Compressor decisions with possible motor swap, please help decide.

[ybnormal70]

I have 2 Speedaire compressors.

First is a 60 gallon 5z628b with brand new Century 5hp 3450rpm motor 13.7cfm @ 175psi 2-stage.

Second is an 80 gallon 3z745a with an older Dayton 3hp 1740rpm motor 17.1cfm @ 175psi 2-stage.

The 80 gallon cfm rating seems to be for a 5hp version. I can not find any cfm ratings for the 3hp version which I have. It does show on this compressor 3-5hp to be acceptable range. My question is, should I swap the 5hp motor from the smaller compressor to the larger before I sell the smaller one? I know I would have to purchase different pulleys for both to get the rpm proper but would this be a good idea?

Thanks,

Kevin

 

[ybnormal70]

After doing some calculations with online pulley calculators, I have found....the motor & pulley setup on the 80 gallon is not correct. Looks like I am only turning about 448rpms when the pump is rated for 700. It also looks like to put the 1740rpm motor on the 60 gallon compressor would require a 15-3/16" pulley, HA! Looks like I have now answered my own question and need to just purchase the correct pulley for the 80 gallon 3hp motor.

Thanks,

Kevin

 

[larry_g]

If I were you I'd suggest that you run one of the fill time tests to confirm what the real life CFM of each setup is. Your numbers don't make sense. Are you sure that each one has the original motor on it?

I have the 5z628b in my old catalog and it is sold as a light duty compressor with a 60% duty cycle and a 1600 rpm pump speed. I suspect that it is quite noisy.

On swapping motors- I wouldn't. It would be fine to put the bigger motor on the one but that would leave the second underpowered unless you went through the drill of slowing down the pump to 3/5 the current speed, or around 1000 rpms.

Do you have the specs on the three hp pump? I suspect that it is the better compressor. I found the below on ebay, http://www.ebay.com/itm/3Z745-REPLA...T-COMPRESSOR-PUMP-WITH-FLYWHEEL-/271393105582 , so it MAY be the correct spec and it is rated 17.4 cfm with a 5hp motor. With a 3 hp I would suspect it is in the 10 cfm range.

Good luck with your venture. In the end on your quest it all comes down the current draw on the motor and if your spinning the pump within its designed RPM range. To slow and you can have oiling problems.

lg
no neat sig line

 

[ybnormal70]

According to the calculators I found online the 60gallon is at 15.5cfm and the 80gallon is at 11cfm.

The 60 gallon from 0-175psi takes 6m4s
The 80 gallon from 0-180psi takes 11m57s

The 60 gallon seems to be correct HP, pulleys & pump speed. The 80 gallon is only turning the pump at 448rpm of it's 700rpm rating. I'm not sure if this is just because of the 3hp vs 5hp motor or because someone put the wrong rpm motor or wrong pulley size. The motor is 1740rpm right now but if it was a 3450rpm motor it would run the pump at 888rpm which seems to be over the rated RPM. At this point I am assuming the slower speed is just because of the smaller hp motor.

Kevin

 

[larry_g]

The 80 gallon is only turning the pump at 448rpm of it's 700rpm rating. I'm not sure if this is just because of the 3hp vs 5hp motor ....snip... At this point I am assuming the slower speed is just because of the smaller hp motor.

Kevin

I believe your assumption is correct. 3/5 x 700 = 480. So it's running at the speed it should be. I would also assume that the 3 hp may be a replacement at some time or it was sold as a cheaper compressor?? One other rule of thumb is that you can produce ~4cfm per hp.

lg
no neat sig line

 

[ybnormal70]

Thanks larry_g! Guess everything will stay as it is for now and possibly upgrade to a 5hp with proper pulley in the future or maybe even that big boy 7.5hp!

Thanks again,

Kevin

 

[redmondjp]

Good call - too many people don't understand that any given pump can be run over a range of speeds, and in general, the faster the speed, the more horsepower required.

Plus, too many people also don't understand that compressor manufacturers lied about their horsepower ratings for decades. My "5HP" 60-gallon compressor built in 1990 is actually around 3.2 - 3.3HP.

If you obtain a true 5HP 1740RPM motor, you can spin that pump on the 80-gallon faster.

 

[marinusdees]

Good call - too many people don't understand that any given pump can be run over a range of speeds, and in general, the faster the speed, the more horsepower required.

Plus, too many people also don't understand that compressor manufacturers lied about their horsepower ratings for decades. My "5HP" 60-gallon compressor built in 1990 is actually around 3.2 - 3.3HP.

If you obtain a true 5HP 1740RPM motor, you can spin that pump on the 80-gallon faster.

Shame on you for falling for this.

 

[redmondjp]

Shame on you for falling for this.

Care to explain?

 

[ybnormal70]

Just another quick question. Since I was still thinking about replacing the 3hp motor with a 5hp motor I was still studying the HP rpm chart and I'm really thinking I should go with a 7.5hp 3450rpm motor. With that motor and the current pulley I would be at 890rpm at the pump and somewhere around 21cfm correct?

That would be easier than buying a 5hp motor and also having to change the pulley to a roughly 6-3/4" one to get the 700rpm.

Later down the road I could always get a larger pulley for the 7.5hp motor if the increased CFM was ever needed.

Thanks,

Kevin

P.S.
If the 7.5hp sounds like a good plan, how does this Leeson 7.5hp motor seem?

https://www.amazon.com/dp/B00HRXVHB4/?tag=atomicindus08-20

 

[ybnormal70]

Or maybe this Baldor:

http://www.ebay.com/itm/271956786548?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

 

[ybnormal70]

Anyone? Really leaning towards ordering that Baldor....

Thanks,

Kevin

 

[ybnormal70]

Is this thing on? HA

Kevin

 

[larry_g]

Are you going by the chart above I gleaned from ebay? If so then have you confirmed that it is applicable to your pump? If so then what does the C10700 mean after the 7.5hp rating? I would assume that is a different pump head. You should confirm that. You should link to the other charts you are consulting as we, I, have no idea if your getting good input from elsewhere.

As for the motors you linked to either one is a good motor, I just don't have enough information or confidence to say that a 7.5hp is what you need or could utilize efficiently. Me personally I would go with a 5hp motor and spin the pump slower giving it a longer life and be quieter.

lg
no neat sig line

 

[ybnormal70]

From what I can find it seems to be accurate for my pump. The C10700 had me curious as well and when searching for that it came back with a filter. I'm guessing maybe different filter for that HP. I would still be running the pump a little slower than what it says for the 7.5hp. Mainly the the price difference between the 5hp & 7.5hp would get eaten by the new pulley required for the 5hp.

Kevin