Compressor pressure question

[Randpage]

Hi all, I just got a new to me 20 gallon compressor. It is rated at 100 psi. My question is if I run say an impact wrench off it as soon as it drops below 90 psi is it dead in the water until it pumps back up again? In other words, what good is the rest of the tank if only the first 20% or so is usable for an air tool requiring 90 psi? Would I need a compressor that is more like 150-175psi? If so, what does the size of the air tank even matter? I am no expert on fluid dynamics so I was hoping someone could give some schooling. Thank you and forgive my ignorance

 

[aandpdan]

The tank just keeps the compressor from having to run constantly. It's only a reservoir.

You need to determine the SCFM of the air tools you are using and the compressor itself. If the tools require more air than the compressor can supply then you'll have to wait while the pressure builds.

A higher pressure compressor isn't really the answer but you'd have more in the tank in reserve, it really boils down to the SCFM - standard cubic feet per minute - that the compressor itself can deliver at a given psi.

 

[Randpage]

The tank just keeps the compressor from having to run constantly. It's only a reservoir.

You need to determine the SCFM of the air tools you are using and the compressor itself. If the tools require more air than the compressor can supply then you'll have to wait while the pressure builds.

A higher pressure compressor isn't really the answer but you'd have more in the tank in reserve, it really boils down to the SCFM - standard cubic feet per minute - that the compressor itself can deliver at a given psi.

Ok I think I follow you. The rated SCFM of my compressor is 6.3 at 90psi. Now with a 20 gallon tank I did some rough S.W.A.G. math and figured I could run a high performance 1/2 impact at full load which is somewhere around 20-22 cfm or so for about 10 seconds before the tank is empty, that is assuming that remaining air is still around 90 psi or does it drop as the tank empties? If my calculations are anywhere correct that is. Most 1/2 inch impacts are about 6 CFM consumption but that is unloaded. Am I anywhere in the ballpark?

 

[larry_g]

Your tool will run at less than 90psi. It will not have the rated torque or speed but it will run. Years back when I was in a similar situation for rotating tires I would manually break loose the lug nuts and then use the rattle gun to zip them off. If you get to the point you don't have enough pressure in the system to run the tool then you wait.

You'll soon hear from the battery fans...

lg
no neat sig line

 

[nadogail]

You are catching on, as the volume of air is released from the tank the pressure in the tank decreases.

The 6.3 CFM number is good for only 90 pounds, if your pressure switch shuts off your compressor at 100 PSI, IMHO your compressor doesn't have enough capacity to run your impact wrench or any other "air hog".

In my little shop sometimes I parallel my two compressors for the occasional times my 20 gallon compressor just doesn't have enough guts to handle the job.

IMHO, your options include adding air storage capacity; I have sometimes added refrigerant tanks to store air; Add a larger diameter pipe to your distribution system (this is the same as adding more tank.
Adjusting the pressure switch to raise the cutout pressure so the tank charges to a higher pressure, you might want to adjust the low pressure pump restart pressure to increase the low pressure limit too.
Spin the compressor faster, in a belt drive that means changing pulley sizes.

Get an Industrial Size compressor to handle your Industrial Size loads.

Or you could just deal with the incompatibility of your compressor and your Impact Wrench.

 

[matt_i]

Simply the pressure switch "on" setting is the minimum air pressure you will have.

Flow * Pressure = Mechanical Power which equates to Electrical Power. So you can see in the equation that one solution to more pressure is also more Power (bigger HP motor). Or limited by existing motor HP you can trade off pressure for flow and improve the underdrive of the motor (smaller pulley dia) to cause the compressor shaft to spin slower. Conversely if you wanted to improve flow on the same pump you could spin the pump shaft faster (improve overdrive).

Eventually there are thermal limits and stress limits for the internal parts, there are limits on how slow the compressor crank can be driven and still supply enough splash lube to the moving parts.

I personally would try it as an experiment with my own equipment, if its adjustable, to dial up your pressure switch to 100psi from 90psi. A compressor should have an extra 10% built-in, in my opinion. If you start tripping overloads then you either have to back off the pressure switch or go with the motor sheave's improved underdrive I mentioned earlier.

