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Reciprocating Compressors
Reciprocating compressors were the first of the modern air air compressor designs. Reciprocating compressors utilize a piston moving within a cylinder to compress low-pressure air to high pressure. They are available in single-acting and double-acting configurations. Each of these configurations utilizes a variety of cylinder layouts.
This style of reciprocating air compressor utilizes automotive type pistons, connecting rods and crankshaft (fig. RC-2.1). All compression takes place on the top of the piston. Single-acting air compressors are available in either single-stage or multi-stage compression and can either be air-cooled or liquid (water)-cooled. Single-stage and two-stage are the most common reciprocating compressors utilized in 100 psig (7 bar) plant air applications. Typical pressure, flow and horsepower ratings are shown in table RC-2.2.
| Single-Stage | Two-Stage |
| Discharge pressure, psig (bar) |
25 to 125 (1.7 to 8.6) |
100 to 250 (7 to 17.2) |
| Flow rating, cfm (m3/min) |
4 to 20 (0.11 to 0.57) |
8 to 105 (0.23 to 2.97) |
| Horsepower, hp (kW) |
1 to 5 (0.75 to 3.73) |
1.5 to 30 (1 to 22) |
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| Table RC-2.2 |
Lubrication of single-acting compressors is accomplished with either a positive pressure oil pump or splash rod. Compressors with oil pumps are rated for continuous duty while splash lubricated units are suitable for intermittent duty operation.
Single-acting air compressors are available in lubricated, non-lubricated and oil-less designs. Non-lubricated cylinder designs utilize PTFE style rider and wear rings on the pistons. Oil-less designs, utilizing sealed-for-life bearings, are frequently applied on applications that cannot tolerate air compressor lubricants in the process or product.
Single-acting air compressors are most frequently purchased as either a base-mounted or tank-mounted compressed air package. The package (fig. RC-2.3) normally consists of the compressor, drive motor or engine, control pressure switch, air receiver tank (tank-mounted units only) and tank drain valve. A belt guard style aftercooler may also be included.
Typical applications for single-acting air compressors can be seen in figure PD-2.1.
This style of reciprocating air compressor utilizes a double-acting piston (compression takes place on both sides of the piston), piston rod, crosshead, connecting rod and crankshaft (fig. RC2.4). Double-acting compressors are available in single and multi-cylinder and single and multi-stage configurations. Single-cylinder, single-stage and double-cylinder, two-stage configurations are the most common styles found in industrial applications. Most double-acting air compressors are available either water-cooled or air-cooled. Typical pressure, flow and horsepower ratings are shown in table RC-2.5.
| Single-Stage | Two-Stage |
| Discharge pressure, psig (bar) |
25 to 125 (1.7 to 8.6) |
100 to 250 (7 to 17.2) |
| Flow rating, cfm (m3/min) |
100 to 750 (2.8 to 21.2) |
450 to 4500 (12.7 to 127.4) |
| Horsepower, hp (kW) |
10 to 125 (7.5 to 93) |
75 to 1000 (55 to 746) |
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| Table RC-2.5 |
Lubrication of double-acting compressors is accomplished with a positive pressure oil pump. Compressors are normally rated for continuous duty operation.
Double-acting compressors are available in lubricated and non-lubricated designs. Non-lubricated cylinder designs utilize PTFE style rider and wear rings on the pistons. Distance pieces are normally added between the cylinder and crosshead to prevent oil migration into the oil-free cylinder.
Double-acting air compressors can be purchased as a packaged air system, however larger horsepower units are usually field installed on a concrete foundation. Although double-acting compressors can still be purchased, oil-injected rotary screw compressors have effectively become the standard choice for 100 psig plant compressed air systems.
Typical applications for double-acting air air compressors can be seen in figure PD-2.1.
Reciprocating compressors, being positive displacement, operate as constant-capacity and variable pressure machines. Since most air air compressors are oversized for actual system air flow requirements, the unit will continue to increase the air system pressure until the safety valve(s) begin to vent the excess air compressor capacity. Obviously this is not a desirable operating condition. Capacity controls are therefore used to regulate the compressors output to match air system consumption. Capacity control systems also contribute to efficient air system operation since the power required operating the compressor falls when the compressor's capacity is reduced. The amount of power reduction depends on the type of control system.
A variety of methods are used to regulate the capacity of reciprocating air compressors.
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| Figure CC-1.1 Pressure Switch with integral unloader valve |
Start - Stop Control - a pressure switch (figure CC-1.1), installed on the outlet of the compressor or on the air receiver, senses the air system pressure. As the compressed air pressure increases, the pressure switch will open when the air pressure reaches the "stop" or "off" setting. The compressor motor then shuts off. When the air pressure falls in the air system, the pressure switch will close at the "start" or "on" setting. The compressor then restarts. This is the most efficient air compressor control system since the compressor does not consume power when it is not running. Although efficient, air systems utilizing start/stop controls, must be designed to limit the number of compressor motor start/stop cycles per hour. As a rule, most NEMA rated motors must not be started more than six (6) times per hour. Utilizing a wide pressure band between the start and stop settings combined with adequate air receiver storage capacity can prevent frequent cycling.
Start - Stop control systems are frequently used on small reciprocating compressors that run on a very low duty cycle.
Constant Speed Control - This style of control incorporates a method of unloading the air compressor without stopping the compressor motor. Unloading devices are typically fitted to the inlet (suction) valves of the compressor. When the air system pressure reaches the desired setting, pneumatic (or hydraulic) pressure is applied to the unloaders. The unloaders hold the suction valves open thereby preventing the air compressor from compressing the air within the cylinders. Operation of the unloaders is done with a pneumatic valve or with an electro pneumatic (pressure switch with solenoid valves) system.
Constant speed control will provide two (2) step regulation of a single-acting or double-acting compressor. Three (3) step control is frequently used on double-acting compressors. Two (2) step regulation will allow the compressor to operate at either 100% or 0% capacity. Three (3) step regulation will provide 100%, 50% and 0% capacity.
Constant speed control will also reduce the operating power requirements of the air compressor, however power is consumed to overcome mechanical losses associated with running the compressor.
The amount of time the compressor runs at 0% capacity or unloaded must be minimized since the air contained within the cylinder is being heated by its' movement through the inlet valves. Excessive unloaded run time, generally over 30 minutes, can lead to compressor overheating.
Both start/stop and dual control should include a means of unloading the compressor when starting. Starting air compressors fully loaded can overload or overheat most standard NEMA rated electric motors.
Dual Control - combines both start/stop and constant speed control into a single control system. Manual dual control allows the compressor user to select either control method depending on actual operating conditions. Auto dual control automatically selects the most desirable control method. Auto dual control systems normally run the air compressor in constant speed control. When the compressor unloads, an unloaded run timer energizes. The unloaded run timer normally has a time range of 5 to 60 minutes. If the compressor does not re-load, the timer will shut the compressor off. The compressor will restart and reload when the pressure switch senses low pressure.
Modern, microprocessor based controls are available to provide auto dual compressor control, monitor critical compressor operating parameters, and provide annunciation and display of alarm conditions.
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