If you are in the market, looking for an air compressor then you know that they come in many different variants, basis on the capacity, pressure requirement, different technologies, sizes, and varieties. It is just like we find many options in the market when we look for cars. But most of us do not know, how do we decide which air compressor is the right choice for our given requirement?
So while going for it, one should essentially know certain things like, where you will be using the compressed air? What is the application? How much compressed air capacity do you actually need? What is the quality of the air you need for your process? And what kind of installation of the compressor you would be looking for?
Should you go for a reciprocating piston compressor or a rotary screw compressor? How are they different and when one option is better than the other?
The main criteria always shall be the total life cycle cost which consists of energy cost, investment and maintenance.
How an air compressor works
Broadly speaking there are two type of compression system designs from which all commercially available air compressors are derived. They are dynamic compression and positive pisplacement compression. In positive displacement compressors type, there are two air compressor designs: the reciprocating piston technology and the rotary screw technology.
The very basic premise of an air compressor is that it draws in large volumes of air to the machine. The air is then compressed inside the machine so it can be later released at elevated pressure and compressed volume.
The pressure is how hard the air comes out of the machine. The volume is the amount of air that comes out of the machine.
Let's take a closer look at these two types.
What is a piston compressor?
A piston compressor which is also known as the reciprocating compressors, has many moving parts involved in order to bring-in the compressed air out. The piston compressor has major parts like:
- Crankshaft
- Connecting rod
- Piston
- Cylinder
- Channel Valves
- Valve springs
- Bearings
- Packing etc.
- V belts
- Belt- pulleys
What is a rotary screw compressor?
The rotary screw compressor is also the technology that falls under the positive displacement compression method. Here, the air gets trapped between the two rotors namely male & female rotors & then move forward under pressure condition.
There are further two types of screw compression. Oil injected or lubricated screw compressor & second is an oil-free screw compressor. Let’s talk about both these types.
Oil injected screw compressor, compresses the air between the two rotors wherein the oil is been splashed over the rotors to act as coolant, sealant & lubricant. Here, the oil forms a thin layer over the rotors however, gets mixed with the air which is compressed in the compression chamber. With the technological limitations, one cannot have compressed air 100% free from the oil when it comes out of the compressor. Although one can provide the filters to remove the oil content from the air, however, you will still have some amount of oil carried-over along with the air in the process.
Air compression in oil-free screw compressor technology is also very similar to that of oil-injected screw compressor however, here the lube oil is not being used for cooling, sealing & lubrication purposes. Both the screw rotors are coated with anti-rust material which as well acts as a lubricant. The specially designed cooling jackets are provided in order to maintain the temp. inside the screw chamber. The quality of the air you get out of this compressor is 100% meet the class 0 standard in accordance with ISO8573-1:2010 requirements.
Comparison: Piston vs Rotary Screw Air Compressor
Now you know how piston and rotary screw technologies work, you will read further about the direct comparison between the two. How do you decide which air compressor is right for your given application?
Wear and tear and maintenance
Piston compressors have more moving parts as compare to screw compressors. Since there are more parts that are moving, there is more wear and tear on those parts which end up requiring more & frequent maintenance. But because the piston compressor is simple is design, one should find it easier to maintain than that of a rotary screw. The frequency of carrying-out the maintenance in case of a screw compressor is less than that of the piston compressor.
Temperature, noise and vibration
A piston compressor is running much hotter than that of a rotary screw compressor. The rotors used in the screw compressor are not touching each other directly. With the piston compressor, the piston rings are always in contact with the cylinder walls. Because of the contact, there is more friction and this ultimately results in higher temperatures from the machine.
For the same reason, the piston compressors have more vibrations and run with louder noise. One mostly needs to provide a foundation for the piston compressor in order to minimize the vibrations which also attracts the additional cost of construction. The rotary screw compressors are much quieter as they are provided with anti-vibration pads. Screw compressor doesn’t require the foundation & can be placed on the finished leveled floor.
Space and energy efficiency
As you know, the piston compressors occupy lots of space in order to achieve the desired compressed air flow & the pressure. It is also very necessary to have a bigger & stronger foundation to take care of the bulkiness & size of the piston compressor.
The screw compressors require much lesser space for a similar capacity machine than that of the piston. The design of the screw compressor is very compact & therefore requires less space. Moreover, no foundation is required for these compressors.
Oil issues
As already explained, the oil-injected piston compressor will have more oil carry-over in the compressed air than an oil-injected rotary screw compressor does. However, the piston compressor will release some oil in the compressed air, more than a rotary screw compressor does.
Piston compressors will have more moving parts and more frictional losses as compared to screw. But if your application doesn’t need the clean (oil-free) air, then one can look for the option of piston compressor as they are more cost-effective & the initial cost of investment would be lesser.
Conclusion
Each compression technology have it’s own best operating range and for piston compressors they are best fit for high pressure compression for pressures above 14bar , while screw compressors are the most common reliable efficient solution for medium pressure application.
Also having both technologies mixed to generate medium pressure and high pressure by having screw compressor to generate medium pressure coupled with piston booster is the best way to get advantage of the low SER for screw compression for pressure below 14bar and low SER for piston compressors up to 40 Bar.
