These controls for rotary-screw compressors all match output to demand by modifying or controlling the effective length of the rotor compression volume. The inlet pressure remains the same throughout the turn down, and the compression ratio stays relatively stable. This method of reducing flow without increasing compression ratios has a power advantage over modulating and/or 2-step controls in the operating range from 50% to full load.

The two most common of these unloading controls are the spiral-cut high lead valve and the poppet valve. Both methods open or close selected ports in the compressor cylinder, thus changing the seal-off points. These ports are located at the start of the compression cycle where pressure is very low. Opening them even a small amount prevents compression from occurring until the rotor tip passes the cylinder bore casing that separates the ports. This effectively reduces the trapped volume of air to be compressed and consequently the horsepower needed to compress it.

Pro: very efficient part-load performance from 50% to 100%.
Pro: maintains set pressure at minimum system pressure. Pro: very responsive.
Con: at higher loads, some units lose efficiency due to increased leakage.
Con: the mechanism is complex.
Con: still must run 2-step or modulation in lower operating range.

Variable-speed drives

Variable-speed drives (VSDs) control the speed of the prime mover. In theory, the performance unloading curve for compressors powered by VSDs is very attractive. Depending on the type of compressor, model, conditions, etc., unloading can be almost optimal in the range from 50% or 60% to 90% of load - i.e.: 75% power could produce close to 75% flow. Variable-speed turbines and engines have proved effective for years on all types of compressors. These drives maintain system pressure at the minimum set point and will modulate back as soon as the sensed system pressure raises.

In the world of electric motors, the most commonly applied VSD has been the variable-frequency driver (VFD) - usually as a retrofit or part of a special package. VFDs convert 60-Hz alternating current to direct current, and then reconvert it to AC at the frequency required to turn the motor at the desired speed. This conversion usually consumes about 2% to 4% more energy, and therefore VFDs are less efficient at full load than other types of controls.

Many VFDs have been installed successfully on lubricant-cooled rotary-screw compressor packages over the years, but there are some areas of concern that have limited their economies relative to cost and overall performance - particularly in retrofits. First, the design of some rotary-screw compressors causes efficiency to drop at less than full-load speed. Second, changing speeds can produce harmonic amplification problems that were not considered at the original design speed. Third, the motor itself may have efficiency problems at the low end of the speed range, possibly because of inadequate heat rejection and cooling capacity. Compressors with air ends designed specifically for VFDs will eliminate or minimize many of these potential problems.

Switched-reluctance VSDs

Another type of VSD being offered is the switched-reluctance system. This electrical control converts standard 3-phase AC power into 2-phase DC. The rectified AC voltage is passed to a bank of capacitors where it is increased to 600-V DC and stored. The bank then supplies the power required by each phase of a brushless motor, eliminating surge currents in the main power supply. The brushless motor has the inherent ability to survive an unlimited number of starts and stops per hour because the absence of inrush current surges keeps its operating temperature low.

The true application for any compressor with a VSD should be as a trim machine, not as the plant air system's base-load unit.

Where to put it

Industrial air compressors are rugged machines that will perform under adverse conditions, but it always is advisable to provide proper operating conditions to maximize reliability at minimum operating cost. Traditionally, compressors have been located in separate rooms to isolate their noise. Such locations are almost mandatory today to meet OSHA requirements. However, it still is important that the compressor room have an adequate foundation (particularly for reciprocating machines) as well as ample space so that the machine is easily accessible for inspection and maintenance. Stairways and catwalks can assist these procedures on larger compressors.

The compressor room ideally should be clean and dry. Auxiliary equipment, piping, and wiring should be arranged so that it does not interfere with routine inspections. Instruments should be located within easy view of operators.



Partial summary of air compressor selection factors - 100 psig service
Type Capacity in scfm Horsepower Cooling medium Lubrication
Reciprocating <1 to 3,018 <1 to 600 <100 hp - Air
>75 hp - Water
For some models
Single-stage,
lubricated rotary
14 to 3,000 5 to 700 Air or water Yes
Two-stage,
lubricated rotary
560 to 3,100 100 to 600 Air or water Yes
Dry rotary 75 to 4,200 40 to 900 Air or water No
Centrifugal 400 to 25,000 125 to 6,000 Water only No


Applicability of air compressor unloading controls
Type of
control
Lubricant-cooled
rotary screw
Oil-free
rotary screw
Reciprocating
(single-acting)
Reciprocating
(double-acting)
Centrifugal
Automatic start-stop Yes Yes Yes Yes Yes
Two step
Three and five step
Yes
No
Yes
No
Yes
No
Yes
Yes
Yes (dual)
No
Throttled inlet
Variable displacement
Yes
Yes
No
No
No
No
No
No
Yes
N/A
Variable speed Yes No No No No

This information was contributed by Hank van Ormer, president of Air Power USA, Pickerington, Ohio.

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