Compressed Air Basics Part 8: Rotary Vane

Rotary vane compressors have been around for a long time and are used for many things other than just compressed air.  The first known description of a sliding vane pump was in a book by an Italian engineer, Agostino Ramelli, written in 1588.  The rotary vane pump was patented in 1876 by Charles C. Barnes, a Canadian engineer, and he is generally credited with inventing the modern design.

They've been produced since the early 1900's, and have been used ever since.  It would take a book to list every application rotary vane pumps are used for (in fact, there is a book).  They are very popular in automotive and hydraulic applications.  The power steering in your car probably uses a rotary vane pump.  In the compressed air and vacuum world, they are still common, but they are not a popular as they used to be.

Here is how they compress air:

You have a cylindrical rotor inside of a housing, and this rotor has slots where the vanes are. The vanes can slide in and out.

The rotor is set eccentrically (off-center) in the housing so that on one side it almost touches.  As the rotor spins, the vanes are thrown out by centrifugal force, until they touch the housing.  If the pump is oil-lubricated, there will be a thin film of oil in between the vane and housing.  

As air enters through the inlet, it's trapped between the vanes.  As the rotor turns, the volume of the trapped air is gradually reduced.

Again, the reduction in volume raises the pressure.

The vanes are often small pieces of carbon fiber or graphite composite, but they may be made of different materials, depending on the application.

They are mostly rectangular.  They sometimes have an angle cut on one of the long edges.

If you're looking a vane pump, you'll have the choice of oil-lubricated or oil-less.  The choice will depend on your application, duty-cycle, and your preference on maintenance.   From a maintenance standpoint, with an oil-lubricated pump you'll replace oil, separators and maybe an oil filter on a regular basis.  With a non-lubricated pump, you'll replace the vanes as maintenance.  It's more expensive to replace the vanes, but you replace them a lot less often than changing the oil and filters on a lubricated pump.  Your duty cycle will determine which is more cost effective in the long run.

Rotary vanes are an older technology that is still common today in compressed air and vacuum.  However, there are now other technologies that are more efficient.

In higher pressure applications (above 80 psi), a rotary screw compressor can usually do the same thing more efficiently.  The initial cost is usually very close on both and the maintenance is approximately the same, so in most cases, you should opt for a rotary screw.

In the lower pressure and vacuum applications, a claw can usually do the same thing more efficiently.  However, at this time the claw pumps are much more expensive than the vane pumps.  You have to make the calculations if the energy savings and lower maintenance of the claw pump has the return on investment you are looking for.

So if you are looking to buy a rotary vane pump for compressed air or vacuum, contact your local compressed air expert.  Maybe there are better alternatives.  Remember to look past the initial cost of the equipment.  Just comparing the price of two pieces of equipment that you might buy is extremely short-sighted and will end up costing you more money.  You need to compare total cost of ownership, including maintenance and energy costs.

Don't get me wrong - vane pumps have their uses and sometimes they are the exact product that is right for the job.  Their role in the automotive world and hydraulics is solidly entrenched.  However, as newer, more efficient technologies emerge, their role in compressed air and vacuum is rapidly diminishing. 

Compressed Air Basics Part 7: Side-Channel

Before I learned about them, I never remembered seeing a side-channel blower or vacuum pump.  However, now I see them all over the place.  Many of them are so small and quiet, that you never notice them unless you're on the lookout for them.

Elmo Rietschle makes one as small as a CD that weighs less than a 1/2 lb. They also make them as large as 700 lbs. 

So let's look at how they compress air - I'll just copy verbatim from Rietschle's literature, because they explain it very well:

The gas is taken in through the inlet (1).  

As it enters the side channel (2), the rotating impeller (3) imparts velocity to the gas in the direction of rotation.  
 
Centrifugal force in the impeller blades accelerates the gas outward and the pressure increases.  
 
Every rotation adds kinetic energy, resulting in further increase of pressure along the side channel.  

The side channel narrows at the rotor, sweeping the gas off the impeller blades and discharging it through the outlet silencer (4) where it exits the pump.


