|04-22-2011 08:44 AM|
This brings something else into the 60 vs 80 gallon equation that I have avoided because it is already confusing enough and besides the only way to make a meaningful comparison between two different tank sizes is if all else is equal, change the CFM input or pump type and there can be no real comparisons. A two stage 60 gallon tank compressor with 175 PSI is going to have a lot more air stored in it than a 60 gallon tank with only 125 PSI having the same effect as a bigger tank. This is all beside the point anyway but it is just another reason why looking at the size of the tank is useless unless it is a special use compressor with an extreme tank volume one way or the other. An example of that was our most popular rig when building service trucks, a 32 CFM compressor with a 30 gallon tank. The small tank was used to save valuable space and the operator depended on CFM to make the thing useful.
|04-22-2011 07:02 AM|
|Jim Rockford||also the tank to do try and let the air cool so you can draw the moisture out of the air, You wouldn't want to run a paint gun or a D/A right off the pump the air is pretty hot, I know on the secondary side of my old pump its about 200 degrees, but even with continuous use the air is almost room temp coming out of the regulator. and I have a 60 gallon horizontal tank, that draws right out of the top of the tank. so It makes a difference in cooling, If I had a 12 gallon tank with my pump It wouldn't bee nearly as cool and would have a ton of moisture it but Yes, always buy on CFM totally ignore the HP rating in bit Numbers on the label... also always go with the @90PSI if it has two cause that is what 90 % of all air tools run at. 140PSI really has no use on you as a homeowner.|
|04-21-2011 11:14 AM|
EXACTLY! If the CFM is not there then a bigger tank, bigger HP numbers nor ANYTHING else will make up for that lack of CFM. No matter how big the tank is the CFM going into the tank vs how much is being used is what determines how well a compressor will keep up and it is pure folly to dwell on the size of the tank.
|04-21-2011 11:09 AM|
Jim you are correct but the confusion starts when someone simply says "get the biggest tank you can" and does not even mention what really matters, it was even stated that "air storage is (the) key to large usage" when clearly the tank is the wrong thing to make the top priority when choosing a compressor, air storage certainly is not the key to high usage! More than once I have seen people buy a compressor with less CFM than a comparably priced model just to get a bigger tank! When the usual debate between a 60 gallon tank and an 80 gallon comes up then any disagreement is almost pointless since there will be no real difference between performance of the two anyway. What I like to point out is that if a compressor with a 60 gallon tank is already running low on air at an annoying rate with the demand placed on it then just how long would it take to run out of air if it had only a 20 gallon tank? Of course it would not take long at all but that is all that is gained over the 60 gallon and then even that is lost to the proportionally longer recharge time so what is gained? Exactly nothing! A compressor with an 80 gallon tank and more CFM than a similar 60 gallon model will certainly perform better but it is the extra pump capacity (higher CFM rating) that makes the difference and not the bigger tank as some mistakenly believe. All too often however the choice can boil down to a compressor with a bigger tank than one with more CFM but a slightly smaller tank or one of lesser quality so choosing the bigger tank over more CFM and quality is a serious mistake! The bottom line is choose the compressor on what really matters and that is CFM, the amount of air going INTO the tank vs what is being taken out that determines how well a compressor will keep up with a tool not how much air is stored at any given time.
For those who would consider adding an extra tank you might want to consider this,
Using round figures for simplicity an example would be a compressor and a demand condition that was allowing 2 minutes of run time before needing three minutes of recharge time to catch up meaning a 5 minute cycle, these are imaginary figures for the example but entirely realistic. If this compressor was used over a 30 minute work period then it would have to cycle 6 times during that time for a total of 12 minutes of work time vs 18 minutes of recharge time or 30 minutes total. Now the owner decides to add another tank to double his capacity (I have seen this done more than once!) so that now he is getting 4 minutes of run time before recharging, the time of which of course is also doubled so that now it takes 6 minutes to do. So the same 30 minute work period now only takes 3 recharge cycles of 4 minutes run time vs 6 minutes recharge time for a total of- 12 minutes of run time and 18 spent waiting on the recharge or EXACTLY the same amount spent with the smaller tank capacity! Of course in extreme examples like this things like duty cycles come into play, although that won't matter much with a 100% duty cycle pump, and can lead to actual losses with the bigger tanks due to loss of efficiency from overheated pumps. However the point is the difference between a 60 gallon tank and an 80 gallon is negligible, assuming the same CFM, and it would make no difference in keeping up with an air hungry tool, that simply is not the purpose of the tank.
