1. Air Compressors
Capacity
Horsepower
Pressure
Duty Cycle

2. Air Compressors Compressors can be classified into two types:
Positive Displacement
Reciprocating (piston type)
Vane
Rotary Screw
Lobe
Dynamic

3. Positive Displacement Compressors
Reciprocating Compressors
Single Stage, Single Acting

4. Positive Displacement Compressors
Reciprocating Compressors
Single Stage, Single Acting

5. Positive Displacement Compressors
Reciprocating Compressors
Single Stage, Double Acting

6. Positive Displacement Compressors
Reciprocating Compressors
Single Stage, Double Acting

7. Positive Displacement Compressors
Intake and Discharge Valves
Work very similar
to check valves
Can open and close
720,000 time a day
The number 1 reason
for compressor downtime
is valve failure
Check the temperature of
the valves regularly

8. Positive Displacement Compressors
Reciprocating Compressors
Multi Stage Piston Compressors

9. Multi – Stage Piston Compressors

10. Cooling Reciprocating Compressors
Heat is generated when air is compressed
With air-cooled compressors, ambient air is circulated
around the housings of the finned cylinders
With water-cooled units, water is circulated through
passages that are cast into the cylinder housings

11. Cooling Reciprocating Compressors
Air-Cooled Intercoolers
Use finned tubing that is connected from one stage to the other
A fan mounted on the flywheel is commonly used to blow air over the fins.

12. Cooling Reciprocating Compressors
Water-Cooled
Use a shell and tube type heat exchanger to transfer the heat from the air to the water.
More efficient than air – type heat exchangers

13. Lubrication There are 2 areas that require lubrication in a
reciprocating compressor.
Compressor Housing
Main cross head, connecting rod and motor bearings
Cylinder Area
Valves, packing and compression rings

14. There are 2 different methods of lubrication.
Splash Lubrication
Most common in small compressors.
Use “dippers” on the connecting rod to scoop oil out of the crankcase.
Pressurized Lubrication
Usually used on very large compressors.
A gear pump is used to force oil through
orifices in the connecting rods.

15. Unloading Valves

16. Unloading Valves Small Reciprocating Compressors
A pressure switch is used to turn the compressor
on and off
When the pressure switch is actuated, a
mechanical lever shifts the unloading
valve open. This allows the compressed
air in the compressor pistons to vent to
atmosphere.
When the low setting is reached, the unloading
valve closes and the compressor is started.

17. Unloading Valves Larger Reciprocating Compressors
Air Pilot Operated Control
Once the pressure in the receiver reaches the
“A” valve’s spring setting, the valve shifts to
direct pilot pressure to open the intake valve.
No compression will occur so long as the
intake valve is open.
When the pressure in the receiver drops
below the “A” valve’s spring setting, the
cycle will repeat.

18. Unloading Valves Larger Reciprocating Compressors
Solenoid Valve Unloading
When the compressor is first started, the
solenoid is energized and a second
timer is started.
Pilot air is directed to shift and hold the
intake valve open. No compression will
occur so long as the intake valve is open.
This allows the compressor to start up at
little or no load.

19. Unloading Valves Larger Reciprocating Compressors
Solenoid Valve Unloading
When the timer elapses, the solenoid is
de-energized and the intake valve is
closed.
When the pressure reaches the high
setting of the pressure switch, the
solenoid is energized and the compressor
is unloaded.
When the pressure drops below the
low setting of the pressure switch,
the solenoid is once again de-energized.

20. Rotary Vane Compressor
Vanes
Drive Shaft
Rotor

21. Rotary Vane Compressor

22. Rotary Screw Compressors
May be of the “dry” or “wet” design.

23. Dry Rotary Screw Compressor
Has a male and female screw.
The male screw is driven by the electric motor.
Timing gears synchronize the rotation of both screws .

24. Dry Rotary Screw Compressor
Limited to approximately 50 PSI unless a
multi stage design is used .
Driven at high speeds between 3,000 and 12,000 RPM.

25. Wet Rotary Screw Compressor

26. Wet Rotary Screw Compressor
No timing gears are required.
Oil is injected into the compression chamber.
Lubricates the screws.
Acts as a sealant between the screws.
Absorbs the heat of compression

27. Wet Rotary Screw Compressor

28. Wet Rotary Screw Compressor
The outlet air flow first
flows through an air/oil
reservoir, which removes
95% of the oil.
The air then passes
through an oil separator
to remove the remaining
oil.

29. Wet Rotary Screw Compressor

30. Wet Rotary Screw Compressor
A check valve (minimum 36 PSI) is installed in the
outlet line of the separator to build back pressure so
that adequate lubrication is provided at all times.

