Got questions about compressed air usage?

While I sure don’t know everything about compressed air, a few decades of using it, and selling it, gives me enough background to be what…. a little dangerous?

If you are using, or are contemplating using compressed air, and you aren’t sure about something, feel free to click here, and ask me a question. I’ll do my best to answer it for you, and share your question with the thousands of readers that visit the site each month. If I don’t know the answer, someone else will, for sure.

Cheers,

Bill

What happens to the compressed air?

Compressed air is generated by electro-mechanical means. Usually an electric motor will use electricity to turn a shaft, a pulley (sheave) will turn a crank of some sort, pistons will move up and down, and larger volumes of free air in the cylinder piston get scrunched down into smaller sizes. How’s that for technical?

If you jam enough air from the cylinders into a tank (receiver) then the pressure in that tank will start to rise. And it will continue to rise, until such time as the pressure inside that tank reaches a set point. For a DIY home compressor that’s usually in the 90-120 PSI range. An industrial compressor might have a higher set point, depending on the pressure that plant needs for their air consuming applications.

It’s what happens to the air as it’s compressed that I want to address here.

As your compressor jams air into the tank the space available between each molecule of air gets smaller. This forces those molecules of air to “rub” against each other more often, increasing friction. As we all know, friction creates heat.

So as your compressor compresses air into the tank the process generates heat. And there could be lots of heat generated, depending on how big the compressor is and how long it runs. Even if it’s a small DIY home compressor, after a while, you can feel that the tank is warmer as the air gets compressed into it.

So why care?

The issue with heat generation has a couple of ramifications.

One of those is that the compressing of air can generate enough heat to provide it to other applications or uses in the plant. There’s lots of information on the internet about using compressor generated heat. Here’s some information to get you started.

Another by product of heat in the compressed air is water.

The hotter the compressed air in the tank, the more water can be held by that air. When that water-laden air moves out into the air lines or air mains on it’s way to your application, that air cools. As the air cools, it’s ability to hold water lessens, and the water vapor condenses into free water in the air lines. Sometimes, depending on the size of the compressor and lines, that water generation can be measured in gallons!

It’s this water, the vapor that condenses in the air mains and air lines, that can wreak havoc on your compressed air equipment. Regardless of what you are doing with compressed air the odds are that water spraying out along with your air won’t be good for your valves, cylinders, tools, air brush etc.

That’s why you need to have end-of-air-line water removal equipment. That equipment can be as simple as a compressed air filter, or more complex, such as compressed air dryers.

Plastic pipe for plumbing compressed air

An earlier blog commented on how I wouldn’t use plastic pipe (except for P.E. (polyethylene) and urethane tube). That blog got lots of attention.

Even though the pressure rating for smaller diameter PVC, CPVC and PVDF may appear to suit the pressure requirements for compressed air, they are not approved for that use.

If a plastic pipe fails, it may shatter, with catastrophic results.

If you are using plastic to plumb your air, or if you are considering doing so, may I suggest that you contact the pipe manufacturer and ask them if they will sanction the use of their pipe for plumbing compressed air? And keep a copy of the documentation, just in case.

Here’s lots of information on compressed air. 

1 gallon or 5 gallon compressor, which is more powerful?

I am pleased to respond to questions about compressed air here.

A recent question had to do with how the power compared between a 1 gallon and 5 gallon air compressor.

The answer is simple. There may be  no difference in the power of these two compressors. Rather, one (the 5 gallon) will run a little longer before low pressure cut in is reached, and the compressor starts to regenerate compressed air in the tank.

What matters when comparing compressors is the horsepower of the electric motor, and the amount of compressed air that the compressor will deliver at the required pressure.

When selecting a compressor, assuming that you need 90 PSI to run your air tools or air cylinders, what amount of air will that compressor deliver at that pressure? That figure will always be a lot smaller than what is often claimed by the compressor manufacturer. At very low pressure, the CFM’s delivered will always look impressive, but it’s air at the higher pressure that you need.

I need 10 CFM @ 100 PSI

One of the visitors to my site asks the following:

“I am working on a model airplane and I need a compressor with about 10 CFM at about 100 PSI, as light as possible. Your product seems to have additional components such as the cover and gauge that are not needed for my purpose. Could you please advise me of what compressor I should get?”

Answer:

Hello, and thanks for writing.

You can remove the gauge from your compressor should you not want it, but no compressor comes without one. The gauge shows what the downstream pressure will be from the regulator, which too is almost always a component that comes with every compressor.

You want the regulator so that you can lower the pressure coming from the compressor discharge to try to eliminate pulsations as the compressor turns on and off with air consumption and to use the absolute minimum of air pressure necessary for the application, reducing compressor run time and power consumption.

As far as the cover goes, different manufacturers cover the compressor head differently, but in most cases it’s not necessarily for appearance, but to keep persons from accidentally touching the compressor head, which can become quite hot when the compressor is running.

You say you need 10 CFM @ 100 PSI. Is that continuous, or for a few seconds at time?

That’s quite a bit of flow for a DIY type compressor. Frankly, it will be pushing it to get that with a 110 VAC compressor. Regardless of what horsepower the compressor is rated for, with 110 VAC in a 15 amp circuit, you will generate about a maximum of 2 HP of compressor power, and that might give you 8 CFM @ 90-100 PSI.

In order to get continuous 10 CFM you might have to go to a larger compressor and 220 VAC power.

So, until I have a better understanding of your needs I can’t be more specific about what compressor. If I can help further, write back.

