Air Compressors



Air Compressors

Compressed air is air forced to pass through a smaller space, thus the pressure increases. The power available from compressed air is used in many applications as a substitute for steam, in operating rock drills, shop tools and engines.

A compressor is an equipment driven by any prime mover that compresses air into a receiver to be used at a greater or lesser distance. The system is not subject to loss by condensation in the pipes, as is the case were carrying the steam in pipes for long distances.

Heat of Air Compression

This subject has probably received more considerations in air compressor design than any other. The principal losses in the earlier compressors were traceable to this source.

It should be noted that the heat of compression, as already explained, represents work done upon the air for which there is usually no equivalent obtained, given that the heat is all lost by radiation before the air is used. The selection of any air cylinder lubricant is, of course, governed to a considerable extent by knowledge of the cylinder temperature it must withstand.

When the air pressures are known, the corresponding temperatures are ascertained fairly accurately.


Cylinder Temperatures at End of Compression
Air Pressure
( lb gauge )
Final Temperature,
Single Stage ( °F )
Final Temperature,
Two Stage ( °F )
10
145

20
207

30
255

40
302

50
339
188
60
375
203
70
405
214
80
432
224
90
459
234
100
485
243
110
507
250
120
529
257
130
550
265
140
570
272
150
589
279
200
672
309
250
749
331

Single-Stage Compressors

The air is compressed to the desired pressure in one operation. The air is taken into the air cylinder at zero gauge pressure and compressed with one stroke of the piston to the desired pressure. It is then discharged directly into the air receiver.



Two-Stage Compressors

The desired pressure is reached in two operations and two separate cylinders are required. The air is taken into the low pressure cylinder and compressed to an intermediate pressure. Then, it is passed through an inter-cooler to the high pressure cylinder in which the compression to the desired pressure is completed. The advantage of compound compression over simple compression is the reduction of losses due to the heat of compression. This is due to the fact that more time is taken to compress a certain volume of air and that this air, while being compressed, is brought into contact with a larger percentage of jacketed surfaces.



Advantages:
  1. Cooler intake air
  2. Better lubrication
  3. Reduction of clearance losses
  4. Lower maximum strains and nearer uniform resistance

Air Compressor Types

Air compressor can be classified into two general categories: positive displacement and dynamic units. The positive displacement type includes reciprocating and rotary compressors. In the dynamic type it includes centrifugal and axial flow designs.

Cylinders in reciprocating machines may be either single or double acting. The single acting arrangement is one in which the compression of air takes place in only one end of the cylinder as it moves back and forth. In this design, the inlet and discharge ports are located at the compression end of the cylinder. Such a compressor may be single or a two stage unit using single acting cylinders in either or both stages of compression.

Compressors defined as units having double acting compression cylinders compress air at both ends of the cylinder as the pistons moves back and forth. The view below shows the internal arrangement in a typical double acting horizontal compressor. This design requires that both inlet and discharge ports be located at each end of the cylinder. The double acting compression cylinder is used on single stage and multistage compressors. In machine with more than one cylinder, the cylinders can be arranged in a vertical/horizontal combination, “V” or “L” fashion, radial and similar patterns in tandem and parallel.

The three principal styles of rotary positive displacement compressors are the sliding vane, impeller and piston. The rotor in the sliding vane compressor is positioned off-center to the cylinder bore. In operation, the rotor slide in and out in the slots in accordance with the varying clearance in the cylinder. Air is trapped ahead of the vanes as they move through the intake area. Due to the off-center position of the rotor with respect to the cylinder, the size of the chambers rapidly decreases as they move toward the discharge port. During this movement from intake to discharge, there is a smooth, continuous compression of the confined air.


Section view of a horizontal double-acting compressor



Off-center rotor in a sliding vane compressor

In the impeller design of a rotary positive displacement air compressor, two impellers called lobes are rotated in opposite directions within the housing. They are designed to mesh as they rotate. A minimum of operating clearance prevents leakage during compression. This types can handle large volumes of air per unit time at relatively low pressure. They are widely used for dry conveying systems of all types, where large volume, steady, low pressure airflow is required.
Two impellers called “lobes” rotate in opposite directions

Another design of a rotary positive displacement air compressor is a screw cycloidal or twist type. The flow of air is axial rather than radial as it is in the lobe type. Because of its simple design, its pulsation free discharge, quite operation, low discharge air temperature, the use of this design is increasing rapidly. The principal components of a screw compressor are housing, two screws, bearings and bearing support.


Twisted type rotary positive air compressor

Horizontal Compressors

The horizontal type reciprocating air compressor, derived its name from the horizontal arrangement of the cylinders. Air enters the compression chambers through the inlet port at the bottom of the cylinder and is discharged through the outlet directly above on top of the cylinder.

Vertical Compressors

Similar to the horizontal compressor, the vertical style is designed primarily for use where space is at a premium. The complete unit is supported by the frame vertically. Inlet and outlet ports are located identically with respect to the cylinder.


L-Frame Compressors

The L-frame cylinder arrangement is designed for heavy duty, two stage, double acting types of air compressors. Inter-cooler is built into the unit and air flows from the first stage discharge, through passages within the frame and cylinders. These units are engineered and built for maximum efficiency to obtain the service requirements. They are manufactured in both positive displacement and dynamic styles.


