Compressors

Standard Air

• Standard air has the following characteristics– 14.7 psia (101 kPa abs)– Density 0.0725 lb/ft3 (11.8 N/m3)– Temperature 68 F (20 C)– Relative humidity 38%

• Pressure lower than 14.7 psi (101 kPa) absolute creates a vacuum pressure

Rating Compressors• Compressors are rated in terms of cfm or scfm

when the inlet is drawing from atmospheric conditions

• (101 KPa, 20oC)• (14.7 psia and 68°F )

21

1221

21

1221

*

*

Tp

TpQQ

Tp

TpVV

Compressors

• While air enters at ambient conditions it must be converted to operate at pressures of 6 to 12 bars, and temperatures of 20-100 C

• When conversion takes place it produces variation in the pressure, temperature, and volume (typically it is reduced)

Compressor Categories• Positive displacement

– Piston, Screw, and Vane

• Compressors having more than one cylinder are called multi-stage;

the pressure increases are done through the use of multiple cylinders to achieve desired pressure

Power requirements

• How much horsepower do we need to run the system? Q(sm3/min) P(kPa Abs)

11.17

14.65

286.0

286.0

in

outin

in

outin

P

PQPkW

P

PQPHP

Compressor types

Single cylinder compressor

Multi-staging

• Up to 4 bar single stage

• Up to 15 bar double stage

• Over 15 bar treble or multi-stage

Two-stage compressor

Multi-staging with inter-coolingfor high pressures

Screw compressors

Screw compressors

Vane compressor

Receiver pressure control via motor start/stop

Compressor unloadingpressure switch

Compressor unloadingcentrifugal unloader

Pressure settings

Conditioning Air• The key components to condition compressed air

are:– coolers– Moisture separators– Air dryers & receivers– Filters, regulators, lubricators– Mufflers

Conditioning components

• After coolers– Used to condense moisture in air prior to operating

components (80-85% effective)

• Moisture separators– Used to collected and discard condensation from air

• Air dryers– Secondary measure used to extract any remaining

moisture from system (uses desiccants)

Conditioning components

• Air receivers– Storage vessel used to equalize the pulsation of

incoming air and allow for uniform flow or air

• Filters, regulators, lubricators– Used to clean, regulate, and lubricate

• Mufflers– Placed at exhaust to reduce noise and the possibility

of airborne particles

Moisture and Dew point• Depending on

atmospheric conditions air will contain varying degrees (percentages) of moisture

• Saturation point (dew point) is the temperature at which moisture begins to condense of the air