Purchasing New Shop Air Compressor Advice

I currently have an 80 Gal, 5HP air compressor, which is enough for what we do. We don't use any air tools, just a blow gun, but I have several pieces of equipment that operate off the compressor. We will be adding a plasma table in the near future, and I'm concerned the compressor won't be able to keep up with the plasma and all the other equipment. By my calculations, with everything running, the compressor will not be able to keep up with everything.

I've been looking at the rotary screw compressors, mainly because many of them have a dryer included, and operate much quieter. So there's three options I'm considering:

1) Keep the compressor, and buy another compressor dedicated to the plasma table.

2) Buy a larger compressor, still a piston type, larger better suited for operating everything in the shop.

3) Buy a rotary screw compressor.

I'd like to hear some advice on which option would be best. I read somewhere that rotary screw compressors are most efficient when they operate 24/7, so it is important they are sized appropriate for the shop. This would be an issue, since the air flow used fluctuates so much at an given time depending on what's being used (shop of 8 people). If this is the case, is it BAD for a rotary screw compressor to start and stop like a standard piston type? That would be the only reason why I would consider a larger piston type compressor instead of a screw type.

If a screw type compressor is best, anything in particular to look for if a good deal turns up used? I don't mind buying new, but I see some good deals on these types of units, especially the ones that are 480V only (I have 480V supply). Though I have also noticed many of these units have a lot of hours, like 18,000+ hours, which is like 3 years of non stop running. I don't have any experience at all with screw type compressors, are they pretty bullet proof, or do they require regular maintenance? Are they easy to repair?

Just installed a Polar Air unit from Eaton compressors here in Ohio. Price is close to that of used, all maintenance parts are off the shelf items. They have a real live tech on staff to assist. Seems to be all the right stuff so far.

Econdron said:

I currently have an 80 Gal, 5HP air compressor, which is enough for what we do.

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Stop right there! You just told me that you do not know much about compressed air for shop use, or anywhere for that matter.

First, the size of the receiver (tank) doesn't matter.

Second, though horsepower plays an important role it is not the sole decider.

Now, the real figure you should be concerned about and is all that really matter because it will determine the first two. It's called CFM! Another way to say it is Cubic Feet per Minute.

Your five horse comp probably puts out around 15 CFM. Figure under the most efficient method of generating compressed air, usually a screw type or hydrovane, you will get a max of about 4 CFM per horsepower under ideal conditions. Your compressor should be sized appropriately for your requirements. This is especially important when considering a screw/rotary type. If you requirements are 10-15 CFM at 120 PSI, you don't want to overbuy and get a 25-30 CFM. You will get no more air because of your requirements and your electric bill will soar.

Here's what you do. Have a qualified sales person (or independent specialist) come in and evaluate your needs and then recommend an appropriate solution. It amazes me how shop owners just buy a machine, throw it in there and never really have a handle on what they need. They, for some reason think that the bigger the tank the better. WRONG. It's CFM. If you don't have enough CFM that big ol' tank will eventually run out because you are using more air than the pump can produce.

Bottom line. The air compressor is probably THE most important machine in the shop yet it is treated like a red headed step child. No air, no production. It's that simple.

Another thing to consider is the air treatment such as chillers and dryers to keep the air dry at all times, machines last longer that way. Also, your air delivery lines. They should circle the shop in a continuous loop from the compressor and back with at least two dips (low points) with valves (automated or manual) to drain any moisture on a daily basis. Your air delivery lines should not have any low points that cannot be drained. Also, figure your main delivery lines should be 1-1/2" dia. (I use copper, never use PVC!). This should get you going in the right direction.

Good advice above. A place that deals air system stuff for a living will be happy to come in and either fit a flow gauge on your setup to see what you need or do some calculations to figure it out.

