30079_61.pdf

CHAPTER 61

AIR COMPRESSORS

Joseph L. Foszcz

Senior Editor, Plant Engineering Magazine

Des Plaines, Illinois

61.1

INTRODUCTION

1865

61.4 SELECTION

1876

61.2 TYPES

1865

61.5 COST OF AIR LEAKS

1876

61.3 SIZING

1875

61.1 INTRODUCTION

Compressed air provides power for many manufacturing operations. Energy stored in compressed air

is directly convertible to work. Conversion from another form of energy, such as heat, is not involved.

Compressed air can be supplied by several different types of compressors (Fig. 61.1). The choice

depends on the amount, pressure, and quality of air a plant system requires.

The reciprocating compressor is manufactured in a broad range of configurations. Its pressure

range is the broadest in the compressor family extending from vacuum to 40,000 psig. It declined in

popularity from the late 1950s through the mid-1970s. Higher maintenance costs and lower capacity,

when compared to the centrifugal compressor, contributed to this decline. The sudden rise in energy

cost and the downsizing of new process plants have given the higher-efficiency, though lower-capacity,

reciprocating compressor a more prominent role in new plant design.

Rotary compressors as a group make up the balance of positive displacement machines. This

group of compressors has several features in common despite differences in construction. Probably

the most important feature is the lack of valves as used in reciprocating compressors. The rotary is

lighter in weight than the reciprocator and does not exhibit the shaking forces of the reciprocating

compressor, making foundation requirements less rigorous. Though rotary compressors are relatively

simple in construction, their physical design can very widely. Rotor design, both multiple and single,

is one of the main items that distinguishes different types.

For certain applications, compression chamber lubricant oils cannot be tolerated in compressed

air. The demand for oil-free air in processes where compressed air comes in direct contact with

sensitive products, such as electronic components, instruments, food, and drugs, has increased the

need for non-lubricated or oil-free air compressors.

Compressors are normally lubricated for a variety of reasons: to reduce wear, provide internal

cooling, and effect a seal between moving parts. In reciprocating compressors, lubricant is distributed

by a pressure or splash system to connecting rods, crank and piston pins, and main bearings. Rotary

screw compressors inject oil into the screw to seal and cool the compressing air. Centrifugal and

liquid ring compressors are, by design, oil-free.

Reciprocating, non-lubricated air compressors substitute low friction or self-lubricating materials

such as carbon or Teflon for piston and packing rings. Oil-free screw and lobe type compressors are

available with a design that does not require lubrication in the compression chamber for sealing and

lubrication. Centrifugal air compressors are inherently nonlubricated.

Generally, nonlubricated compressors have a higher initial cost due to special designs and ma-

terials. Nonlubricated, reciprocating compressors have higher operating costs due to the increased

maintenance of valves and rings, which tend to have short lives.

61.2 TYPES

Reciprocating single-acting compressors resemble automotive engines, are generally of one- or two-

stage design, and are constant-capacity, variable-pressure units. They are very popular because of

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Fig. 61.1 Pressure-capacity chart showing the effective ranges of most compressors. 1

their simplicity, efficiency, compactness, ease of maintenance, and relatively low price. In a single-

stage compressor, air is compressed to the final pressure in a single stroke. This design is generally

used for pressures from 25-100 psig. Units can be air- or liquid-cooled.

The two-stage design compresses air to an intermediate pressure in the first stage (Fig. 61.2).

Most of the heat of compression is removed as the air passes through an intercooler, which is air or

liquid cooled, to the second stage, where it is compressed to the final pressure. Two-stage compressors

are generally used for pressures from 100-250 psig.

The reciprocating compressor is a positive displacement, intermittent-flow machine and operates

at a fixed volume. One method of volume control is speed modulation. Another, more common,

method is to use clearance pockets with or without valve unloading. With clearance pockets, cylinder

performance is modified. With valve unloading, one or more inlet valves are physically opened.

Capacity may be regulated in a single- or double-acting cylinder with single- or multiple-valve

configurations.

Lubrication of compressor cylinders can be tailored to the application. The cylinders may be

designed for normal hydrocarbon lubricants or can be modified for synthetic lubricants. The cylinder

may also be designed for self-lubrication, in which case it is generally referred to as nonlubed. A

Fig. 61.2 Single-acting, two-stage compressor. 1

compromise lubrication method that uses the nonlubed design but requires a small amount of lubricant

is referred to as the mini-lube system.

Another feature necessary to the reciprocating compressor is cylinder cooling. Many compressors

are furnished with water jackets as an integral part of the cylinder. Alternatively, particularly in

smaller-size compressors, the cylinder can be designed for air cooling.

Reciprocating compressors can be classified into several types. One is the automotive piston type.

The piston is connected to a connecting rod which is in turn connected directly to the crankshaft.

This type of compressor has a single-acting cylinder and is limited to smaller air compressors.

Another common type of compressor for nonlube service is the crosshead type. The piston is

driven by a fixed piston rod that passes through a stuffing or packing box and is connected to a

crosshead. The crosshead, in turn, is connected to the crankshaft by a connecting rod. In this design,

the cylinder is isolated from the crankcase by a distance piece. A variable-length or double-distance

piece is used to keep crankcase lubricant from being exposed to the compressed air.

Reciprocating compressors usually will not tolerate liquids of any sort, particularly when delivered

with the inlet air stream. A suction strainer or filter is mandatory for keeping ambient dirt and pipe

scale out of the compressor. Fines from pipe scale and rust make short work of the internal bore of

a cylinder and are not good for other components. The strainer should be removable for cleaning,

particularly when it is intended for permanent installation. Under all circumstances, provision must

be made to monitor the condition of the strainer.

Discharge temperatures should be limited to 30O 0 F, as recommended by API 618. Higher tem-

peratures cause problems with lubricant coking and valve deterioration. In nonlube service, the ring

material is also a factor in setting the temperature limit. While 30O 0 F may not seem all that hot, it

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 cyl-

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

lubricating oil in the crankcase.

Reciprocating double-acting designs compress air on both strokes of the piston and are normally

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).

Rotary screw compressors use one or two rotors or screws and are constant-volume, variable-

pressure 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 com-

pressed 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 recip-

rocating 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.

Fig. 61.3 Various cylinder arrangements used in displacement compressors.

Some are suitable for single-acting compressors, while others are double-acting

and require a crosshead and guide. 1

Fig. 61.4 Diagram showing the operation of the rotary, single-screw compressor. 1

In dry screw compressors, the rotors are synchronized by timing gears. Because the male rotor,

with a conventional profile, absorbs about 90% of the power transmitted to the compressor, only 10%

of the power is transmitted through the gears. The gears have to be of good quality both to maintain

the timing of the rotors and to minimize noise. Because the compressor will turn in reverse on gas

backflow, keeping gear backlash to a minimum is important. A check valve should be included

because compressors are sometimes subjected to reverse flow. To control backlash in the gears, a

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: