Compressed Air Basics: Clean Air Treatment Part 2

Last week we looked at removing the liquid water out of your compressed air system.  However, if you are using more than just shop tools, then there are other things in the air that you need to be concerned about.

Particulates

In layman's terms particulates are dust.  They're little particles suspended in the air.  You'll also hear this term when it comes down to the cleanliness of your oil.   The oil filter in your car is used to trap the particulates that get into the oil.  In the same way, you often need to trap that particulates in your compressed air. 

Particulates, and the filters that trap them, are measured in microns.  A micron is another word for micrometer, which is one millionth of a meter. 

To the right you can see a perspective on how small a micron is. Human hair is 50-70 microns wide.  When we talk about filtering out particulates in compressed air, we start out at "big" particles, which are 3-5 microns.  Filtration down to .01 micron is common.  Special applications can often demand filtration beyond that.

Think of how small that is.  A human hair is 50-70 microns, basic filtration traps particles 20 times smaller than that, and common advanced filtration can trap particles that are 5000 to 7000 times smaller than with width of your hair, or 9000 times smaller than a typical grain of sand.  The largest particle a human eye can see is in the range of 40 to 50 microns.

Just in the air around us there are thousands of different types of particulates suspended in the air.  In industry, there are many other contaminates as a byproduct of what the company does.  Obvious examples are sawdust in a cabinet shop, concrete dust at a concrete plant, or grain dust at a mill.  However, there are much smaller particles that you can't see in high concentrations, and there are too many to list here.


The first line of defense against particulates is your compressor's inlet air filter.  Many of the things we have mentioned will mess up your compressor if allowed to continually get in.  Most compressor inlet filters will be between 1 and 25 microns.  The manufacturer will put on the air filter that they think will protect the compressor.  Make sure you check in often and keep it clean.


The second line of defense is lubricant. The oil in your compressor (assuming it's oil-lubricated) will trap a large amount of particulates.  If your compressor has an oil filter, then many of these will get trapped in oil filter.  Again it's important that you keep your oil (and oil filter if applicable) changed on a regular basis. 

Filters are the last line of defense for particulates.  When talking about particulates, filters will have a micron rating.  That will tell you what size particles it traps.  If you have a 5 micron filter, then it will trap a large majority of particles 5 microns and above, usually around 99% or greater.  The filter may also have a ppm rating, and that tells you how many parts per million of particles of that size (the micron rating) or greater will pass though the filter.  Filters have changeable elements inside of them.  You need to change them on a regular basis as part of your maintenance plan.  For the most part you need to change them every 6 months to a year, depending on how many hours you run per year.

Vapors & Mists

Particulates are solid particles suspended in the air, but there are also vapors and mists suspended in the air around us.  Most of it is water, which we discussed last week, but that's not all. 

A mist is liquid suspended in the air.  A vapor is a gas mixed in with the air.  Whether a substance will be a particle, mist or a vapor is determined by pressure and temperature of the air it's in.  Every substance has a pressure and temperature thresholds where it changes states. 

Many processes are sensitive to mist, such as painting.  A few are sensitive to vapor, such as breathing air.  A good example is oil mist and oil vapor.  A common misconception is that the oil mist in compressed air is solely generated from the oil in the compressor, and that using an "oil free" compressor will give you oil free air.  While an oil-lubricated compressor will have a small amount of oil carryover, there is already a higher hydrocarbon concentration in the air around us and, if you want oil-free air, you must filter that out even with an "oil-free" compressor.

An oil-lubricated compressor that is well maintained will pass hydrocarbons downstream in the amount of 1-3 parts per million (ppm).  However, in relatively clean atmospheric air, there is already 6-10 ppm of hydrocarbons.  If you're near a well-traveled road, an airport, or an industrial area, then the air will average 10-30 ppm of hydrocarbons in the air.  You're breathing that in right now as you're reading this.

So remember, it's not the compressor that gives you oil-free air or even clean air.  The filtration after the compressor gives you clean air; nothing else does.  The junkiest, nastiest compressor that's passing a lot of oil, with just a moderate amount of filtration, will give you cleaner and dryer air than the most pristine "oil-free" compressor with no filtration.  Also if you put that same exact moderate amount of filtration after the pristine compressor, the quality of air will be the exact same as the quality of air after the junky compressor.
 
