Pump RPM

[f575gtc]

I'm thinking of buying a new EMAX pump that produces 14.1CFM @100PSI.

I did the math and found out my pump is putting out about 8.5CFM @ 95PSI. The problem is I have a older motor that runs at 3510rpm with a 3 inch pulley.

If I were to mount the Emax to this motor with the 10 inch flywheel it comes with I would be spinning the pump at 1053rpm, the manual states the pump runs at 950rpm.

My question is how much damage can I possibly do to the pump by increasing its rpm by 103? Would it produce a negligible amount of wear and tear? What about CFM increase would it make any big difference there?

 

[zkling]

What are the specs on the pump (max RPM)? CFM output is roughly linear with RPM of the pump. Without looking it up IIRC for rotational machines wear is proportional to speed squared. Going from 950rpm to 1053 rpm would be a 23% increase in wear on the pump. You should probably be using a 1725rpm motor to avoid speed and belt slip issues, but....

 

[f575gtc]

emax doesn't had a max rpm post on their site unfortunately.

 

[A_Pmech]

Pump damage? Very unlikely.

Overloading the motor is possible though. The faster you spin the pump, the more power it will consume by compressing more air.

 

[DXN1EL]

8.5 CFM sounds like a 2 or 3 hp motor
14CFM is more like 5hp
The motor is not going to get you 14 CFM you would have to slow the pump, slow enough to get you 8.5

How big is your tank?

 

[larry_g]

I'm thinking of buying a new EMAX pump that produces 14.1CFM @100PSI.I.

The problem is I have a older motor that runs at 3510rpm with a 3 inch pulley.

What is the rated HP, or amp rating (FLA) of this motor you have? Do you have a link to the pump you intend to use?

 

[f575gtc]

Motor is a 21amp motor

60 gal tank

http://emaxcompressor.com/?fuseacti...productID=B22890C1-F4DD-3FBE-5536FED1FB82DA88

 

[larry_g]

So your pumphead is rated for 5hp at 950 rpm 145psi. Your proposing a 10% increase in speed so your going to have to do something to accommodate that. You can increase the HP of the motor, slow the pump speed down using the recommended 2.75" pulley, or reduce the cutout pressure to something less than 145psi. Bottom line here is not to overcurrent the motor.

lg
no neat sig line

 

[f575gtc]

I don't run past 125psi anyways, its only a single stage.

so currently my tank takes 6 min and 55 seconds to fill from empty

and from 95 psi to 125 psi it takes 1 min 23 seconds, after doing the calculations that translates to about 8.5CFM at @95psi when the tank kicks on.

Now if I do the math of a empty tank to the full cut off, it gives me ~10CFM free air, so it can't possibly be giving me more than 8CFM under pressure.

The the pump is a older Sanborn V210 @ 850rpm dual cylinder design, I tried and tried finding info on it but had no luck.

BTW this is for a home garage, I do very little work, spray an occasional car, and do some sandblasting very seldom, I would say I run this compressor on average 2-3 hrs total per week.

 

[f575gtc]

It does seem odd that I have such a low CFM with a motor and pump of this size, but unless my math is wrong, it seems like the pump is just not as powerful or just older and not performing as well.

 

[zkling]

and from 95 psi to 125 psi it takes 1 min 23 seconds, after doing the calculations that translates to about 8.5CFM at @95psi when the tank kicks on.

I get ~12cfm for that interval. If you want the most accurate assessment take it in steps of 10psi and plot the trend. I think there is an issue with the data you present above, maybe check the numbers.

 

[f575gtc]

What are you doing there to calculate the CFM?

I followed these steps http://www.truetex.com/aircompressors.htm

and keep getting 8.5

 

[zkling]

What are you doing there to calculate the CFM?

I followed these steps http://www.truetex.com/aircompressors.htm

and keep getting 8.5

I derived the equations myself, thus far they seem to have been pretty accurate.

 

[f575gtc]

hmm so that emax pump won't be much of an upgrade.

 

[zkling]

What are you doing there to calculate the CFM?

I followed these steps http://www.truetex.com/aircompressors.htm

and keep getting 8.5

I think you may have miss used that equation. That is essentially the same one I use and with that, real quick I get ~12cfm from 95-125psi 83sec on a 60gal tank.

 

[DXN1EL]

60gal tank is a good size. Why not just buy a 10hp motor? that can get you 40-50CFM @ 100 PSI

10hp motor with TWO 5hp pumps?
I have seen 10hp motor on ebay go for cheap

 

[f575gtc]

60gal tank is a good size. Why not just buy a 10hp motor? that can get you 40-50CFM @ 100 PSI

10hp motor with TWO 5hp pumps?
I have seen 10hp motor on ebay go for cheap

Way out of my budget and requirements.

 

[f575gtc]

would ~15CFM be about the maximum a 5hp motor will be able to produce on most pumps?

If I in fact am running aprox. 12CFM @90PSI, I think going to 14.1@100PSI will not be much of an upgrade.



I found a 2 stage pump that is more money about double, but it is rated for 17cfm at 175PSI with a 5hp motor, but my tank is stamped for 150PSI max and I think going to 175psi is risking it no? If I go with a 2 stage pump and cut out the pump at 135-140psi will I still be exceeding 15 cfm at 90psi?

The highest PSi tool I use is my sandblaster which calls for an average of 10CFM @100PSI. Right now if I use the air compressor and sandblaster, once the pump kicks on at 95PSI, the pump keeps on running until I give it a 20-30 second break. I just want to have a little more power to allow the pump to catch up to my sandblaster, DA, hvlp, etc and not have it run consistently while I am running those tools, but I don't use these tools often enough to warrant spending money on a 80 gal $3000 compressor.


Also thanks for all your help on this issue.

 

[f575gtc]

Thanks for the help again guys!

I pulled the trigger on a 2 stage pump today, was able to get it as a best offer for $280 it was normally $360, overstock/closeout prices could have probably gone cheaper.

It is rated for 5HP and produces 17CFM @ 175PSI, so I should be able to get a good 18-19 at least at 90PSI from it, and I plan to have it cut out at 155psi and back in at 125psi.
Best of all the pump spins at 780rpm, so it should be quiet.

I will eventually upgrade to a slower rpm motor once my current one burns up, but for now I should be able to keep up with sand blasting and not have this thing run all the time.

Edit for spell check...

 

[Todd.Brock]

A lot of Cfm ratings are published at 90 psi. You could be closer to 21-22 at 90 psi

 

[f575gtc]

Please don't get me too excited.

I figured if I am going to upgrade, might as well make it a worth while upgrade.

Should be a much better upgrade than my current single stage pump.

Are there any preparations I should make to the tank to go from a single stage to 2 stage?

I already know I need to upgrade my emergency release valve, I think mine is only rated to 135psi.

 

[f575gtc]

Hey quick question about working PSI.


My tank has Max W.P 150 PSI stamped on it, I assume that stands for max Working Pressure.

The 2 stage pump I ordered is capable of 175PSI, but it seems like I won't be maxing out its potential on this tank. My plans were to run the tank to a 155PSI cutoff and a cut in of around 120PSI, with a emergency blow off valve of 165psi.

The tank is an older tank circa 1985, but it was well maintained, drained all the time, and has no rust on the outside, and when I shine a flash light into the tank, it looks like it is in good shape.

Am I running into safety hazards running 5psi over the max limit of the tank? I would figure these tanks are tested well over the stamped PSI for safety.