 

[Randpage]

Simply the pressure switch "on" setting is the minimum air pressure you will have.

Flow * Pressure = Mechanical Power which equates to Electrical Power. So you can see in the equation that one solution to more pressure is also more Power (bigger HP motor). Or limited by existing motor HP you can trade off pressure for flow and improve the underdrive of the motor (smaller pulley dia) to cause the compressor shaft to spin slower. Conversely if you wanted to improve flow on the same pump you could spin the pump shaft faster (improve overdrive).

Eventually there are thermal limits and stress limits for the internal parts, there are limits on how slow the compressor crank can be driven and still supply enough splash lube to the moving parts.

I personally would try it as an experiment with my own equipment, if its adjustable, to dial up your pressure switch to 100psi from 90psi. A compressor should have an extra 10% built-in, in my opinion. If you start tripping overloads then you either have to back off the pressure switch or go with the motor sheave's improved underdrive I mentioned earlier.

The cut off pressure for my compressor is 100 psi. So I guess I only have 10 psi worth of force to drive my impact and as soon as the tank goes below that 90 psi the rest of the air volume is effectively useless? I was thinking of adding a second 20 gallon tank to give me 40 gallons overall but it seems like that wouldn't really gain me anything if you can only use the first 10-20% of volume/pressure. So would I be better off with a little 5 gallon 175 psi compressor then? This stuff is much more complicated then I originally thought lol

 

[HotrodHR]

I'm just curious, what's the HP rating on the motor, and what's the brand/model of this compressor? What are you using this for mainly?

 

[HotrodHR]

The cut off pressure for my compressor is 100 psi. So I guess I only have 10 psi worth of force to drive my impact and as soon as the tank goes below that 90 psi the rest of the air volume is effectively useless? I was thinking of adding a second 20 gallon tank to give me 40 gallons overall but it seems like that wouldn't really gain me anything if you can only use the first 10-20% of volume/pressure. So would I be better off with a little 5 gallon 175 psi compressor then? This stuff is much more complicated then I originally thought lol

Air compressor rule of thumb number one - buy the biggest unit your budget allows that can do the jobs you intend to use it for. That, said you don't need a 7.5 HP, 80 gallon tank unit if all you plan to do is air up bicycle tires...

 

[Randpage]

I'm just curious, what's the HP rating on the motor, and what's the brand/model of this compressor? What are you using this for mainly?

It is a 2 hp, 100psi 20 gallon oil lubricated craftsman, model 919-176120. It's an older unit but in very good condition, when I bought it I was looking for any rust on the tank and I was going to pass if it did (thinking if I see rust on the outside then the inside is compromised). As for uses, just using it in my garage for blowing out stuff, airing up my 10 ply truck tires, occasionally running a 1/2 impact for the odd recalcitrant bolt, maybe an air hammer when I need to persuade something to come apart (I live in the heart of the rust belt!). Not looking to spray cars or anything crazy like that.

 

[HotrodHR]

It is a 2 hp, 100psi 20 gallon oil lubricated craftsman, model 919-176120. It's an older unit but in very good condition, when I bought it I was looking for any rust on the tank and I was going to pass if it did (thinking if I see rust on the outside then the inside is compromised). As for uses, just using it in my garage for blowing out stuff, airing up my 10 ply truck tires, occasionally running a 1/2 impact for the odd recalcitrant bolt, maybe an air hammer when I need to persuade something to come apart (I live in the heart of the rust belt!). Not looking to spray cars or anything crazy like that.

I owned one of the Craftsman oil less years ago, I believe it was a 30 gallon. Used it to paint a few motorcycles, way too noisy.

I have a quadzilla ( CH 4 cylinders) 80 gal, I think it's 6 or 7 HP that's about 20 years old and still runs OK. I have replaced a couple of head gaskets though. When it needs to be replaced I think I'll look into the Eastwood scroll compressor. It's supposedly quiet, mobile, and puts out steady cfm... I think they going for around $1700.