Filtration for vapors and mists will have a micron, ppm rating, or an ISO rating.  The manufacturer of your equipment will usually specify a micron rating and they should specify whether that applies to particulates, mist, vapor or all three.  If it's a real concern to the equipment, they should specify an ISO rating.

ISO 8573.1:2010

ISO is the International Organization for Standardization.  They publish standards about many different things in all different types of industry.  ISO 8573.1:2010 is the ISO standard about compressed air quality.  There are actually other parts to this standard and there is another standard that has to do with how the air and filters are tested.  However, when you buy filters ISO 8573.1:2010 is what you need to look at.

There are three parts to this standard:  Particulates, Water, and Oil:

The ISO rating will have 3 digits with a period between each number.  Let's look at a common ISO rating:  2.4.2. 

The first "2" represents the particulate requirement.  It basically means a .1 micron filter.  The chart above shows you how many particles are allowed to pass by of different sizes.

The "4" represents the water requirement.  This is the simplest one.  4 means you need a dew point o 38F (or 3C).  A properly sized refrigerated dryer will meet this standard.

The last "2" represents oil.  Now this one is the trickiest....sometimes manufacturers need you to meet only the oil mist standard and not the oil vapor standard.  An oil mist filter is different from an oil vapor filter.  For breathing air and medical air you need to meet the complete standard (mist and vapor).  For painting you'd only need to meet the mist standard. 

Beyond ISO Ratings

Some processes require filters that go beyond the above ratings.  There are even compressed air filters that are designed to filter out bacteria.  Going into the "other" category is beyond the scope of this post.  99% of you reading this will never need or even encounter that level of filtration.

FILTRATION ORDER

If you're going beyond just standard shop air, you'll need more than one filter.  You can't just stick an ISO 1 oil vapor filter in line and call it a day.  A filter that gets out oil mist is different from one that filters out water, and they're both different from one that takes out oil vapor.  An oil vapor filter will be ruined if it gets wet.  A oil mist filter (a coalescer or coalescing filter) will get won't work properly if used to catch water.

The basic order is:
1.  Catch the big particulates.
2.  Filter out the water (can sometimes be combined with #1 above in the same filter)
3.  Catch the smaller particulates and mists (if needed).
4.  Filter out the vapors (if needed). 
5.  Bacterial filter (if needed).

The exception is if you have a desiccant dryer or a membrane dryer.  Those dryers will filter out the water, but if any oil gets on the desiccant or membranes, it will ruin it.  If you have a desiccant or membrane dryer, then order is this:

1.  Filter to catch big particulates and most of the water.
2.  Coalescing filter
3.  Desiccant or membrane dryer to lower the dew point.
4.  If you have a desiccant dryer, after it you need a particulate filter to catch the desiccant dust.
5.  Vapor filter (if needed).
6.  Bacterial filter (if needed).

You have to gradually step up the filtration as you go down the line.  The filters that catch the smaller particles will get destroyed or clogged up by bigger particles or water.  Thing of trying to catch cannonballs with butterfly net - it won't work. Catch the bigger particles first (the cannonballs), then catch some of the medium sized ones (basketballs maybe?), and then you can catch the small ones (your butterflies).

Consult an Expert

If you need advance filtration you need to talk to your local compressed air salesperson.  If you just have shop tools, most of the time you can just stick in a water filter and go.  However once you go beyond that, it gets complicated.  There is a wide variety of filters available for you to buy.  Even sizing filters is not always simple. 

Just like the dryers we looked at in the last post, filters have correction factors, and you must take those into consideration when sizing them up.  You can get your compressed air thousands of times cleaner than the air you breathe, and you can dry your air well beyond the driest any desert on earth has ever been.  However, excessive filtration causes large pressure drop, additional maintenance, and greater energy costs.  It's not recommended to go far beyond the level you need; you'd just be wasting your money and running up your electricity bill. 

Your local compressed air salesperson should have hours of training and years of experience on filtration.  They'll take into account the level of filtration you need, the flow you need it at, the correction factors based on the ambient conditions, the pipe size you have, and do all this while taking into account that you want to have the least amount of pressure drop possible.  They should also take into account your duty cycle and maintenance considerations for later.  Let them guide you toward the right decision.

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. 

Compressed Air Basics, Air Compressor Sizing Part 1

Imagine you went to the pharmacy and told the pharmacist,

"Give the cheapest medicine you have that comes in 250mg pills."

Fever's not gone, but......

