how to size compressor?

[docbarb]

I have the miller spectrum 375 cutter and am using my 2hp emglo nail gun compressor. It runs most of the time.
So what size compressor do I need?
I am guessing that capacity is more important than HP
12 gal big enough?
Oil/Oilless?
2 stage (whatever that is)?
Which ones are quietest, perhaps I will install it outside...

[enlpck]

You are correct: The important thing is capacity, but not so much storage capacity as production capacity. The plasma cutter has a specified air consumption (4.5CFM at 90PSI). A small compressor guns is probably about at (or beyond) its limit. If the compressor is powered by a standard 120V outlet, it may be drawing up to 2HP of electric power to start it (about 1500W), but is probably only running at about 1HP (about 750W), which, in an efficient machine, will get you about 4CFM at 90PSI (the spec for EM790+CH4V 2HP Emglo is 3.6CFM at 125PSI, which is about 4 to 4.5 at 90PSI... http://www.amazon.com/Emglo-M790-HC4V-Electric-Master-Compressor/dp/B00004TRBT )

This means that the compressor will run constantly, and will not have much of a lifetime since it isn't rated continuous duty-- it will run hot and cook eventually. In addition, the air will not have time to cool and condense out moisture, and the consumables on the plasma unit will have short life as well.

You can do several things: Parallel another compressor (not really recommended for these small units. One will still do most of the work) or upsize to something in the ballpark of 10CFM.

You also want to be sure to have sufficient reservoir for the air to cool and let moisture condense and drop out, drain the tank regularly, and, if you can, add a drier and filter near the plasma unit. The little condensate traps with the plastic bowls help a lot and can be mounted right on the back of the plasma unit (moisture that condenses in the air line gets caught), but a good moisture seperator/filter is better, if expensive.


Adding another reservoir in series will also help cool the air, and provide some reserve capacity if you arn't constantly cutting: the compressor will run to charge the system, then stop. As you use air, the pressure drops and the compressor kicks on. The air will get more cooling time and more moisture will drop in the second tank. This will not lengthen the compressor life, especially if the compressor is not sufficient to begin with, as it needs to do just as much work overall. It will lengthen the life of the plasma cutter consumables.

[docbarb]

ouch! a quick google finds this size compressor costs $500 - $1,100 and up and 240v.
Looks like I'll be lurking in craig's list again, and do some favors for my electrician friend

[Zrexxer]

Just get ya one of these puppies and you're set for anything... mine makes 26 cfm of air at 175 psi :D:D Of course, there's two things it ain't: quiet or light.

http://img.photobucket.com/albums/v58/Zrexxer/Tools/ChampionCompressor009-800-1.jpg

[Northweldor]

ouch! a quick google finds this size compressor costs $500 - $1,100 and up and 240v.
Looks like I'll be lurking in craig's list again, and do some favors for my electrician friend

What kind of work are you planning on doing? What range of thicknesses do you cut? If you are planning on production work and long periods of cutting, then you should get production equipment.
On the other hand, I have been using (for maximum portability) a 2.5 HP oil lubricated 10 gal. Briggs + Stratton (Cheapo dryer filter added) for about a year and a half, with my Powermax 30, and it is still running strong. Cutting 3/8" material with 4" to 12" cuts on my latest fabrication job, it usually ran for about 5 mins. after cutting stopped (continuous, while cutting), and never over heated. It has paid for itself several times, and when it quits, I plan to salvage the tank for more portability and buy another.

[docbarb]

northwelder
This isn't production
This is a learning experience

my technique could be condensed to:
search for tape measure (STM):mad:
sketch idea
find materials
revise sketch
(STM):mad:
clamp materials
sip
Scratch my butt
think about it (the project not my butt)
(STM):mad:
Reclamp
sip
cut
rest
find another scrap piece
repeat until dinner
I don't expect to make a whole lotta money on this
unless I can get paid by the hour;)

But in serious response to your post:
I will be doing hobby things right now a few cuts not very long less than 1/4" mild steel.
Since my 2hp emglo seems to run 80% of the time it is obviously undersized and I don't want to ruin it for the carpentry.
I will get another compressor but perhaps my needs aren't quite as severe as I imagined and I can get by on a 110V at 6CFM or so. That may happen sooner money wise.
The $500-$1,100 scared me since the cutter itself was quite a splurge in itself.
I'll make it all work. eventually.