 

[larry_g]

The cut off pressure for my compressor is 100 psi. So I guess I only have 10 psi worth of force to drive my impact and as soon as the tank goes below that 90 psi the rest of the air volume is effectively useless? I was thinking of adding a second 20 gallon tank to give me 40 gallons overall but it seems like that wouldn't really gain me anything if you can only use the first 10-20% of volume/pressure. So would I be better off with a little 5 gallon 175 psi compressor then? This stuff is much more complicated then I originally thought lol

Did you not read post #4? Your tools don't magically stop when the pressure goes below 90psi, they just loose some power.

lg
no neat sig line

 

[Lenny B]

It is a 2 hp, 100psi 20 gallon oil lubricated craftsman, model 919-176120. It's an older unit but in very good condition, when I bought it I was looking for any rust on the tank and I was going to pass if it did (thinking if I see rust on the outside then the inside is compromised). As for uses, just using it in my garage for blowing out stuff, airing up my 10 ply truck tires, occasionally running a 1/2 impact for the odd recalcitrant bolt, maybe an air hammer when I need to persuade something to come apart (I live in the heart of the rust belt!). Not looking to spray cars or anything crazy like that.

The compressor you have is designed for what you are using it for, blowing up car tires (as long as the bead is seated), bike tires, nail gun, blowing things off with an air nozzle, and short run air tools like a ratchet. It has a low cfm output, probably less than 4 CFM, and the tank is small. It's fine for the above but not good for tools requiring higher CFM (air flow). One way to look at the compressor is it should be able to operate the tool with no tank at all. The tank is just a shock obsorber to soften the ups and downs of the pump. If you where trying to use a spray gun or sander that had a requirement of 10 CFM you would want a pump that delivered at least that. In this example, even an 80 gallon tank would be depleted so fast that you would either have to stop and let it build back up or you would need a pump with enough air output to operate the tool while refilling the tank at the same time.

 

[bobabuee]

craftsman, model 919-176120 , should be able to run it at 240 volts it runs so much better
atleast it did for me also adding another 20 gallon tank would help youout you find on craiglist or facebook markeplace, you dont want to spend much here ...because you can pick up bigger older unit for 350.00 - 500 with bigger tank 5hp motor vertical to save space or space is not concern then horizantial would fit bill they are more plentiful

 

[Randpage]

craftsman, model 919-176120 , should be able to run it at 240 volts it runs so much better
atleast it did for me also adding another 20 gallon tank would help youout you find on craiglist or facebook markeplace, you dont want to spend much here ...because you can pick up bigger older unit for 350.00 - 500 with bigger tank 5hp motor vertical to save space or space is not concern then horizantial would fit bill they are more plentiful

i was already toying with the idea of adding another 20 gallon tank since the compressor can fill its own tank in under 2 minutes. Think it would be worth while to do it? Space isn't really a concern anyway.

 

[sberry]

90 is at the tool. A heavy 1/2 air gun draws 25cfm. That 6 rating is an industrial avg number, some hat based on 100 units, it's for sizing large comps for plants. A 2 stage 175 is made for running g air tools, 3 hp for mechanics and 5 for body work. It's just turning on when a good single is turning off, let's it have enough head room to make up for hose and reel losses, 1/2# or more loss per ft on 3/8 with 1/2 gun

 

[sberry]

You aint getting 90 when it's full if you have more than 10 ft hose.

 

[Randpage]

You aint getting 90 when it's full if you have more than 10 ft hose.

I actually made up an end of line pressure tester with a spare gauge and a tee to see what I was getting at the end of my 15' 3/8'' line with high flow 1/4'' fittings. Getting 90 psi no problem, even 100 psi. Considering turning up the cut off pressure by maybe 15 psi. Not sure how safe that would be though. Would be nice if I could turn it to 115-120 cut off psi and an auxiliary 20 gallon tank. Would give me 40 total and wouldn't be too bad for the little bit of money I've got in the set up.

