AIR COMPRESSORS

7 Jun

The purpose of an air compressor is to provide a continuous supply of pressurized air.  This chapter will describe the various types of compressors and their basic operation.

Introduction

Air compressors of various designs are used widely throughout DOE facilities in numerous applications.  Compressed air has numerous uses throughout a facility including the operation of equipment  and  portable  tools. Three  types  of  designs  include  reciprocating,  rotary,  and centrifugal air compressors.

R eciprocating C om pressors

The reciprocating air compressor, illustrated in Figure 1, is the most common design employed today.

The reciprocating compressor normally consists of the following elements.

a. The compressing element, consisting of air cylinders, heads and pistons, and air inlet and discharge valves.

b. A  system  of  connecting  rods,  piston  rods,  crossheads,  and  a  crankshaft  and flywheel for transmitting the power developed by the driving unit to the air cylinder piston.

c.         A  self-contained  lubricating  system  for  bearings,  gears,  and  cylinder  walls, including a reservoir or sump for the lubricating oil, and a pump, or other means of delivering oil to the various parts.  On some compressors a separate force-fed lubricator is installed to supply oil to the compressor cylinders.

d.         A regulation or control system designed to maintain the pressure in the discharge line and air receiver (storage tank) within a predetermined range of pressure.

e.         An unloading system, which operates in conjunction with the regulator, to reduce or eliminate the load put on the prime mover when starting the unit.

A section of a typical reciprocating single-stage, single-acting compressor cylinder is shown in Figure 2.  Inlet and discharge valves are located in the clearance space and connected through ports in the cylinder head to the inlet and discharge connections.

During the suction stroke the compressor piston starts its downward stroke and the air under pressure in the clearance space rapidly expands until the pressure falls below that on the opposite side of the inlet valve (Figures 2B and 2C).  This difference in pressure causes the inlet valve to open into the cylinder until the piston reaches the bottom of its stroke (Figure 2C).

During the compression stroke the piston starts upward, compression begins, and at point D has reached the same pressure as the compressor intake.  The spring-loaded inlet valve then closes. As the piston continues upward, air is compressed until the pressure in the cylinder becomes great enough to open the discharge valve against the pressure of the valve springs and the pressure of the discharge line (Figure 2E).  From this point, to the end of the stroke (Figures 2E and 2A), the air compressed within the cylinder is discharged at practically constant pressure.

Rotary Compressors

The rotary compressor is adaptable to direct drive by induction motors or multicylinder gasoline or diesel engines. The units are compact, relatively inexpensive, and require a minimum of operating attention and maintenance. They occupy a fraction of the space and weight of a reciprocating machine of equivalent capacity.  Rotary compressor units are classified into three general groups, slide vane-type, lobe-type, and liquid seal ring-type.

The rotary slide vane-type, as illustrated in Figure 3, has longitudinal vanes, sliding radially in a slotted rotor mounted eccentrically in a cylinder. The centrifugal force carries the sliding vanes against the cylindrical case with the vanes forming a number of individual longitudinal cells in the eccentric annulus between the case and rotor.  The suction port is located where the longitudinal cells are largest.  The size of each cell is reduced by the eccentricity of the rotor as the vanes approach the discharge port, thus compressing the air.

The rotary lobe-type, illustrated in Figure 4, features two mating lobe-type rotors mounted in a case. The lobes are gear driven at close clearance, but without metal-to-metal contact. The suction to the unit is located where the cavity made by the lobes is largest. As the lobes rotate, the cavity size is reduced, causing compression of the vapor within. The compression continues until the discharge port is reached, at which point the vapor exits the compressor at a higher pressure. The rotary liquid seal ring-type, illustrated in Figure 5, features a forward inclined, open impeller, in an oblong cavity filled with liquid. As the impeller rotates, the centrifugal force causes the seal liquid to collect at the outer edge of

the oblong cavity.  Due to the oblong configuration of the compressor case, large longitudinal cells  are  created  and  reduced  to  smaller  ones. The  suction  port  is  positioned  where  the longitudinal cells are the largest, and for the discharge port, where they are smallest, thus causing the vapor within the cell to compress as the rotor rotates. The rotary liquid seal compressor is frequently used in specialized applications for the compression of extremely corrosive and exothermic gasses and is commonly used in commercial nuclear plants as a means of establishing initial condenser vacuum.

C entrifugal Compressors

The centrifugal compressor, originally built to handle only large volumes of low pressure gas and air (maximum of 40 psig), has been developed to enable it to move large volumes of gas with discharge pressures up to 3,500 psig. However, centrifugal compressors are now most frequently used for medium volume and medium pressure air delivery. One advantage of a centrifugal pump is the smooth discharge of the compressed air. The centrifugal force utilized by the centrifugal compressor is the same force utilized by the centrifugal pump. The air particles enter the eye of the impeller, designated D in Figure 6. As the impeller rotates, air is thrown against the casing of the compressor. The air

becomes compressed as more and more air is thrown out to the casing by the impeller blades. The air is pushed along the path designated A, B, and C in Figure 6. The pressure of the air is increased as it is pushed along this path. Note in Figure 6 that the impeller blades curve forward, which is opposite to the backward curve used in typical centrifugal liquid pumps. Centrifugal compressors can use a variety of blade orientation including both forward and backward curves as well as other designs. There may be several stages to a centrifugal air compressor, as in the centrifugal pump, and the result would be the same; a higher pressure would be produced. The air compressor is used to create compressed or high pressure air for a variety of uses. Some of its uses are pneumatic control devices, pneumatic sensors, pneumatic valve operators, pneumatic motors, and starting air for diesel engines.

