Brake horsepower is the input power required at the compressor input shaft for a specific speed, capacity, and pressure condition.

Motor or engine horsepower is the nominal rating of the prime mover.

The 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.

Water-cooled rotary screws

Another version of oil-free rotary-screw compressors is a single-stage design that uses water injection to cool and seal the rotors during compression. The bearings and drive gears are lubricated with oil and sealed from the compression chamber. These units serve a selected market and are a special design. In some applications, care must be taken to avoid the build-up of bacteria in the water.

Dynamic air compressors

Dynamic, or centrifugal compressors, Figure 3, are dissimilar to the positive-displacement machines already discussed because they raise the pressure of air by converting the energy of its velocity into pressure. First, rapidly rotating impellers (similar to fans) accelerate the air. Then, the fast flowing air passes through a diffuser section that converts its velocity head into pressure by directing it into a volute.fig. 3. cutaway view of single-stage, single-inlet centrifugal compressor with closed-type impeller. electric drive motor can be seen at left center.

Because the centrifugal is a mass flow compressor, it has a limited stable operating range. This has a large effect on economic operation or bhp/100 cfm delivered at part load. Minimum turn-down capacities for centrifugals may vary from 20% to 30% of full load, depending on impeller design, number of stages, etc.

There are limits to the pressure rise that can be achieved in a single stage by a centrifugal compressor - due to both physical and economical restraints - so two- to four-stage units are built that incorporate one to three water-cooled intercoolers. Cooling the air between stages reduces the power required to compress the air further, resulting in more efficient operation. Intercooling actually may permit the desired compression to be accomplished in fewer stages.

The centrifugal compressor is definitely a continuous-duty unit because its service life is unaffected by full-load operation. However, it is also a relatively sensitive machine because it operates at high speeds - often as high as 50,000 rpm. Ambient factors which affect flow are altitude, inlet air temperature, and the relative humidity of inlet air. The operating life of this type of unit is primarily determined by the amount of entrained liquids and solids carried into the unit at the inlet - and the quality of the cooling water. As in all machinery, correct installation and maintenance is critical to the efficient production of compressed air and reaching a satisfactory operating life.

When a facility requires a continuous-duty, high-volume (2,000 to 25,000 cfm) supply of non-lubricated air, the centrifugal compressor is one of the best choices. In fact, it is the only choice in sizes above 1,000 hp. Whether or not it fits the installation best is another question to be answered after analyzing the job conditions. In any event, when correctly applied, installed, and maintained, a centrifugal compressor offers a reliable, continuous source of compressed air.

Advantages and disadvantages

After reviewing the comments on air compressors in this article, one conclusion is fairly obvious: each design has advantages and disadvantages which must be matched to a specific application. The table on this page summarizes a number of selection factors for the most common basic designs. Other factors, such as air quality and installation requirements, are difficult to quantify. The unavoidable cost factor - initial, operating, and maintenance - is noted with them in the following text.

Double-acting reciprocating - Advantages: highest efficiency, longest service life, field serviceability. Disadvantages: highest initial cost, high installation cost, high maintenance cost.

Oil-flooded, single-stage rotary screw - Advantages: low initial cost, low maintenance cost, packaged design. Disadvantage: low efficiency.

Oil-flooded, two-stage rotary screw - Advantages: higher efficiency, simple packaged design, same low maintenance cost. Disadvantage: higher initial cost.

Oil-free rotary screw - Advantages: high-quality air, moderate efficiency, simple packaged design. Disadvantage: higher initial cost.

Centrifugal - Advantages: the only type available above 600 hp, high-quality air, moderate efficiency, longer service life than other rotaries. Disadvantages: higher initial cost, must be water cooled, air flow is sensitive to changes in ambient conditions.