[ptsideshow]

The most important thing to remember is the specs on most compressors and other tools are fudged a little.
all compressors a couple years ago were rated in CFM @ the highest pressure and again @ half the pressure. Which was a good rating.

Now they are rated as scfm which means sustained cubic feet/minute. Which isn't gives them a little fudging room as it would change with atmospheric conditions. If you can find the spec's buried in the fine print. You want to pay attention to duty rating of the electric motor.
Always buy a bigger compressor than you need, and you can't go wrong when you get the blast cabinet next year.:D

The correct definition of SCFM (Standard Cubic Feet per Minute) is the volumetric flow rate of a gas corrected to "standardized" conditions of temperature, pressure etc, but it has been corrupted by the advertising boys and girls :rolleyes:

All the doubling up on compressors and staging them, etc is a PIA in operation.and has been said you never seem to get it to even out.

[mwoh419]

Doc,

Campbell Hausfeld makes a good air compressor. They have 60 gallon units in the 10cfm range for $400-$500 range. Make sure you get the "cast iron series". Check out your local TSC, but stay away from the Farmhand brand. The big blue and orange stores don't sell to bad of a unit either. Just don't get anything "oiless", it won't hold up. ---Mike---

[mwoh419]

Here,

http://www.lowes.com/lowes/lkn?action=productDetail&productId=102750-1126-VT6275&lpage=none&cm_mmc=search_gps-_-gps-_-gps-_-Campbell%20Hausfeld%2060-Gallon%20135%20PSI%20Electric%20Air%20Compressor

Sold my 15 year old version of this last year for $100, and it still worked good.

---Mike---

[docbarb]

Hmmm my searches reveal that a compressor putting out 6.75+ CFM or more needs 240V.
So I either need to go with a smaller unit like 20Gal/5CFM or install a 240 line.
I am going to have my electrician friend price it out for me cuz 110 V is limiting a lot of choices now so it will only get worse. Seems like I can get a 240V machine with higher CFM f 10CFM or a little less money than the 110V that puts out 5CFM.
How does gallon capacity affect operation. Can I get by on a 20-30 Gal?

[enlpck]

Reservoir capacity (the 'gallon capacity') affects operation three main ways:

It smooths the pulses of the pump so you get a steady air supply. With no tank, the air pressure would pulsate. A lot. Not good for the gear on the line. It takes a LOT of pipe to match the volume of even a fairly small tank. The tanks are generally sized to match the compressor, in the rough ballpark of one to two minutes or so worth of compressor volume (a 30CFM compressor will have roughly an 80 to 120 gallon tank, or about 12 cubic feet. Remember compressors are sized based on UNcompressed-- free air-- volume)

They provide reserve for intermittent use, high draw, tools like a large impact gun. Smaller compressors often have proportionally larger tanks to help here.

They let the compressed air cool and let the moisture condense and drop out. This is important for most tools, especially a plasma cutter. Adding a second tank after the main one can help remove moisture in high volume applications, especially if there is a moderately long metal (steel or copper) line between them to help reject heat. The second tank doesn't need to be big, as it is only there to collect moisture. As the air enters it, the velocity goes down and condensate can drop out of the air stream. It helps if the outlet from a second tank is near the top and the inlet is low from the side so the air has to change direction, as well.

A rough rule is that at 100PSIG (115PSI absolute from 15PSI), a single stage compressor will get you about 4.8CFM per MECHANICAL horsepower at best. This is based on adiabatic compression (minimal heat rejection from the compressed air during its time in the pump, which is a reasonable approximation for real pumps), and is based on the thermodynamic properties of air, right from the table (machinery's 25ed, Pg397) and a reasonably efficient pump. The electrical power required will be more than this, due to the less-than-100% efficiency of the motor. Small compressors are often a lot less than this, as the small single phase motors are often not real efficient. Might get 3.5CFM or less per electrical horsepower on a small 120V unit, versus 4CFM per electrical horsepower on a larger three phase single stage (the one at work is measured to make roughly 4.1CFM during the winter when the air is cool and dry. Nominal 15HP, 60CFM 4cyl Quincy)

The 20gal, 5CFM unit will likely be too small in practice, as another drawback to the small units is that they don't like to run 100% duty. Some will overheat fairly quickly at 50%.

[docbarb]

Thanks Enlpck

Thats a lot of information!
May sound stupid but How do I add a second tank? in series? or are you referring to the twin tank units?

