Types of Air compresser

Centrifugal

in service. Where capacity or horsepower rather than numbers is considered as a measure, the centrifugal,

without a doubt, heads the compressor field. During the past 30 years, the centrifugal compressor,

because of its smaller relative size and weight compared to the reciprocating machine, became

much more popular for use in process plants, which were growing in size. The centrifugal compressor

does not exhibit the inertially induced shaking forces of the reciprocator and therefore does not need

the same massive foundation. Initially, the efficiency of the centrifugal was not as good as that of a

well maintained reciprocating compressor. However, the centrifugal established its hold on the market

in an era of cheap energy when power cost was rarely, if ever, evaluated.

The smaller compressor design was able to penetrate the general-process plant market, which had

historically belonged to the reciprocating compressor. As the compressor grew in popularity, developments

were begun to improve reliability, performance, and efficiency. With the increase in energy

cost in the mid-1970s, efficiency improvements became a high priority. Initially, most development

had concentrated on making the machine reliable, a goal that was reasonably well achieved. Run

time between overhauls currently is three years or more, with six-year run times not unusual. As

plant size increased, the pressure to maintain or improve reliability was very high because of the

large economic impact of a nonscheduled shutdown.

Centrifugal compressors are dynamic types with rotating impellers that impart velocity and pressure

to air (Fig. 61.6). Their design is simple and straightforward, consisting of one or more highspeed

impellers with cooling sections. The only lubrication required is in the drive system, which is

sealed off from the air system.

Integral gear-type centrifugal air compressors are generally used in central plant air applications

requiring volumes ranging from 1000-30,000 cfm and discharge pressures from 100-125 psig.

Centrifugal air compressors are normally specified on the basis of required air-flow volume.

However, there are several ways to calculate volume and serious problems can result unless both

user and manufacturer use the same method. At the very least, the user can have problems comparing

bids from competing manufacturers. At worst, he may choose the wrong compressor.

These problems can be avoided by specifying capacity in terms of actual inlet conditions and by

understanding how compressor capacity is affected by variable ambient conditions such as inlet

pressure, temperature, and relative humidity. Factors such as cooling water temperature and motor

load must be considered before a compressor and its drive motor can be sized.

A multistage arrangement for integral gear-type compressors is shown in Fig. 61.7. The flow path

is straight through the compressor, moving through each impeller and cooler in turn. This type of

centrifugal compressor is probably the most common of any found in process service, with applications

ranging from air to gas.

compressors are second only to reciprocating compressors in numbers of machines

Sliding-vane

61.8). As the rotor turns, the vanes slide out against the stator or housing. Air compression occurs

when the volume of the space between the sliding vanes is reduced as the rotor turns. Single- and

multistage versions are available.compressors consist of a vane-type rotor mounted eccentrically in a housing (Fig.

• Oxidation

• Condensation

• Viscosity

• Outgassing in the inlet

• Foaming

• Separation performance

• Chemical reaction

Some problems can be solved with specially selected oil grades. Another solution is synthetic oils,

but cost is a problem, particularly with silicone oils. Alternatives need to be reviewed to match

service life of the lubricant with lubrication requirements in the compressor.

One consideration for flooded compressors is the recovery of liquid. In conventional arrangements,

the lubricating oil is separated at the compressor outlet, cooled, filtered, and returned to the compressor.

This is fine for air service, where oil in the stream is not a major problem, but when oilfree

air is needed, the separation problem becomes more complex. Because the machine is flooded

and the discharge temperature is not high, separation is much easier relative to compressors that send

small amounts of fluid at high temperature down stream. Usually part of the lubricant is in a vaporized

form and is difficult to condense except where it is not wanted. To achieve quality oil-free air, such

as that suitable for a desiccant-type dryer, separators that operate at the tertiary level should be

considered. Here, the operator must be dedicated to separator maintenance, because these units require

more than casual attention. Separation by refrigeration is not as critical if direct expansion chillers

are used. In these applications, the oil moves through the tubes with the refrigerant and comes back

to the compressor with no problem, if the temperature is not too low for the lubricant.

Advantages of helical screw compressors include smooth and pulse-free air output, compact size,

high output volume, low vibration levels, and long life.

Air Compressor

material is also a factor in setting the temperature limit. While 30O

should be remembered that this is an average outlet temperature and the cylinder will have hot spots

exceeding this temperature.

Lubricated compressors use either a full-pressure or splash-lubricating system with oil in the

crankcase. Oil-free compressors have a crosshead or distance piece between the crankcase and cylinders.

Nonlubricated compressors use nonmetallic piston rings, guides, and sealed bearings with no

lubricating oil in the crankcase.

0F may not seem all that hot, it

Reciprocating double-acting

used for heavy-duty, continuous service. Discharge pressures range from above atmospheric to several

thousand psig. The largest single application is continuous-duty, supplying air at 100 psig. This design

is available with the same modifications as single-acting compressors.

Double-acting crosshead compressors, when used as single-stage, have horizontal cylinders. The

double-acting cylinder compressor is built in both the horizontal and the vertical arrangement. There

is generally a design tradeoff to be made in this group of compressors regarding cylinder orientation.

From a ring-wear consideration, the more logical orientation is vertical; however, taking into account

size and the ensuing physical location as well as maintenance problems, most installations normally

favor a horizontal arrangement (Fig. 61.3).

designs compress air on both strokes of the piston and are normally

Rotary screw

machines. Oil or water injection is normally used to seal clearances and remove the heat of

compression. Oil-free designs have reduced clearances and do not require any other sealing medium.

In single-screw designs, the rotor meshes with one or two pairs of gates (Fig. 61.4). The screw

and casing act as a cylinder, while the gates act like the piston in a reciprocating compressor. The

screw also acts as a rotary valve, with the gates and screw cooperating as a suction valve and the

screw and a port in the casing acting as a discharge valve. Single-stage sizes range from 10-1200

cfm with pressures up to 150 psig. 250-psig designs, supplying 700-1200 cfm, are available.

Dual rotor designs use two intermeshing rotors in a twin-bore housing (Fig. 61.5). Air is compressed

between the convex and concave rotors. The trapped volume of air is decreased along the

rotor, increasing pressure. Single- and multistage versions are available with and without lubrication.

The power consumption of rotary screw compressors during unloaded operation is normally higher

than that of reciprocating types. Recent developments have produced systems where the unloaded

horsepower is 15-25% of loaded power. These systems are normally used with electric motor,

constant-speed drives. Use as a base load compressor is recommended to avoid excessive unloaded

power costs.

A dry screw compressor may be selected for applications where a high air-flow rate is required

but space does not allow a reciprocating compressor, or where the flow requirement is greater than

can be supplied by a single-unit, oil-flooded screw compressor. Packaged versions of dry screw

compressors require a minimum of floor space.

Dry screw compressors generate high frequency pulsations that affect system piping and can cause

acoustic vibration problems. These would be similar to the type of problems experienced in reciprocating

compressor applications, except that the frequency is higher. While volume bottles work

with the reciprocator, dry-type screw compressors require a manufacturer-supplied proprietary silencer

to take care of the problem.

There is one problem this compressor can handle quite well: unlike most other compressors, it

will tolerate a moderate amount of liquid. Injection for auxiliary cooling can be used, normally at a

lower level than would be used in a flooded compressor. The compressor also works well in fouling

service, if the material is not abrasive. The foulant tends to help seal the compressor and, in time,

may improve performance.compressors use one or two rotors or screws and are constant-volume, variablepressure