31. Wet Rotary Screw Compressor

32. Wet Rotary Screw Compressor
Thermostatic Valve
Regulates the oil flow through the oil cooler

33. Wet Rotary Screw Compressor

34. Wet Rotary Screw Compressor
The air is ported through an aftercooler before
being directed to the system.

35. Wet Rotary Screw Compressor

36. Wet Rotary Screw Compressor
Solenoid Unloading Valve

37. Rotary Lobe Compressors
High Volume
Low Pressure

38. Rotary Lobe Compressors

39. Dynamic (Centrifugal) Compressors

40. Dynamic (Centrifugal) Compressors
Advantages
Smooth air flow
Very high volume
Disadvantages
More horsepower per CFM is required
High drive speeds required (3000 – 50,000 RPM).
Cannot achieve high pressures without using multiple stages.

41. Dynamic (Centrifugal) Compressors
Air Flow Control
Speed Variation
Suction Throttling
This is the most common method,
but the compressor becomes inefficient
at slower speeds.
A butterfly valve or other device is
used to restrict the inlet air flow.
Discharge Throttling
Recirculation
The outlet air is restricted with a throttling
valve.
The outlet air is directed back
to the inlet port.
Lubrication
A separate pump is used to supply lube oil through a filter and heat exchanger.

42. Compressor Horsepower and Energy Savings
For every 10 PSI, 5% more electrical horsepower is needed.
100 HP will be required to drive a compressor that delivers 520 CFM at 110 PSI
If the pressure is reduced to 100 PSI, then only 95 HP is required.
1 HP = 746 Watts
5 HP = 3,730 Watts or 3.73 kilowatts
If the compressor operates 24 hours a day, 7 days a week
(8,760 hour per year):
Kilowatt hours = 3.7 kilowatts X 8,760 hours
Kilowatt hours = 32,412
Electrical Cost = 32, 412 kilowatt hours X .10
(10 cents per kilowatt hour)
Electrical Cost = $3241 per year

43. Troubleshooting Air Compressors
Checking the Compressor Output
Close the valve in the receiver outlet line to block flow to the system.
Turn the compressor off and bleed the air in the receiver tank to 30 PSI.
Turn the compressor on and record the amount of time it takes for the pressure
to build from 50 to 100 PSI.
Oc = .45 X VR / T
VR = Receiver volume in gallons
T = Time (in minutes) required to build from 50 to 100 PSI
OC = Actual compressor output in CFM

44. Troubleshooting Air Compressors
Checking the Compressor Output
If the receiver tank volume is 400 gallons and the time required to fill it is
30 seconds (.5 minutes) then the actual flow can be found.
OC = .45 X VR / T
OC = .45 X 400 / .5
OC = 360 CFM
If the original flow rating of the compressor is 450 CFM, then we can find
the current capacity.
Present Capacity = Current CFM Flow / Original CFM Flow Rating
Present Capacity = 360 / 450
Present Capacity = .80 or 80%

45. Troubleshooting Air Compressors
Common Compressor Problems
Low Pressure at the Compressor Outlet
Dirty intake filter
Defective intake valve
Worn or broken valve, vane or screw
Improper adjustment or failure of unloading control
Improper pressure switch setting
Large leak in the system

46. Troubleshooting Air Compressors
Common Compressor Problems
Compressor Overheating
Restricted air flow through the intake filter
Dirty fins on an air type intercooler
Restricted water flow through water
jackets or an intercooler.
Low oil Level
Thermostatic valve not working
Restricted oil flow due to contamination
High ambient temperature

47. Troubleshooting Air Compressors
Common Compressor Problems
Oil in Compressed Air
Oil separator saturated (rotary screw)
Oil level too high or viscosity too low
High temperature (reduces oil viscosity)
Worn piston rings or cylinder (reciprocating)

48. Compressor Reliability Checks
Oil Analysis
The condition and performance of the oil has the greatest effect on compressor
cost and reliability!
Particle Count (ISO Cleanliness Code)
Water Content
Usually %
PH Level
TAN (Total Acid Number)
4.50 or below is considered normal
Only used for polyglycols and
synthetic fluids.
Represents the amount of oxidation
in the oil.
Viscosity
Low viscosity can be caused by high temperature or adding the wrong type of oil.

