OT: Buying a Rotary Screw Compressor - Educate me first, please.

I am coming to a point in my shop where air usage is going to be a huge problem.

I have a Haas VF-0 that requires a minimum of 95 psi, and I have a new VF-2SS coming in, in a few weeks that will require 100 psi. Right now I'm not even sure if the air compressor I have will even kick in before dropping below 100 psi. I have a Husky 7hp 60 gallon compressor and a Campbell Hausfeld 6hp 60 gallon compressor.

Air usage with air guns has always been a problem as well. If the air compressor is on the bubble for kicking in to make air, and someone uses the air gun for an extended period of time to clean off parts, it will throw an alarm on the VF-0 and shut it down due to low air pressure. It's a huge pain in the ass.

Moving into the new shop we came into another air compressor so one will be for the machine only and the other for air guns only. That will eliminate that problem, but not so sure it will be enough to run both machines if they are changing tools at the same time using the air blast and the compressor is on the low end of the psi before kicking in.

This is one of the first ones I saw on ebay just now. Have no clue if it's a piece of crap or not, and I don't have money growing on tree's like david, so a Kaiser is probably untouchable for me right now. Plus, to be honest....I would not know good brand names from lousy ones, so please........educate me about rotary screw compressors so I can make an informed decision before buying and being stuck with something I will regret.

Best Regards,
Russ

Brand New Eaton 10 HP Single Phase Rotary Screw Air Compressor | eBay

I can't comment on the screw compressor, but I have a 7.5hp recip from eaton and a refrigerated air dryer and have not touched it other than clean filters and change oil twice with mobile rarus 427.

I can easily run 3 haas machines with air misted bearings and side mount tool changes, air bearing cmm, and some sand blasting in a small cabinet with no issues. I am sure you have already checked all of this but do you have any significant air leaks. We found several on all of our haas machines seemed like most fittings leaked.

But on the other had I wish I had purchased a screw machine just because on the noise. I would also be curious as the when they become better than a recip as I assume they run all the time and just load and unload as needed.

Well, I know that if I was doing it again I'd buy a Kaeser.
Compare them with the cost of adding a receiver tank and dryer on the one in your link.
I don't think its something I'd really buy off ebay, surely there's a few good dealers of a few brands in your area that offer service and all that? (hopefully reasonably priced...)

SND said:

Well, I know that if I was doing it again I'd buy a Kaeser.
Compare them with the cost of adding a receiver tank and dryer on the one in your link.
I don't think its something I'd really buy off ebay, surely there's a few good dealers of a few brands in your area that offer service and all that? (hopefully reasonably priced...)

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It's just something I'm beginning to kick around, so I know of none at this point. However, that doesn't mean that there's not one right in the next town, I've never checked into it.

Want to learn all I can first.

If I do get the compressor I would have two 60 gallon tanks I could use off the two piston type ones.

Later,
Russ

csharp said:

I can't comment on the screw compressor, but I have a 7.5hp recip from eaton and a refrigerated air dryer and have not touched it other than clean filters and change oil twice with mobile rarus 427.

I can easily run 3 haas machines with air misted bearings and side mount tool changes, air bearing cmm, and some sand blasting in a small cabinet with no issues. I am sure you have already checked all of this but do you have any significant air leaks. We found several on all of our haas machines seemed like most fittings leaked.

But on the other had I wish I had purchased a screw machine just because on the noise. I would also be curious as the when they become better than a recip as I assume they run all the time and just load and unload as needed.

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I understand what you're saying, but how do you adjust the low end of the psi before the compressor kicks in?

Your issue isn't pressure, it's volume. The compressors can't put out enough CFM to keep up with demand from the description of the problems your are having. A Screw will put out a lot more volume (usually).

Used to work for a machine shop that had 11Haas' and paint booth, sand blaster, and 2air presses plus blow off parts several air wrenches in assembly. Used to use piston compressors, had 2 25hp and a 125 hp. Moved to a new shop and put in a large after cooler, receiver, and for the Haas a dryer. Sized the dryer for 100cfm that cycles on and off as not all cnc are on are on the line on all shifts. Figured 5 cfm per machine, for the dryer so they have room for expansion. Didn't want to fire up the big compressor as it was rather overkill now and didn't want the demand on the new building. (Big compressor was built in 1952 so parts were hard to get and $$$$.). Talked them into getting a new screw compressor and the rep sized it at 50hp to allow for more usage later and VFD drive. They set the pressure to 112-105 and it ramps down to meet the load then if low usage ie, over lunch, it will shut completely off and then back on on demand, hi usage gets it to spool up but--it is quiet. Less than 50db, quieter than a VF-2. Use it 24-6 and service it 1-2 time a year. I would say figure 5cfm/machine add extra for blo off and go 3ph with a VFD, your 2tanks and you can't go wrong.

