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Help me choose an air compressor for sandblasting and shop.


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#136 NUTNDUN OFFLINE  

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Posted February 24, 2011 - 04:30 PM

Alright, if my calculations are correct, at 550 rpm it will be right around 20 cfm.

2.5r x 2.5r x 3.14pi x 3.25stroke = 63.78 cu in. cylinder volume

63.78 cu in x 550rpm = 35,080 cu in per minute / 1728 cu in. per sq ft = 20.3 cfm

#137 NUTNDUN OFFLINE  

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Posted February 24, 2011 - 04:33 PM

At 750 rpm that puts me around 27 - 28 cfm. Which is about where I would like to be. I know one thing, for how big that compressor is I would have thought it would have made more cfm. I guess that is where technology comes in LOL.

#138 RailmanB110 OFFLINE  

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Posted February 24, 2011 - 05:00 PM

Something don't sound right. I'm guessing the stroke to be in the balpark of 5" on both cylinders, & the bores to be about 4.5", & 2.5". If I'm not mistaken, the strokes are usually the same, & the bores change to keep the pump in balance.
Of course I could be wrong....but?

If you scale your picts, the cylinders are nearly 1ft tall. It's hard to believe that the stroke is only 3 1/2".
Joe
Joe

#139 thecoater OFFLINE  

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Posted February 24, 2011 - 05:06 PM

its not the technology its the piston motors themselves,the eaton I run in my shop is a 4 cylinder 2 stage and am about 40cfm


do you have shop air lines run already and an air dryer etc capable of keeping up with the cfm ?, the air system in my shop was 1 of the biggest investments I have made and definitly was more than I thought it would cost --flexable air hose right off the compressor will cause tons of issue with water and crap in your lines

what is the npt on the tank going out ,the bigger you have it and run your airlines it also gives more air reserve (I use 3/4" black pipe about 100ft)
for blasting it doesnt have to be clean air just dry

the price of clean dry air is expensive,

24-28cfm sounds about right

#140 NUTNDUN OFFLINE  

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Posted February 24, 2011 - 05:50 PM

Something don't sound right. I'm guessing the stroke to be in the balpark of 5" on both cylinders, & the bores to be about 4.5", & 2.5". If I'm not mistaken, the strokes are usually the same, & the bores change to keep the pump in balance.
Of course I could be wrong....but?

If you scale your picts, the cylinders are nearly 1ft tall. It's hard to believe that the stroke is only 3 1/2".
Joe
Joe


Joe, I have a feeling you might be right about the measurements being wrong. When she told me the measurements she just told me 5" x 3 1/2" x 3 1/4" and that was it. So it more then likely is 5" stroke but the cylinder diameter doesn't seem right either.

Here are some pics I just took with a tape measure to show the dimensions. The primary cylinder is 8 1/2" tall from the base of the cylinder to the top of the cylinder(below the head). It is also 5 1/2" in diameter at the narrowest point(not at the fins).

The second stage cylinder is the same height but the diameter is about 3 3/4" - 4".

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#141 NUTNDUN OFFLINE  

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Posted February 24, 2011 - 05:57 PM

I am pretty tempted to yank one of the heads off just to be sure.

I think Joe nailed the dimensions. If the cylinder has an outside dimension of 5 1/2" and the piston is 4 1/2" that would mean it has a 1/2" cylinder wall. I would think if the cylinder wall was too much thicker that it wouldn't transfer heat well enough.

#142 olcowhand OFFLINE  

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Posted February 24, 2011 - 06:00 PM

NO.........I would not mess with the compressor unless absolutely necessary! Just my opinion, but I would leave it alone. You're gonna have more air than you'll need no matter what size the bore & stroke is.

#143 caseguy OFFLINE  

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Posted February 24, 2011 - 06:10 PM

NO.........I would not mess with the compressor unless absolutely necessary! Just my opinion, but I would leave it alone. You're gonna have more air than you'll need no matter what size the bore & stroke is.


:ditto::ditto::iagree:

#144 RailmanB110 OFFLINE  

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Posted February 24, 2011 - 06:30 PM

Ok,
How fast should he spin the pump with the motor he bought?

#145 NUTNDUN OFFLINE  

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Posted February 24, 2011 - 06:42 PM

The manual that Caseguy was able to get a hold of for me has a good bit of good info in it. There are a couple of models listed in the same manual. In the one section it states how they are 7.5hp, 10hp and 15hp pumps and range from 30cfm to 75cfm. I was hoping I had the manual here at the house but I have it on my work computer. I will post it tomorrow.

If I go by the sizes the lady gave me except using the 5" for the stroke it comes out to 20cfm at 750 rpm. For the size of that pump it doesn't seem right.

If I go with the stroke being 5" and the primary cylinder being 4.5" diameter at 750 rpm that comes out to be: 34.5 cfm.

