Screw Compressor Size

I am looking at buying a Kaeser 15hp (SK15 with drier and large-ish tank) screw compressor that is available for a good price used. I believe it is rated for around 70 CFM. Right now I only have one 5-axis cnc mill that needs about 10 CFM and a small manual CMM. In the future I’ll have more equipment using air but that may be a few years until I can justify the next purchase.

I have heard it is bad to oversize compressors and that a compressor that is too large can be very expensive to run. Is purchasing this compressor a bad idea? How much will it cost me per year to run this? It will sit on almost all the time but the mill will not be running 24 hours a day. I don’t have any real leaks in my air system.

Thanks!

A rotary compressor has many advantages over a piston type, but they are more expensive. However, it is a wives tail to think it is too expensive to run under low volume requirements, IF you use large storage capacity. There is no doubt that a reciprocating compressor is less expensive. I also am a low volume user, but I will get a rotary when a deal presents itself. However, I have increased my air storage volume to 700 liters in anticipation. I will also point out that large rotary compressors are far more available used than the smaller ones, so the prices reflect that.

I have a modern Kaeser 5HP air compressor running one Mazak VMC, pick and place machine, wave solder machine, and general shop duties. I've also had 15 HP, 30HP, and 60HP Kaeser screw compressors that have landed in my lap over the years.

The modern Kaeser screw compressor won't shut off the motor until the oil temperature is up to something like 220 degrees F. To boil off the water. So the first run of the day will be 5 minutes even though it brought tank pressure above the threshold in 30 seconds. We'll also see this if we are just running shop stuff and not one of the machines that uses air continuously.

If you have the 15HP compressor and 3HP of demand, make sure you have enough tank so that the compressor can run long enough to heat up the oil. Then, as far as I can tell, it won't matter* if it only runs a few times a day.

* ignoring demand metering. If you are demand metered you could possibly be paying 3x as much demand to run the 15HP motor versus a 5HP motor if its run time exceeds the demand period (15 minutes where I am).

I had an 80 gal recip compressor (220 single phase) before switching to Kaeser 7.5hp 3 phase w/200 gal tank.
No noticeable difference in my electric bill.

Trevor360 said:

I am looking at buying a Kaeser 15hp (SK15 with drier and large-ish tank) screw compressor that is available for a good price used. I believe it is rated for around 70 CFM. Right now I only have one 5-axis cnc mill that needs about 10 CFM and a small manual CMM. In the future I’ll have more equipment using air but that may be a few years until I can justify the next purchase.

I have heard it is bad to oversize compressors and that a compressor that is too large can be very expensive to run. Is purchasing this compressor a bad idea? How much will it cost me per year to run this? It will sit on almost all the time but the mill will not be running 24 hours a day. I don’t have any real leaks in my air system.

Thanks!

Click to expand...

Your looking at buying a compressor that is 7X capacity of your load.

The biggest issue I see that you have is that your load is just too small for almost anything. With such a small compressed air load, the compressor that you are looking at will run only about 8.5 minutes per hour. The problem with the short run time is that the compressor will seldom be at operating temperature leading to condensation issues.

You did not say exactly what type of drier that this new system has. A refrigeration type drier will not be running long enough to do much of anything vs. a desiccant type will work sort of okay. Not sure about the regeneration cycle of the desiccant though.

You also did not say what you currently have for a compressed air supply. If your current system is only a cheap recip unit without a drier, this new system will not be much of an improvement as far as water condensation goes until you get more air load from more machines.

If condensation issues in your air supply are the real issue you are trying to address, you would likely be much better off making modifications to that to improve the condensation drains and drip legs.

If you are going to add another machine in 3mo., then that is a different situation and the bigger compressor would be more beneficial.

Call your Kaeser rep/service center and ask them for advice. Mine is very helpful.

They make a lot of air very efficiently, but you gotta use that air cuz they need to make more.

That's too much compressor for your needs...................it will never reach temp to boil off the water..............I have an SX7.5 and it has a hard time gettin all the moisture outa the oil.............and I'm runnin 3 VMC's and 3 CNC lathes(I am a one man shop with a few part timers though).....................Just had it serviced and the tech said run it hard................rotaries need to be sized to your air requirements.

We have an SK15 with 2X 200 gallon tanks.

It's too big for our load requirements: four horizontals, a vertical, a lathe, a WEDM, and various blowguns/support equipment around the shop. The horizontals bleed 10+ CFM continuously each (positive pressure spindles).

I would [hard] pass on that compressor.

Trevor360 said:

I am looking at buying a Kaeser 15hp (SK15 with drier and large-ish tank) screw compressor that is available for a good price used. I believe it is rated for around 70 CFM. Right now I only have one 5-axis cnc mill that needs about 10 CFM and a small manual CMM. In the future I’ll have more equipment using air but that may be a few years until I can justify the next purchase.

I have heard it is bad to oversize compressors and that a compressor that is too large can be very expensive to run. Is purchasing this compressor a bad idea? How much will it cost me per year to run this? It will sit on almost all the time but the mill will not be running 24 hours a day. I don’t have any real leaks in my air system.

Thanks!

Click to expand...

We have an atlas gx15 running one 5 axis, 4 verticals, 1 lathe and a welding area with lots of air powered angle grinding happening. Thats about max for that compressor.
Edit: our 5 axis uses ALOT of air.