Positive Displacement Compressors
Positive displacement compressors draw in and capture a volume of air in a chamber. They then reduce the volume of the chamber to compress the air. Reciprocating Piston Compressors, Rotary Screw Compressors, Rotary Vane Compressors, and Scroll Compressors are all positive displacement compressors.
Reciprocating Piston Compressors
There are two main divisions of Piston compressors: single acting and double acting.
Single Acting means that air is drawn in and compressed on one side of the piston. The other side is exposed to the crankcase of the compressor. In this case, the downward stroke of the piston draws in the air, and the upward stroke compresses it.
Single acting machines may have one, two or more stages (see below). They are typically air-cooled and sized 25 Horsepower or below. Home, hobbyist, and automotive service compressors typically fall in this category, though sometimes light industrialists can use as well.
These small, air-cooled reciprocating compressors are limited in duty cycle to approximately 50%. They should not be run more than 30 minutes per hour. This gives the pump time to cool off. Without this break, the pump will run too hot, causing excessive wear and short life. Efficiencies are typically low to moderate at full load, but part load efficiency is typically good because the machines use start/stop control. A pressure switch starts the compressor when pressure drops to the cut-in setpoint. The compressor pumps at full capacity until reaching the cut-out pressure and shuting off the compressor. This typically requires about a 30 PSI swing in pressure to prevent the motor from starting and stopping too often.
These machines are typically inexpensive to purchase and maintain. However, their performance gradually degrades over time, contain high noise levels, and possess low air quality. With high levels of oil (about 50ppm), and high discharge temperatures, these machines may cause moisture to travel down line.
Double Acting reciprocating compressors have compression chambers on both sides of the piston. On the down stroke, air is drawn in on the top of the piston while air is compressed on the bottom side. On the upstroke, air is drawn into the bottom side while air is compressed on the top side. Double acting machines require sealing of the piston rod, so they use a crosshead to eliminate the angular movement of the rod.
Double acting reciprocating compressors may have one or more stages and are typically water-cooled. Sizes range from around 40 Horsepower to over 1000 Horsepower. The continuous flow of water through the cylinders and heads provides better cooling to these machines than their air-cooled counterparts. Therefore, they can operate fully loaded for long periods (100% duty cycle). Full load efficiency is typically excellent on these machines, as is part load (particularly when three- or five-step unloading is used). In machines with multi-step unloading, multiple valves per cylinder exist. The valves are ported such that controlling certain valves varies the amount of the cylinder where compression is occurring. Typical multi-step unloading schemes allow the machine to operate at 0, 50%, and 100% load, or 0, 25%, 50%, 75%, and 100% load. Double acting piston compressors without multi step unloading typically operate in the load/unload control scheme, without shutting off. Pressure controls on these machines typically operate over about a 10 PSIG band.
Very few manufacturers still produce the double acting reciprocating compressor because it is quite expensive to produce, requires special foundations to handle vibration, and requires frequent extensive maintenance.
Rotary Screw Compressors
Rotary screw compressors are presently the industry standard in plant air compressors from about 25 to 300 horsepower. They are rapidly expanding into both smaller and larger markets. Many manufacturers offer size ranges as low as 3HP and as high as over 600HP.
Rotary screw compressors draw air and lubricant into a void created as two helical rotors mesh together. Once the rotors pass by the inlet port of the pump (called an airend), the cavity decreases in size for the remainder of the rotation, compressing the air-oil mixture.
The oil introduced into the airend along with the air leads to the terminology of “oil flooded screw compressor” or “oil injected screw compressor”. This oil performs numerous functions in a rotary screw compressor, including lubrication, sealing of the air pockets, collection of contaminants, and absorption of heat. The compressor oil (called ”coolant” by some manufacturers) has a much higher specific heat than the air, allowing it to soak up about 80% of the heat of compression. This leads to far lower operating temperatures than reciprocating and centrifugal compressors. The lower temperature, in turn, allows the oil flooded rotary screw compressor to operate at 100% duty cycle without adverse effects.
The oil, however, must be removed from the air before it leaves the compressor. These compressor types require a large coalescing oil removal filter, often known as the oil separator cartridge. This regular maintenance item must be replaced periodically, or it will suffer from high pressure drop, or excessive oil passage. When operating properly, oil carryover from the rotary screw compressor is from 2 to 5 ppm.
Early rotary screw compressors exhibited poor efficiency in comparison to the double acting piston compressor. However, modern rotary screw compressors typically have very good full load efficiency. Part load efficiency, however, is greatly dependent upon control method. Many rotary screw compressor control methods are available, as discussed in the controls section.
Typical rotary screw compressors come completely packaged, with all capacity and motor controls, oil and air coolers, and safety devices pre-installed. Most new machines (outside the smallest units) include microprocessor controllers.
Rotary Vane Compressors
Rotary sliding vane compressors operate similar to an air motor, with an off-center rotor turning sliding vanes. As the vanes near the area where the distance between the rotor and casing is small, the air compresses.
Other than the geometry, rotary vane compressors are very similar to rotary screw compressors. As oil injected machine, they also require the same separators and oil system components. Generally, separators of a more marginally size, lead to increased oil carryover on vane compressors than on similar piston compressors.
Full load efficiency of vane compressors is typically moderate, with part load performance highly affected by control scheme. Control methods for vane compressors are largely the same as rotary screw compressors, with the exception of variable displacement. Vane compressors do not use it.
Oil-Free and Oil-Less Compressors
Most positive displacement compressor types are available in oil-free or oil-less designs, where no lubricant is injected into the air. Oil-less typically means that the machine contains no oil, whatsoever, while oil-free refers to designs that have a lubricated crank case or gearbox that is isolated from the compression chamber. Oil-free and oil-less machines typically have 10 to 20% lower efficiency and require more maintenance than the lubricated counterparts.
Both oil-free or oil-less single- or double-acting piston compressors use Teflon or similar piston rings which require frequent maintenance, but are otherwise similar to lubricated counterparts.
Oil-free screw compressors can be dry or water-flooded types. Dry running screw compressors operate at very high speed (often 20,000 RPM or more). They use a lubricated gearbox with a bull gear and high speed pinion gears to run the two airends. Sometimes called stages or elements, depending upon the manufacturer. Each stage also includes lubricated bearings and timing gears to prevent the rotors from touching. PTFE or similar product coats the rotor and creates a seal between them. The coating is typically a wear item, requiring replacement of the stage every 5 to 10 years. Control methods for these machines are limited to load/unload and variable speed, though turndown range on variable speed models is often poor.
Water flooded oil-free compressors are typically single stage, and use water to seal the compression chamber and to absorb heat. However, bacteria growth can be a significant issue. Most machines now incorporate a reverse osmosis water filtration system and change the water frequently. Control methods are similar to oil-flooded screw compressors.