At the risk of sounding like the classic "broken record", buy based on CFM and unit quality and just don't worry about whether the tank is 60 gallons or 80 because you simply won't be able to see any difference! NEVER, EVER pass up higher CFM just to get a big tank!
|04-21-2011 10:42 AM|
|JohnnyK81||You don't even 'need' a tank, except for cooling. If you have enough cfm, you have enough cfm. If you don't, you don't.|
|04-21-2011 09:54 AM|
You have to factor Duty cycle in with CFM's , storage is so you can run longer without the pump running, but you have to fill the tank back up. But if you have the correct size and type of compressor , if you have a good compressor you can have it run constantly while you work oilless compressors are not for any type of constant work. Oil filled aluminum pumps with a cast iron cyl is fair if you are willing to stop and let the pump catch up and cool off, again its not for constant use. Heavy cast iron slow rpm pumps are the best for constant use and if the CFM is correct you can basically run the tool completely off the pump, if you have excess cfms it will slowly fill the tank back up wile you work.
V style pumps tend to last longer because each cyl is cooled evenly, vs a inline where the front cly is blocked from the fans airflow by the rear cyl. I have a old (AKA 1964 ) IR t30 compressor that has been in constant use for a long time and even now will run a D/A for as long as I can stand to run it with no problem.
|04-20-2011 10:09 PM|
There is far more at play there than just the size of the tank! You just can't compare a little 5 gallon tire pumper to a full size compressor and say "see there the tank does make a difference", that's just plain ridiculous because the CFM is not there. CFM is what runs tools and if the compressor has enough CFM theoretically you would not even need a tank! The tank size does not determine a compressor's ability to keep up with tools and it makes no difference to the tool if the tank is a 60 gallon or an 80 gallon (the usual discussion) if all else is equal. This has been explained dozens of times but once more, ANY gains in run time between a small tank and a larger one is going to be offset by the proportionally longer recharge time, no way around it it's just simple laws of physics. The tank size is chosen to balance the run time or duty cycle for a given CFM rating and it's expected demand, adding tank capacity only gets fewer but longer off/on cycles and does NOT change the amount of air delivered to the tool in any given work period past the first start cycle! By your analogy the little pancake compressors could be used to paint a car if only they had a bigger tank?
You never even mentioned the most important thing to look for when selecting a compressor to do a given job, it's CFM rating! Maybe you don't understand but the fact is a compressor with 16 CFM with a 60 gallon tank will outperform a 14 CFM compressor with an 80 gallon tank by a wide margin but you are saying to buy the bigger tank? That is exactly the type of thing some manufacturers use to sell cheap compressors, stick a cheap small pump and motor on top of a big useless tank and give the illusion of a big compressor when in fact a smaller tank would be better.
The example of the 14 CFM 80 gallon is a real one made by Campbell Hausfeld and that thing would be better with a smaller tank because the prolonged recharge times overheat the pump and actually decrease performance due to loss of efficiency from the hot pump. The bottom line is a bigger tank DOES NOT MAKE A BIGGER COMPRESSOR but a bigger pump and the power to turn it does! CFM makes the compressor keep up with tools not the tank!
I am not saying to avoid large tanks or that a big tank is bad, although it can be with a small pump, but the tank size should not be used to determine compressor selection. Let the designer select the tank to match the pump/motor and select the compressor by the CFM rating and quality of the outfit. Too much tank for a small compressor means an overworked pump and too small a tank for a large pump could mean too fast a cycle rate but that is just not going to happen with a properly designed compressor. Once more consider the CFM rating and overall quality and forget the size of the stinkin tank!
|04-20-2011 08:06 PM|
can't tell you how many times a guy would bring in a pancake or 5 gal compressor that is melted down. They can't run continues to keep up with painting or air tools.
