Page 39 - Reciprocating Compressors Operation Maintenance

26 Reciprocating Compressors: Operation and Maintenance

earth's atmosphere decreases. This is reflected in the barometer and in
absolute intake pressure, which decreases with altitude. This fact is well
understood and allowed for with process compressors.
At higher altitudes, the low-pressure cylinder size is increased to pro-
vide greater inlet capacity and to bring the power imposed on the frame
and running gear closer to normal values.
Single-stage reciprocating and other positive displacement compres-
sors are limited somewhat by the allowable compression ratio and dis-
charge temperature. Frequently, they must be materially derated for alti-
tude operation.
Although the power required by a given compressor decreases as the
altitude increases, the ability of engines and electric motors to safely
develop this power usually decreases even more rapidly.

BRAKE HORSEPOWER

Reciprocating units are calculated on the basis of theoretical adiabatic
horsepower modified by compression and mechanical efficiencies which
result in the brake horsepower (bhp). Compression efficiency depends
on many factors—effectiveness of valving, compression ratio, gas com-
position, compressor size, etc. Mechanical efficiency varies with
machine type and size.
For preliminary estimation of sea-level air compressors for general
power services, the data shown in Figure 1-15 are reasonable but subject
to confirmation by the manufacturer. Information is based on 100 cfm
actually delivered intake air and heavy-duty water-cooled compressors.
For altitude installation, the performance will differ. Figure 1-16 also
gives approximate altitude correction factors for bhp/100.
Multistage machines may be approximated by using equal compression
ratios per stage and multiplying the single-stage bhp/million by the num-
ber of stages. A compression ratio per stage of over 3.5 should not nor-
mally be used, although there will be exceptions. If involved, compress-
ibility must bellowed for separately, stage by stage. Interstage pressure
drop, imperfect intercooling, and vapor condensation between stages that
reduces the volume handled, must also be allowed for in this manner.

CHARACTERISTICS OF RECIPROCATING COMPRESSORS

Reciprocating compressors are the most widely used of all compres-
sion equipment and also provide the widest range of sizes and types. Rat-