Does my compressor leak?

Sometimes compressors will leak air from the receiver, back through the check valve, to atmosphere. Or there may be a pinhole leak in the receiver itself. In either case, it’s a waste of air and the energy used to compress it.

With your home compressor, simply disconnect the air line from the discharge line coupler and watch the gauge upstream from the coupler on the regulator.  For an in-plant application, turn the valve off in the line from the compressor that feeds the plant and watch the tank gauge.

If the pressure falls, you’ve got a leak somewhere in your compressor head or tank, and air is bleeding out of the receiver to atmosphere through that leak. The faster the pressure drops, the more money you’re wasting compressing air that’s never going to be used.

Understanding compressed air CFM, PSI, Force & Flow

When it comes to applications for compressed air, a user must consider both CFM and PSI when they are determining if they have sufficient force their application. In every application for the use of compressed air, that air has to be able to provide a certain force at a certain flow rate to do the work expected.

When it’s a simple use, such as a blow gun in a blow off application, if the air didn’t have sufficient force, it wouldn’t blow anything away. In a spray painting application, if the compressed air lacks sufficient force, the paint won’t spray. Similarly in an air actuator. The air has to deliver force to move the piston.

Force is calculated as Pressure Times Area. In all three examples above, there is an “area” involved. In the blow gun and spray application, it’s the area of the nozzle or the surface area of the paint in the spray can. In an actuator, the “area” is the surface area of the piston.

The pressure part of F=PxA (force equals pressure times area) is measured in PSI. PSI is an acronym for Pounds per Square Inch.

The discharge port on your compressor should have an air regulator on which you can select the pressure level reaching your downstream application. That pressure setting is independent of the pressure that’s actually inside the compressor tank, unless the regulator is set higher than the pressure in the tank. In this case, the downstream pressure will match the tank pressure.

If you set the compressor regulator to 5 PSI, then air will reach your application with 5 PSI of pressure, and if the area of your application (eg: the surface area of the paint in the spray reservoir) was, for example, 10 square inches in size, then that 5 PSI would generate 50 LBS. of force on the surface of the paint.

The specifications for the air using device (air tool, cylinder, spray gun etc.) will tell you what pressure that the device requires, and it should also tell you the flow that that device will require to operate properly.

That’s where CFM comes in.

CFM is an acrynym for Cubic Feet per Minute.

Some folks measure flow from the air compressor in SCFM, but in my opinion, that’s incorrect. SCFM refers to “Standard” Cubic Feet per Minute of air, and a “Standard” Cubic Foot of air is at 68 deg. F, at sea level, with a specific humidity level, circumstances far removed from the condition of the air discharging from your compressor. For measuring the pressure coming out of the compressor, I use CFM.

You can have compressed air delivered to your application from your compressor at the correct PSI level, and the device may not work properly.

In order for blow guns, spray guns, air cylinders etc. to work satisfactorily, the compressed air has to be delivered to these devices at the correct PSI (that to generate the force required) and also at the correct flow rate ( so that your force is delivered within the acceptable time frame of the air using device).

If the air brush requires 4 PSI of air to work properly, and you supply that air through a pin hole sized tube, then the air brush is getting the correct pressure, but not enough flow to make the paint spray properly. Similarly, 30 PSI delivered to your car’s tire through that same pin hole tube will eventually fill the tire, but it will take unacceptably long time to do so.

So, when you are looking at supplying compressed air for your application remember that you have to consider the compressed air flow (CFM) and that you must have that flow at the specified pressure (PSI) to be sure to generate enough force for your device to work properly.

Need more info on using compressed air? Here’s the spot.

An interesting use of compressed air….

I’m spending some time on vacation in the Florida Keys….sorry about that to all readers from the frozen north. We’re from the north as well, and our at-home email contacts are telling us continuously about the miserable winter weather currently being experienced from mid-continent to the eastern seaboard. Just so you know, I broke out in a real sweat walking over to the local internet hot-spot a short while ago so I could post this blog. . Anyway, enough about how hot it is.

Here’s information on the compressed air application.

The Key we’re on has many man-made canals, so the various sites almost all have access to the Gulf or Ocean, depending which side of the Keys they are on.

In an effort to reduce the ingress of fish to the canals, bubblers are placed across each opening. The noise of the compressed air surfacing from the submerged pipe, along with the visible bubble curtain is supposed to startle incoming fish and to drive them away.