In this case, the impeller is just a fancy fan. It spins really fast inside the housing and creates pressure or vacuum, depending on which side you hook it up to.  Actually, every compressor creates both pressure and vacuum, but side-channel is one of the few technologies where you can use the same pump for both with no modification.  For instance, a piston compressor creates a vacuum on its inlet, but you can't just hook a pipe up to it and use it in your process.   With a side-channel pump, you can.

This the first dynamic compressor that we have spotlighted.  As we mentioned before, a dynamic compressor uses a rotating element to accelerate the air and then forces it into a smaller space.  I like to think of it as a traffic jam on the interstate that happened because some lanes are blocked off, and the cars are the air molecules. The cars are jammed tighter together because of the lane blockage.  More cars jammed together in a tighter area is the higher pressure.

Unlike a bad day on I-95, however, side-channel technology is very useful.

Here are some applications where you'll find side-channel blowers:

Central vacuum systems	Packaging industry
Degasification of food	• Assembling and folding packaging materials
Dental vacuum	• Vacuum packaging
Drying out buildings	Plastics industry
Filling bags / bottles / silos	• Contact free plastic film redirection
Fishpond ventilation	• Cooling and drying extruder products
Gas analysis	• EPS foaming
Industrial vacuum cleaners	• Granulate conveying
Laser printers	• Plastic welding
Pneumatic conveying	• Thermoforming
Swimming pool technology / jacuzzis	Printing and paper industry
Letter sorting / enveloping	Textile industry
Lifting and holding parts using vacuum suction	Ventilation of sewage treatment plants
Soil remediation

Side-channel blowers and vacuum pumps have a lot of benefits:

1.  Very little or no maintenance
2.  100% duty cycle
3.  If sized correctly, they last a very long time. 
4.  Very clean, no oil or lubricants at all in the pump
5.  The same pump can be used for pressure or vacuum.
6.  They are usually quieter than other technologies that offer the same pressure and flow

A good example is the Rietschle Velocis series.  It will run 40,000 hours flat out before you need to do any maintenance.  There are 8760 hours in a year, so it could run 4-1/2 years non-stop without you needing to touch it.  Of course, your usual work week for a compressor like this is 40-80 hours, so that's between 9 and 19 years before any maintenance or parts are needed.

Of course, many of you haven't heard of these before, so there must be some drawbacks.  Here they are:

1.  The range of vacuum and pressure they can supply is very small.  The most pressure out of the ones I've seen is 15 psi, and that's a special 3-stage version.  Normally they operate between 3 and 10 psi.  That's not a lot of pressure.  The same is true for using it to create a vacuum.  They don't create a deep vacuum.  This limits their use to special applications that need very little pressure or a very light vacuum.

2.  They are very tricky to size properly.  Most of these blowers are used by OEM manufacturers as part of an engineered package.  The sizing was done by the engineers at the factory.  Unless you are extremely knowledgeable about compressed air or vacuum, you're not going to pick one out of a brochure and put it in your shop.  Well, actually you can, but the odds of you choosing the wrong one are high.

Those are really the only cons, and they aren't bad points - you shouldn't use any technology outside of the limitations it was designed for.  If you need to supply a continuous flow of under 10 psi, side channel blowers are usually the way to go.  However, you should have a professional to help you pick one out.  Once sized correctly, side channel compressors will give you years, maybe decades, of  worry-free service.

Compressed Air Basics, Part 6: Claw Compressors and Vacuum Pumps

In most compressed air training classes and seminars, the rotary claw pump is usually only mentioned in passing, if at all.  In most of the applications where claw compressors and vacuum pumps can be used, rotary vanes and rotary lobes are much more common.  However, a claw pump can often provide a large amount of energy savings when sized correctly.

We decided that instead of just lumping them in a post under "other," we'd highlight them in a separate post.



Here is how a claw compressor works.  It is a rotary compressor with two claw-shaped rotors. They turn in opposite directions in the housing.  They don't touch but the clearance is very, very small.  As the claw moves over the inlet, air is sucked into the compression chamber. As the rotors revolve, it reduces the volume of the trapped air, which causes the pressure to rise.