Of course all of this is hearsay until I know the actual dimensions.

#146 NUTNDUN OFFLINE  

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Posted February 24, 2011 - 06:57 PM

Another wrench to throw in to the mix. I was trying to do some more googling which lead me to a new 10hp IR pump on northern tool: FREE SHIPPING — Ingersoll Rand Two-Stage Type 30 Compressor Pump — 10 HP, Model# 2545V | Air Compressor Pumps | Northern Tool + Equipment

In the key specs area of the description it says the bore diameter is 5" on the main and 2 3/4" on the secondary and they both have a 3 1/2" stroke.

I could just size the pulley to 750 rpm and be done with it and whatever it produces it produces.

#147 olcowhand OFFLINE  

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Posted February 24, 2011 - 07:57 PM

Another wrench to throw in to the mix. I was trying to do some more googling which lead me to a new 10hp IR pump on northern tool: FREE SHIPPING — Ingersoll Rand Two-Stage Type 30 Compressor Pump — 10 HP, Model# 2545V | Air Compressor Pumps | Northern Tool + Equipment

In the key specs area of the description it says the bore diameter is 5" on the main and 2 3/4" on the secondary and they both have a 3 1/2" stroke.

I could just size the pulley to 750 rpm and be done with it and whatever it produces it produces.


I used the Q&A at Northern to find the operating speed of this pump. Here is the whole conversation:

Thank you for choosing Northern Tool. A representative will be with you shortly. Hi, my name is Holly. I am a Northern Tool + Equipment Sales Specialist. How may I help you today?
dan: what is the operating rpm of this pump, item #1592033
Holly: I would be happy to help you Dan!
Holly: Please allow me 1 to 2 minutes to research this for you.
dan: ok
Holly: I'll be right with you.
Holly: 1050 rpm
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#148 RailmanB110 OFFLINE  

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Posted February 24, 2011 - 09:03 PM

After rereading George's orig numbers, I'm thinking the lady told him big bore, stroke, small bore. Since most notations are bore x stroke, it would make sense that the first two would be the big cylinder bore x stroke. Since the stroke is the same on both cylinders, she would have given the small bore last.

After you ms'd the ouside diameter of the big cyl, it probably has 1/4" wall, & hence the 5" bore.
It looks like your volume computations are probably right.

Here's my best guess based on comparable compressors pumping 32 cfm (pist volume) & 7 1/2 hp motor. I've looked at several, & seems to be in the ball park.
If you use a 8" motor pulley to 16" pump pulley @ 1725 rpm you get 863 pump rpm.
Bore = 5" x 2.5 x2.5 x 3.25 = 63.78 cid (= George's numbers above)
63.78 x 863 rpm / 1728 = 31.84 cfm piston volume
AVG net cfm @ 175 psi = about 75 % of piston volume = .75 x 31.84 = 23.88 cfm @ 175 psi

This should give you camparable values to what's out there. If you want to be a little conservative, I'd go with a little smaller motor pulley.

A typical 5hp pump will be 17 cfm @ 175 psi, (23 cfm pist disp).

If you went with a 7.5" motor pulley (809 pump rpm) , you'd end up with 22.38 @ 175 psi (29.85 cfm pist disp). This would give a little margin for safety, given it's an old pump, & might eat up a little more hp than the newer stuff.
To make a long story short I'd go with a 7 1/2" to 8" on the motor.
Joe
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#149 NUTNDUN OFFLINE  

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Posted February 24, 2011 - 09:13 PM

Sounds like a plan. I will definitely get a bushing / sheave pulley setup that way if anything would have to be changed later on to accommodate a different compressor it will make things a little easier. Without knowing how the compressor will do, time will tell. If I can get 20+ cfm of useable air I will be happy. Plus since I went ahead and bought the motor starter and the higher hp motor I am pretty much set for good now other then a pump change once this one gives out. :D

#150 RailmanB110 OFFLINE  

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Posted February 24, 2011 - 10:32 PM

George,
After looking at your picts again, & looking at the pump Olcowhand linked, I'm thinking the stroke is 3 1/2". It kinda adds up better the way your picts & measuments went. The t-30 linked, was 5" x 3 1/2" stroke, & sure looks like the same crankcase as yours.
Anyway, if it is in fact a 3 1/2" stroke, then the cfm changes a bit.
With a 7 1/2" pulley, you'd get 32.14 piston disp (24.11 cfm@ 175psi).
With an 8", It might overload your motor.
With a 7.25" pulley you'd get 31.07 cfm piston disp (23.30 cfm @ 175psi).
From what I can figure, you can get from 3 to 4 cfm @175 psi, per hp.
With a 7.25" pulley, you'd be at 3.11 cfm per hp.
The other consideration though is that you don't plan on running at 175psi. In that case you might be able to go with the 8".
Joe




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