Compressed Air Basics: Clean Air Treatment Part 1

In the previous posts we focused on what's going on in the compressor, but what happens to the air after it leaves the compressor?  The compressor is just a component in your air system - it's not the only thing you have to pay attention to.

Think of an air system as a body, and the compressor is the heart.  Your heart may be working fine, but it's not the only part of your body that you need.  To have a healthy body you need all of parts working correctly.  It's the same with your compressed air system.

 ....but we need to back up a little.  Before we can talk about the air leaving the compressor, we need to talk about the air coming into the compressor.

What is "air"?

You breathe it every day of your life, but do you really know what's in it?  Air is mixture of many different things.  Perfectly clean atmospheric air consists of about 78% nitrogen, 21% oxygen, and about 1% of other gases.  However, no air is perfectly clean.  There will always be water vapor in it, and there will always be suspended particles, known as particulates, in it.  A more common term is dust.  There is a lot of dust in the air around us.  There are also vapors and mists of different substances, such as oil or gas, suspended in the air.

All of these things are sucked into the inlet of the compressor and, unless they get trapped in the compressor oil, they are concentrated and shot down the pipe at high speed.  Humidity in the air, which is water vapor, gets squeezed into liquid water .

If you don't get rid of the water and particulates it will get into your equipment, and often destroy it.  If you're painting, not only do you have to get rid of water and particulates, you must remove the mists in the air, like oil mist, or they'll give you fish eyes.  Additionally there are many other applications, such as breathing air, medical air, pharmaceutical, food and beverage, that need a higher purity where you have to take out water, particulates, vapors and mists.
 
First let's talk about the water vapor.

The amount of water suspended in the air as a vapor is often described as humidity.  You've heard this term on a weather report, and you generally know that when it's very humid it's usually miserable outside.  In the same way, high humidity in your air system is miserable to your equipment.  Many pieces of machinery are very sensitive to the moisture content of the compressed air they receive.

 The important term when discussing the amount of water in compressed air is dew point.  Dew point is the temperature where water vapor in the air will condense into liquid water.  Dew point is highly dependent on pressure.  Think of a sponge full of water.  When you squeeze the sponge, you squeeze more water out of it.  It's the same principle when you compress air; you squeeze the water out of the air.  It's normally the liquid water that destroys your equipment, ruins your paint job, and rusts your pipes. Another way to think about dew point is that hot air will hold more water vapor.  When air cools, the water condenses into a liquid.


So cooler air holds less water vapor - remember that for later.

When the air comes out of the compressor it's very hot.  When it goes down your pipe, it cools.  As we discussed before cooler air holds less water, so water condenses (turns from a gas to a liquid) as the air goes downstream.  More liquid water condenses as the air goes down the pipe, and it will keep condensing more water until the air cools to the ambient temperature - the temperature around the pipe.    On a 75F day with 75% humidity a 100 CFM compressor will take in about 18 gallons of water in a 24 hour period.  That water is going to hit your equipment, much of it as a liquid, if you don't do anything about it.  So now we'll talk about what you can do about it.

Aftercoolers

Usually the first line of defense against the onslaught of water is an aftercooler.   There are two-types:  air-cooled and water-cooled.

An air-cooled aftercooler looks like the radiator on your car.  All it does is run the compressed air through its tubes while air is blown across the tubes by a fan.  If sized appropriately this will cool the compressed air to within 20F or less of the ambient air temperature.  So if your room is 85F, an air-cooled aftercooler will cool the compressed air to 105F.

A water-cooled aftercooler uses water, instead of air, to cool the compressed air inside.  The temperature of the water will determine how much you can cool the compressed air.

These work much better than the air-cooled versions, but you have to have a chilled water system available - it's not something that every business has. 

Additionally you have to make sure that your chilled water system has the additional capacity to handle the heat of the compressed air.  You can also use municipal water to run through it, but that gets expensive.


