The rotary screw elementLet me explain how a rotary screw element is built up. See  this example picture of an oil-free screw element.

Compressor element (oil-free type). Photo: Atlas Copco

Of course, we see the two rotors (male rotor on the bottom, female rotor on the top) and the housing (the gray part).

As we see the rotors have different kinds of bearing son both sides so they run smoothly for years without any maintenance. There are usually two pairs of bearings on both sides; bearings for radial loads (loads because of the turning of the rotors) and axial bearings.

Because the screw 'push' are to one side (the high pressure side) the rotors want to move to the opposite direction. The axial bearings take on this load.

We can also see that the male rotor has an axle that sticks out with a gear on it. This is the driving gear. Sometimes it's a pulley. The two rotors are also connected to each other by gears (on the left side in the picture) these are the synchronization gears.

The element is water-cooled, for this purpose there are water cooling pockets in the element housing (the green parts). The gears are lubricated with oil, as is indicated by the yellow/brown parts. Oil-injected screw elements don't have this, since they are cooled by the injected oil.

There is a sealing between the oil and the compressed air compartments, to prevent any oil from contaminating the compressed air (specific for oil-free compressor elements).

The housing can be disassembled for maintenance.

What a screw element looks likeAir-ends come in many differten sizes, but they all look basically the same. Here are some photos of air compressor elements.

Air compressors element. This one is on a brand new compressor, as you still it's stillclean and shiny.

Air compressor element on variable speed compressor. This is what a compressor typically looks like: dirty!

Compressor element on portaable air compressor.

How it worksHow does it work? Inside the compressor element are two screws (called 'rotors') that turn in opposite direction.

The rotary screw compressor is a 'positive displacement' compressor. Which simply means that the air is physically squeezed together by an external force (like piston- and scroll-compressors, which are also positive displacement compressors).

Trapped air between the roters

In the case of the rotary screw, the air gets trapped between the two rotors. The rotors have a special design for optimal efficiency and performance.

One rotor is called the 'male' rotor, the other one is called the 'female' rotor.

As can be seen on the picture: air gets sucked in on one side (cold, low pressure), gets trapped between the rotors, and is discharged on the other side (hot, high pressure).

This compression requires power, which is usually supplied by a big electro motor.

Types of screw compressor elementsThere are two basic types of screw compressor: oil-injected and oil-free.

Oil-injected rotary screw compressors are the most common, since they are the cheaper ones of the two types.

Oil-free screw compressors are only used in applications where to compressed air must be 100% oil free (usually in food-processing plants, chemical plants, etc).

I will explain later why the oil-free type is more expensive.

Rotors / helical screwsThe rotors have the shape of what is called an "helical screw". Yes, it looks like a screw. There is a male rotor and a female rotor.

Male and female helical screw rotors.

The male rotor is the 'thick one', it has lobes. The female rotor is the 'thin one' and has grooves or 'flutes'.

The air gets trapped between the male and the female rotor and is transported to the exhaust side of the element in 'air pockets', pockets of air that are trapped between the rotors.

Mostly, the male rotor has 4 lobes, while the female rotor has 6 grooves. But this is not set in stone. Manufacturers are always looking to improve the screw design. They look for a design that gives the best efficiency. In other words: how to pump the most are with as little possible power.

The exact design and manufacture of the screw is one of the most best-kept secret of any air compressor manufacturer. It's a no- go / no photographs area of the factory.

Drive shaft and synchronization gearsThe female rotor is driven by gears off the shaft of the male rotor. When the male rotor turns 1 time, the female rotor turns exactly 1.5 times.  They are synchronized. The gears that drive the female rotor are called synchronization gears.

The male rotor is driven by an electro motor or sometimes a diesel engine (which is the case with portable compressors). They run anywhere between 1000 and 6000 rpm.

Pressure ratioBecause of the compression, the air heats up. The hot air will also heat up the rotors and the metal housing of the compressor element.

This is a problem, because hot metal expands, it becomes bigger. When it expands too much, the two rotors will touch each other and/or the housing… this will usually result in a completely messed up screw element (costly!).

For this reason, we can't make an unlimited high pressure this way; it would simply get too hot.

The maximum pressure a screw element can create is called the pressure ratio. That is the maximum output pressure divided by the input pressure.

For oil-injected types, the pressure ratio is normally maximum 13. For oil-free types, this compression ratio is about 3.5 max. We'll see later why.

More on oil-free and oil-injected screw compressors in the following paragraphs.

Design limits in creating the best screw elementThe rotary screw element is an example of a highly engineered part, with thousands hours of research into it. There are many variables to think about when designing the best screw element.

As said before, oil-free screw compressors used two stages, with an intercooler to reach the desired end pressure. But why is it so difficult, in this age of computer-aided design, robot controlled cnc-machines and complicated mathematic models, to create a single-stage oil-free air compressor?

The problem is that many factors influence each other. 

A higher pressure-ration means more internal air leakage (at a higher pressure, more air flows through the same gap). This higher leakage reduces the overall element efficiency.

