Positive Displacement Compressors
Positive displacement compressors draw in and capture a volume of air in a chamber, 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
Piston compressors are further divided into two main categories – 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), and are typically air-cooled and sized 25 Horsepower or below. Home, hobbyist, and automotive service compressors typically fall in this category, though some are used for light industrial use as well.
These small, air-cooled reciprocating compressors are limited in duty cycle to approximately 50%. This means they should not be run more than about 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. This means that a pressure switch starts the compressor when pressure drops to the cut-in setpoint, and the compressor pumps at full capacity until the cut-out pressure is reached, shutting 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, but performance gradually degrades over time, noise levels are high, and air quality is typically low, with high levels of oil (about 50ppm) and often high discharge temperatures that cause moisture to travel down line to the end users.
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 a crosshead is used 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, allowing them to 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, there are multiple valves per cylinder, and 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, with many manufacturers offering size ranges as low as 3HP on the low end, and over 600HP on the high end.
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. This requires a large coalescing oil removal filter to be included in the compressor package. This is commonly referred to as the oil separator cartridge. It is a regular maintenance item that 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, but 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 are completely packaged, with all capacity and motor controls, oil and air coolers, and safety devices 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 is compressed.
Other than the geometry, rotary vane compressors are very similar to rotary screw compressors. They are also oil injected, and require the same separators and oil system components. Separators are generally more marginally sized on vane compressors, leading to increased oil carryover similar to a piston compressor.
Full load efficiency of vane compressors is typically moderate, with part load performance highly affected by control scheme. Control methods for vane compressors are the same as rotary screw compressors, with the exception that variable displacement is not used in vane compressors.
Positive Displacement Oil-Free Compressors 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), and are typically two stage machines for most applications. They use a lubricated gearbox with a bull gear and high speed pinion gears to run the two airends (called stages or elements, depending upon the manufacturer). Each stage also includes lubricated bearings and timing gears to prevent the rotors from touching. Rotors are coated with PTFE or similar coatings to seal between them, but 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, so most machines now incorporate a reverse osmosis water filtration system and change the water frequently. Control methods are similar to oil-flooded screw compressors.