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370 Applied Process Design for Chemical and Petrochemical Plants

the engineer. Bid evaluations must include detailed analysis Compressibility is expressed as the multiplier for the per-
of performance, power drivers, and materials of construc- fect gas law to account for deviation from the ideal. At a
tion. given set of conditions of temperature and pressure:

PV ZNRT (12-1)
General Considerations for Any Type
of Compressor Flow Conditions
where Z compressibility factor, usually less than 1.0
N number of lb-mol
In establishing specifications, the first important items to
R gas constant, depends on units of pressure, volume,
identify from the plant process material balance are normal, and temperature
maximum, and minimum intake or suction flow rates T absolute temperature, °R °F 460
together with corresponding conditions of temperature and P pressure, absolute, psia
pressure. The required discharge pressure must be estab- V volume, ft (see paragraph to follow)
3
lished. If it is necessary or important to be able to operate at
reduced or over-normal flow rates, these should be identi- Gas volumes are corrected at the intake conditions on the
fied for the manufacturer, together with the length of time first and each succeeding stage of the compression step, and
of such expected condition; e.g., full time at one-half rate, compressibility factors are calculated or evaluated at these
20 minutes out of every hour at 10% over normal, etc. These individual intake conditions. Some manufacturers use the
operating requirements may separate the types of equip- average value between intake and discharge conditions.
ment. Because it is uneconomical to purchase horsepower
that cannot be used by the fluid system, ask that the manu- Corrosive Nature
facturer state the maximum load and/or conditions that will
Corrosive fluids or contaminants must be identified to the
fully load the available horsepower of the compressor-driver
manufacturer. The principle gas stream may or may not be
unit.
corrosive under some set of circumstances, yet the contami-
nants might require considerable attention in cylinder
Fluid Properties
design. For example, considerable difference exists between
Fluid properties are important in establishing the perfor- handling “bone-dry” pure chlorine gas and the same mater-
mance of compression equipment. Whenever possible, fluid ial with 5 ppm moisture. The corrosiveness of the gas must
analysis should be given, and where this is not available due be considered when selecting fabrication materials for the
to lack of complete information or secrecy, close approxi- compression parts as well as seals, lubricants, etc.
mations are necessary. Under these last conditions, actual
Moisture
field performance may not agree with the design data due to
the deviation in values of the ratio of specific heats and the
Moisture in a gas stream might be water vapor from the
average molecular weight. Identify, as to composition and
air or a water scrubber unit, or it could be some other con-
quantity, any entrained liquids or solids in the gas stream.
densable vapor being carried in the gas stream. It is impor-
No manufacturer will design for entrained liquids or solids,
tant in compressor volume calculations to know the
although some machines will handle “dirty” gases. Solids are
moisture (or condensable vapor) condition of the gas.
always removed ahead of any compression equipment, using
suitable wet- or dry-scrubbing equipment, and liquid sepa- Special Conditions
rators are recommended for any possibilities of liquid carry-
over. Often the process may have conditions that control the
flexibility of compression equipment selection. These might
Compressibility include limiting temperatures before polymer formation,
chemical reaction, excess heat for lubrication materials,
Gas compressibility has an important bearing on com- explosive conditions greater than a certain temperature, etc.
pressor capacity performance. Therefore, it is good practice Any limiting pressure drops between stages should be
to state compressibility values at several temperature and specified, in which the gas and vapors are discharged from
pressure points over the compression range under consid- one stage, pass through piping, cooling equipment and/or
eration. When possible, a compressibility curve or reference condensate knock-out equipment, and are then returned to
thereto is included in the inquiry. Where specific informa- the next higher stage of the compression process. Usually a
tion is not available, but compressibility is anticipated as reasonable figure of 3—5 psig can be tolerated as pressure
being a factor to consider, approximate values should be drop between stages for most conditions. The larger this
established and so presented for further study by the manu- drop is, the more horsepower required. Special situations
facturer. might hold this figure to 0.5—1 psig.