Compressor Coolers

The amount of moisture that air can hold is inversely proportional to the pressure of the air.  As the pressure of the air increases, the amount of moisture that air can hold decreases.  The amount of  moisture  that  air  can  hold  is  also  proportional  to  the  temperature  of  the  air. As  the temperature of the air increases, the amount of moisture it can hold increases.   However, the pressure change of compressed air is larger than the temperature change of the compressed air. This causes the moisture in the air to condense.  Moisture in compressed air systems can cause serious damage.  The condensed moisture can cause corrosion, water hammers, and freeze damage; therefore, it is important to avoid moisture in compressed air systems.  Coolers are used to minimize the problems caused by heat and moisture in compressed air systems.

Coolers used on the discharge of a compressor are called aftercoolers. Their purpose is to remove the heat generated during the compression of the air. The decrease in temperature promotes the condensing of  any moisture present in the compressed air. This moisture is collected in condensate traps that are either automatically or manually drained.

If the compressor is multi-staged, there may be an intercooler, which is usually located after the first stage discharge and before the second stage suction.  The principle of the intercooler is the same as that of the aftercoolers.  The result is drier, cooler, compressed air.  The structure of a particular cooler depends on the pressure and volume of the air it cools.  Figure 7 illustrates a typical compressor air cooler.  Air coolers are used because drier compressed air helps prevent corrosion and cooler compressed air allows more air to be compressed for a set volume.

Hazards of Compressed A ir

People often lack respect for the power in compressed air because air is so common and is often viewed as harmless.  At sufficient pressures, compressed air can cause serious damage if handled incorrectly.   To minimize the hazards of working with compressed air, all safety precautions should be followed closely.

Small leaks or breaks in the compressed air system can cause minute particles to be blown at extremely  high  speeds. Always  wear  safety  glasses  when  working  in  the  vicinity  of  any compressed air system.  Safety goggles are recommended if contact lenses are worn.

Compressors can make an exceptional  amount of noise  while  running. The noise of the compressor, in addition to the drain valves lifting, creates enough noise to require hearing protection. The area around compressors should normally be posted as a hearing protection zone.

Pressurized air can do the same type of damage as pressurized water.  Treat all operations on compressed air systems with the same care taken on liquid systems.  Closed valves should be slowly cracked open and both sides should be allowed to equalize prior to opening the valve further.   Systems being opened for maintenance should always be depressurized before work begins.

Great care should be taken to keep contaminants from entering air systems. This is especially true for oil. Oil introduced in an air compressor can be compressed to the point where detonation takes place in a similar manner as that which occurs in a diesel engine. This detonation can cause equipment damage and personnel injury.

Summary

The important information in this chapter is summarized below.

Air Compressors Summary

The three common types of air compressors are reciprocating, rotary, and centrifugal.

The single-stage reciprocating compressor has a piston that moves downward during the suction stroke, expanding the air in the cylinder. The expanding air causes pressure in the cylinder to drop.  When the pressure falls below the pressure on the other side of the inlet valve, the valve opens and allows air in until the pressure equalizes across the inlet valve. The piston bottoms out and then begins a compression stroke.  The upward movement of the piston compresses the air in the cylinder, causing the pressure across the inlet valve to equalize and the inlet valve to reseat. The piston continues to compress air during the remainder of the upward stroke until the cylinder pressure is great enough to open the discharge valve against the valve spring pressure.  Once the discharge valve is open, the air compressed in the cylinder is discharged until the piston completes the stroke.

A ir Compressors Summary ( Cont.)

The centrifugal force utilized by the centrifugal compressors is the same force utilized by the centrifugal pumps.  The air particles enter the eye of the impeller.  As the impeller rotates, air is thrown against the casing of the compressor.  The air becomes compressed as more and more air is thrown out to the casing by the impeller blades.  The air is pushed along the path  on the  inner wall  of  the casing.  The  pressure of  the air  is increased as it is pushed along this path.  There could be several stages to a centrifugal air compressor just as in the centrifugal pump, resulting in higher pressure.

Rotary compressors are driven by a direct drive that rotates a mechanism (impellers, vanes, or lobes) that compresses the air being pumped.   The actual compression of the air takes place due either to centrifugal forces or a diminishing air space as the impellers rotate.

Cooling systems are required in compressed air systems to remove any heat added by the compression.  The advantages to cooling the compressed air are that cool air takes less space and holds less moisture.  This reduces corrosion and allows more air to be compressed into a given volume.

Hazards associated with compressed air are similar to hazards of any high pressure system.  Three general hazards include the following.

Small leaks or breaks can cause minute particles to be blown at speeds high enough to cause damage.  Goggles or safety glasses should be worn when working around compressed gas.

The compressors, especially larger ones, can be quite noisy when running. The cycling of automatic drain valves contributes noise as well.  Hearing protection should be worn around compressors.

Pressure  swings  may  cause  system   damage.      Closed  valves  in  a compressed air system should be slowly cracked open and the pressure should be allowed to equalize prior to opening the valve further.  Systems should be depressurized prior to opening for maintenance.  Oil should be kept out of air systems to prevent possible explosions.

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