Congrats on your move and you commentary on your shops. Warren

First thing is definitely to figure out your CFM demand and duration of peak demand followed by how long of normal/low demand, it helps size storage capacity, min/max psi and steady output required to keep it all happy happy happy.

Tonytn36 said:

Your issue isn't pressure, it's volume. The compressors can't put out enough CFM to keep up with demand from the description of the problems your are having. A Screw will put out a lot more volume (usually).

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There is no free lunch. You need more power to get more volume.

Actually, a recip compressor will make a little bit more CFM for the same amount of power.

I have been going through the same thing and it is driving me nuts trying to figure it out. I have an old Kaeser SK19 15HP screw that I am using as my main unit right now. Makes a crapload of air but uses a lot more electricity than the old recip. When the receiver tank gets low the screw starts, brings the pressure back up, but then goes into an unloaded mode. Loaded it pulls about 45 amps, unloaded about 15 amps. The bad part about this is that once it comes on and reaches pressure, it "idles" for about ten minutes before it shuts down. The service guy said that this is to keep motor starts under six per hour and to bring the oil up to operating temperature. I know it makes the compressor last longer, but damn it is expensive to run. Ours pretty much runs ALL the time that the shop is going.

A piston compressor is not as efficient at making air but from what I can tell but is a lot more forgiving to the wallet if you have a wildly varying load (like a lot of shops have). Starts up, does its work, and then shuts off. I am leaning towards a high quality recip for the shop.

If you want to get a screw, the only under 20HP unit to get is a Kaeser. From what I can tell, all of the other smaller units are made in China or as the Quincy made in Italy out of Chinese parts. The other ones cost a little less than the Kaesers but you get what you pay for.

Mickey_D said:

I have been going through the same thing and it is driving me nuts trying to figure it out. I have an old Kaeser SK19 15HP screw that I am using as my main unit right now. Makes a crapload of air but uses a lot more electricity than the old recip. When the receiver tank gets low the screw starts, brings the pressure back up, but then goes into an unloaded mode. Loaded it pulls about 45 amps, unloaded about 15 amps. The bad part about this is that once it comes on and reaches pressure, it "idles" for about ten minutes before it shuts down. The service guy said that this is to keep motor starts under six per hour and to bring the oil up to operating temperature. I know it makes the compressor last longer, but damn it is expensive to run. Ours pretty much runs ALL the time that the shop is going.

A piston compressor is not as efficient at making air but from what I can tell but is a lot more forgiving to the wallet if you have a wildly varying load (like a lot of shops have). Starts up, does its work, and then shuts off. I am leaning towards a high quality recip for the shop.

If you want to get a screw, the only under 20HP unit to get is a Kaeser. From what I can tell, all of the other smaller units are made in China or as the Quincy made in Italy out of Chinese parts. The other ones cost a little less than the Kaesers but you get what you pay for.

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They sure put out a crap load of heat and wasted energy, mine was factory set at first to unload for 15 minutes, so it would shut off for about 1-5 mins and then it would need to restart and fill up again, quickly had enough of that and wasting that much power and having the shop at 90°.
So now I mainly run on tank capacity and adjusted it lately to go from 70 to 135psi, unloads for 1 minute and shuts off for 15-20 of cnc run time, seems to work ok for my demand(which is rather low). The time it takes it to fill the tank back up brings it well into operating temperature, and being in a dry/warm building I doubt its as big an issue as those who have them outside or real damp places... hopefully so anyway. I don't see how someone with more demand couldn't do similar with just more storage capacity which would give it more time at operating them, then shut off for a while. Having a big receiver tank also helps a lot at getting more of the moisture out, if the dryer is downstream from it.

I have a Kaeser 3 1/2 HP screw compressor on a 60 gallon tank in my garage. I don't have a dryer, but I rigged up a cheap sorta dryer in the airline. I ran a piece of 3/4 inch PVC pipe up tp the ceiling in the garage and then back down to about 3 feet off the ground, then a 90* elbow and a Home Depot water trap, then another 90* elbow back up to the ceiling and then I run all my air lines from there.

Before I got the Kaeser Compressor, I was using a 2 HP Home Depot compressor, and when it came on, everyone stopped talking because all you could hear was that compressor. This Kaeser is bitchin' because when it comes on, I can be standing right next to it, talking to you on the phone and you can't hear it run.

When it comes on, it runs for pess than 1 1/2 minutes and then shuts off. I've had it for about 3 months. It was a lot of money, but oh so worth it.

I paid about $3,500.00 for it, and Kaeser has an office near by so I picked it up at their warehouse thus, no shipping.

I keep looking at them too, where I have single phase coming in and no hope of getting 3 phase the dealer said I would be better off to stick with single phase on the comp. Rather than have the energy loss in converting to 3ph to run a 7.5hp that could just run on 1ph. The downside is that the VFD models only seem to be available in 3ph.