The claw pump is another "oil-free" technology.  There is no oil in the compression chamber, only in the gearbox.


Usually claw pumps are used as vacuum pumps or as blowers.  What I mean by the term "blower" is a compressor that puts out a low pressure at a high volume.  Of course low pressure and high volume are relative terms.  What we're comparing it to in this case are the technologies we reviewed before - piston, screw, and scroll.  For the same amount of energy a blower will give you a lot more volume, but at a considerably lower pressure.  Piston, screw, and scroll compressors are mostly used for pressures above 80 psi.  When used as a pressure pump, the claw is usually for pressures below 30 psi. Every technology has their "sweet spot" where they perform well compared to the competition.

A lot of customers don't know that the claw pumps exist, but they're great for certain situations.  Rotary vane and rotary lobe tend to dominate the market for these low pressure applications, because those pumps have been around for a long time and the customers are familiar with the technology.

However, if you're in the market for a vane or lobe pump, you may want to consider a claw pump, because there may be some benefits.

Here are the benefits of a claw:

1.  In most cases you use less horsepower to provide the same amount of air or vacuum that you would from the claw's major competitors - the vane and lobe pumps.

2.  The maintenance is extremely low.

3.  100% duty cycle.

A good example of this is the Rietschle VLR-301 vacuum pump.  At 7.5hp it can produce the same amount of volume that Rietschle's 10 hp vane pump can do (and the 10hp vane pumps of the competitors).  Additionally with the vane pump, you're replacing about a gallon of oil at least twice a year - sometimes as much as four times per year.  With the VLR-301, you replace the gear oil every 20,000 hours of running time and it's only about a quart.

So that sounds great, what's the catch?

The drawbacks are:

1.  Higher initial cost.  In the example above the 7.5hp VLR-301 costs a couple of thousand more than Rietschle's 10hp vane pump.  However, that's made up later in energy savings and maintenance costs.  If you look long term, or even medium term, this isn't a drawback, because over the course of a few years, your overall cost is less.

2.  In the instance above, the VLR-301 cannot pull a full vacuum.  It can pull a lot, but it can't pull the vacuum that a vane pump can.  If you needed a full 29 inHg, then you'd have to go with the vane pump.  For a more general drawback of claw technology as a whole, it's designed around its sweet spot of power, flow, and pressure/vacuum.  When you hit that sweet spot, it's usually the best option.  If you're too far outside that sweet spot, it may not be able to do what's asked of it.  It needs to be sized correctly by a professional.

So in what situations should you look for a claw pump?  Here's a list of good applications for it:
• Central vacuum systems
• Chemical industry - gas compression
• Aeration
• Drying
• Dust extraction
• Gas compression
• Soil remediation
• Drying systems
• Dust extraction systems
• Industrial furnaces
• Medical vacuum
• Packaging
• Pneumatic conveying
• Printing industry
• Woodworking industry
• Clamping
• Dust extraction
• Holding

Claw pumps seem to be exceptionally good fits for the printing industry, soil remediation, medical vacuum, bulk material handling, CNC machines and vacuum hold-down. 

Again with any technology, it has a sweet-spot where it performs the best.  Claw sizing is more advanced than your typical compressor installation.  I highly advise you contact the experts before you buy one.  Also, if you ask for a claw pump, and the distributor doesn't ask a bunch of questions on why you need that pump and what it's used for - you should probably get a quote from somewhere else.  That company just wants to sell you equipment with no regard on whether it's right for you or not. They just want to make a quick buck and run - they're not looking to be your compressed air partner.

Compressed Air Basics Part 5: Scroll Compressors

The scroll compressor was invented in 1905, but but the metal casting technology of the period was not advanced enough to make them.  Scroll compressors require a really tight tolerance.  They started being made just after WWII, and started being commercially produced in large volumes beginning the in 80's, as refrigeration compressors.  They still are very popular in the refrigeration industry, and in the last ten years or so, scroll compressors have become a major player in compressed air - an industry that's been dominated by reciprocating and screw compressors.