The pharmacist then asks you, "What type of medicine?  Do you want aspirin, Tylenol, NyQuil, Claritin, or maybe some antacid?  What are your symptoms?"

"That doesn't matter....just give me the cheapest one that comes in 250mg pills," you respond.  So you grab the cheapest medicine on the shelf that comes in 250mg pills, and you confidently walk out of the store with Viagra, even though you have a fever.



That seems a little ridiculous, but we see this every day in the world of air compressors.  Several times per day, we have a customer that asks us for a compressor in a certain horsepower. When you ask for a compressor based on horsepower, what you are doing is very similar to pharmacy situation described above. 

Horsepower (hp) is not an accurate way to size an air compressor.  Horsepower is a number assigned by a manufacturer to represent how much power the engine or motor can deliver.  However, that number can be creatively altered by manufactures to show what they want it to show, and more often than not that is the case.  Additionally the hp rating only tells you rating of one component of the package.  It doesn't tell you what's going on with the other components. 

This creative horsepower labeling is extremely common consumer world.  Go to Home Depot and Lowes, and you might see compressors that plug into your wall that say 5hp or 6.5hp.

If you see this plug, it's 1.5 hp or less

The plug in your wall is on a 20 amp breaker and it's a 120V circuit.  Watts = volts x amps, so the biggest motor you can run on that plug is 2400 watts.  2400 watts is 3.2 hp.  Sometimes it's just a 15 amp breaker (1800 watts or 2.4 hp).  Compressors are "hard starting" which means the starting amps are over double the full load amps listed on the motor.  Also, you have to factor in motor efficiency.  That means any air compressor bigger than 1.5 hp will blow the breaker when it starts.

So any compressor that plugs into your wall at home with a regular plug is 1.5hp or less, no matter what it says on the label.

So what about the 230V compressors that they sell at a big box store or automotive store?

If it says 5 hp or 7.5 hp, then that's probably the actual hp or close to it. If you see some other hp rating than that, like 6.5 hp or 9 hp, then you know some creative labeling has been applied.

You can look at the amps to see the correct horsepower on it.

A 5 hp will be around 21 amps, and a 7.5 hp will be around 35 amps.


It's not just compressors where you'll see this, there have been several class action lawsuits where manufacturers were giving "peak hp" ratings.  The horsepower on vacuums, lawnmowers, and cars have all been creatively labeled. There was one which involved air compressors, as well.  If you see the words "peak hp," on anything you can completely ignore that horsepower rating - it means absolutely nothing.

It's not just the consumer world either.  Horsepower labeling in the industrial world often goes the other way with horsepower.  Let's look at some popular industrial brands and their 100 hp compressors.  I'm going to link you to the CAGI data sheets, and look at line 11, which is the input kW at full load.

Kaeser CSD-100S:  77.0kW - 417 SCFM at 115 psi
Kaeser CSD-100:  90.6kW - 494 SCFM at 115 psi
Ingersoll-Rand R75I:  89.4 kW - 455 SCFM at 115 psi
Gardner Denver Saver II ST 100:  85.9kW - 450 SCFM at 125 psi
Sullair 7509:  90.3 kW - 444 SCFM at 125 psi
Atlas Copco GA-75:  87.3kW - 424 SCFM at 125 psi
Atlas Copco GA-75+:  88.2kW - 483 SCFM at 125 psi

Input kW is the actual amount of power you use when running fully loaded.

Now let's convert kW to hp.  100 hp is 74.6 kW.  However, no motor is 100% efficient.  That's physically impossible with today's technology.  To to calculate the real hp, we need to multiply the kW by the motor efficiency and then convert to hp.  As an example, a 100hp motor and 95% efficiency would need input kW of 78.5kW.   The motor efficiency rating is line 7 in the CAGI data sheets.

77kW x .941 = 72.5kW = 97.2 hp
90.6kW x .941 = 85.3kW = 114.4 hp
89.4kW x .954 = 85.3kW = 114.4 hp
85.9kW x .954 = 81.9kW = 109.8 hp
90.3kW x .954 = 86.1 kW = 115.5 hp
87.3kW x .945 = 82.5kW = 110.6 hp
88.2kW x .945 = 83.3kW = 111.7 hp

All of those compressors are "100 hp" compressors.  There's really only one that's truly close to 100hp, the Kaeser CSD-100S.  Among these compressors you have a 15% variance in the amount of power they consume and nearly 16% variance in the amount of air they can give you.