Air Compressor Troubleshooting

The various symptoms for which corrective action may be taken. In most cases the remedy is apparent once the cause of trouble has been determined, the following troubleshooting guide is set up in terms of problems and causes.

Delivery Below Rated Capacity
  • Excessive system leakage
  • Restriction of intake
  • Intake filters clogged
  • Broken or worn valves
  • Valves not seated in cylinder
  • Leaking gaskets
  • Unloader defective or improperly set
  • Worn or broken piston rings
  • Cylinder or piston worn or scored
  • Leaking rod packing or safety valve
  • Slipping belts
  • Speed too low
  • Excessive system demand

Below Normal Discharge Pressure
  • Excessive system leaks
  • Restriction of intake
  • Intake filters clogged
  • Broken or worn valves
  • Valves not seated in cylinder
  • Leaking gaskets
  • Unloader defective or improperly set
  • Worn or broken piston rings
  • Cylinder or piston worn or scored
  • Leaking rod packing or safety valve
  • Slipping belts
  • Speed too low
  • Excessive system demand

Noisy or Knocking Compressor
  • Valves worn or broken
  • Piston rings worn, stuck or broken
  • Cylinder or piston worn
  • Pulley or flywheel loose
  • Anchor bolts loose or foundation uneven
  • Piston to head clearance too small
  • Excessive crankshaft endplay
  • Piston or piston nut loose
  • Bearings loose
  • Liquid in cylinder
  • Inadequate lubrication

Extra Long Operating Cycle
  • Excessive system demands
  • Intake filter clogged
  • Valve worn or broken
  • Valves not seated in cylinder
  • Leaking gaskets
  • Defective unloader or control
  • Piston rings worn, stuck or broken
  • Excessive system leakage

Motor Overheats
  • Motor too small, wrong connections and low voltage
  • Excessive number of starts, belts too tight and speed too high
  • High ambient temperature and poor ventilation
  • Inadequate lubrication or incorrect lubricant
  • Unloader setting incorrect, pressure setting above rating
  • Mechanical trouble, valves, pistons, packings and gaskets

Compressor Overheats
  • Pressure setting above rating
  • Restricted intake or clogged filter
  • Broken or worn internal parts, pistons, rings, cylinders and valves
  • Insufficient water or clogged jackets
  • Inlet water temperature too high
  • Speed too high, belts misaligned
  • Bearings need adjustment or renewal
  • Inadequate or incorrect lubricant
  • Poor ventilation
  • Wrong rotation

Excessive Valve Wear and Breakage
  • Insufficient or incorrect lubrication
  • Liquid carryover
  • Dirt or foreign materials entering the cylinder
  • Incorrect assembly or installation
  • Springs broken
  • New parts combined with worn parts

Excessive Compressor Vibration
  • Speed too high
  • Belt misalignment
  • Pulley or flywheel loose
  • Foundation bolts loose, base inadequately supported
  • Improperly piped or piping unsupported
  • Pressure setting above rating

Excessive Wear: Piston and Cylinder
  • Piston rings worn, stuck or broken
  • Cylinder or piston worn or scored
  • Inadequate or incorrect lubricant
  • Dirt or foreign materials entering the cylinder
  • Defective air filter

Excessive Wear: Rod and Packings
  • Inadequate or incorrect lubricant
  • Defective air filter
  • Dirt or foreign materials reaching wearing surfaces
  • Packing rings worn, stuck or broken
  • Rod worn, pitted or scored

Excessive Oil Pumping
  • Intake filter clogged or restricted
  • Piston rings worn, scored or broken
  • Cylinder or piston worn or scored
  • Too much or wrong type of lubricant
  • Oil pressure too high
  • Piston ring gaps not staggered or ring drain holes plugged

Low Oil Pressure
  • Defective gauge
  • Insufficient or wrong type of lubricant
  • Defective oil relief valve, pipe leaks
  • Pump suction air leaks
  • Worn or defective oil pump
  • Excessive clearances

Low Inter-cooler Pressure
  • Excessive system demand or system leakage
  • Restricted intake or clogged intake filter
  • Valve worn or broken in low pressure cylinder
  • Piston rings worn, scored or broken in low pressure cylinder
  • Cylinder or piston worn or scored in low pressure cylinder
  • Unloader defective or incorrect set

High Discharge Temperature
  • Discharge pressure setting above rating
  • Intake filter clogged or restricted
  • Valves worn, broken or not seated in the cylinder
  • Unloader or control defective
  • Piston rings worn, stuck or broken
  • Cylinder or piston worn or scored
  • Excessive speed
  • Too much or incorrect lubricant
  • Poor ventilation, airflow blocked
  • Insufficient cooling water, jacket clogged
  • Water temperature too high, cooling lines restricted

High Inter-cooler Pressure
  • Discharge pressure set above rating
  • Valves worn, broken or not seated in high pressure cylinder
  • Cylinder or piston worn or scored in high temperature cylinder
  • Unloader or controls defective or improperly set
  • Inter-cooler passage clogged
  • Water insufficient, temperature too high or passages clogged

High Outlet Water Temperature
  • Discharge pressure set above rating
  • Dirty cylinder head or inter-cooler
  • Water temperature too high, water jackets or inter-cooler dirty
  • Air discharge temperature too high, inter-cooler pressure too high
  • Speed too high