like about a bigger tank making for more performance, those cheapo compressor manufacturers love those who think that! :D



The argument that 80 only lasts a few seconds longer than a 60 can be extended - get a 40 or 20 then - you're only getting a few seconds! I don't buy it...

You may not be "buying it" but it's still true! If you run your 60 gallon dry in a minute or two then just how many seconds would another 20 gallons get you? If your 60 gallon is not running dry then you don't have a problem in the first place. Besides it's a moot point anyway because then you have to wait a proportionally longer time on the recharge so please explain how you gain?

gnm109
05-18-2012, 01:15 PM
I am NOT saying a small tank is better, just that a larger tank is not necessarily better either!

You might be surprised to know that some high CFM screw and rotary compressors DON'T have a tank or even if they do it can be very small. As far as your "get a 40 or 20 gallon then" example you are missing the point, I said within reason but even those small tanks would still provide the same average run vs recharge times with all else being equal! Of course if the tank is too small it creates other problems such as too frequent cycling even if available use times remain the same but in the case of the 40 vs 60 it may not be as bad as you seem to think! One popular tank option we had was to use a truck bumper, because of available space, as the tank on some service trucks (6" diameter x 3/4" wall) which was about 40 gallons capacity with a 24 CFM compressor, there was no complaint about tank capacity. Believe what you like about a bigger tank making for more performance, those cheapo compressor manufacturers love those who think that! :D




You may not be "buying it" but it's still true! If you run your 60 gallon dry in a minute or two then just how many seconds would another 20 gallons get you? If your 60 gallon is not running dry then you don't have a problem in the first place. Besides it's a moot point anyway because then you have to wait a proportionally longer time on the recharge so please explain how you gain?


That's the sticking poiint in this discussion with which I'm having trouble. You say that a larger tank takes a proportioinally longer time to pump back up. That's not the case with my compressor which has a two stage, twin cylinder pump that weight about 150 pounds. It stops at 150 psi and will start again at 130 psi. I never run more than 90 psi. It will shut down again in less than 30 seconds.

Moot? Is that lawyer talk....careful. Black Forest doesn't like lawyers.

As to where did the term "nut shell" come from. I don't know. I did hear where the term "making your nut" came from. I heard that the circus would come to town and rent a space from the local mayor. The mayor would remove the nuts from some of the circus wagon axles to make certain that the circus couldn't leave town until the rent was paid, presumably after the first few days when the circus took in some money. Thus, if you made enough money, you got your nut back, i.e. "made your nut".

This is different from modern marriage where the man never gets them back. LOL. :D

(Trying some levity to make you forget that you really, really like small compressors.....)

radkins
05-18-2012, 01:52 PM
That's the sticking poiint in this discussion with which I'm having trouble.

[You say that a larger tank takes a proportioinally longer time to pump back up.]

[That's not the case with my compressor]....)


Umm, I think you had better think about the laws of physics here! Are you saying that your compressor could pump up a big tank just as fast, or nearly so, as a small one? :confused:

I think not and it most certainly would take a proportionally longer time to pump up any extra tank capacity, just simple laws of physics!


The time it takes to recharge a tank is solely determined by the CFM it's capable of vs the storage capacity of the tank, decreasing or increasing either CFM or tank capacity will affect the recharge times proportional to the increase or decrease of either.

Of course a compressor with a higher CFM rating (larger pump/motor) could recharge an 80 gallon tank as fast as a lesser rated compressor with a 60 gallon tank but that's comparing apples to oranges! (I know where that one comes from BTW :) ) Obviously a larger compressor as a unit is going to outperform a smaller compressor with less CFM, no one has suggested otherwise. The whole point of my post is that far too often (and I mean FAR too often!) a compressor is chosen by the buyer being impressed by a useless large tank while a much better performing model with more CFM but a smaller tank gets passed over for that big tank! No matter how the argument gets twisted the tank simply cannot put out more air than gets pumped into it and extending performance is not the reason designers choose the tank capacity. Ideally a compressor will have a balanced run/cooling cycle ratio and the tank is chosen to maintain this balance as much as practical within a compressor's expected demand, if the tank is being exhausted due to too much demand the only two solutions are either reducing demand or a larger compressor as a UNIT not just a larger tank! The larger tank by itself simply changes the cycling rate of the compressor and does not change the available run time during a given work period.