Cheers,

Bill

Compressor Air Line Freezing

We’ve discussed many times how compressing air generates free water and water vapor in the compressor tank and in the air lines. What happens if a line goes outside?

If your air compressor is in the sub tropics or tropics….nothing special.

If you happen to be in northern latitudes however, there’s a real likelihood that the water that’s getting into your air lines, where those air lines exit a warm building to travel to another through exposed winter time temperatures, will freeze. Get enough of it in the lines, and it will freeze solid and block your air line.

Sometimes you cannot help but have an air line go outside to get to another location, and if you are one of the unfortunate folks that lives where the air freezes, here’s what you can do.

First choice…run a heater wire the exposed length of the air line, and then insulate it so that the temperature in the line doesn’t get below freezing. Be prepared to have a lot of water traveling along that line, so have a drop leg with an auto drain where the line re-enters a warm building.

Next choice…reduce the dew point of the compressed air just before it leaves the warm building  so that the dew point of the air entering the cold line outside is well below the lowest temperature that the compressed air might experience as it traverses the outside air line. If the air is dry enough, there won’t be any water in the air to freeze, and you won’t have a problem.  Warning, this is an expensive and maintenance prone solution.

Last…don’t do it! Determine the requirements of compressed air in the building to which the exposed air line was traveling, and install a compressor there to supply that part of your operation.

Don’t forget to drain the receiver

Though draining the compressor receiver is important for any compressor user to do regularly, those of us with compressors in unheated garages in the winter get to experience another problem.

If (after warming up your compressor for a while) you only run it for a little while, and you don’t drain the receiver, then any water accumulating in the tank will freeze. Freeze solid, and block the drain cock.
The next time you use your compressor, if that ice dam doesn’t get warm enough to melt, then more water will be added, and more ice will be created in your compressor tank as the compressor cools below the freezing point.

Each similar cycle will add more water, and ice will continue to block the drain so that even it it’s left open, water can’t get out. Eventually that ice will thaw (spring does come, though sometimes it’s hard to believe) and you’ll have quite a puddle of water in your compressor tank.

Water in the receiver will, in time, rust it out. It will also mix with any airborne dust ingested by your compressor intake (yes, I know there’s a filter, but dust will migrate through in time) and create a sludge that you definitely don’t want getting out the discharge port and into your air line.

For the DIY (do it yourself) home workshop compressor user, drain your compressor tank after every use.

For the plant receiver, an tank automatic drain is the recommended way to go.

Here’s more information on compressor water.

There’s plastic….and then there’s plastic

From time to time I’ll get asked about using plastic pipe to plumb compressed air. As a general rule, I advise against it. Plastic, when it fails, will shatter, obviously creating a serious health hazard for anyone in the vicinity as shards of plastic ricochet around the workshop or plant.

The plastics I don’t recommend for plumbing compressed air are PVC, ABS, CPVC or even PVDF. Though, depending on the diameter, some of these will show an acceptable pressure rating, one that appears to be able to handle the discharge pressure of a compressor, these plastics are not formulated to handle compressed air.

Some other plastics that are suitable for use in plumbing compressed air; Polyethylene or Polyurethane being the most common. The latter is more expensive than the former, but provides better flexibility.

These two plastic tubing products are safe to use with compressed air, are relatively inexpensive, malleable, readily available, and when and if the compressed air pressure exceeds their pressure rating they simply blow up like a balloon and will ultimately pop….not shatter. These I recommend for use universally, assuming they suit the application.

I know of one company that makes a hard plastic pipe and fittings for use in compressed air lines. Nibco, supplying a product called Chem Aire. Information on their product is available here.

I do know that the Nibco product is not approved for use universally, so if you are inclined to use it, make sure it’s approved for compressed air use wherever you are.

When plumbing around your plant or home workshop I recommend Polyethylene or Polyurethane, rubber air hose or copper pipe, in that order. Last, and the least desireable (only considered if other options cannot provide large enough diameter pipe for your plant’s air mains) is black pipe.

Not every air compressor is imported from China.

Wandering around on the net today, I stumbled across this site, a compressor manufacturer in Cambridge Ontario, Canada.

I don’t know a lot about them, except that they manufacture, among other things, industrial reciprocating compressors from 3 – 15 HP. And they manufacture these units in North America, which is becoming increasingly rare.

If you aren’t sure about what reciprocating compressors are, let me invite you to see further information here.

This is the link to the company manufacturing in Canada: Air Power Products Ltd. web site.

A dry air compressor?

I heard some folks were looking for the driest air compressor. Sorry all…no such thing.

Every compressor using free air will generate water. It’s the nature of the beast.

Free air (the stuff we breathe) has water in it, in a vapor form. Some days it’s got lots of water, other days less, but it almost always has water. Some days it seems like it’s 100% water…remember hot, humid summer?

Take about eight cubic feet of that free air at 14.7 PSI (one atmosphere) and compress it to one cubic foot in size, to generate normal compressed air pressure (about 120 PSI) . You now have eight times the water vapor in one single cubic foot.

That concentration of water vapor in the compressed air exceeds the capacity of that air to hold water and the extra water vapor that the air can no longer hold condenses into free water. It pools in the bottom of your compressor reservoir (receiver) and it blows out along your air lines to your application.

The amount of water you get depends on how high you pressurize the free air, how much air you compress, and the relative humidity of the free air that you are compressing. You will always get free water  when you compress air.

Dry air compressor? No such thing. Want more info?