Every compressed air system begins with a compressor — the source of air flow for all the downstream equipment and processes. The main parameters of any air compressor are capacity, pressure, horsepower, and duty cycle. It is important to remember that capacity does the work; pressure affects the rate at which work is done. Adjusting an air compressor's discharge pressure does not change the compressor's capacity — even though many people seem to believe it will.
There are a number of basic air compressor designs — and variations of them — on the market today. They all fall into two general categories: positive displacement and dynamic. Although the operating specifications for two different types of air compressors may be very similar on the surface, other installation and performance factors can make one design superior to the other in a real-world application. Let's review some of the basic designs and terminology.
Power and efficiency
Motor or engine horsepower is the nominal rating of the prime mover. Service factor is the additional power built into an electric motor above its nominal rating — expressed as percent. Within the service factor, the brake horsepower driving an air compressor can be higher than the motor’s nominal horsepower.
The power efficiency of a compressor is the ratio of the air delivered by the compressor and its input electrical requirements. Efficiency usually is expressed as brake horsepower per 100 cfm of delivered air.
Reciprocating compressors are positive-displacement units that trap a charge of air and then physically reduce the space that confines it, causing its pressure to increase. Reciprocating units, often called piston compressors, use a piston, cylinder, and valve arrangement. Their operation is very similar to the familiar internal-combustion engine, but they simply trap and compress the air without adding fuel to explode it. Note that whenever air is compressed, heat is generated. Proper cooling of the internal parts of any air compressor is a critical part of its design.
There are three basic selection decisions that must be made about reciprocating compressors:
• single- or double-acting operation,
• single- or multi-stage configuration, and
• air or water cooling.
In a single-acting piston compressor, the piston only compresses air in one direction of its stroke. In a double-acting model, the piston compresses air with both directions of its stroke. Obviously, because both strokes perform work, a double-acting compressor is more efficient (in moving a volume of air per input hp) than a comparable-size single-acting unit.
A single-stage unit compresses air from inlet to discharge pressure in one operation. A multi-stage unit compresses from inlet to discharge pressure in two or more operations — generally passing the air through an intercooler to remove some of the heat of compression between each stage. This saves power and keeps the compressor's internal operating temperatures lower.
In air-cooled compressors, ambient air circulates around the compressor cylinders and finned heads to provide cooling. Heat transfers through the metal to the air. Air-cooled units are generally designed for 50 to 75% duty cycles, depending on the particular units and their application. In water-cooled compressors, integral water jackets surround the cylinders and heads. Heat transfers through the metal to the water — more effectively than through metal to air. Thus, water-cooled reciprocating units reduce internal temperatures more efficiently than comparable air-cooled units.
Most air-compressor manufacturers promote the two-stage compressor as the optimum machine for producing 100-psi class air — the base pressure level in most industrial plants — providing the best efficiency per dollar cost with adequate reliability of internal working parts. For a reciprocating compressor to be categorized as continuous duty, it is generally agreed that it must be double acting and water cooled. Double-acting, water-cooled reciprocating compressors are offered in a variety of styles that combine efficient air compression with durability and reliability. However, they also are heavy and bulky, making them relatively expensive to install. They generally have more-significant unbalanced forces, which combines with their size to require a special foundation and support.
When they meet selection criteria such as capacity, weight, size, and price, single- and two-stage single-acting reciprocating units are a good choice — particularly in the 50- to 150-psig pressure ranges. (Three-stage reciprocating units are offered, but generally are used for pressures above 250 psig.)
Oil-cooled rotary-screw types
The rotary-screw compressor is another positive-displacement machine. In an analogy with the reciprocating compressor, Figure 1, the male rotor is like a piston, pushing air along the female rotor, which is like the cylinder. The sealing strips are like piston rings, and air is compressed against the stationary end plate, which is like the bottom of the cylinder. This design has been around for about 50 years. However, until the mid 1970s, it was considered suitable only for engine-driven portables and small-horsepower electric-motor units because of low efficiency (the ratio of compressed-air delivery to power cost).
In the 1970s, development began on two-stage rotary screw compressors for pressures up to 250 psi. Rotor-profile development during the 1970s, 1980s, and early 1990s has led the oil-cooled rotary-screw design to become the significant choice in electric-motor-driven, lubricated, industrial air compressors, particularly in sizes from 20 to 300 hp.
Then, a significant breakthrough in air-end design occurred. The introduction of the unsymmetrical profile resulted in an efficiency improvement of approximately 15%. This improvement was significant enough to make the oil-cooled rotary-screw compressor competitive in the larger-horsepower sizes for continuous duty. It has almost the same efficiency as the single-stage double-acting units and smaller centrifugal compressors.
Two-stage rotary-screw compressors can approach and sometimes equal the full-load performance of two-stage reciprocating units in 100-psig class service. Today, two-stage oil-cooled rotary-screw compressors are frequently used in the 150- to 400-psia pressure range. They also are used for 100-psi service with significant power savings. Two stages offer advantages associated with lower compression ratio per stage. Reduced pressure differential across the rotors minimizes blow-by and significantly reduces thrust-bearing loads. (Obviously two-stage units require two air ends, which increase the initial cost.)
Water-cooled rotary screw types
The unique characteristic of this compressor is that it is cooled by oil. Oil injected into the air stream absorbs the heat of compression while it is being generated. The heated oil then is taken to an air- or water-cooled heat exchanger for cooling. Because the cooling takes place right inside the compressor, the working parts are never subjected to extreme operating temperatures. The cooling oil never is cracked nor burnt. No matter what the load on the compressor is, there are no hot spots inside the air end. The resulting absence of wear produces trouble-free service and high efficiency. In other words, oil-cooled rotary-screw compressors can run at full load and full pressure —twenty-four hours a day, seven days a week. This compressor's useful life in operating hours and its maintenance cost per hour will be the same as under any other load condition.