Another great example of this is the Kaeser AS-25, which is called 25 hp.  The new version can give you 120 SCFM at 125 psi.  Compare that to the Ingersoll Rand 30 hp UP6-30, which gives you 117 SCFM at 125 psi.  The Kaeser "25 hp" gives you more air output than the "30hp" Ingersoll-Rand.  Just to check the kW on both:  the UP6-30 is 28.7 kW and 92.4% efficiency (35.5 hp), and the AS-25 is 23.3 kW and 91.7% efficiency (28.7 hp).

So the Kaeser "25hp" is closer to 30hp and the Ingersoll Rand "30hp" is really 35 hp.  Also, the Kaeser gives you more air with about 7 less hp.  This is a great example of how useless the horsepower rating is.

Let's add more confusion to he horsepower ratings - gas engines and electric motors of the same hp won't do the same amount of work.  If I were running a compressor pump that normally takes a 5 hp electric motor, I would need about a 10 hp gas engine.

So basically it takes double the amount of hp for a gas or diesel engine to do the same amount of work that an electric motor does.  Here is a good explanation of why.    The short story is that an electric motor just handles the power given to it by the electrical system and translates that to physical force, but a gas engine actually produces power by its combustion. 

About 90% of customers who contact us looking for a compressor ask for a certain horsepower, but as you can see sizing up a compressor based on horsepower is not a good idea.  It will get you in the ballpark range, but that's it.  Air compressors for home use and small business tend to greatly exaggerate their hp ratings, while air compressors targeted toward larger industrial customers tend to give you more horsepower than what's on the label.

Additionally the horsepower of the motor doesn't tell you how efficient the pump is or anything about any of the other components that may affect the compressor output.  There is far more variance in these components from compressor to compressor than there is on the motor, and their impact on compressor output is far greater.

So if you size air compressor by horsepower, you'll be in same position as the person who walked out the pharmacy  - you'll know what size pills you have, but you won't know what's in them.

So what is the correct way to size a compressor?

....we'll cover that next week.

Compressed Air Basics, Part 11: Other Technologies

In the previous posts, we went several different technologies in the compressed air world.  There are other technologies that exist, but they are a small percentage of the air compressors/blowers/vacuum pumps out there. 

Centrifugal Air Compressors

Centrifugal compressors use the same technology as jet engines.  They are dynamic compressors.  One or more impellers spin really fast, accelerating the air.  Then the air is forced through a diffuser, which is a smaller area.  The speeding up of the air and then slowing it down causes the pressure increase.  These are usually huge air compressors, hundreds of horsepower, and they're designed for large, continuous duty air use.   They're sometimes called radial compressors.




Below is a picture of a multistage centrifugal with several impellers and diffusers.  The pressure gets higher as it goes through each diffuser.

Axial Air Compressors

 Much like the centrifugal compressors above, these are large dynamic compressors and this technology is also used in jet engines.  They tend to be even larger than centrifugal compressors.  This technology is also common in steam and natural gas use.

The compressors are called axial because the air flows down the axis of the compressor.  You can see a good example of the air flow in this video.

Instead of using diffusers to slow down the air, the axial compressor has alternating rows of rotating and stationary elements.  The air gets sped up and then slowed down over and over.

 

Peristaltic Air Compressors

From the biggest air compressors (centrifugals and axials), we now go to one of the smallest compressors, the peristaltic.

Basically a rotor with one or more lobes squeezes a tube with air in it, moving the air from the inlet to the discharge.  These are more commonly used for juice and pizza sauce than they are for compressing air.

Helical Lobe Compressors

This technology mixes the rotary screw with the rotary lobe.  There is a gap in psi between a rotary lobe blower's maximum  psi and the psi where a rotary screw is no longer energy efficient (on the lower range of its psi).  This technology is designed to fill that gap.  Here is what the rotors look like.

Image courtesy of Aerzen USA

Linear and Vibrating Armature Compressors

Image courtesy of Gast Mfg

These are very similar to diaphragm compressors.  They are positive displacement compressors that use alternating current to move a piston, diaphragm or shuttle.  It compresses the air in the same manner that a diaphragm pump does.  It just uses electromagnetic force to move the components. 

Guided Rotor Compressors

Image courtesy of Combined Heat & Power, Inc.

This is another positive displacement rotary compressor, and this one is based upon an envoluted trochoid geometry.  Wikipedia says that "the compression volume is defined by the trochoidally rotating rotor mounted on an eccentric drive shaft."