Scroll compressors use two interleaving scrolls, one is moving and one is stationary.  The moving one doesn't actually rotate - it orbits.  Air gets trapped in the inlet, and ask the orbiting scroll moves, the spiral volume the air is trapped in gets smaller and smaller.  As we discussed before, when you decrease the volume, you increase the pressure.  In the picture below you can see the air in yellow.  In each following step, the space the air is trapped gets smaller and smaller.







Scroll compressors are very popular in medical air field and laboratory air.  The reasons are:

1.  Extremely low maintenance requirements.  Scroll compressor are very easy and inexpensive to maintain.

2.  Scroll air compressors work well in both low duty cycles and high duty cycles.  With a reciprocating air compressor, you don't normally want a 100% duty cycle.  With a rotary screw air compressor, you normally don't want a duty cycle below 50%.  Scrolls operate well in all duty cycles.  The amount of air needed by a medical air system can vary greatly from day to day, because it depends on the number of patients they have and what type of procedures are being done.  A scroll compressor works well in all of these situations.

3.  Very, very clean - there's no oil to leak out.  Even with compressors that are "oil-free" there is usually oil in a gear box.  The scroll compressors have no oil anywhere whatsoever. 

4.  Quiet - given the same type of sound-proofing, a scroll compressor will be more quiet than a similar sized reciprocating compressor.  Of course you can make any compressor quiet with the proper sound-proof enclosure, but with a scroll it's easier and less expensive for the manufacturer to keep the dB to an acceptable level. 

5.  Small size - the airend is very small, compared to a reciprocating pump that delivers the same volume.  A rotary screw airend is approximately the same size, but a greater amount of controls and valves are needed for a rotary screw.  The reason this is important to a medical air application is that per the NFPA 99 requirements (the rules that dictate medical air), you must have redundancy.  It's a lot easier to build in redundancy when the airend is small and you need less valves and controls.  Additionally there is often limited space in a lab, clinic or hospital.

So what's the drawback?  These sound great - why doesn't everybody have a scroll air compressor?

1.  The initial cost is pretty high.  These are advanced, well-built, premium machines, and you usually pay a premium for that.  However, that can be made up later by increased efficiency and lower maintenance.  For a place like a lab or a hospital, a premium compressor is needed - you can't compromise on quality, and they have the capital to pay that premium.  If you're a small business or home user, it's probably out of your price range.

2.  They don't scale well for large volume demands.  These scroll pumps, just until recently, only came in 3hp, 5hp, and 7.5hp versions.  If you needed about 40hp worth of air, you'd have to get a compressor that has eight scroll pumps and eight motors.  That drives the price up.  A system with one 40hp rotary screw airend and one 40hp motor is usually easier to produce and less expensive to manufacture than a system with eight 5hp pumps and eight 5hp motors.  However, Powerex Compressors, just announced that they're now producing 10hp scroll pumps.  With this announcement, that will allow them to compete on a larger scale. 

3.  For most industrial applications, a rotary screw compressor will be more efficient.  A factory doesn't always need the extra benefits that a scroll compressor provides.  Not every technology is right for every situation.  Scroll compressors are usually a perfect fit for a lab or hospital, while a rotary screw is usually the best fit for a big factory, and a reciprocating compressor is the best fit for a small body shop.  However, every situation is different - we have found a few factories where a scroll compressor was perfect for what they were doing, and we even have found a place where a scroll was perfect for a guy's garage (he had a really nice garage, though!).  We've found that scroll compressors are usually great for boats, too.

Again with any compressor, the best way to know what's right for you is to contact the experts. 

That's it for this week.  Next week we'll go over claw compressors and vacuum pumps.

Compressed Air Basics Part 4: Rotary Screw Compressors

Sorry for the delay on this one.  Last week I was out of town.  This week we are talking about rotary screw compressors.

Rotary screw compressors are the workhorses behind a majority of manufacturers worldwide.  If you see a big building and they make stuff there, there's a good chance there is a rotary screw air compressor powering their manufacturing process. 

