Saturday, October 16, 2010

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

1 comment:

  1. The largest single application is continuous-duty, supplying air at 100 psig. authorised ingersoll rand dealers