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