In the main picture above you can see the line of bubbles surfacing. The sign floating in the middle of the canal opening asks boaters to raise their engines to try to prevent props or skegs from cutting the plastic pipe. Unfortunately, many boaters disregard this request, and this makes a lot of work for local divers to go down and fix the cut pipe underwater.

In the small inset picture, you can see the plastic discharge pipe allowing the compressed air to flow down onto the sea bed. The blue device just behind it is the intake filter, and the intake pipe is in line with the discharge pipe, so it can’t be seen in the photo.

The compressor itself is housed in a concrete enclosure to the right in the inset photo. I couldn’t see into the enclosure to identify what type of compressor it was, but audible for quite some distance from that enclosure was the characteristic whine of a continuous duty, demand type, rotary screw compressor.

As to the efficiency of the whole system, there are a few fish in the canals, but not too many, and there is the odd lobster. Please don’t capture the lobster is also a common sign around the canals.

Yesterday, a couple of manatee appeared in one of the canals, making an interesting, but not exciting few moments for the locals. For this northern boy, it sure was pretty interesting.

Want more information on using compressed air, selecting a valve, a cylinder, a compressor…whatever. Please click here.

How compressed air filters work

The cap in the compressed air filter directs the incoming compressed air down into the filter bowl, and against the filter bowl, which forces the air into a tight, cyclonic spiral inside the bowl.

The compressed air, reputedly moving at nearly the speed of sound (I say reputedly as I’ve never actually measured it’s speed) literally throws free water and debris against the side to the bowl. This contaminate runs down the inside of the bowl to the bottom quiet zone, where it’s either manually or automatically drained off, depending on the type of drain in the filter bowl.

The air continues spiraling inside the bowl while it also drives through the filter element, back up to the cap, and out downstream to the application. Any debris that hasn’t been thrown out will be captured by the filter element, as long as the debris is larger than the holes in the element, the holes being measured in microns of size.

Here’s a bunch more information on compressed air filters.

What it is is what it is….

A visitor to my compressed air information site (it’s here if you want to visit it) has a compressor that generates about 5 CFM of compressed air. He didn’t indicate at what pressure this flow was, but I suspect it was in the 30-40 PSI range. That flow is fairly typical for a small DIY home compressor.

He wanted to know if it was possible that this same air compressor could generate 12 CFM. Again, he didn’t indicate at what pressure that higher flow was required, so it was difficult to comment with any accuracy.

Just so you know, a compressor will produce all of the compressed air it can “out of the box”. The discharge rate of any compressor will be predicated on the power of the electric motor, the size of the piston/cylinder, and the required pressure of that discharge. The higher the pressure required, the lower the discharge flow. That’s physics, and there’s really nothing you can do about it, unless you change one of the parameters…ie: larger motor, larger compressor pistons, etc.

If you need a greater flow than what is specified in the compressor’s manual or information plate on the compressor you have, then you’ll either have to modify the compressor, or get a larger one.

Unique compressed air application

Wandering around the web today led me to an article describing how compressed air is used to treat the open ulcer wounds of severely diabetic persons.

The diabetic ulcer wound and surrounding area is aerated using 1 bar (14.7 PSI) compressed air for a period of time each day. The results were pretty impressive.

If you would like more information about the industrial and home use of compressed air, please click here.

For more information on the use of compressed air to treat diabetic ulcers, please click here.

How many gallons in my compressed air tank?

For some reason that doesn’t make a lot of sense to me, manufacturers of compressed air tanks – receivers – air pigs, etcetera, measure their capacity in U.S. gallons. However, you don’t use gallons of compressed air, you use CFM of air, at a specific pressure for your application.

So, is there a conversion between the two?

In his white paper #5 entitled Air Receivers, Thomas Kreher offers the following for us.

“Receivers, tanks, reservoirs are used to store a volume of compressed air. The sizes of these receivers are often rated in gallons. To readily convert from gallons to cubic feet, divide the number of gallons by 7.48 (7.48 gallons = 1 cubic foot) Wow! We could pour 7 ½ gallons of liquid into a 12” cubed tank. Also you may multiply gallons by 13.4% (.1337) to get cubic feet. 300 gallons = approximately 40 SCF.”