In simple English, the thing in the middle spins and reduces the volume of the air, which causes an increase in pressure.


I've never personally seen one for compressed air use yet, but they do exist.  According to the literature of the companies that make them, their target applications are hydrogen and natural gas compression.  Like the hybrid screw-lobe above it's an emerging technology in the compressed air world, so time will tell how it fits in.

Trompes

A trompe was a way to compress air and other gases prior to electricity.  You can read more about them here.  In fact trompes helped power some of the first electricity generating plants.

It uses falling water to compress air.

One of the more recent ones that was used industrially is now a tourist attraction, and you can see it here.


Just because this blog, compressor classes or compressed air seminars may group a compressor technology into the dreaded "other" category,  it doesn't mean that technology is useless.  They are not any better or worse than the technologies I highlighted in previous weeks.  Every technology has its use.  These compressors are made for a reason and they often are the best choice for special applications.  I grouped them here because they are a small percentage of the air compressors in use today. 

There are many situations where one of the above technologies will be either be the most efficient or most effective way to compressor air - even a trompe .... if you happen have a waterfall handy and need to ventilate a mine or smelt iron.

Compressed Air Basics Part 10: Rotary Lobe

Rotary lobe pumps have been in industrial use since the late 1800's.  They were invented by the Roots Brothers around 1860.  The brothers were looking for a way to build a more efficient "water motor" for a mill.  When they were working on it, one of the brothers rotated the shaft, and it blew the other brother's hat off.  A superintendent of an iron foundry saw this, and said that would be helpful in smelting iron.

From there the Roots Blower Company was started and it sold "Roots Blowers" to iron foundries and for mine ventilation.  The applications that rotary lobe blowers has since expanded to many different applications, and hundreds of different companies make them.  They're often still called "Roots Blowers," and that company still exists today as a part of General Electric. 

Rotary lobe pumps are used in a wide variety of applications, and not just in compressed air.  The first engine supercharger was a rotary lobe pump.  Interestingly enough, one of their most common uses is in mills, though not as the original Roots Brothers had intended.  Rotary lobe blowers are nearly in every industrial grain, rice or flour mill, but they're used for pneumatic conveying.


Rotary lobe pumps are positive displacement pumps.  However, unlike the technologies we've covered before, no compression happens inside of the pump.  There is reduction of volume to create pressure, but it's not inside the pump.

So how do they compress air?


Inside the pump housing you'll have two rotors that each have either two or three lobes.  If the rotors have two lobes, they are shaped like the number 8.




 

 If the  rotors have three lobes they look like this:

Rotary lobe pumps with more than three lobes do exist, but as far as I know they are not used in compressed air.

 
One of the rotors is driven by the motor or engine and the other is geared to the driven one.  When one spins, the other spins in the opposite direction with very precise timing and clearances.

Air is sucked into the inlet, and it is forced around by the lobes, and then pushed out of the discharge.  A very small amount can escape back through the clearance in the rotors, and this is called the "slip."  The slip is why blowers are only used for very low pressures.


When the air is discharged out of the pump, this is when the volume reduction occurs occurs.  The air gets forced down the pipe.  Unlike the other positive displacement pumps we've covered, that take a fixed amount of air and gradually reduce its volume to increase pressure, the rotary lobe pumps takes a fixed volume and continually forces more air into it to increase pressure.

The amount of pressure a rotary lobe pump can produce is very small compared to what a piston or screw pumps can do.  They're referred to as blowers, because they produce low pressure at a high volume.  They're also used for high volume, light vacuum applications.

Pros and Cons:

Pros:
1.  Can produce a very high volume of air.

2.  Very little maintenance - the compression chamber is oil-free, so you only have gearbox oil and maybe an air filter or silencer to change.

3.  Plug and play - most manufacturers make the pumps to fit in the same spot as the other brands.  Additionally these standard designs have been around for decades.  So you might have a blower system from the 50's, and most likely you can buy a new pump and it will drop right in the system with no modifications and perform the same.

4.  Durable.  The pumps can take a pretty good beating.  They're often used in harsh environments.  Just make sure if it's extremely hot, to change the gearbox oil more often.


Cons:

1.  Limited pressure range.  They can only give you about 15 psi.

2.  They're not always the most  energy efficient, due to the slip.  Sometimes other technologies can out perform it.

3.  Loud.  Usually it's not big deal, because these are in industrial environments where the noise doesn't matter.  However, you can put a silencing box around them or add silencers to reduce the noise.