About 99% of stationary rotary compressors already have a built-in aftercooler.  Only about 2% of piston compressors have one.  If you are looking at getting one, you'll see the term "approach temperature."  Approach temperature is how close you can get the compressed air to the temperature of the cooling air or cooling water.  You size aftercoolers by the CFM of the compressed air flow and the approach temperature you want.

Okay, so we cooled the air with an aftercooler and it dumped a bunch of water into the system - what do we do with water?  Either you have to drain it right then and there or run it through a filter or dryer.

Filters

If you've seen a compressed air system, then you probably have seen compressed air filters.  There are different types of filters, but for now we'll talk about the ones that take out water.

The filters designed to take out water either use centrifugal separation, mechanical separation or both.  In the interest of not making this post too lengthy we won't get into how that works.  What you need to know is that filters will take out 95-99% of the liquid water present. 

However, they do so at the temperature of the air going through it, so the dew point of the air is the temperature of the air when it goes through the filter.  That means if the air cools later down the line, more water could drop out.  You can't just put a filter after a compressor and then expect the air to be completely dry after 100' of pipe or hose.  It will be mostly dry, but you might get a drop or two of water occasionally.  This is fine for most shop tools, but not for other processes or equipment.

That's why filters are called "point of use." Unless there is a dryer before them, they are best installed right before you use the air.  The basic rule of thumb is that you have about 25-50' of hose or pipe after a filter before you see more water.  You can install drip-legs in the pipe down the line to catch any additional water that may form.

Make sure you're checking the drains on the filters.  If you have a manual drain, make sure you remember to open it to drain out the water.  If you have an automatic drain, check the site glass to make sure it's working.  Most filters have internal elements that need changing between 6 and 12 months.  If you don't change the elements when they need to be changed, then you'll have problems.

Dryers

Dryers are machines that dry compressed air so much that they actually lower the dew point.  There are a few different types, but the main ones are refrigerated, desiccant and membrane dryers.

Refrigerated dryers are very simple.  They use a refrigeration system to cool the air, and then they run it through a filter.  Refrigerated dryers give you a dew point somewhere between 35F and 60F, depending on their design.  Normally you get around 38F when sized correctly.  That means if I run the air through a refrigerated dryer and it gives me a dew point of 38F, then there will be no liquid water that will form downstream unless the air around the pipe is less than 38F - no matter how far you go (there are some rare exceptions to this, such as adiabatic expansion).


For a large majority of industrial applications a dew point of 40F is more than enough.  If you are up north, and your pipe will be exposed to temperatures lower than 35F, then a refrigerated dryer won't work.  If you have a specialized process that requires a lower than 35F dew point, then you also have to use a different type of dryer. 

Please notice that I emphasized when sized correctly.  Refrigerated dryers are some of the most commonly missized equipment in the industrial world.  Most refrigerated dryers are air-cooled and their capacity is greatly affected by the ambient air and the temperature of the air coming into it.  All dryers are rated at a "standard".  Usually that standard is 100F inlet temperature, 100F ambient temperature, and 100 psi (high temp and high pressure dryers use different standards).  Higher ambient temperatures and inlet temperatures make the dryer perform worse, and higher pressures make it perform better.  In the literature of every dryer brochure they show you "correction factors" which will tell you how to calculate the actual CFM of the dryer in the conditions you’ll put it in.  It looks like this:

So let’s say you have a 100 CFM compressor and you need a refrigerated dryer, so you would want a 100 CFM dryer.  However the dryer you’re looking at that says 100 SCFM may not be 100 CFM in your compressor room.  If you had a 90F compressor room, then a compressor with an aftercooler will give the dryer air about 20F over the ambient, which is 110F.  So the inlet temperature of the dryer is 110F, and let’s say the compressor is going into the dryer at 125 psi.  Looking at the first chart, you can see the correction factor is 0.80.  Then look at the second chart at 90F, which is 1.06.  So in that 90F compressor room the 100 SCFM dryer is actually 100 x .80 x 1.06, and that comes to 84.8 CFM.  That means if you had a 100 CFM compressor and bought that 100 SCFM dryer, it would not work correctly.  You would need a 118 CFM or greater dryer. 

That’s just for compressors with an aftercooler.  If your compressor does not have an aftercooler, then you need a “high temp” dryer, which has its own aftercooler, or you can buy a stand-alone aftercooler.

Most customers would have just bought the 100 SCFM dryer, because it appears to match up with the 100 SCFM compressor they have.  Then in the summer months it would break or not work properly.  The “S” in SCFM means standard.  The manufacturers cannot list every possible condition a customer could put it in, so they give you the standard and the correction factors.  Then it’s up to you to do the math. 

If you need a lower dew point than 35F, then you’d have to go to a membrane dryer or a desiccant dryer.  A membrane dryer uses osmosis to bring the dew point to as low as -40F.  A desiccant uses desiccant beads to absorb the water in the air.  They can dry the air to as low as -100F.  Most of you will never need that low of a dew point.  However, if you do, there’s a price to pay.  All membrane dryers and most desiccant dryers use “purge air.”  Long story short- they blow about 10% of your compressed air to atmosphere as part of the drying process.  This is 10% of your air compressor lost, just because you need a lower dew point.   Because of that, it’s not advisable to use them unless you absolutely need that low of a dew point.
 
There are some desiccant dryers that don’t use purge air, but they either are very expensive or they’re the small paint booth ones.  The small paint booth ones can be effective for small installations, as long as you change the desiccant on a regular basis.  

Desiccant and membrane dryers also have correction factors for pressure and temperature,.  Make sure you pay close attention to the chart before you buy. 

There are some other kinds of dryers, such as deliquescent dryers, but they are rare and only used for special situations.  We won't cover them in this post.

So, as you can see, an air compressor can and will send a lots of water down your line.  If you don’t catch it, it can ruin your equipment.  Luckily you have many different ways of removing that water from the system.  Ask your equipment manufacturer what dew point they require.  If they don’t have a dew point requirement, and just say “dry air,” then a refrigerated dryer is usually sufficient. 

Remember to size the dryer or filters correctly.  The SCFM rating is just the rating at standard conditions.  Find out what the correction factor for your conditions are and adjust appropriately.

Water is just one part of clean air treatment.  Next week, we’ll talk about how to remove other contaminants from your compressed air.

Compressed Air Basics, Air Compressor Sizing Part 2

Last week we left you hanging.  We showed you how not to size up an air compressor and then made you wait a week for the correct way.

So what is the correct way to size a compressor?

The correct way is to determine the flow, pressure, duty cycle and ambient conditions.

1.  Flow & Pressure

In the USA flow is normally measured in cubic feet per minute (CFM).  You may also see the flow rating in m3/min, l/s, or m3/hr.  Basically it's just the volume of air the compressor is supplying and how fast.  A cubic foot is 7.48 gallons, so 1 CFM gives you 7.48 gallons of air every minute.

Pressure is normally measured in pounds per square inch (psi), but you may also see it as bar, mbar, kg/cm3, or MPa. I think most people reading this already understand the concept of pressure.  If not, please read more here.

I mentioned flow and pressure together, because you need CFM delivered at a pressure.   Knowing your psi without your CFM is useless, and knowing your CFM without knowing your psi is useless.  A compressor's CFM rating is dependent on the psi it's delivering.  You must determine both.

Flow and pressure are inversely proportional, so with the same compressor as your pressure goes up, your flow will go down. You need to make sure the compressor produces the correct CFM at the correct pressure.  Make sure you don't look at the CFM "displaced" rating.  You need the "delivered" CFM rating.  So a compressor brochure might say 125 psi max and it delivers 10 CFM at 40 psi.  If you needed 10 CFM at 90 psi for your application, then that compressor probably wouldn't work.  You have to call up the distributor or the factory to see what the delivered CFM at 90 psi would be.