Secondly when do you see a high cfm pump on a small tank. To get higher cfms you will need to buy a larger tank.
|04-20-2011 09:03 AM|
You may have repaired compressors but you obviously have little understanding of what determines performance and we have spent a great deal of time here trying to clear up the bigger tank myth! A BIGGER TANK DOSE NOT HELP, that is not the purpose of the tank and to tell someone to buy a compressor based on the size of the tank is ridiculous. You never even mentioned what really matters and that is the CFM. Air storage absolutely is NOT the key to performance and anyone who is truly familiar with compressed air supplies and how they function would know that. This has been explained in detail time after time here but buying a compressor based on the size of the tank is about the biggest mistake a buyer can make.
|04-19-2011 08:48 PM|
Retired compressor repairman here. Oiless compressors are for pumping up tires. Period. They have a leather seal and aluminum cylinder that can't take extended use. Most oil type compressors will do the job. For painting and air tool use get a large tank. Air storage is key for large usage. Home depot etc have 60 galion compressors for a good price. These are still homeowner grade but will work if you need to paint a car or to.
Large storage and oil type.
|04-19-2011 07:47 AM|
|DanTwoLakes||In case anyone is wondering, the 746 divisor is the number of watts in one HP. The 5 horsepower they refer to is "peak" horsepower which is electrical power measured at the instant of starting. They can fudge the "peak" horsepower even more by measuring the electrical power at the instant of starting under a load. Like Lizer pointed out, the only way to determine the true horsepower is through the formula he used. If they use the start up amperage and it's 38 amps for a split second, using his formula 120 volts times 38 amps times .82 efficiency divided by 746 watts, the "peak" horsepower calculates out to 5 horsepower.|
|04-18-2011 10:36 PM|
I had that Craftsman when I started the hobby, used it for a year, then upgraded to a two-stage 60 gallon to finish the hobby. Since then the Craftsman sits in a corner as a backup. We also had a Craftsman oil less upright in the shop at my old job. With that being said, I haven't had any problems with them, but then again mine only got used good for a year before it retired (thought I bought it used). It is absolutely grating on all senses however, when it kicks on. I was just wondering the other day how a motor can work like that and sustain itself being oil free. I just wanted to give you a confirmatory +1 on ditching the Craftsman. Though it would make a nice compressor for the guy who just needs to use it every once in awhile.
BUT the notion of either of these being truly 5 hp is laughable. In the following formula we determine calculate horsepower from a motor with known current, voltage, and efficiency.
HP = (V x I x Eff)/746
HP = horsepower
V = voltage
I = curent (amps)
Eff. = efficiency
The motor is 120V, and we'll give it the benefit of the doubt and assume it has a large power draw, say 12 amps (since you say it trips the breaker), and an efficiency of 82% (an assumed value since it is unknown). This gives us:
(120 x 12 x .82)/746= 1.5 hp
Even if the motor were 100% efficient, it would still only be capable of producing 1.9 hp! What a sham.
The judgement should have been handed out against these manufacturers a lot earlier than it was.
|04-18-2011 04:09 PM|
Thaks guys thats exactly the information I was looking for. It makes sense that the Devilbiss ProAir is a far more expensice compressor. I did notice that the craftsman, oilfree, single cylinder is a bit louded (Dryer sounding like metal to metal like)
I just hope any potential buyers of the craftsman do not read this post. Anyone interested in a Craftsman 5 hp 30 Galon compressor?!! $100
|04-18-2011 02:22 PM|
|DanTwoLakes||I had two Craftsman air compressors, including one exactly like you have, both of them the "oil free" kind, and both of them literally blew up. I got an oil lubricated Sanborn and have never had a minute's problem with it. I'm sure the problem is the oil free feature, and is not specific to Craftsman.|
|04-18-2011 12:53 PM|
|KMatch||My vote goes for belt driven nearly every time. They're quieter and usually more heavy duty. I've run my Ingersol into the ground without a hiccup. I ran my Craftsman beltless a bit much one time and destroyed a piston. Rebuilding it was cheap, but NOT rebuilding the other was cheaper!|
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