Rotary Screw vs Piston Compressor: What’s the best solution? (Part 1 of 2)

If you've been in the market earlier, 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? & 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?

We have the answers to the questions you have in your mind about air compressors. Read on to learn all about piston and rotary screw technologies. 

Before we can look at the differences between the piston compressor and the rotary screw compressor, let's look at what an air compressor actually does. Once we know the basics of how air compressors work, it'll be easier to understand how these two types are different than each other.

An air compressor is a machine that can be run a few different ways. Most industrial compressors are operated through an electric motor driven, although you will also find the compressors having prime-mover that is fuel-driven too. 

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.  

The two major technologies that are most prominently being used to compress the air are either a piston or a rotary screw type. These two types although are under the same technology of positive displacement, work differently inside the machine to compress the air.

Let's take a closer look at these two types.

A piston compressor which is also known as the reciprocating compressor, 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

The crankshaft is driven by prime-mover that is either an electric motor or a fuel-driven engine. The machine will come with an option to have an air tank to hold the amount of compressed air.

The crankshaft operates and forces the piston to move up and down inside the machine. There is a valve at each end of the cylinder. When the piston moves one-way air enters the cylinder. Then, when it moves the other way the air inside the cylinder is compressed. As the volume decreases, pressure increases.

The full compression cycle is the process by which the air comes into the cylinder and then gets compressed to the required pressure. Depending on the pressure requirement, the number of stages is decided by the manufacturer.

In a single-stage piston compressor, the piston will compress the air in one stroke. One stroke means the piston has a full-motion with one rotation of the crankshaft. In layman's terms, a single-stage piston will travel forward & backward directions with one rotation of the crankshaft.

A single-stage unit can operate with lower RPMs and noise levels. A two-stage unit uses two pistons. One crank uses one piston to bring the air into a cylinder. It's then transferred and a second piston for the further air compression & in turn for pressure elevation

In the case of an oil-lubricated piston compressor, the piston and crankshaft need lubrication to operate effectively. When the piston moves in the cylinder the lubrication oil gets mixed with compressed air. This means that the compressed air coming out of the compressor will have some oil in it. 

In the case of an oil-free piston compressor, the crankshaft needs lubrication to operate effectively whereas the piston won’t be lubricated with oil. This means that the compressed air coming out of the compressor will not have any oil in it.

However, it is very essential to understand that, in piston-type compressor the compressed air output shall be pulsation type (not constant) & unbalanced vertical forces cause cylinder ovality, acting along with the piston weight & also an unbalanced couple necessitating heavy foundation. They are also very bulky in size & weight.

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

In the case of oil-free screw compressors, as both the screw rotors (male & female) are not touching to each other, the wear & tear of these elements is very minimal & also the noise emission is limited to the standards. There won’t be any oil carryover in this type of compressor which increases the overall uptime of the system

The coolers designed for these compressors are much efficient although, they are compact in size. The compressed air delivered by the screw compressor is continuous without having any pulsations.

One has to select the type of screw compressor based on the application & the quality of air required for the given process. 

Now that, you have understood the working principles of both technologies. In our upcoming blog, you will see the comparison between the piston and rotary screw compressors. How do you decide which air compressor is right for your given application? So, stay tuned!