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.