I guess the point of it all is I need a larger compressor which means I need 240V which means money. So the plasma machine may have to sit awhile...:(
Why do they make the plasma cutter run on 110V when you need 240V to run the compressor anyway?
The good news is I learned a new word today: adiabatic:cool:

[mwoh419]

Enlpck,

Does output hose size, (to tool), affect cfm output?

---Mike---

[enlpck]

Adding a second tank: Put it in series after the first tank, preferably with a decent length run of metal pipe (steel of copper) from the first to the second. 3/4" should be sufficient for a 20CFM compressor. At 100PSI from the primary tank the compressor feeds and 20CFM flow, this would give a drop of about 3PSI to the second tank for a 20ft run (calculator-- watch the units if you use it: http://www.pipeflowcalculations.com/pressuredrop/index.htm) The pipe is for cooling. The second tank need not be large... it is primarily to collect condensate, and a vertical run of larger diameter pipe will serve the same purpose. (see pic for one way to do it) This GREATLY reduces the load on an air drier or point-of-use condensate trap further down the line.

Why will the plasma run on 120V? One good reason is that if you are on a job site with a fueled compressor (gasoline or diesel), you can run the plasma unit on a good size extension cord. Or in a shop with piped air, but not lots of 240V outlets around. Also, a you have seen, you CAN use it with a smaller compressor run on 120V. The compressor will just be near or at its limit, especially if you run long cuts or lots of short ones without stopping to let the compressor catch up then cool.

Hose size: doesn't affect CFM capacity of the compressor, but too small a hose will increase the pressure drop, as will too long a hose. For most small tools, not a big concern. For example, in one of the shops I work in: the Spectrum375 on a 100PSI system is fine with 50Ft of 3/8 hose with interchange-type quick connects. A small cheapie die grinder is fine on 50ft of 1/4 hose, but you notice it. The better IR die grinder doesn't notice it at all. A good 1/2" impact is fine on a 25ft 3/8" hose. All of these are fed by about 100ft of 3/4 steel pipe from the compressor. The heavy 3/4 (CP) impact is much happier with 1/2" hose and large quick connects... won't make full force on 25ft of 3/8 hose.

[docbarb]

Maybe this would work for me:

http://sfbay.craigslist.org/eby/tls/1102851244.html

Then I wouldn't have to bring 220V to the barn,
I could put this just outside and build a little shed around it, have the air come thru the wall.
Any pros or cons to a gas compressor?
It's as cheaper than less powerful electric ones.
Would 8 gal. be enuf?

[Sberry]

There are no good substitutes for adequate 240V service, anything else is a poor cobble job. If you are using the 120V circuit what powers the plasma?

[Northweldor]

You can get this same make and model with 110v power, brand new, for a little more than $150 over this price. You would also have to fuel. change oil, service, start, etc, with this, as well as build a shed. I think you would far sooner spend the time using your plasma cutter with some of the more elegant solutions proposed above.
Also, with your workstyle described above, (no disrespect) my previously recommended solution will never be overworked! (and will save you $400 and some effort).


Maybe this would work for me:

http://sfbay.craigslist.org/eby/tls/1102851244.html

Then I wouldn't have to bring 220V to the barn,
I could put this just outside and build a little shed around it, have the air come thru the wall.
Any pros or cons to a gas compressor?
It's as cheaper than less powerful electric ones.
Would 8 gal. be enuf?

[docbarb]

Yes but the electric one has only 5.7 CFM
My plasma cutter is a miller 375 that runs on 110V

[Hotfoot]

Just train yourself to not cut with the Plasma while the compressor is running, and you'll be OK...and don't neglect that all important "Air Charge" with Compressor Air!


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[docbarb]

Then do I send my old air to ?! Technologies to get re-prefiltered?:confused:

[GSSFC]

Maybe this would work for me:

http://sfbay.craigslist.org/eby/tls/1102851244.html

Then I wouldn't have to bring 220V to the barn,
I could put this just outside and build a little shed around it, have the air come thru the wall.
Any pros or cons to a gas compressor?
It's as cheaper than less powerful electric ones.
Would 8 gal. be enuf?

they are loud, the fumes can kill you, if you run out of gas you're done until you refill, they are loud and will annoy your neighbors, they are loud and will annoy you, you'll be selling to buy electric...save yourself the hassle, and they are loud.

