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The Best Parts Of The Rotary Screw Compressor

Rotary Screw Compressor

Now, if you’ve been around compressors for at least a week, you will have heard of a rotary screw air compressor, you know, the one you have if you don’t have a reciprocating compressor. Both of these compressors are positive displacement compressors; the main difference between the compressors is their applications and the way the units are built. Rotary Screw Compressors do not use valves like the reciprocating compressors do to move the air downstream.

These compressors have become an industry staple due to their reliability, noise level, and efficiency. Through the use of two motors, pressure is created to compress the air. The simplicity of their design makes these some of the easiest compressors to use and maintain. The standard enclosure features technology to reduce the noise of an air pump that is already much quieter than a reciprocating piston compressor. The way that the rotary screw compressor starts and stops uses less energy than the reciprocating piston would in the same environment.

Now how did this machine come about? Is it a modern revelation or a cumulation of time and energy going into making a machine of that caliber?  Well you may be surprised to know that the original patent for a “Screw Blower” #4121, was obtained on March 24th, 1878 by a German engineer named Heinrich Krigar. He would then go on to receive two more patents for his 2+2 helical compressor that was capable of making less than 2 psig. So this technology, although nowhere near as refined as it is now, was incredibly groundbreaking.

These compressors have come a long way in 145 years from just under 2 PSIG to machines that are powerful enough to power an entire manufacturing facility. So just how far have these compressors come and how do they work now?


How Does The Rotary Screw Compressor Work?

The main components of these compressors are the male and female rotors. In order to create pressure, these two rotors turn in opposite directions. As the air is moved through the rotors, the space between the individual rotors, as well as the housing for them, is decreased to create compression. Each individual screw component has a built-in pressure ratio that is fixed based on the length and pitch of the screw as well as the shape of the discharge port. These fixed ratios are crucial to making sure the built in pressure ratio is properly adapted to the working pressure.

Now that the more technical explanation is out of the way, let’s take a look at the step by step process of how rotary screw compressors create compressed air:

  1. Gas is sucked into the compression chamber, which consists of the two screw rotors.
  2. As they rotate, the air is isolated in the cavities and moved down the chamber through the rotors.
  3. The overall size of the chamber decreases as it moves away from the opening. This process decreases volume and increases the pressure.
  4. The pressure builds and the air is condensed.
  5. Once the pressure has reached the threshold, the discharge valve will open and move the air into the next step of the process, whether that be dryers, storage, or filtration.

The process of compression is streamlined and requires less internal movement of the air in comparison to the reciprocating compressors. The air is pulled in and as it moves through the compression chamber pressure is applied to compress it. No matter the variation, this process remains the same across all rotary screw compressors. These aptly named compressors are known for their consistency and efficiency and they can come in oil lubricated or oil-free.


Wait What Needs Oil

Rotary Screw compressors are typically divided into two main technologies when it comes to their oil usage: Oil-free and Oil-injected. Both of these options cover all of the needs for rotary screw compressors. If there are high air quality standards, you might need an oil-free, but for everything else the oil-injected is going to work just fine. These two technologies are often referred to as dry and wet screws respectively.

The main difference, besides oil content, is how the rotors spin:

  • Oil-Lubricated Screws the male rotor drives the female rotor
  • Oil-Free Screws a timing gear is responsible for ensuring a calculated clearance between the two rotors.


Oil-Free Rotary Screw Compressors

This variant of compressors comes with an asymmetric screw profile to boost energy efficiency and reduce internal leakage. These compressors rely on the timing gear to prevent any contact between the rotors. This results in high-speed compressors with unmatched efficiency that also delivers extremely clean air on demand; however, in order to reach this level of quality the attention to detail needs to be extreme.

External gears are the key component for making sure that the screws are synchronized to prevent any unwanted contact between the two. By preventing contact between the rotors and the housing from occurring, there is no need for lubrication to be present on the rotors. This allows the byproduct to be completely oil-free. Without having any oil in the compression chamber, these machines are able to deliver air that is completely oil-free.

These units typically work in several stages due to the built-in pressure ratios limiting the components. Through multiple stages and intercooling between them, these compressors can reach higher pressures while still not using oil to help cool the air internally. This helps to limit temperature differences between the air being brought in and the final product. Temperature is important to monitor in oil-free compressors due to the way they heat up rapidly.

These compressors do not have the oil in the compression chamber to help reduce the temperature of the air back down to recommended limits. This creates a need that is not present in your standard oil-injected screw. It requires adequate lubrication on bearings and bearing surfaces to ensure reliable and efficient operation. Overall these machines come at the cost of needing more mindful maintenance, but the product these machines create is unlike any other compressed air.

Oil-Injected Rotary Screw Compressors

Completely opposite of the oil-free model, the oil injected/flooded/lubricated goes by many names for the same process. These machines use oil that has been injected into the compression chamber to lubricate the components, as the male rotor is responsible for driving the female rotor in these machines. Friction is impossible to avoid when one component is responsible for turning another and lubrication is needed to make sure that friction does not wear down the parts.