 

[Lenny B]

i was already toying with the idea of adding another 20 gallon tank since the compressor can fill its own tank in under 2 minutes. Think it would be worth while to do it? Space isn't really a concern anyway.

If you add another tank it will not increase your CFM or PSI. The stored air will last a little long before the compressor kicks back in but will take that much longer to refill and kick off. If what you currently have is not enough PSI or Volume, then get a bigger pump and tank. Sounds like your in the trap of trying to save some bucks, but after all is said and done, it is almost always cheaper to get a new unit that is capable of doing what you want instead of tinkering with old units that can't. You'll spend more time and money fixing this, rebuilding that, etc.... than it would have cost to get a new capable unit. That's been my experiance in the compressor department.

 

[nadogail]

After considering my earlier response to your post; I may have been a little harsh.

Can you imagine a house with several children, 5 bathrooms and a 30 gallon water heater?

I think that would describe your compressed air system.

 

[Randpage]

If you add another tank it will not increase your CFM or PSI. The stored air will last a little long before the compressor kicks back in but will take that much longer to refill and kick off. If what you currently have is not enough PSI or Volume, then get a bigger pump and tank. Sounds like your in the trap of trying to save some bucks, but after all is said and done, it is almost always cheaper to get a new unit that is capable of doing what you want instead of tinkering with old units that can't. You'll spend more time and money fixing this, rebuilding that, etc.... than it would have cost to get a new capable unit. That's been my experiance in the compressor department.

Big part of the reason I'm doing this is purely for the fun and enjoyment of tinkering with it and experimenting. No I don't want to spend a ton of money but this isn't a mission critical type deal. More of a learning experience/project to mess with.

 

[ericm]

Try using the compressor and impact wrench for a while first before getting worked up about theory. My compressor is a bit smaller than yours in both CFM and tank size and it runs a 1/2" impact wrench just fine. It wouldn't if the impact wrench was on for long periods like a sand blaster, but that's not how impact wrenches are normally used.

 

[Randpage]

Try using the compressor and impact wrench for a while first before getting worked up about theory. My compressor is a bit smaller than yours in both CFM and tank size and it runs a 1/2" impact wrench just fine. It wouldn't if the impact wrench was on for long periods like a sand blaster, but that's not how impact wrenches are normally used.

I definitely will. I'm just enjoying learning all the ins and outs to the whole thing, like I said it's my first compressor. I'm inquisitive by nature and just enjoy diving into stuff like this. Learned a lot from you guys and I appreciate the helpful responses and insights. Think I'm going to keep an eye out for an extra 20 gallon tank just to Frankenstein it a bit because why not

 

[sberry]

You get 90 with the gun running ? At the gun?

 

[sberry]

I will agree about using it and see,what happens. Most the small comps are 125# which would help the power. There is some variations in air guns too, lighter ones sniz under 20, heavy duty north of 25 cfm.
Old hose can be bigger, got some was left on so long it's as close to 1/2 as it is 3/8.

 

[Randpage]

You get 90 with the gun running ? At the gun?

I haven't tried with the gun running but I highly, highly doubt it. Would probably have to raise the regulator pressure to like 125 psi or even higher would be my guess.

 

[HotelMike]

Another thought. Put a coupler before your regulator so you can get the full tank pressure to your impact. You may also want to use high flow or 3/8” couplers. Try it out. Best way to figure out if it’s enough for you. Your impact will keep running after the tank pressure drops below 90. It will loose its uumph slower or faster depending on you setup and what your doing.

 

[Randpage]

Another thought. Put a coupler before your regulator so you can get the full tank pressure to your impact. You may also want to use high flow or 3/8” couplers. Try it out. Best way to figure out if it’s enough for you. Your impact will keep running after the tank pressure drops below 90. It will loose its uumph slower or faster depending on you setup and what your doing.

I am using all Milton high flow plugs and couplers with a 3/8'' hose. New impact should be here in a day or so and then I can do some experimenting.

