Dynamic Displacement Compressors
Rather than physically reducing the volume of a captured pocket of air, dynamic displacement compressors instead speed up the air to high velocity, and then restrict the air flow so that the reduction in velocity causes pressure to increase. They are oil-free by nature, and some are oil-less. Dynamic compressors include axial and centrifugal types.
Axial compressors use a series of turbine blades, similar in appearance to a jet engine, to force air into a smaller and smaller area. These compressors are not commonly used in industry.
Centrifugal compressors draw in air to the center of an impeller, and then accelerate it outward toward its perimeter. There it impinges upon a diffuser plate and outlet scroll, where velocity decreases and pressure increases. Typical centrifugal compressors used for manufacturing are water-cooled and use two or three stages of compression. The stages are driven at 50,000 to 75,000 RPM by a lubricated bull gear and pinion gears.
Full load efficiencies are good overall (competitive with lubricated compressors) and very good in comparison to other oil-free compressor designs. However, performance can change substantially with inlet conditions. Heat, low barometric pressure, and high humidity have an effect on all compressors, but more so on centrifugal machines. Part load is achieved through throttling of the inlet down to 70 -80% of capacity, at which point the machine either operates using load/unload or blowoff control. It should be noted that due to the fundamentally different operation of the centrifugal compression process, throttling the inlet provides good efficiency, unlike inlet modulation of a positive displacement compressor.
Inlet throttling turndown is limited by the surge characteristics of the compressor. Since the centrifugal compressor has no valves, seals, or piston rings to separate one stage from the next, a certain amount of flow must be maintained through the compressor. If flow is reduced too much by the inlet throttling, then the incoming air is not sufficient to overcome the pressure at the compressor discharge, and air reverses flow backwards through the compressor. This condition is called surge, and may damage the compressor and/or cause nuisance shutdowns. The surge line and aerodynamic limit form the operating boundaries for the compressor, as shown in the following diagram.
At a given pressure, flow can only be reduced to the surge line. Further reduction will cause surge and unstable operation. To allow a centrifugal compressor to meet the full range, two primary control methods may be used: blowoff or load / unload. In blowoff control, the inlet throttles the compressor almost to the surge line. If demand drops lower, a valve is opened to vent air from the compressor discharge to atmosphere, controlling pressure. This is quite inefficient at low load, as air is being wasted that has already been compressed. Alternatively, the compressor can unload when demand drops below the turndown range. When the compressor unloads, it vents the discharge to atmosphere, and closes the inlet valve. Air system pressure is maintained by a check valve between the compressor and system. Load/unload cotrol is typically more efficient, but requires storage volume.
Day to day maintenance is minimal on centrifugal compressors, which may operate continuously for six to twelve months without significant maintenance. Every 4 to 10 years (depending upon manufacturer) an overhaul or “turnaround” service is recommended. This is generally quite expensive and time consuming, requiring teardown, cleaning of impellers and heat exchangers, changing of seals, etc..