Every piece of equipment you have that uses compressed air should have a rating of flow and pressure that it needs.  This information should be in the manual or on the manufacturer's website.  If not, call them and find out.  Some tools, like nailers, give you the amount of cubic feet used per cycle, instead of a CFM.  In that case you must figure out how many cycles you can do in one minute.  If you could put in a nail every 2 seconds, and the nailer is .3 cubic feet per cycle at 40 psi, then you'd be using 9 CFM at 40 psi.  If you had a slow assistant and he can only do a nail every 10 seconds, then when he's using the nailer it's only 1.8 CFM at 40 psi. 

You need to add up all of the CFM ratings and find out what psi you need.  After that you must look at duty cycle.

2.  Duty Cycle.

We went over duty-cycle when talking about reciprocating compressors.  If you missed it, scroll about 3/4 of the way down the page and read the section on duty cycle.

After figuring out the psi and adding up the CFM you need, determine how you're using this air.  Are you using this equipment all day long or are you using it for just a few hours a day?  If you have a constant use application, then either oversize a piston compressor by 40% or get a rotary compressor.

Another part of duty cycle is looking at equipment that occasionally needs large bursts of air.  Let's say you have a machine that needs a large amount of CFM for just 30 seconds every hour, but the rest of your machines use much less.  Do you need to buy the compressor for the CFM of the biggest machine?  Probably not.  That kind of problem can be solved with extra tank storage.  Get a tank large enough to supply the air for that 30 seconds, and make sure the compressor can fill it up in less than an hour with all of the other things running, and you should be okay.

Vehicle maintenance shops that use diaphragm pumps to deliver oil and other fluids are good examples of this.  Your double-diaphragm pump may need 60 CFM, but you only use that 60 CFM for 90 seconds at the most.  Then it's another 2-3 minutes before you use the next pump for the other fluid.  So that's 90 cubic feet used (60 CFM at 90 seconds) every 120-180 seconds.  You'd only need a 30-45 CFM compressor with a properly sized tank to do that - you wouldn't need 60 CFM.  Pretty much all of the other applications for a maintenance bay are 10 CFM or less.  Getting a 60 CFM compressor to run all day just for that is a waste.   A smaller compressor with a bigger tank would do the job, cost less up front, and use less electricity.

As I stated before, duty cycle is one of the most important things to look at when sizing a compressor, but it's often the most overlooked.   How often you're using the air is just as important as how much.

3.  Ambient Conditions.

I just said that duty cycle is often overlooked, but it's not even in the ballpark of being overlooked when you compare it to ambient conditions.  Have you ever been inside of a typical compressor room?  People stick air compressors in some of the hottest, dirtiest rooms you can imagine.  The only comparison I can think of is a boiler room, and I've seen compressors in boiler rooms (please don't do that).

I've been in compressor rooms that were 140°F, I've been in compressors rooms where the dust in the room was over a foot thick (grain dust, concrete dust, or saw dust), and I've been in compressor rooms where the chemicals were so strong you could barely be in the room for 5 minutes without getting sick.

We understand that you may have to stick the compressor in these extreme conditions, because you have no other choice.  However, if you're going to put the compressor there, at least take that into account when you buy it.

The CFM rating on all air compressors is SCFM, which stands for standard CFM.  A compressor manufacturer cannot list all of the possible conditions customers may have on its brochure.  Because of this, the manufacturers got together and agreed upon a standard.  That standard is 68°F, sea level (or 14.5 psi ambient pressure), and 0% humidity.  If your conditions vary, and they probably do, then you must take that into account.

If you go up in altitude, higher in temperature, or higher in humidity, the compressor will give you less CFM than the SCFM rating on the brochure or tag.  Altitude affects it the most.  If you are high in altitude, then you need to oversize the compressor.  A 100 SCFM air compressor in Denver will only give you 80-85 CFM.

