Drive method is probably the simplest method of differentiating compressors.
The compressor, whether reciprocating or rotary, can be v-belt driven, direct coupled, or either of the above accompanied by a variable frequency drive. The simple v-belt drive is most commonly seen on the air cooled reciprocating compressor, and smaller rotary screw compressors. V-belt drives provide for versatility in selecting optimum speeds for the desired flow and pressure. Direct drive (coupled) compressors require that the air end be constructed to maximize the flow (CFM) for a given horsepower motor, or use internal gears to adjust the speed of the compressor to achieve this maximum flow.
Variable speed (frequency) drives adjust the speed for optimum flow and pressure by adjusting the frequency of the incoming power to the compressor.
The final method of compressor differentiation is by duty cycle. This determines the amount of time the compressor can safely run without shutting down for cooling.
Small air-cooled reciprocating air compressors generally are allowed to run 75% of the time and be idle for 25% of the time. This allows the components to cool between cycles. Rotary compressors and water cooled reciprocating compressors can run continuously 24/7, shutting down only for periodic maintenance.
Centrifugal compressors are also designed to run continuously 24/7.
Finally, cooling method be a determinant of compressor type. This is simple, as there are only two. Cooling can be either air-cooled or water-cooled. Rotary screw compressors, if air cooled, use a radiator-style cooler (heat exchanger) for both oil cooling and after-cooling. If water cooled, shell and tube type heat exchangers are typically used. Plate-fin coolers can also be used.
Air-cooled reciprocating air compressors will typically use a finned tube intercooler and the same for after-cooler. Some will use a radiator type for after cooler. Water cooled reciprocating compressors will typically use shell and tube heat exchangers for both intercooling and for the after-cooler.
Centrifugal compressors will generally be water cooled, and use shell and tube type coolers.
“I want a quote on an air compressor”. This is how the conversation and subsequent negotiation begins. Our first response is “what size?” This refers not only to the CFM required, but also to the pressure required. Once these points are determined, the next question is “what type compressor?”
Compressor manufacturers offer a range of compressor types, each with pros and cons, depending on the customer’s application, specifications, and budget restraints. The compressors are broken down into various categories defined by: 1) compression method, 2) lubrication, 3) drive, 4) duty cycle, and 5) cooling method.
Compression method refers to the way the air is compressed. The earliest devised type is reciprocating or piston-type. This incorporates a piston sliding up and down within a cylinder compressing the air within the cylinder, and exhausting out the discharge valve. These reciprocating compressors can be air cooled or water cooled, single stage, or multiple stage, single acting or double acting. They can range from fractional horsepower up to several thousand horsepower.
The second type is rotary screw. This method uses helical screw rotors, male and female, in a continuous rotary motion, compressing the air within the housing. Most commonly, screw compressors are lubricated with oil flooding the rotors for sealing, cooling, and lubrication of the rotors, bearings, and gears (if used). The rotors are not timed by gears, but they are separated by a film of lubricating oil. These are used in heavier manufacturing where continuous duty is required. Rotary screw compressors are designed to run continuously 24/7 with shut down only for maintenance.
A third type of compression is by rotary vane. This method utilizes a series of vanes or blades arranged on a cylindrical rotor, rotating in an eccentric cylinder or housing. As the rotor rotates, the blades slide in and out of slots, allowing the volume between the vanes to be compressed. These units are also used in manufacturing facilities and are designed for continuous duty.
The fourth major classification by compression method is centrifugal. These compressors use an impeller with blades, curved to accentuate the air flow within a volute casing. They may be single or multiple stage depending on pressure requirement, and are typically oil-free. They are usually larger capacity and can be provided up to 10,000+ HP.
There are other compression methods available (diaphragm, rotary piston, etc.) but they are specialty/niche compressors.
More discussion on compressor types will follow in coming articles.