There is a good reason for this.  An industrial rotary screw compressor has a 100% duty cycle.  It can run 24/7 without a break, and in fact it usually works better and lasts longer when it's used that way.  A piston compressor normally works better when it can take a break - it likes a intermittent duty cycle.  However, the rotary can go all out, all day without stopping - it doesn't like starting and stopping constantly.

Another reason is that when sized correctly, rotary screws can be some of the most energy efficient compressors on the market.  The keys are correct sizing, proper air system design, and intelligent compressor control.  You can throw the most efficient compressor in the world in an air system, but if the system and control scheme are poorly designed, the compressor won't be efficient.

Let's talk about how they compress air.  

 A typical rotary screw air compressor has two interlocking helical rotors contained in a housing.  Air comes in through a valve, typically called the inlet valve and is taken into the space between the rotors.  As the screws turn, they reduce the volume of the air, thus increasing the pressure.

There are rotary screw air compressors with just one screw, as well.  However, they're not very popular when it comes to compressing air.  You'll see them more in refrigeration applications.  Their principle of operation is beyond the scope of this blog, but if you're interested, you can read more here.  For the rest of this blog post, it can be assumed that we are talking about compressors with more than one screw.

The assembly that includes the rotors and the housing they're in is called an "air end" or airend.  This is actually the correct terminology for all rotary compressors, whether they be rotary vane, scroll, screw or lobe - the part that compresses the air is called the airend. 

Rotary screw compressors can either be oil-flooded or "oil-free."  Oil-free is in quotation marks because oil-free compressors don't provide oil-free air (there's oil in the air around us).  However the difference is that with oil-free rotaries there is no oil in the compression chamber. 

In an oil lubricated rotary screw compressor the male rotor is driven by the motor or engine, and the female rotor is driven by the male rotor, or actually by the thin film of oil that's between them.  The oil also seals the compression chamber and acts as a coolant.

In an oil-free rotary screw compressor a set of gears controls the timing between the male and female rotor.  There is no oil to seal the chamber, so without multiple stages you cannot achieve as high as a pressure as you can with an oil-lubricated one.  Additionally there's no cooling oil, so they run hotter, and that decreases the efficiency.  Because of this oil-free rotary screw compressors are usually limited to special applications.  There are some oil-free compressors that use water as a coolant, but those are rare. 

There is so much more to a rotary screw compressor than the airend.   Let's take a look at the typical oil-lubricated rotary screw:

The airend doesn't just compress air; it compresses an air/oil mixture.  That mixture then flows into a tank called the separator tank or sump.  The oil is separated out of the air by centrifugal force - as the air spins around in the tank, the oil drops out because the oil particles are heavier than the air particles.  Usually there are baffles in the tank that assist with this.  There is also a separator element that takes out nearly all the remaining oil - all but a few parts per million (usually 3 ppm). 

From there the oil and air take two separate paths.  The air then goes out through a cooler and then out to your application.  The oil will either go back into the airend or through an oil cooler.  There is usually a thermostatic valve that directs the oil one way or the other, based on the temperature of the oil.  You don't want the compressor to run too hot or too cold.  If you run too hot you'll fry the oil, decrease efficiency and burn out other components.  If you run too cold, you'll never get hot enough to boil off the liquid water that dropped out of the air when it was compressed.   Too much liquid water in the oil will cause airend failure.

Usually there is a minimum pressure valve or a minimum pressure check valve which doesn't let the air out into the air system, until there is minimum pressure for the compressor to lubricate itself.  There is an oil filter that filters out contaminants in the oil.  There is also an air filter to keep large contaminants from getting in.  Another common component is a blow-down valve (or unloading valve) that blows the excess pressure in the sump to idle pressure when the compressor is idling. 

An oil-free rotary has different components.  Normally there are two airends and the air is cooled with an intercooler between them.  Typically the gears for both airends are housed in the gearbox and that gear box is lubricated.  An oil seal and positive pressure are used to keep the oil from the gearbox out of the airend.  There's no separator tank, oil cooler, or thermal valve, but the other components are usually there. 

That's it for the basics on rotary screw air compressors.  Next we'll go over the basics of scroll air compressors.