Humidity and temperature affect the air treatment more than they do air compressors, and the effect is not as dramatic as going up in altitude.  However, extremes need to be looked at.  Here in South Florida seeing a compressor room with 100°F temperatures and 90% humidity is not uncommon.  In that situation a 100 SCFM compressor will give you about 93 CFM.  That's only about a 7% derating, but if the compressor was sized for 90 CFM, and you had some pressure drop in filters or piping, or maybe there's a small leak somewhere, then that 7% could make or break you.

If you like doing math, here is the formula:

Ps  = standard pressure, psi

Pa  = atmospheric pressure, psi

Ts  = standard temperature, °F

Ta  = ambient temperature, °F

PPwv  = partial pressure of water vapor at ambient temperature

rh  = relative humidity

ACFM is the actual CFM that you will get in the conditions you have.

An easier way is to contact your local compressor salesperson and have him/her figure it out.  In addition to doing the math, they can use their experience and training to possibly find a better solution or know of a trick you can do to make sure everything works properly.  Speaking for our company, we have guys that have been doing this over 30 years, have months of factory training and multiple industry certifications.  Whatever you're facing, they've seen it before and they know what works and what doesn't.

4.  Other Factors.

There are a few other things to check for.

1.  Air treatment.  We'll cover compressed air treatment in different post, but filtration inherently has pressure drop associated with it.  If you size them correctly, the pressure drop should be minimal, but take it into account anyway.

2.  Tank size.  The bigger, the better.  You can read more about that here.  If you're buying a piston compressor, then you probably have very little choice in tank size, and usually the tank it comes on is more than adequate.  If you have applications that need a large burst of air you may want to get a surge tank.  If you have a rotary screw compressor, then the rule of thumb is four gallons of tank storage per CFM of the compressor.  You may need more or less, depending on duty cycle.

3.  Backup.  If the compressor is essential to your business, you should always have a backup.

4.  Multiple compressor solutions.  If you are a business that runs multiple shifts and those shifts vary in their air demand, then usually a multiple compressor solution is best.  If you need over 100 CFM of air, then you also get in the area where a multiple compressor solution may be better than just one.

5.  Fudge Factor.  You should always give your compressor a 10-15% "fudge factor."  What I mean by that is that things don't always turn out as planned.  You might get pressure drop in your piping; you might spring a leak; a filter could get clogged - who knows.  As your equipment gets older, it may consume more compressed air (this is actually fairly common).  Getting a compressor with 10-15% more capacity than you calculated is often a good idea to cover for the unexpected.   It's a safety cushion.  Now we always preach energy savings and using the most energy efficient compressor you can get, and oversizing goes somewhat against that.  However if your facility is big enough for that amount to matter, then advanced compressor controls and large tanks can make it so that the compressors only run in their most efficient state, and you won't waste that 10%.


"....But it's so much easier just to say I need a 20hp compressor!"

What's easier - taking an extra hour or two to do things the right way or buying the wrong thing and then having your compressor break on you in the middle of production?  If you have kids, I'm sure they may complain about doing their homework sometimes, but you make them do it.  You know that doing their homework now results in deeper understanding of the subject matter later, plus they if they don't do the homework, they might fail the class.

You reap what you sew.  People spend days deciding what car they want to get.  Sometimes they'll put in hours of research.  That's a good thing, because your car is very important to your life.  In the same way your air compressor is essential to your business.

Put in the extra work before you buy it, and you will reap the benefits of having the correct air compressor.  It's an expensive piece of machinery.  Why wouldn't you want to make sure you're getting the right thing?

Additionally there are trained air compressor sales personnel, who have been to multiple week long classes on how to correctly size and configure compressed air systems. You should contact them and rely on their experience and training to get you the correct air system.