One thing I'll say about Hydrovane compressors is they're sort of irritating to listen to. I have a 10hp and a 15hp model at work and you wouldn't want to work next to them. Newer ones may have sound baffle boxes, the ones I deal with are older without that stuff. They also make a considerable amount of heat so ventilation is important. The older 10hp two stage piston pump we have is far nicer to listen to, but since it has no internal heat exchanger it makes the air hot. We have a refrigerated air drier, it does wonders for keeping equipment from failing. When the drier died, within 2 weeks parts were failing from rust and corrosion inside them.

you need a technical salesman. Most of what you have read on screw compressors is wrong. I just installed a Kaeser 15 hp, a bunch more storage, ( 800 gallons in my case ), and tied into existing shop/ lines. I came from a constant on 40hp screw ( we used to do a lot more blasting), went to a cycling ( set up like your piston compressor but a screw ) rotary. pressure range in the tanks is 103-125 psi- regulated out of the tanks to 100 psi shop air. You want the compressor to run when it runs..... 2 minute on 60 off won't do it much good but if it runs 40 -50 minutes per hour ( quietly ) you are getting good efficiency. You want it to run no less than 30 minutes when it kicks in.... so put in enough tank. I prefer a separate dryer, it will generally die before the compressor, rule of thumb is 3-4 dryers in a compressor's life as I recall.

in my area the kaeser sales team was the best hands down.... knew their stuff and steered me well.

I bought one of the variable drive screw from Polar. The full kit with dryer, chiller, etc. Clean dry air. In 2 years maybe a couple of ounces, maybe less, from the drain drops. No gearbox shifting issues on the Haas VF-2. Life is good. I had the QT 7.5 3-phase piston before, and dryer etc, but it was running a lot (noisy too). Electrical bill did not go up with the rotary compressor. AND - it's quiet!

Are you really going to run all of your equipment at the same time? Very often? If just occasionally, your tank may take up the slack.

Are you going to use a CNC (i.e. continuous cut) plasma cutter, or a handheld cutter?

I'd be tempted to try it with what you have first and see if it's really going to be a problem.

steve45 said:

Are you really going to run all of your equipment at the same time? Very often? If just occasionally, your tank may take up the slack.

Are you going to use a CNC (i.e. continuous cut) plasma cutter, or a handheld cutter?

I'd be tempted to try it with what you have first and see if it's really going to be a problem.

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CNC plasma. Hypertherm consumes about 6CFM. I think I will be fine for the most part, but it could be a huge problem if all the equipment is running at the same time and the tank gets depleted. I think I'll call up a local sales team and see what they have to offer.

What I did is that I have two compressors, the nice old Quincy 17.3 CFM guy that I have always had, and when I started to beat that down because I added air blast, I got a second cheapie that is like 7 CFM that I plumbed into the system and then set my regulators so that the Quincy runs from down low to 140 and the the cheapie doesn't kick in until I hit 120. The shop is regulated at 120PSI, and we are small enough that I don't have OSHA concerns... but in any case, I almost never see that little guy kick in, the big guy runs less frequently for longer, because of the additional capacity, and on Mondays the system gets up to pressure faster as a bonus. All for like 250 bucks from craigslist.

I secound the noise thing, piston compressors all thumb - have a real base like sound thats hard to stop, the screw - vane compressors all whine more higher pitched. But being a higher pitch its a lot easier to box em in enough to really reduce it.

Screw or vane both need to run long enough to get to operating temp and dry any moisture out of there oil. That means bloody hot to the touch like 60-80 celcius is common, not always anything like 30 minutes of run time either, or even running every hour, but it needs to be more than filling a small tank just a couple of times a day with but 1-2 minute run times.

adama said:

..... but it needs to be more than filling a small tank just a couple of times a day with but 1-2 minute run times.

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If your compressor is only running a couple times a day then your compressor is too big for your requirements.

Get rotary screw. Get a Kaeser.

John Welden said:

Get rotary screw. Get a Kaeser.

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^^^THIS^^^

Great machines, a little on the pricey side but you get what you pay for.