No doubt there may be a few times where that extra capacity might help, such as managing to get that last lugnut off before the compressor has to recharge but this rarely happens enough to make any real difference and in fact if the tank is too large performance can actually suffer. This is rarely a problem with a factory built unit but some home assembled compressors or factory compressors that have had extra tank capacity added could indeed suffer problems. Adding an extra tank in the futile attempt to increase the performance of an inadequate compressor is the most common example, what happens is the run/recharge cycles become so unbalanced that pump overheating occurs. A hot pump losses efficiency rapidly meaning that the extra tank capacity is in fact reducing available use time rather than increasing it.

gnm109
05-18-2012, 02:31 PM
Umm, I think you had better think about the laws of physics here! Are you saying that your compressor could pump up a big tank just as fast, or nearly so, as a small one? :confused:

I think not and it most certainly would take a proportionally longer time to pump up any extra tank capacity, just simple laws of physics!


The time it takes to recharge a tank is solely determined by the CFM it's capable of vs the storage capacity of the tank, decreasing or increasing either CFM or tank capacity will affect the recharge times proportional to the increase or decrease of either.

Of course a compressor with a higher CFM rating (larger pump/motor) could recharge an 80 gallon tank as fast as a lesser rated compressor with a 60 gallon tank but that's comparing apples to oranges! (I know where that one comes from BTW :) ) Obviously a larger compressor as a unit is going to outperform a smaller compressor with less CFM, no on has suggested otherwise. The whole point of my post is that far too often (and I mean FAR too often!) a compressor is chosen by the buyer being impressed by a useless large tank while a much better performing model with a smaller tank gets passed over for that big tank! No matter how the argument gets twisted the tank simply cannot put out more air than gets pumped into it and extending performance is not the reason designers choose the tank capacity. Ideally a compressor will have a balanced run/cooling cycle ratio and the tank is chosen to maintain this balance as much as practical within a compressor's expected demand, if the tank is being exhausted due to too much demand the only two solutions are either reducing demand or a larger compressor as a UNIT not just a larger tank! The larger tank by itself simply changes the cycling rate of the compressor and does not change the available run time during a given work period.




No doubt there may be a few times where that extra capacity might help, such as managing to get that last lugnut off before the compressor has to recharge but this rarely happens enough to make any real difference and in fact if the tank is too large performance can actually suffer. This is rarely a problem with a factory built unit but some home assembled compressors or factory compressors that have had extra tank capacity added could indeed suffer problems. Adding an extra tank in the futile attempt to increase the performance of an inadequate compressor is the most common example, what happens is the run/recharge cycles become so unbalanced that pump overheating occurs. A hot pump losses efficiency rapidly meaning that the extra tank capacity is in fact reducing available use time rather than increasing it.


My compressor pump is, indeed, quite large and was sized to work properly to recharge the 80 gallon tank in a reasonable period of time. Neither does it overheat, even when running the bead blaster.

The extra tank capacity does come in handy as well. For example, only this morning, I was putting the finishing coats of lacquer on an electric kit guitar The gun I'm using is an old style one that requires about 6 CFM to spray. I started working and put two entire coats of lacquer on a neck and a body. I worked for nearly 15 minutes before the compressor started and it only did so twice in a half hour's work. Admittedly, I'm only using about 40 psi but it's very pleasant to work without hearing the compressor running constantly.

I got this compressor out of frustration with an endless succession of smallish Sears compressors which were running literally all of the time, often while letting the pressure fall below 90 psi when I needed it.

I remain unconvinced that a small tank will have as much capability as a large one, given a properly sized compressor. You are obviously sold on small compressor tanks. We can agree to disagree, nicely, of course.

:)

radkins
05-18-2012, 03:13 PM
I fail to see why you think I am "sold on small compressor tanks" :confused: I am simply saying that tank capacity does not make up for a lack of CFM! And yes given the proper compressors a small tank vs a larger one does have as much capability as a large one from the standpoint of available time during any work period.