 

[Jvvmusme]

Another thought. Put a coupler before your regulator so you can get the full tank pressure to your impact. You may also want to use high flow or 3/8” couplers. Try it out. Best way to figure out if it’s enough for you. Your impact will keep running after the tank pressure drops below 90. It will loose its uumph slower or faster depending on you setup and what your doing.

which are the high flow couplers ?

 

[CraigStu]

I am surprised your pressure is so low. My 30+ year old Craftsman shuts off at 125# and it runs my impact just fine. Not as well as it did at 150# when I used it at work, but I don't have any problem in all the car work I do. Can you verify what the pressure should be for your model? Is it possible that someone reduced the pressure setting?

 

[Showkey]

I haven't tried with the gun running but I highly, highly doubt it. Would probably have to raise the regulator pressure to like 125 psi or even higher would be my guess.

I am surprised your pressure is so low. My 30+ year old Craftsman shuts off at 125# and it runs my impact just fine. Not as well as it did at 150# when I used it at work, but I don't have any problem in all the car work I do. Can you verify what the pressure should be for your model? Is it possible that someone reduced the pressure setting?

This^^^^^^^^^^ YES ^^^^^^^^[[[

Instead of buying a 5hp 60 gal.........Turn the pressure switch up to 125 psi ?

If you question the integrity of the tank or motor/compressor head and 20 Psi is making a difference ......it’s time to move on.

 

[Randpage]

I am surprised your pressure is so low. My 30+ year old Craftsman shuts off at 125# and it runs my impact just fine. Not as well as it did at 150# when I used it at work, but I don't have any problem in all the car work I do. Can you verify what the pressure should be for your model? Is it possible that someone reduced the pressure setting?

It's marked 100 psi on the unit itself. I'm going to turn it up a bit to 115-120. Waiting on a new gauge to come because I suspect the one on it isn't accurate. I can't figure out how to post a picture but the model number is 919-176120

 

[Randpage]

which are the high flow couplers ?

All of them. Plugs and couplers are all hi flow

 

[sberry]

As for the couplers. I got it screwed all the way and basically only 1 coupler in the system, at the tool. If I am piping,,, depending on scale might have only 1 valve at the tank, pipe screwed all the way to the filter regulator, a t after, a hose or even to a single hose reel. Only time I add more pieces is if on a manifold, remote regulators where I valve for local disconnect, service, etc.

 

[sberry]

If a guy is a buy sell type and doesn't mind some trading part of the hobby can be find a better compressor. I think Olsen, one of the inspiratory here did something like that. Got a 3 hp, was single stage but he can use short hoses and doing limited work but it's just 2x the power of 120v units, makes it fast.
I use air every day. As a hobby type might rig it up a little, somewhere you can use ball valve disconnect. If I wanted it portable still might make for a coupler disconnect to building system, plumb around any cheap factory equipment, might not need reg for portable or add inline.
I recall once painting a big truck. Had it parked a couple hundred ft from air. Took and used 2 hundred foot 3/8 hoses, hooked it to 2 stage and put regulator at the user end, worked well as so a guy wouldn't notice it.

 

[csp]

All of them. Plugs and couplers are all hi flow

Pretty sure he's asking what brand/style are high flow.

 

[Randpage]

Pretty sure he's asking what brand/style are high flow.

I apologize, I misunderstood. They are all Milton S-217 High-flow v-style plugs and couplers.

 

[ericm]

Those are good couplers. I like how they let you just plug in without moving the sleeve. But even cheap HF couplers flow more than enough for a 1/2" impact driver. I've used both and there's no difference in gun performance.

 

[sberry]

Most of the auto rated air stuff is 35, those A and B type things. Every time a guy puts one in it adds to it a little but I doubt they make much difference most of the time to a 1/2 gun. Heavy ones are 25 cfm. Maybe a pound or so at that rate, not sure.
I got a box of H at a sale at one point and went to them, it lets me use it for all the tools and at one time used a little 1/2 hose on my service truck and a 3/4 gun. They are old school but 74 cfm rated, about the same as a modern hi flow.