Air Compressor Types and Controls
There are two basic types of air compressor:
Positive Displacement
In the positive displacement type, a specified quantity of air trap in a compression chamber. And the volume which it occupies is mechanically reduced, causing a corresponding rise in pressure prior to discharge. Rotary screws, vane, and reciprocating air compressors are the three most common types of air positive displacement air compressor found in small and medium size industries. air compressors
Dynamic
Dynamic air compressor include centrifugal and axial machines and are used in very large manufacturing facilities. These units are beyond the scope of this document.
a. Rotary Screw air Compressor
Rotary screw compressor have gained popularity and market share (compared to reciprocating compressors) since the 1980s. These units are most commonly used in sizes ranging from about 5 to 900 HP. The most common type of rotary compressor is the helical twin, screw compressor. Two mated rotors mesh together, trapping air, and reducing the volume of the air along the rotors. Depending on the air purity requirements, rotary screw compressors are available as lubricated or dry types.
Cross Section of a Representative Rotary Screw air Compressor
The biggest advantage of screw compressors over small air cooled reciprocating units is that they can run at full load continuously where the reciprocating compressors must be used at 60% duty cycle or below. Rotary screws are also a lot quieter and produce cooler air that is easier to dry. Be aware that rotary screw compressors may not be the most efficient choice compared to start/stop reciprocating compressors. Please refer to Case 3: On/Off vs. Load/No Load Control on page 101 for an example.
Lubricant Injected Rotary Screw.
The lubricant injects rotary screw compressor is the dominant type of industrial compressor for a perse set of applications. For lubricant inject rotary screw compressors, lubricants may a hydrocarbon composition or a synthetic product. Typically a mixture of compress air. And inject lubricant exits the air end. And is passed to a sump where the lubricant is removed from the compressed air.
Directional and speed changes are used to separate
quid. The remaining aerosols in the compress air then separate by means of a separator element within the sump. Resulting in a few parts per million (ppm) of lubricant carryover in the compressed air. With two stage compressors, interstage cooling and the reduced internal losses due to a lower pressure across each stage increase the compression efficiency. Consequently, less energy require to compress the air to the final pressure.
Dry Type Rotary Screw
In the dry type, the intermeshing rotors do not contact one another. And their relative clearances maintained to very close tolerances by means of external lubricated timing gears. Most designs use two stages of compression with an intercooler and an aftercooler. Lubricant free rotary screw compressors have a range from 25 to 1,200 HP or 90 to 5,200 cfm.
b. Reciprocating Compressors
Air compressors have a piston that driven through a crankshaft and by an electric motor. Reciprocating compressors for general purpose use are commercially obtainable in sizes from less than 1 HP to about 30 HP. Reciprocating compressors often used to supply air to building control and automation systems. Large reciprocating compressors still exist in the industry. But they now no longer commercially available. Except for use in specialize processes such as high pressure applications.
c. Vane Compressors
A rotary vane compressor uses an elliptical slotted rotor situated within a cylinder. The rotor has slots along its length, each slot contains a vane. The vanes force outwards by centrifugal force when the compressor is rotating, and the vanes move in and out of the slot. Because the rotor is eccentric to the casing. The vanes sweep the cylinder. Sucking air in on one side and ejecting it on the other. In general, vane compressors used for smaller applications where floor space is an issue; however, they are not as efficient as rotary screw compressors.
d. Compressor Motors
Electric motors widely use to provide the power to drive compressors. As a prime mover. The motor needs to supply sufficient power to start the compressor, accelerate it to full speed. And keep the unit operating under various design conditions. Most air compressors use standard, three phase induction motors. For new or replacement air compressors, premium high efficiency motor should specified over a standard ones. The incremental cost of the premium high efficiency motor usually recover quickly from the consequential energy savings. For additional information about energy efficient motors, please refer to the Electric Motors Energy.
e. Compressor Controls and System Performance
As air systems seldom operate at full load all of the time, the ability to efficiently control flow at part loads is essential. Consideration should placed to both compressors AND systems. Control selection as they are important factor affecting system performance and energy efficiency.
Various inpidual compressor control strategies exist including the following:
Start/Stop. This is the simplest. And most efficient control strategy. It can apply to either reciprocating or rotary screw compressors. Essentially, the motor driving the compressor turn on or off in response to the discharge pressure of the machine. For this strategy, a pressure switch provides the motor start/stop signal. Start/Stop strategies generally appropriate for compressors smaller than 30 horsepower in size. Repeated starts may cause the motor to overheat. And place greater maintenance demands on compressor components. For this reason. Care should take in sizing storage receivers. And maintaining wide working pressure bands to keep motor starts within allowable limits.
Rotary screw compressor
Load/Unload. This control mode sometimes called online/offline control. It keeps the motor running continuously, but unloads the compressor when the discharge pressure adequate. Unloaded rotary screw compressors typically consume 15-35% of their full load power demand, while producing no useful compressed air output. Optional unload timers available that will save energy by automatically turning off the compressor and placing it in standby if the unit runs unloaded for a period of time. Load/unload control strategies require significant control storage receiver capacity for efficient part load operation.
Modulating Control
This mode of control varies the compressor output to meet flow requirements by adjusting the inlet valve. Resulting in air restrictions to the compressor. Even fully modulated at zero flow rotary screw compressors typically consume about 70% of their full load power demand. The use of pressure switch activated unloading controls. It can reduce the unloaded power consumption to 15 to 35%. Modulating control unique lubricate screw compressors. And the least efficient way to operate these units. Compressor controls have a significant impact on energy consumption, especially at lower flows. Where start/stop controls generally the most energy efficient.
Variable Displacement
Some lubricated rotary screw compressors vary their output capacity using special capacity control valves, also called spiral, turn or poppet valves. With a variable displacement control scheme. The output pressure and compressor power consumption can closely control without having. To start/stop or load/unload the compressor. This control method has good efficiency at points above 60% loading. Use of pressure switch activated unloading controls at flows below 40% capacity can significantly reduce power consumption at lower flows.
Variable Speed Drive
This control method varies the speed of the compressor to respond to changes in air demand. Both lubricate and oil free screw compressors can purchase equip with variable drive controls.
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