It looked like you will more than likely have true 3 phase before long so a 3ph comp would be the way to go then. Once I have the spare money I'm deciding between an Atlas Copco and a Kaeser. I think David N just picked up a Kaeser and said it was only a little bit more than the Copco but probably is a better machine.

In the preshure switch there is a cut in adjustment and a cut out
How-To Adjust A Pressure Switch Video

I think you have plenty of compresser now just need to get them set right.

wrustle said:

I understand what you're saying, but how do you adjust the low end of the psi before the compressor kicks in?

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ewlsey said:

There is no free lunch. You need more power to get more volume.

Actually, a recip compressor will make a little bit more CFM for the same amount of power.

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And where's your proof to back that up?

I have a 10hp Kaeser, on-off rotary screw. It makes some noise (and draws some power I don't know how much) when "off", but even when "on" it's vastly less noise than the piston compressor I had before. Mine is a nice combined package with compressor, tank, dryer. 40cfm on about 10hp.

It can be adjusted for low and high switch settings, and to run on or not run on to get/keep warm. Service folks turned that on last time and I turned it back off just for less noise.

They now have variable speed compressors that can ramp up and down according to load, and (according to their rep at fabtech) are very soft start. It's the same technology attached to a VFD. How much (if any) savings is a function of what your air demand looks like.

From talking with various people about various installations, the technology has been changing more than you think. (So apparently say 20 years ago big rotary screws where "always on" drawing high power, and needed to be left on like 168 hours a week. That's obviously not true of modern ones.)

The other thing to have in mind is that at least kaeser, and surely others, have controls such that you can have say 2 compressors and one pretty large tank. One compressor runs a fair amount of the time, the second compressor kicks on when loads gets above some threshhold. (That trick can surely be done with piston compressors too though?)

Do or will any of your machines have air blast? My DMU can eat 27cfm with the air blast on, and the direct air nozzle I put on the atrump must be close to that. If you never want to use air for cooling/chip cleaning no biggie.

If and only if your ongoing CFM load is less than your compressor makes, you may be able to reduce alarms and such with surge tanks.

Do you have a good compressor rep local (for any reputable brand?)

http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1349&context=icec

Things have changed a little sence than but a screw is still 10% less efficient at full load and about 50% at partial load than a 2 stage reciprocating.

david n said:

And where's your proof to back that up?

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david n said:

And where's your proof to back that up?

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Just look at the specs from the manufacturers. The CFM for a 10HP screw and recip will be pretty much identical, or the recip will be just a tad higher.

ewlsey said:

There is no free lunch. You need more power to get more volume.

Actually, a recip compressor will make a little bit more CFM for the same amount of power.

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Thats not entirely true.

The higher the pressure the more power you need. On the same size pump with the same size motor you will have to slow down the pump to compress air to a higher pressure. Example a 5 hp compressor will make more cfm if setup to run at 125 psi vs 175psi because the pump can be spun faster (pulleys) at 125 psi.

There is no general rule of thumb that says that recips produce more cfm per hp than rotarys. It might seem that way because the cheap recip compressors are typically overrated like shopvacs. Don't forget that a rotary can run at 100% duty cycle and the typical recip compressor has a 50% duty cycle. That means a rotary thats sized the same as a recip will produce roughly twice as much air. Some recips have higher duty cycles and some can run 100% and unload like a rotary but those are typically more expensive compressors that cose as much as a rotary, not cambell hausfields or huskys.

Wrustle, don't shop based on purchase price, but based on total cost of ownership. Whether you choose rotary or recip buy the best quality compressor you can. Compressors (especially rotarys) can be expensive to service and you don't want a cheap pos that breaks down. Downtime in the shop is also expensive. I'd look at Kaeser for rotarys and Quincy (QR-25 pump) for recips.

Russ,
Here's what you need:



That's my set up and I love it. IIRC the whole set up was in the mid/upper $7k range. That was 3 yrs ago and I'm sure they have gone up. I know for a fact it has been saving me between $60-$80/month on juice over my old comp and giving me almost double the cfms. Heck, that's an average savings of $840/yr. The machine will pay for itself in juice savings in about nine yrs. And being a rotary, she'll run for 70K-80K hrs. I'll be in my late 70s before it wears out. My ol girl was a heavy duty, 5hp 15cfm, two stage Rollair. Indestructable beast that was noisy as all get out. She still runs, but I was sick of the noise and I needed a write off. I would not get an all in one unit(comp/dry/tank). If something goes down, I can just hook up ol Bessy and still have dry air. The main reason I went with Kaeser is their air ends are made in Germany. Everyone elses small unit rotaries' (unded 10hp) air ends where made in China, Atlas, Qnincy, IR, Chicago Pneumatic, and all the no name units. Cheap electronics too. Kaeser has Siemens controls. In comps, YOU GET WHAT YOU PAY FOR!