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.