You seem to be missing the point, once more I am not saying a small tank is better just that a bigger tank is not necessarily better either when selecting a compressor because it does not make a bigger compressor, and also that buying a compressor based on the size of the tank is often a huge mistake but still it happens all the time. Whether you understand my point or not it's still a fact that extra tank capacity will not off-set a lack of CFM and that's what this whole thread has been about! The OP was talking about a bigger tank and did not even mention the real concerns when selecting a compressor and this very thing is all too often the case. I fail to see what's so hard to understand, like I said a tank can only put out as much air as is being pumped into it so it should be obvious that a unit with more CFM going into the tank is what matters, not how much is stored in it at any given time.


Let's take a look at your example here and then go back to a couple of things I said earlier.

"The extra tank capacity does come in handy as well. For example, only this morning, I was putting the finishing coats of lacquer on an electric kit guitar The gun I'm using is an old style one that requres about 6 CFM to spray. I started working and put two entire coats of lacquer on a neck and a body. I worked for nearly 15 minutes before the compressor started and it only did so twice in a half hour's work. Admittedly, I'm only using about 40 psi but it's very pleasant to work without hearing the compressor running constantly."


First off I have said repeatedly that if you are not exhausting your tank then there is no supply problem, which is apparently the case here, and attempting to solve a supply problem by using a larger tank is what we have been discussing. You say your spray gun is using 6 CFM so you are not running out of air and your work time would not be affected whether you had an 80 gallon tank or an 8 gallon tank, this compressor would run more but shorter cycles but you still would not run out of air! Of course that is an impractical extreme using only 8 gallons as an example but it would be impractical only in the sense the compressor would have to cycle too fast, as far as having air available there would theoretically be no difference between 8 or 80 gallons stored in the tank. In your case your compressor was exceeding demand and theoretically it would not even need a tank in order to keep up so the size of the tank, big or small, does not matter from an air supply standpoint. The problem is when it's the other way around and demand exceeds the supply capabilities of the compressor and people opt for a bigger tank instead of more pump CFM in a futile attempt to solve the problem. This is what we have been discussing and simply opting for a larger tank with all else being equal accomplishes little or nothing! An even bigger mistake, and this happens all too often because of this "bigger tank is better attitude", is when someone passes up a compressor with more CFM which actually would help just to get a bigger tank which will not!



Granted the lack of having to listen to the compressor may be a plus for you but air supply is what the discussion has been about and listening to the compressor has until now never been part of the discussion, still the same principle applies and it may not matter as much as you think anyway. You use as an example that your compressor only kicked on twice during the time you were using it but did it occur to you that when it does start up you will have to listen to it for a longer time each time while recharging that 80 gallons vs 60? The fact is, and this has been my point all along, you had to listen to the compressor for the same amount of time with that 80 gallon tank as you would have had it been only 60 gallons! Just as with having to wait on the air to recharge in a bigger tank the time you will be exposed to the noise will be exactly the same with a big tank vs a small one, shorter but more frequent time periods with the small one vs fewer but longer time periods with the larger one. So with that 80 gallon you were exposed to two long noise cycles as opposed to maybe three shorter ones but the time still remained the same. Certainly one could now argue that having to listen to the two longer cycles might be preferable to three shorter ones but that would be "splitting hairs" (nope don't know where that one comes from! :D ) and really has little to do with the point I have been trying to make about selecting a compressor based on tank size.

gnm109
05-18-2012, 09:08 PM
Radkins: Too much to quote. I'll just say that I've noticed that the larger tanks are almost always associated with higher CFM, greater pressure, larger motors, etc., especially when considering professional equipment.

My friend down the road who is a professional CNC machinist, has a Curtis compressor that has been running daily for more than 30 years with nothing more than oil changes, belts and an occasional draining. It's got a horizontal tank with 120 cubic feet. The compressor is two stage with a 10 hp three phase motor. It shuts off at 175 psi and starts at 125 psi. I've been there many times when he runs parts and it hardly ever starts up. It operates a system of pneumatic cylinders for changing parts.

I'm really not sure what you are advocating. I'll just say that if you want a compressor that is going to serve your needs for many years, it pays to buy a good quality one with proper size, CFM, power and capacity. That would be what I and my friend both have. Surely you can't disagree with that premise.

oldtiffie
05-18-2012, 10:01 PM
Some very good advice from Radkins and gm109 here.

Some do buy smallish compressors with smallish tanks that have the motor start-stop switch set as 90psi (6 bar) "LO" and 115psi (7 bar) "HI".

