James R. Stringer
Though many new modern buildings are turning to electronic controls, many buildings and process plants use pneumatic controls requiring an air compressor. Air compressors not only provide air for the pneumatic control system but also provide the added benefit of a compressed air connection that can be used for tools, cleaning, and equipment. I also believe that pneumatic control systems are easier to diagnose, repair, and seem to have less planned obsolescence (jury is still out on those new screw compressor stations!).
Compression Ratio is the volume at the start of a compression stroke to the volume at the end of the stroke (low pressure to high pressure).
Capacity is the quantity of air compressed and delivered in a time period expressed as m3/min of free air. Free air is also known as free air delivered (FAD) or free air capacity and is at atmospheric pressure. In other words, capacity is the volume of atmospheric air compressed in one minute.
Staging is compression carried out in more than one stage. High compression in a single cylinder results in low volumetric efficiency and a high discharge pressure. With staging, each cylinder in stages discharge into the next high pressure cylinder. This means a more moderate compression ratio in each cylinder and an improved volumetric efficiency.
Compressor Displacement is the volume swept out by the piston; it is expressed in m3/min. If there is more than one stage, the displacement is the volume swept out by the first stage piston only as the same air passes through all stages and no new air is added.
Intercoolers cool compressed air between compressor stages. Intercoolers increase the density of air and improve efficiency. It also causes compression to reach isothermal (constant temperature) instead of adiabatic (change in temperature) compression. Intercooling reduces power required for compression and increases capacity, while also potentially causing moisture to condense, helping to prevent free moisture from entering the next stage.
Aftercoolers cool compressed air before it is delivered to the air receiver or the point of use. It removes moisture and oil vapour from the air stream; oil vapour must be removed or it will have detrimental effects on air instruments or can accumulate and cause an explosion if ignited.
Note: A compressor installed at a higher altitude must have a higher compression ratio to produce the same discharge pressure as it would if it was at sea level. The compressor will deliver less mass of air the higher the elevation above sea level.
Reciprocating Compressors are suitable for all pressure ranges and compression is caused from pistons moving in a reciprocating manner, back and forth. Reciprocating compressors are positive displacement.
Single Acting compression takes place at one end only of a piston. One compression stroke for each crankshaft rotation; compression occurs on the outward stroke.
Double Acting compression occurs on both sides of the piston. One revolution of the crankshaft equals two compression strokes.
Single Stage Compressors may have one or more cylinders and all the cylinders will be of the same size. Multiple cylinders increase capacity and help smooth discharge pressure.
Two Stage Compressors have two or more cylinders with the first stage having a larger cylinder diameter than the second stage cylinder. Air is compressed in the low pressure cylinder and then further compressed in the high pressure cylinder to the final discharge pressure. Cylinders are either single or double acting.
Tandem Compressors have cylinders in pairs connected to a common crankshaft; movement of pistons oppose each other. The load is able to be divided evenly between outward and return strokes providing smoother operation.
Inlet Air Unloaders keep the inlet valve open, preventing the compression of air in reciprocating cylinders and allowing it to pass freely through the inlet. The unloader allows a reciprocating compressor to start without a load.
Rotary Compressors provide energy to a fluid being compressed through an input shaft of rotating elements. They do not have inlet or discharge valves and are considered positive displacement.
Rotary Sliding Vane Compressors have a cylindrical rotor in which flat sliding vanes fit into radial slots and may be water jacketed. The rotor has supportive bearings that allow it to be eccentric to the casing. Pockets of gas are trapped between the vanes as the vanes are forced outward against the casing. The pockets decrease in volume and compress the gas. Most often there are two stages in this type of compressor.
Rotary Lobe Compressors have two rotors with a figure 8 cross section revolving in opposite directions. Rotors intermesh so no air can pass between them. Timing gears maintain the rotor position and casing lubrication not needed as the rotors do not touch. These compressors are compact and provide an even flow of oil free air.
Rotary Screw Compressors have two intermeshing rotors machined into an axial screw. The rotors do not contact and thus internal lubrication is not required. The convex rotor drives a concave rotor and provides a pulse free delivery. These compressors are compact, vibration free, smooth flow, and are oil free air. They can have a large capacity at low pressure.
Centrifugal Compressors accelerate the velocity of the gas which then increases the kinetic energy of the air and converts it to pressure. The impeller rotates at a very high speed with backward/radial blades and has a volute casing potentially with diffuser vanes. The straighter the impeller blades coincides with less pressure converted in the impeller. Air passes through the diffuser vanes of the casing and some of the kinetic energy is converted to pressure. Air further passes through the casing and more of it is converted to pressure energy. Centrifugal compressors provide large volumes of oil free air, have simple rugged construction, and have low maintenance requirements BUT lower efficiencies than positive displacement and unsuitable for low capacity work. Centrifugal compressors are considered dynamic displacement.
Power Engineering Third Class Edition 2.5; Part B2; Chapter 8: Compressor Theory and Designs