Because of the lower efficiency, the rotors will need to run in at a higher speed. This gives extra problems with vibrations and life time of the rotors and bearings.

The high compression ratio will result in a higher exhaust temperature. Steel has the bad habit of expanding when it gets warmer. The high temperatures result in high thermal expansion of the rotors.

This all adds to the problem of making the clearance as small as possible (too small and the rotors will touch each other when warming up!).

Oil-free vs oil-injectedWhat are the differences between oil-free and oil-injected rotary screw elements? And why is n oil-free element more expensive compared to an oil-injected element?

The injected oil has several functions; one of those functions is to seal any gaps and clearances between the male and female rotors and between the rotors and the housing.

Gaps and clearances allow the compressed air to flow back the 'wrong way'. It lowers the efficiency and performance of the air-end.

Because the oil-free type compressors don't have any oil, the clearance between the rotors and the housing must be much smaller compared to oil-injected type screw elements. Because of this the price of an oil-free air-end is much higher.

The needed maximum clearance depends on the rotor diameter and is about 1 thousand of the diameter, max.  So if the diameter of the rotor is 200 mm, the clearance would be only 0,2 mm, which is pretty small for such a complicated shape.

Besides this, the oil-free elements require extra pockets and channel for cooling water. The oil-injected type element is cooled by the injected oil and doesn't need extra cooling water.

Pressure

Pressure switches are pressure actuated electrical switches. Wich simply means, that there is an electrical switch, which is forced open or closed by air pressure.

The air pressure at the inlet port acts on a flexible membrane. The pressure in transformed into a force. How much force depends on two things: the area (square mm's) of the membrane and the pressure (bar) of the compressed air. Since the area is constant, the force generated is directly proportional to the air pressure.

The other side of the membrane is connected to a spring and a lever that acts on the electrical switch. As long as the downward force of the spring is higher than the upward force of the compressed air, the membrane (and lever) stay down.

But when the pressure rises, there comes a point where the upward force of the compressed air is higher than the downward force of the spring. The membrane (and lever) will move upward at this point. When the membrane moves upward, so does the lever. The lever in turn opens the electrical contacts and the compressor stops.

Now this isn't a direct connection, but there is an ingenious mechanism inside, that makes the switch flip-over at once, at a certain pressure, and not slowly slowly ("snap-action"). This mechanism is also connected to the second set-screw which sets the differential pressure. For the inner workings of the pressure switch, this simply means that the switch closes again at a lower pressure than the pressure where the switch closes (built-in hysteresis).

I will try to explain it with the pressure switch I have here on my desk. I will simulate the air pressure with a screwdriver (instead of the compressed air pushing up, it's me with the screwdriver pushing up :).

This is not recommended when you still want to use your pressure switch, as it will damage the membrane. I bought this pressure switch just for the purpose of illustration, and have no intention of using it on an air compressor, so here it goes...

Pressure switch with low air pressure

Pressure switch inner workings at low pressure: the big spring is forcing the membrane (green circle) downwards (red arrow), while the air pressure tries to push it upwards (yellow arrows).The spring 'wins' and the membrane stays down. The electrical contacts are closed (blue circle)

Pressure switch with high air pressure

Pressure switch inner workings at high pressure: the big spring is forcing the membrane (green circle) downwards (red arrow), while the air pressure tries to push it upwards (yellow arrows).Now the pressure is high enough and the membrane moves upwards. The electrical contacts open (blue circle) and the compressor stops.

Setting the pressure.

To be able to change the pressure setting, the downward force of the big spring can be adjusted (red arrow in above pictures).

The adjustment is done by compressing the spring using a long screw. When you turn the screw clockwise, the spring is compressed more. It will now generate a larger downward force on the membrane, so we need an higer air pressure to overcome this force. In other words: we raised the pressure setting.

When you turn the screw counter-clockwise, the spring becomes less compressed. It will now be easier for the compressed air to push against the spring. In other words: the pressure setting is lowered.

The main spring is almost not compressed. The pressure switch is set at a very low pressure.

 

The spring is highly compressed now. The pressure switch is set at an high pressure.

The off/on/auto switch

Some pressure switches have a switch or push button for on/off control. On some switches it says "ON / OFF", but the words "AUTO / OFF" would be more correct.

In the AUTO ('ON') position, the pressure switch works as expected. It opens and closed automatically according to the pressure. In the OFF position, the pressure settings are overruled and the electrical contacts are always open. The compressor is stopped.

(an ON postion wouldn't make much sense, as this would make the compressor run indefinately, which of course would result in a way too high pressure).

How does it work? The on/off buttons or switch are connected to a plasitc pin. When the pressure switch is set "OFF", the pin pushes down on a plastic block. The plastic block in turn opens the electrical contacts. The compressor will stop and stay stopted.

Auto / off switch action. The white pin pushes down on the black plastic block (red circle).The black plastic block forces the electrical contacts open and the compressor stops.