Even if the regulator is set to 115psi (static) there will be at least a 10>15psi pressure drop accross the regulator in the "demand" state with a further pressure drop (in the hose/line) between the regulator and the tool.

If this happens, the tank and regulator pressue will continue to drop as the demand exceeds the capacity of the compressor.

Eventually the compressor will run continuously and the pressure will drop to a state of equilibrium while or if the load/demand is applied continuously.

In this state the compressor will never stop or "catch up".

If the load/demand is stopped the compressor will rise to and pass through "Start" until it reaches its upper limit ("Stop").

With a fully "topped up" large tank the demand will be kept supplied for quite a while while the pressure in the larger tank "drops" but once the tank pressure drops to the "LO" (start) level it will take that much longer to get to the "HI" (off) switch - if it ever does while under load.

If a tank had been emptied or "blown down" it wil take a lot longer for a compressor to fill a larger tank than it does a a small tank.

Compressors are electricity hogs and every time the compressor switches on and is running it is costing you money - and perhaps lots of it.

radkins
05-18-2012, 11:05 PM
Radkins: I've noticed that the larger tanks are almost always associated with higher CFM, greater pressure, larger motors, etc., especially when considering professional equipment.

That's mostly true but not always and therein lies the very point I have been trying to make!

Allow me to present a hypothetical scenario that might explain my point,

A home shop owner decides to buy a new compressor so he asks the all too often asked question should I buy an 80 gallon tank or just settle for 60 gallons, he hears a lot of advice to buy the biggest tank he can afford which is what he was assuming any way, bigger is better right? Well no not necessarily, here is the problem with that and this is where a lot of people mess up big time! He goes into a store and looks at compressors in his price range and he finds, for example one rated at 14 CFM mounted on an 80 gallon tank and sitting next to it is a better built model with a slightly better pump/motor unit rated at 16 CFM but only a 60 gallon tank, so which one is the better buy?

Far too many will take the big tank version when in fact the higher CFM compressor would be a much better buy that would be far better at keeping up with their tools. It's CFM that matters, a tank's purpose is to buffer the difference between demand from the user and supply from the pump, much like a capacitor in an electrical circuit, it is not to extend performance! If, as in your earlier example of the paint gun, the compressor pump output is exceeding demand then the tank makes no difference in how much air is available and the user will not run out of air regardless of the size of the tank. If however the demand exceeds the pump capacity then the compressor will run out of air and will no longer be able to maintain pressure so the user must stop and wait for it to catch up and THAT is where the myth about a bigger tank becomes important! People will buy a bigger tank thinking that the bigger the tank then the less chance they will run into this problem but that's rarely the case, "Reserve air" stored in the tank is, for the reasons it is mostly used for, a nonsense term. If a compressor is running out of air at an annoying rate a bigger tank is not going to help, what is needed is more CFM but all too often people buy based on nearly useless tank capacity rather than what really matters and that of course is pump and motor capacity! Take two otherwise identical compressors with the same CFM rating but with one on an 80 gallon tank and the other on a 60 which would give the user the most available air during, for example a 30 minute work period? It would be the same between the two, if demand exceeds supply and the compressors are running out of air then the one with the smaller tank would run more but shorter cycles than the one with the 80 gallon which would have fewer but longer cycles, air produced during that 30 minute work period and thus the time available to use the compressor will be the same regardless for either tank! CFM from the pump determines how well a compressor keeps up with tools not air stored in the tank!







who is a professional CNC machinist, has a Curtis compressor that has been running daily for more than 30 years with nothing more than oil changes, belts and an occasional draining. It's got a horizontal tank with 120 cubic feet. The compressor is two stage with a 10 hp three phase motor. It shuts off at 175 psi and starts at 125 psi. I've been there many times when he runs parts and it hardly ever starts up. It operates a system of pneumatic cylinders for changing parts.

Umm, you lost me here, of course a bigger outfit will outperform a smaller one so I don't understand your point?




I'll just say that if you want a compressor that is going to serve your needs for many years, it pays to buy a good quality one with proper size, CFM, power and capacity. That would be what I and my friend both have. Surely you can't disagree with that premise.


Exactly! You are 100% right about that and I agree wholeheartedly!



