In my last article, the different types of positive displacement air compressors have been discussed with the apt pictures. In this article, the roto-dynamic type of air compressors and other compressor classification types will be discussed.
A Centrifugal type of air compressor has a simple rotating element, mounted onto the shaft which is usually connected directly to the prime mover. These types of compressors are usually used for gas compression in oil platforms and LNG/LPG storage and transport installations. A typical example for the use of centrifugal air compressors are in turbochargers of diesel engines. The compressor in a whole has inducer, which sucks the air in, the impeller, which rotates at high speed, and the diffuser, which increases the pressure of the compressed air. The centrifugal compressors are best suited for constant load requirements. One of the remarkable characteristics of a centrifugal compressor is as the speed of the impeller reduces the capacity of the compressor increases. The air is sucked in axially by the impeller and the blades of the impeller impart additional energy which is converted into pressure energy and delivered out of the compressor casing. The centrifugal compressor may be of a single-stage or two and some times three-stage, which is more efficient than some screw and other positive displacement types.
Axial Flow Air Compressors:
These are similar to centrifugal compressors except the direction of air flow is axial. The blades of the compressor are mounted onto the hub and in turn onto the shaft. As the shaft rotates at a high speed, the ambient air is sucked into the compressor and then gets compressed (high speed of rotation of the blades impart energy to the air) and directed axially for further usage. An axial flow compressor, in its very simple form is called as axial flow fan, which is commonly used for domestic purposes. The pressure built depends on the number of stages. These are commonly used as vent fans in enclosed spaces, blower ducts, etc. One can find its main application in the aerospace industry, where the gas turbines drive the axial flow air compressors.
Types based on Number of Stages of Compression:
If the required delivery pressure is low, a single-stage of compression is sufficient to achieve that pressure. If the required pressure demand is say 10 bar and more, then it becomes more difficult for a single-stage compressor to deliver to match as per the demand. Thus a two stage compressor would be preferred and as the delivery pressure increases, the number of stages also increases. It may be a reciprocating compressor or a rotary centrifugal compressor, the number of stages will increase if the demand on the delivery pressure is more.
Types based on Cooling Method and Medium:
Air Compressors are cooled by either ambient air, water or sometimes with oil. Smaller capacity compressors are usually cooled by the atmospheric ambient air, which can sufficiently take the heat away from the cylinder units to achieve near isothermal conditions. If the number of stages increases or the compression pressure ratio increases, the amount of heat generated while compressing air is so high that ambient air cannot cool it sufficiently. Thus in this case, the water cooling method is adopted. Water is forced under pressure around the cylinder walls to extract considerable amount of heat to achieve near isothermal conditions. Some of the special purpose compressors where there is no provision for water cooling; Oil is used for cooling from the in-built sump.
Types based on Drives/Driving Methods/Prime Movers:
Air compressors always have the option of choosing from a wide range of prime movers. Prime movers which are generally used are electric motors, diesel engines, and sometimes turbine with reduction gearing and clutch arrangements. However turbine driven is unusual to find as it involves the complexity of the design. Electric motor driven is the most commonly used prime mover, as it can be well suited for automatic starting and stopping, not to forget the unloading and loading sequence.
Irrespective of the prime movers, the compressors are also classified based on the type of movement transmitters. They are belt drives, gear trains, direct coupled and clutches. Depending upon the need, the compressors are suited for the above options. However, air compressors are usually found with direct coupled with the prime mover. Belt drives are popular with portable, small air compressors.
Types based on Lubrication method:
Smaller compressors usually have splash type of lubrication method. As the compressor size increases, the splash lubrication becomes ineffective and thus requires a separate lubricating oil pump driven by the compressor itself. This lubricating oil pump, takes suction from the lubricating oil sump inside the crankcase, through a suction strainer, and supply oil under pressure to various parts of the compressor through drilled holes. This type of lubrication is called as Forced lubrication. Also there are compressors which do not use any oil for its lubrication. These compressors are used for special purpose for filling air in breathing apparatus etc.
Thus these are the various types of compressors that can be found anywhere in any industry. Other minor classifications are also there which low, medium, and high pressure compressor. In my future articles, let us discuss in detail regarding each type of compressor and its features.