Old Air Compressors: Replace or Repair?

      It could be well worth your while to keep that old unit running, rather than shop for a new one. May 14, 2006

I have an opportunity to upgrade my air compressor and I need to learn what to look for in a new one. The one I have is about a million years old. It was in the building when I moved in and it was old then. The only markings on it say Westinghouse. This compressor has never let us down. We have never been short on air. It is a 5 hp two stage. I understand the two stage to mean that one cylinder pounds the air and when it is compressed, it gets pounded a second time. I'm thinking this is why we get such even flow. The compressor itself sits on a 60 gallon tank.

The primary impetus for considering a change right now is that I have a machinery salesman who has offered to remove and cart off the old machine. This seems like a huge EPA type problem and it would probably be best to schedule this change out before the old compressor schedules it for me.

What can I expect from a 5HP compressor? Would a conventional Champion type compressor output air like my old two stage? Is there really anything to the "two-stage" story or is this just some movie I saw in my own head? I have also been told that the new compressors are self-bleeding. How does this work? Assuming that I have to increase the size of this new compressor to match the output of my old one, what is better: two 5HP or one 10HP? As long as I have air where I need it, I don't really care how it gets there. I do care about efficiency. I would like whichever system cycles less and uses less energy.

Forum Responses
(Cabinetmaking Forum)
From contributor M:
I'm no compressor brain, but your decision will be based on what you'll do with it. We will be likely looking at a 10 hp screw-type compressor soon due to lower noise level and high output.

We run a HolzHer 1411 edgebander, a couple spray guns, and all the oddball hand tools right now on a 20 gallon 5hp Craftsman, and it works fine for the time being (the edgebander must monopolize the compressor when running or it peters out).

From contributor P:
You're sort of right about 2-stage. The first stage usually compresses the air to about 30 psi, cools it, then the 2nd stage compresses it as high as 200 psi. A single stage typically tops out at around 125 psi. A 2-stage compressor is supposedly more efficient, too. Most of the 10 HP and over compressors tend to be 2-stage. CFM is more important than HP. Check the CFM requirements for your tools, bearing in mind the compressor CFM ratings are expressed in terms of CFM at a given psi.

From contributor C:
I suggest you run your current compressor until it dies, or until you outgrow it. If a 5 HP, 2 stage machine meets your current needs, I doubt any new model will bring significant efficiencies.

From contributor J:
Two years ago we installed a Kaeser rotary screw compressor and drier. I am very happy with this unit, even with the high yearly maintenance costs. I think there are 2 issues with compressors - adequate CFM to meet your needs and clean, dry air. Most new stationary machines use compressed air in one way or another along with DA sanders, nail guns, spray guns and other hand tools. Put dirty or wet air into any of these and itís going to cost more than a new compressor.

From contributor E:
They don't build 'em like they used to. If you keep the oil clean and up to level, that old Westinghouse will outlive you. Put your money in a refrigerated drier.

From contributor G:
I would like to learn more about requirements for below the compressor to clean and dry the air.

From contributor Y:
Get an air dryer, and that old compressor is so simple to rebuild (most any old compressor, for that matter). From your post, I'm assuming you went CNC and you need that air to be dry.

From contributor I:
A sharp guy like you should have no problem figuring this one out. I can't remember all the details, but it goes something like this. Empty your tank, close the valve, and see how long it takes to get up to pressure. There is some way to measure volume at a given pressure with a 60 gallon tank. Now you can determine the CFM of your pump. Use this number to see if it is sufficient for your needs. It seems like I remember it should run around 25% of the time (i.e.- 3 minutes idle for every 1 minute running). This is, of course, while there is a demand for air. If it is running 50% of the time, it is underpowered, and you could benefit from an upgrade.

It seems like those old pumps were built right. I have been told that new castings are lighter and just don't hold up as well. If you do go for a new compressor, consider the speed of the motor. Slower speeds are generally associated with better pumps - less heat, less wear.

Self-bleeding could relate to a couple of things, but check with the manufacturer. Some compressors come with packages where they have a refrigerated dryer, low oil shut-off, and automatic drain. Could be the draining feature. Or it could be a function where you can run the pump continuously and the excess pressure is bled off. I think most gas powered compressors are this way, but my IR has a switch that will allow it to run continuously or shut off at a prescribed pressure.

From contributor A:
You also should consider the cost of running a compressor. They say that a rotary screw is much less expensive to operate than a piston type. We have a Kaeser Rotary Screw Compressor with a drier. It is a 15hp and is quieter than our old 5hp compressor. It does need to be serviced twice a year, but the company brings a backup unit that hooks into our line so we are never down.

From the original questioner:
Thanks for all your input! I went to the Eaton Compressor website. This is an example of how I would like to see all machinery websites. These people put a lot of emphasis on educating their customer. The way they do this gives you a lot of confidence in them as a company.

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