I'm really not sure what you are advocating


Simple, in order to do what you said above use the right criteria when choosing a compressor and running to the biggest, badest LOOKING tank in the store ain't it! All I tell anyone is forget the tank for the most part and let it be of secondary consideration, let the designers choose the tank for the particular pump and motor combo being considered. Buy based on how much air that compressor will actually deliver (CFM), what really matters will be the volume of air coming from the pump vs the volume of air the tool is using. The tank is nothing more than a buffer sized to smooth out the run/recharge cycles and provide a cooling period for the compressor and a bigger one does not make a small compressor bigger! Choose a compressor based on having enough CFM to match the tools it is to run, overall quality and price range and don't worry so much about the size of the stinkin tank! :D



I love a good compressor discussion, since 1977 I built and installed compressors for service trucks and a few shop systems and those trucks were always pressed for space. We learned many years ago what works and what don't and what will satisfy the end user but it is still frustrating to hear this same old myth about bigger tanks over and over! The system I mentioned earlier using the bumper/tank unit was a 24 CFM 175 PSI set-up almost exclusively used for 3/4" and 1" impact wrenches, it usually could not keep up with the 1" tools. I have seen guys add tanks and I have had them hire me to install extra capacity but not a single one of them was satisfied with the results which used up valuable space for no gain in performance. They simply went from waiting a couple of minutes for the compressor to catch up to waiting twice as long but not as often, most opted to go back to the shorter but more frequent wait, overall the waiting time was exactly the same.

gnm109
05-18-2012, 11:33 PM
I keeping the compressor that I have. LOL. :)

rohart
05-19-2012, 03:00 PM
Right. Well. It seems to have quietened down on this thread for a moment !

But if you two, and others for that matter, know so much about compressors, perhaps you could lend an ear to my question.

I have a small, old noisy compressor. I would like a quieter compressor. I understood, a while back, that 'quiet compressors' were all the rage. What I'd like to know is, if I bought something that is advertised as a quiet compressor, does it in fact use a different technology, and is it really much quieter ? And is there a downside to the quiet compressors, apart from cost ?

lakeside53
05-19-2012, 03:23 PM
I find oil sump are quieter than oiless, and low rpm belt driven are MUCH quieter then the screaming direct coupled small units.

On a larger compressor a great deal of the noise comes from the intake - put on a quality intake muffler.

radkins
05-19-2012, 07:06 PM
The technology used to make these quieter compressors is simple, a big slow turning pump. A two stage compressor is generally quieter than a single stage of similar displacement but really those big, heavy low RPM outfits are the quietest but unfortunately they are usually the most costly too. An oil-less direct drive compressor (I hate to even call those things compressors!) are the noisiest with the oil type single stage high RPM outfits being nearly as bad, it's just a different and extremely irritating sound from those oil-less contraptions.


One thing about those direct drive oil-less compressors is that after only a very few hours of operation they tend to quieten down considerably, in fact they tend to become VERY quiet indeed! :)

RTPBurnsville
05-19-2012, 07:09 PM
Hi,

I have been looking for a compressor so here are three 'real world' examples. Anyone have an opinion? I don't exactly have a money tree in my backyard but don't want to end up buying something again in 6 months. I could swing the 2 stage model but not much more $ wise.


http://www.fleetfarm.com/catalog/product_detail/tools/air-tools/air-compressors/magna-force-60-gal-vertical-air-compressor


http://www.fleetfarm.com/catalog/product_detail/tools/air-tools/air-compressors/magna-force-80-gal-hi-flow-air-compressor


http://www.fleetfarm.com/catalog/product_detail/tools/air-tools/air-compressors/magna-force-80-gal-2-stage-cast-iron-air-compressor

Thanks,
Robert

radkins
05-19-2012, 07:33 PM
Believe it or not one of the best bargains out there on home shop type compressors is at Harbor Freight. Harbor Freight has the U.S. General brand compressors in both 3 HP single stage and 5 HP two stage designs and believe it or not these things are actually re-badged American built Belaire compressors, these are not Chinese knock-offs but they are the real thing! They are made in the USA (South Carolina I think) and are the same units sold under the Belaire brand name for about 40% higher cost, the single stage does have an Italian built pump and that pump is really good quality. Don't mistake these for the junky Central Pneumatic compressors, these are the black units that have the U.S. General name and A.O. Smith and/or Square D electric components (some of the two stage compressors actually have Baldor motors).