Tim

[Hotfoot]

they are loud, the fumes can kill you, if you run out of gas you're done until you refill, they are loud and will annoy your neighbors, they are loud and will annoy you, you'll be selling to buy electric...save yourself the hassle, and they are loud.

Tim

Ditto...Ditto!!:)

[mwoh419]

enlpck,

So, would it be accurate to say, that the rated cfm, is the amount of air that the pump will put in the tank, (@ specific temp and humidity), and as long as your air tools don't exceed that rating, (@ a specific psi), then the pump will catch up, but if you exceed the rated cfm with tool usage, then the pump will run continously? ---Mike---

[enlpck]

enlpck,

So, would it be accurate to say, that the rated cfm, is the amount of air that the pump will put in the tank, (@ specific temp and humidity), and as long as your air tools don't exceed that rating, (@ a specific psi), then the pump will catch up, but if you exceed the rated cfm with tool usage, then the pump will run continously? ---Mike---

Pretty much.

Other than in the case of tools that take high volume for brief periods (like an impact wrench), the pump capacity is what matters. You can't make up for an insufficient pump capacity with storage for more than a brief period. If the demand is greater than pump capacity for more than a brief period, then the pump won't be able to maintain pressure in the system and will not catch up.


With regard to the question about an engine powered unit: I agree wholeheartedly with Sberry and Hotfoot that it is probably not a good choice. The only reasons I can come up with for going with a fueled compressor are when you need to use it places without reliable power (such as you would tend to find when working off a truck) and places where power isn't otherwise available. The fuel and maintainance costs will quickly overrun the cost of running any reasonable cable.

The math summary, presuming the cost of the compressor it about the same for the 10CF/min capacity: 100Ft of 8-3UF: maybe 2.50/ft (HD around here.. not the best price available), for $250. 50A subpanel: $80 (again, HD. Not the best deal) Total: less than $350. The gas compressor will take 2gal to 4gal per hour of gasoline (estimate based on rated horsepower), which is $4 to $8/hour. So, just for the fuel, in 50 to 100 hours, the price difference between installing electric and running gas is made up under light use where the electric won't be running much. Under heavy use, the much lower effective cost/KW-hr of commercial power than gasoline shows even faster. (note that the gas powered runs constantly. Less fuel is used when not drawing air, but the idle fuel is not insignificant. Electric units generally only draw power when making air)

[Northweldor]

DocBarb:
What is the answer to Sberry's question?

[docbarb]

DocBarb:
What is the answer to Sberry's question?

As I mentioned a few post up, I use a miller 375 which runs on either 110 or 220
:cool:

[Sberry]

Even the Spec will benefit from running from 240 and I like Enlpk description of the cost, and as he said you can even do better on wire cost, I think 2 quad alum, good for 90A is near 1.50 a foot and when all is done you are left with something besides an empty gas can and the cost of electric to make air and do some cutting is insignificant, so little that I never think about it.

[Northweldor]

We all know that.


"There are no good substitutes for adequate 240V service, anything else is a poor cobble job. If you are using the 120V circuit what powers the plasma?"

Sberry asked you this in post #10. He, if you check his other posts, is an electrician, among many other areas of expertise. The reason he is asking is that you haven't told us whether you have at least two 15 -20 amp circuits available in your "barn," properly wired and breaker-equipped to handle a small compressor start-up and your plasma on 110v.

[docbarb]

We all know that.


"There are no good substitutes for adequate 240V service, anything else is a poor cobble job. If you are using the 120V circuit what powers the plasma?"

Sberry asked you this in post #10. He, if you check his other posts, is an electrician, among many other areas of expertise. The reason he is asking is that you haven't told us whether you have at least two 15 -20 amp circuits available in your "barn," properly wired and breaker-equipped to handle a small compressor start-up and your plasma on 110v.

OK I misunderstood the question.
Yes I have several 20amp circuits w/ breakers installed by an electrician about two years ago.and have run both the compressor and the plasma cutter simultaneously.

[Sberry]

Then I wouldn't have to bring 220V to the barn You likely already have 240 if you have several circuits.

[Grumpy]

I have the miller spectrum 375 cutter and am using my 2hp emglo nail gun compressor. It runs most of the time.
So what size compressor do I need?
I am guessing that capacity is more important than HP
12 gal big enough?
Oil/Oilless?
2 stage (whatever that is)?
Which ones are quietest, perhaps I will install it outside...

Here are a couple of Word articles about electric motors and air compressors. Hope this can help you a little with understanding HP versus CFM.