With friction comes heat, and this heat needs to be removed from the air before it goes downstream and potentially causes problems with the equipment. Can you guess what is used to help remove some of the heat? If you guessed oil you’d be right, and if you didn’t well now you know. By removing heat from the air stream, the amount of vapor present in the air stream is reduced and minimizes leak potential. Although more oil is being introduced into the compression chamber, the air and equipment benefit greatly from this being introduced.

Even with the benefits to compression, it is important to ensure that as much oil has been removed from the air as possible. Compressors will often have a centrifugal separator to help remove and reuse the oil. This method does not remove all of the oil, there will be some carry over downstream as the oil passes through in the air. This oil will be removed downstream by passing through filters and a dryer. The oil that is removed from the air is cooled down and recycled back into the chamber to continuously cool the air being pulled into the compressor.

Depending on the application, compressed air may undergo more or less filtration than others. This is ultimately determined by the air quality standards and specifications like temperature, CFM, and PSI can all create a difference in your performance. It is important to know what your individual application needs when it comes to determining the details of how your compressor operates.


Rotary Compressors Can Run at Whatever Speed You Need With VSDs

When it comes to rotary screw compressors, they have access to technology that no other compressor can compete with: The Variable Speed Drive (VSD). VSDs can go where no other compressor can, they can match the rate of production to your needs.

When it comes down to it:

  • Fixed Speed compressors run at one speed when they are running, they are either on or off
  • Variable Speed Drives run at the speed necessary to fulfill the current demand.


Why Would I Want a Fixed Speed?

Fixed-Speed compressors when properly sized will deliver when they are needed. These machines run at 100% of their capacity or 0% and no in between. The problem here occurs when the machine is winding down and the motor is not producing air but is still rotating and using energy. This means precious time and money is being wasted whenever the machine is not running at full capacity. So if this machine isn’t as efficient as a VSD why would it be something you would want to invest in?

The main differentiator between the two, aside from speed, is the initial cost. VSDs can be a lot more expensive upfront. Fixed Speed compressors can be a valuable asset to your operation if you need a continuous supply of air. IF you just need air consistently, the fixed-speed compressor will be the right fit for you. That being said, you will still be wasting valuable energy and money when the machine is not in use.


Should I Use A VSD Rotary Screw Compressor?

The VSD currently has a monopoly on energy efficiency. No compressor can compete with the energy savings that come with using a VSD. These compressors can save you anywhere from 35-60% of your total costs of ownership. Although the initial investment may be steeper than a fixed-speed rotary screw compressor, the Variable Speed Drive will end up paying for itself in no time.

These machines are the unsung heroes of slow production days, slow second and third shifts, or periods where you might be understaffed. By changing the operational speed based on needs, the energy usage during slow periods can be significantly cut down by a VSD. So in short, on slow days the VSD will save you more money than any other compressor would.

Where Would I Use a Rotary Screw Compressor?

With the rotary screw compressor, there are a multitude of different applications that it can be used for. From manufacturing to pharmaceuticals, these compressors can simply get the job done. They are built to handle whatever you throw at them.

The Typical Applications for Rotary Screw Compressors:

  • Food and Beverage
  • Manufacturing
  • Painting
  • Automotive
  • Agriculture
  • Food Packaging
  • Construction
  • Energy
  • HVAC

Of course this list is not all inclusive of every possible application for a rotary screw compressor, this list is in place to give you an idea if you are still unsure about whether a screw is right for you.


Before You Buy

Rather than delve into the details, let’s go over the benefits to going with a rotary screw compressor. There are five main points we are going to focus on.

  1. Energy Efficiency
    1. Due to low heat emission and the potential use of a VSD, these machines are more energy efficient than their reciprocating piston counterparts.
  2. Long Lifespan
    1. A combination of easy maintenance and low heat emissions make this machine the ultimate workhorse. These machines can run full at full power 24/7 and still experience minimal capacity loss.
  3. Continuous Air Flow
    1. Plan and simple, there is no need for a cooldown period and that allows these machines to keep working and avoid overheating issues. The only time it would take a break is when there is no longer any capacity.
  4. Noise Volume
    1. With these motors not being in direct contact with each other, it creates a quieter environment around the compressor. Not only is the basic unit quieter, these machines can be made nearly silent with the right add ons.
  5. High Capacity
    1. Due to the ability to compress air 24/7, these compressors can make higher volumes of air than other models of compressors.


Now all that’s left is for you to go get your own rotary screw compressor for your own application. These workhorses can be found from the best brands on our website. With access to the latest in VSD technology and compressor technology in general, you won’t be able to find a better machine. All it takes is a few clicks and you can be looking at the best fit machine for your application. Shop with us today.


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