If you have an application that needs below 15 psi of air, a large volume of air, and you have a dirty or dusty environment, a rotary lobe pump is often a good choice.  As always, when sizing any application, contact your local compressed air expert.

Compressed Air Basics Part 9: Liquid Ring Pumps

Liquid Ring pumps are very simple systems, but it's very complicated to explain how they work.  There is a lot going on inside the pump, despite only having one moving part.

While typing this out, I felt I was going into advanced principles of fluid dynamics, instead of just compressor basics.  The pump uses centrifugal force on a sealing liquid, which acts like a piston in an ideal cylinder, to compress a gas trapped between the liquid and an impeller.  Say what? 

Let's step backwards a little bit....

A liquid, for all intents and purposes, is nearly incompressible.  Of course you can put enough force on any liquid and it will compress, but it's a large amount compared to a gas.

For instance, if we wanted to compress water, we would need about 29,000 to 30,000 psi to reduce its volume by 10% (depending on temperature).  If we want do the same thing to air at sea level, that only takes between 1 and 2 psi.  That's a huge difference.

For the pressures that a liquid ring pump can achieve, we can say that the liquid in it does not compress, or it cannot be reduced in volume.  The gas in the pump, however, can be compressed.

So let's look at what's going on in the pump.

This is the pump before it starts up:

Image courtesy of Dekker Vacuum Tech. Inc.

The pump has an impeller, which will spin when the pumps starts.  Similar to the rotary vane pump's rotor, the impeller is eccentric (off-center) to the housing.

There is the liquid in the bottom - could be oil, could be water, or possibly another liquid.  This is called the sealing liquid

At the bottom there's a hole for the sealing liquid to enter the chamber

In the middle of the pump, around the center of the impeller you can see the inlet and discharge ports.  This is where the air will enter and leave.

This cut-away side view gives you a better idea of how the gas goes in and comes out into the compression chamber.

Once the impeller starts spinning, the sealing liquid is thrown outward by centrifugal force.

Because the sealing liquid is denser than the air, it will occupy the outside, while the gas is trapped on the inside.




Here is an animation of the air flow.  The blue is the sealing liquid, the orange space is air, and the white dots are air particles.

You can see that because the impeller is off-center, as the impeller rotates, the space that the air is trapped in gets smaller and smaller.  The reduction in volume causes an increase in pressure.

Some of the sealing liquid gets moved downstream, as well.  This has to be separated out and fed back into the pump.


So why do all this - why not just use another type of pump?

The advantage of liquid ring pumps is that they are very durable.  They can take a beating and keep on going like nothing happened.

There is only one moving part with no metal to metal contact, so there is very little wear.  Also, the liquid in it helps keep the components cool, which also helps out the life span of the parts.

When used for vacuum duty, liquid ring pumps can suck in all types of junk and fluids and it doesn't hurt them all (if they were designed and sized properly).  Here in Florida we have had cases where either a piston vacuum pump or a rotary vane vacuum pump was inadvertently sucking in sea water.  This immediately ruins the vane or the piston pump.  So we put in Rietschle 2BL liquid ring pumps, which use water as the sealing liquid.  The internal parts are bronze or stainless steel.  It often sucks in sea water, and there's no problem.

Liquid ring pumps can be put in bad situations, extreme conditions, and suck in materials that would ruin other pumps.  In addition to being durable, they also have great longevity.  You can expect decades of service from a properly configured liquid ring pump, as long as you do the maintenance.


So what are the disadvantages of a liquid ring pump?

1.  Liquid ring pumps tend to be much less energy efficient than other technologies.
2.  Liquid ring pumps usually have a higher initial cost.

So there you have it.  If you have a situation where other pumps keep failing, put in a liquid ring pump.  Make sure you consult your compressor company on what type of sealing liquid to use in the pump and the materials that the pump is made out of - that can make or break you.   For the sealing liquid, you'll usually have the choice of oil-sealed or water-sealed.  For the internal materials there will be a wide variety of choices.  These choices will be determined by the material compatibility of what's coming into the pump.

On the other hand, if you have a liquid ring pump or are looking at replacing an old one, and you have a situation where there is no extreme condition and nothing but air is going into the pump, then another technology may be a better purchase.  Liquid ring pumps are designed for rough conditions.  If you don't have a rough condition, you're just wasting money and electricity on a pump you don't need.