When it comes to industrial air compressors, “best” is subjective to the specific requirements of your manufacturing environment which varies wildly between industries. The decision is made ever more difficult as many purchases are a decades-long commitment to both the unit and the company you buy from. The first step in making a decision is educating yourself on the different types of air compressors that are available.
Air compressors fall into two different groups: positive displacement and dynamic.
Positive displacement compressors work by forcing air into a chamber. The volume of this chamber is then decreased, which compresses the air. Once maximum pressure is reached within the chamber, a valve opens, and the air is then discharged into the outlet system. Both rotary and reciprocating compressors fall into this category.
Rotary screw compressors are generally used in plants requiring roughly 25-250 HP, although some modern machines can reach up to 600HP. Rotary compressors use two intermeshing helical rotors (screws) to force the air into a smaller and smaller space, thus creating pressure. Oil is used throughout for lubrication, sealing, and heat absorption. Before usable air exits the chamber, the oil must be removed. This process uses oil filters that must be replaced regularly. Although oil-free options exist, trace amounts are commonly found in regular screw compressors.
Reciprocating compressors employ the use of a piston to reduce the volume within a cylinder and increase air pressure. Single-acting reciprocating compressors only compress air on one side of the piston and are very low horsepower (25HP or less), and are generally used in the home or small automotive shops. Double-acting reciprocating compressors have chambers on both sides of the piston and are seen in sizes between 40-1,000 HP. Although more powerful than their single acting brethren, this type is rarely used due to the frequent upkeep needed and expensive manufacture process. Both types of this compressor are generally noisy and have low air quality, suiting non-sensitive environments.
Dynamic compressors pressurize air through the use of rotating impellers which accelerate and decelerate the air. The deceleration, or restriction, of the air is what creates the increase in pressure. Some of these compressors are completely oil-free for highly sensitive environments. Axial and centrifugal are both dynamic displacement compressors.
Axial compressors are not typically used in industrial settings and are traditionally seen in jet engines, high-speed ship engines, and small-scale power stations.
Centrifugal compressors convert energy efficiently utilizing a series of stages which compress and cool air as it continuously flows through the unit. Air is drawn into an impeller and accelerates as it moves outward. This kinetic energy is then converted into potential energy as the flow is slowed down by a diffuser. In each stage of compression, the air is cooled, and moisture is further removed to increase air efficiency and quality. This continuous flow through multiple stages allows centrifugal compressors to thrive in higher capacities and are best suited for applications above 250HP but can reach as high as 6,000HP in more demanding applications. Centrifugals also tout the benefit of being able to run continuously for years without significant maintenance.
A sought-after benefit of centrifugal compressors is their ability to supply oil-free air, referred to as Class 0 (per ISO 8573-1:2010). On the surface, the apparent advantages of oil-free compressors are reduced cost of consumables. Since oil is never injected into the air stream, filters after the compressor discharge are nonexistent. In addition, oil only needs to be changed every two-three years as opposed to 6-12 months for an oil-flooded compressor.
Benefits go beyond simply minimizing maintenance and energy costs though – oil-free air is essential to the manufacturing process of industries including pharmaceuticals, food products, electronics and textiles where the risk of trace oil reaching the finished product is unacceptable.
Each type of compressor has their own benefits and drawbacks, but kicking off your research with a solid understanding of the options available is vital in the decision-making process. We invite you to use the AirCompare℠ compressor costs calculator to further narrow down your options and discover potential savings you may be missing out on in your compressed air system.