Basic Compressors

1
Basic Compressors

• 60-125 PSIG

2
Air Compression

• Air must be compressed in stages
• Air must be cooled between stages
• Compression ratio about 4-5 to 1 to keep
  temperature rise from burning lubricant
• Compression ratio about 3 to 1 in multistage
  compressors.
• Heat is lost energy!
• Circulating air or water used for inter-stage
  cooling.

3
Prime Movers

• Electric motors
• Diesel or gasoline engines
• 1 hp mechanical input will produce about 4 to 6
  SCFM of shop air (125 PSIG)
• Compressors usually deliver 5 SCFM per hp.

4
Air Processing

• Filtering
• Regulation
• Lubrication
• Processing must be done at every machine using
  compressed air.
• Water vapor may have to be removed with a dryer.
• Air is stored in a tank called the receiver.
  Three times the pump capacity.

5
Cycling the Compressor

• Motor turns on at a preset low pressure (cut-in)
• Motor turns off at a preset higher pressure
  (cut-off)
• Compressor should have enough capacity to
  maintain useable pressure by running 50 to 75
  of the time.

6
What pump capacity in SCFM?

• Step 1 Size the Cylinder for Max Performance
• Step 2 Calculate total volume per cycle
• Step 3 Calculate total volume per minute
• Step 4 Convert CIM to Cubic Feet/Minute (CFM).
• Step 5 Convert CFM to SCFM

7
Step 1 Size the Cylinder for Max Performance

• Based on common practice the range of the force
  multiplier to be between 1.25 to 2.00 times the
  load being moved.
• AF/P
• F is the resistance force
• P is the system pressure (F x 2 )
• For a given pressure,
• Larger bore makes piston slower
• Smaller bore makes piston slower also

8
Step 2 Calculate total volume per cycle

• Calculate the extend and the retract volume. Add
  them together.

Example for a 4 bore cylinder with a 1 rod. The
stroke is to be 12 The retract volume is 141.36
cubic inches. The total volume per cycle is
292.20 cubic inches.
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Step 3 Calculate total volume per minute

• Multiply the cycle volume by the number of cycles
  per minute to get CIM (cubic inches per minute)

Continuing the previous example, if there are 30
cycles each minute then the CIM is 8,766 in.³ per
minute (CIM)
10
Step 4 Convert CIM to Cubic Feet/Minute (CFM).

• To get CFM, divide CIM by 1728.

Continuing the previous example, CFM would be
8,766 in.³ per minute (CIM) divided by 1728 or
5.07 CFM
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Step 5 Convert CFM to SCFM

• Ignoring the humidity has little effect on the
  result of calculation.
• Find the compression ratio (System pressure
  14.7 PSI) divided by 14.7 PSI
• Multiply the CFM by the compression ratio.

Continuing the previous example, The compression
ratio is 6.44 so multiply the CFM by 6.44 for
32.65 SCFM. This is the flow-rate needed from
the compressor for just one 4 cylinder.
12
Step 6 Find HP from table

• HP depends on the number of stages of
  compression.
• HP depends on the pressure required
• The table gives HP per 1 SCFM so the tabulated HP
  values must be multiplied by the required SCFM.

Continuing the same example Assume a three
stage compressor and a pressure of 150 PSIG.
Read .190 from the table and multiply by 32.65.
Thus we will need 6.2 hp of compressor power for
this actuator.
13
Air is NOT free.

• It costs between 0.25 and 0.50 to deliver 1000
  SCFM