My Degree May Have to be Rescinded
January 26th, 2009Compressor Confusion
Figure this out for me. I’m elaborating on an earlier post.
The classic air dryer for cheap metalworkers is the Maxhootue/Harbor Freight job. Same compressor, different sellers. “Maxhootue” is an Ebay user who sells them. They are rated up to about 21 CFM @ 140 psi. I keep my compressor’s pressure switch set at 175, and I never raise the output pressure above 125, although I suppose there is no reason why it couldn’t pump more than 21 CFM at the lower pressure. It’s probably at 90, now that I think about it.
I don’t really know how much it blows. It does 17.4 at 175. Presumably, the pressure and volume relationships are linear and so on (Ideal Gas Law), so you would think it would go up to maybe 25 CFM at 125 psi, with nothing connected. I am too lazy to use “calc.”
These things are made in China, but everyone, without exception, seems to like them.
Right now–do not ask me why–Grainger is selling a bigger drying machine for the same price as the Ebay job. It’s a Speedaire. Probably also made in China, but somewhat more reputable, and there is a local dealer I can raise hell with if it craps out. The Speedaire will do 40 CFM at 175 psi.
The odd thing about these machines is that they seem to work better at higher volume rates. It’s as if the big thing limiting the capacity is the compressor, not the dryer. Jack up the psi, and you get more CFM, and the dew point remains the same.
Okay, wait. If that’s how it works, then either compressor is fine for my needs. The figures must describe the unit’s ability to pass air, not its cooling ability. That would explain why more pressure gives more dry air, with no penalty for shoving more molecules through the system.
Now my head hurts. What if my compressor blows LESS air at 125? If it works the same way as the dryer, that’s what you’d expect.
Maybe I need to put bigger fittings and a bigger hose on this thing. Curtis recommends 3/4″ pipe for 20 cfm.
The Curtis materials say this thing blows over 16 cfm pretty much regardless of pressure, so I guess the 125 psi figure (assuming I’m not choking the machine) is about 16. How can that be? You would think it would suck matter in at the same rate, all the time, so it should also blow it out at the same rate, so you would think lower psi would equal more CFM. BUT APPARENTLY NOT.
Hmm…if they’re measuring at the inlet into the tank, it would make sense. No, it wouldn’t. Geez.
I am going to go dig out my diploma and confirm that I really have a degree in physics. Maybe I imagined it. I can’t believe I’m confused by this crap.
Anyway, I’m wondering if there is any point in getting the Grainger machine. The cost is the same, and it will be murderously effective even if I upgrade to a bigger compressor some day, and it probably has a warranty that actually functions. And resale should be high. On the down side, it weighs twice as much and is about the size of a 15,000 btu unit air conditioner. The Ebay thing appears to be roughly the size of a PC tower case.
January 26th, 2009 at 6:06 PM
Each rotation draws in N cubic inches of air and compresses it. That is the only air available. if N cfm goes in, only N can come out. The only way to change that is to change the rotational speed of the motor, or the amount of displacement of the piston. You have your physics right, it’s just being applied differently than maybe you’re used to.
January 26th, 2009 at 6:23 PM
When I had that Wilkerson dryer, I obtained a manual, which told me it was rated at about 21 CFM (IIRC). This surprised me a bit, as the connecting hoses seemed somewhat small for this much air flow. I finally decided that it wouldn’t offer too much restriction because the total length of the air passages in the machine was relatively short.
FWIW: you can figure about 4 CFM/Hp @ 100 Psi. Connections such as elbows, quick disconnect fittings, etc. will really cut the flow rate.
Another FWIW: I watched a tech verify the output capacity of a newly rebuilt 40 Hp compressor, 160 CFM rating, I think it was. He installed a plug in the side of the tank that had a 5/16″ hole, length being only 1/4″ or so. The compressor would JUST hold 100 PSI in the tank with that hole open. Needless to say, it was noisy as hell! Point being that you need largish lines for longer distances, especially if there’s turns, valves, and other fittings, but a short run can be quite small and still pass enough air. Most air tools are rated at 90 PSI, at the tool itself. My own compressor is set for turnon at about 100, turnoff at about 115. More or less.
January 26th, 2009 at 6:55 PM
You should still follow JDunmeyers suggestion. There is a device called an aftercooler that is sometimes used to take some load off the drier. It’s a radiator with a fan. The copper tubing serves the same purpose. Then there’d be less load on the dessicant drier. maybe you wouldn’t need the drier. Speedaire is Grainger’s brand for pneumatics as Dayton for their electrical.
January 26th, 2009 at 7:57 PM
The higher the pressure, the more water you remove (dew point).
Compressors are typically suction volume limited. Otherwise, they would trip out on high amps, as the air density changes (temperature and humidity.)
January 27th, 2009 at 8:10 AM
I’ve used air tools, but have never been responsible for the compressors before. The drying thing, it is new to me, please explain.
January 27th, 2009 at 7:29 PM
Chalkie,
The way I understand it (which may be technically incorrect, but it’s a good analogy anyway) is this:
The room that you’re in has a certain volume of air in it, and the air contains a certain volume of water, in the form of vapor. When your compressor pumps all the air out of that room, it also pumps the water vapor with it. Now, because that air takes up a lot less space, it can no longer hold the water in the form of vapor, so the water condenses as the air cools. If that cooling takes place in the air lines, you’ll have water in the air going to your air tools. If you’ve been around a while, you’ve seen air lines that will blow water out like it was a garden hose; this is very hard on air tools and makes spray painting impossible.
If you can cool the air before it gets to the tools and have a place for the water to drop out, it’ll prevent water from getting into the airlines and tools. A simple aftercooler, either a home-made one as I described above, or a commercial unit, will remove a lot of the water, and as long as the air doesn’t cool any further, the remaining water vapor won’t bother anything.
Besides an aftercooler, there’s a couple of other ways to eliminate moisture in the compressed air supply: a dessicant or deliquesent dryer or a refrigerated dryer. The latter is considered better for general usage, the former is usually cheaper to buy. The refrigerated dryer cools the air to just above freezing, causing almost all the moisture to condense and drop out. As the air exits the dryer, it’s warmed up again, making it unlikely that further condensation will take place.
Here’s more than you ever wanted to know about air dryers:
http://www.hydraulicspneumatics.com/200/TechZone/AirFiltersandFR/Article/True/6459/TechZone-AirFiltersandFR