49. Compressor Reliability Checks
Fluid Levels
Levels should be checked regularly
Flow rates to cylinder injectors must be monitored
Accumulated water must be drained regularly
System Pressures
Pressures should be checked / monitored daily.
Differential pressure of the inlet air filter
Bearing, seal and filter pressures
Suction, discharge and intercooler pressures
Pressures should be compared to the manufacturer’s recommendations.
Temperature
Temperatures should be checked daily.
Suction line, compressor discharge,
intercoolers and aftercoolers
Inlet & outlet water temperatures of coolers
Lube system and crankcase
Compressor valves, bearings and cylinder heads

50. Work Exercises
1. The amount of air flow that a compressor delivers is rated in:
A. GPM B. CFM
C. HP D. HP
2. If 60 CFM is required at 100 PSI, how much horsepower is required?
___________ HP 15
3. A centrifugal compressor is considered what type of compressor?
A. Reciprocating B. Dynamic
C. Positive Displacement D. Rotary Screw
4. True or False: A single stage compressor is normally used with pressures
above 100 PSI.
True False

51. Work Exercises
5. True or False: Single stage compressors usually run relatively cool with
temperatures rarely exceeding 200°F (90°C).
True False
6. What is the number one reason for downtime of a reciprocating compressor?
_______________________________________________________________
Intake and discharge valve failure
7. What happens to air when it is compressed?
A. The air is heated
B. The pressure increases
C. A vacuum is created at the inlet port of the compressor
D. All of the above
E. None of the above
8. True or False: The intercooler reduces the horsepower required to drive the
compressor.
True False

52. Work Exercises
9. The incoming water in a water cooled intercooler should:
A. Be cooler than the inlet air temperature
B. Flow the same direction as the air
C. Be cool enough to turn the water vapor to condensate.
D. Flow in the opposite direction of the air
10. What two areas require lubrication in a reciprocating piston compressor?
1. ________________________
2. ________________________
Compressor housing
Cylinder Area
11. What happens if the check valve fails open in a reciprocating compressor
system where a pressure switch and unloading valve are used?
__________________________________
Air will back flow into the compressor

53. Work Exercises
12. What will happen to a reciprocating compressor if the air operated
unloading valve fails in the shifted position?
A. Pressure will not build up at the outlet port
B. The electric motor will trip out
C. Pressure will build until the relief valve on the receiver tank opens
D. The compressor will continue to operate normally
13. Where would you most likely use a rotary vane compressor?
A. When flow rates over 5,000 CFM are required
B. When pressures below 80 PSI are required
C. When a compressor is needed that is more efficient than a
rotary screw type
D. When the surrounding air is dirty
14. True or False: In a rotary screw compressor, the male screw drives the
female screw.
True False

54. Work Exercises
15. What 3 purposes does the injected oil serve in a wet rotary screw
compressor?
1. ____________________________________________________
2. ____________________________________________________
3. ____________________________________________________
Provides a lubricant between the male and female screws
Acts as a sealant between the screws, preventing blow by
Absorbs the heat of compression
16. What is the purpose of the minimum pressure check valve in a wet
rotary screw compressor system?
A. To dump the air to atmosphere if the outlet pressure builds too high
B. To allow the air to flow through the oil cooler
C. To make sure the air pressure doesn’t build over 36 PSI
D. To make sure that adequate lubrication is provided to the screws
at all times

55. Work Exercises
17. What happens if the oil return filter element in a rotary screw
compressor system becomes contaminated?
A. The oil will flow through the bypass check valve inside the filter
housing
B. The filter housing will be destroyed
C. The thermostatic valve will port all the oil exiting the air/oil
reservoir through the oil cooler
D. The compressor will shut down on overload
18. True or False: You would normally use a rotary lobe compressor when
80 PSI is required.
True False
19. You would normally use a centrifugal compressor when _______ CFM
or more is required.
3,000

56. Work Exercises
20. What 3 components require lubrication in a centrifugal compressor?
1. ______________ ______________ ______________
Bearings
Gears
Seals
21. If 50 horsepower is required to operate a compressor at 90 PSI, how
much horsepower will be required if the pressure is increased to 110 PSI?
A B C D. 55
22. You are called in to troubleshoot a low pressure problem at the
compressor. What is the most likely problem?
A. Improper pressure switch setting
B. Dirty oil filter
C. The fins on the air cooler are dirty
D. The outside air temperature is too high

57. Work Exercises
23. A reciprocating compressor with a water intercooler has shut down due
to over temperature. Name three checks you would make.
1. ___________________________________________
2. ___________________________________________
3. ___________________________________________
Check the intake filter
Check for restricted water flow through the cooler
Check the oil level
24. Oil is showing up in the outlet air of a wet rotary screw compressor. The oil
level and temperature are normal. What’s the most likely problem?
A. Worn piston rings
B. Bad aftercooler
C. The thermostatic valve is bad
D. The oil separator is saturated