Consider the impact wrench. This is a honking big tool that demands a lot of compressed air flow and pressure.
All of us backyard mechanics dream of using one of these, hearing that wonderful brraaaaapp as the wrench starts to turn a too-tight lug not off.
Yet, if you have a .5 HP compressor with a 2 gallon tank on board, you will run the wrench for only a second or two before you are out of air.
When buying an air tool read the specs. Somewhere it will tell you that this air tool requires so many CFM of compressed air at so much PSI.
A typical air compressor will deliver 3-4 CFM of compressed air at around 90 PSI for every HP of electric motor size.
This means that if your air tool needs 10 CFM of compressed air at 90 PSI to run properly, the math tells you that you will need a 2.5 HP air compressor to provide the compressed air for just that one air tool.
Yes, having a large air tank will help you run a high-demand air tool with a smaller compressor. With a smaller compressor bigger-tank combo, this means that your air compressor is going to work very hard to try and keep that tank full as you use your air tool, and you will still exceed the air supply available, and have to stop work and wait for the compressor to catch up.
Bill,
I’m going to be doing a lot of work on my car to save money and I know what I’m doing but have only used hand tools in the past. Now its time to move up to air tools so what type and size compressor would I need in order to use the average back year mechanic air tools.
Thanks,I appreciate any help you can give me.
Dave
Dave, sadly, there is no average. An air fill tire valve will use pressure and low flow. An air ratchet will use pressure and high flow. Both can be run from the same air compressor, but if the air compressor is too small, then you won’t be able to use a ratchet for more than a few seconds before your tank runs out of air, as the small air compressor cannot keep up. Be realistic about use of your backyard mechanic air tool use (how long at a time, and how frustrated you will get when you run out of air and have to wait for the tank to fill). Go to about-air-compressors.com and click the link to SIZING. Much more info there. Bottom line, big air tool air demand and small air compressor, not a good combo.
So Bill, when a compressor’s specs say it is rated at 5.5 cfm at 40 psi, does that mean it will deliver 5.5 cfm at 40 psi continuously in use? Or will it do that until the tank empties and then you stop and wait for the tank to fill again?
I’m trying to understand the ratings on the compressors I see advertised at Harbor Freight.
For spraying a clay-papercrete slurry mix through a hopper, I need a minimum of about 5 cfm at 40 psi continuously and want to buy the least expensive, 120 volt compressor that will do the job.
When I say “continuously,” I mean for about 4 minutes at a time (until the gravity-fed hopper empties). Then I walk over to the mixer and scoop it full again and walk back to the job and spray for another 3 to 4 minutes or so. And this goes on for about a half-hour, then I trowell for 15 minutes, than back to scooping and spraying.
Given that description, do you think a $150, 2 HP, 4-gallon compressor that says “Air delivery: 5.8 SCFM @ 40 PSI, 4.4 SCFM @ 90 PSI, 2 HP rated motor, Oil lubrication” should do the job?
Appreciate your help,
ken
Ken, if it is a continuous duty compressor, then yes, it should give you that flow at that pressure. In reality, I suspect that the performance data of compressors is much like the claims made about the mileage on automobiles. Under controlled conditions, maybe the car will get the mileage claimed, but rarely will it do so under real world conditions. I feel it is the same with compressors.
I don’t know about the $150 dollar price tag, but a 2 HP compressor should give you the flow you need based on the rule of thumb that you get 3-4 CFM of compressed air at 90 PSI for each HP of motor size, and therefore, you should get 6-8 CFM at 90 PSI, meaning you certainly should get the c CFM at 40 PSI.
Err on the side of caution. If you can get a 2.5 HP for the same money, go for it. Not much bigger than that, though, as a 120 Volt supply can only be converted into so much flow, regardless of what a manufacturer says. Much more info about this topic is on the site, http://www.about-air-compressors.com.