Factors that Affect Efficiency

Application factors

•  speed

•  suction/discharge pressures

•  type of refrigerant used

•  capacity control method

Speed

•  hermetic compressors 3600 Rpm. Rated operating 3450 Rpm

- Semi-hermetic comp. 1800 Rpm. Rated operating 1725 Rpm

- Open Drives 500 Rpm. to 150 Rpm.

Formula for calculating motor speed or pulley sizes

Compressor Speed =
Motor Speed x Compressor Flywheel Diameter/ Compressor flywheel diameter

Motor Pulley Diameter =
Compressor Speed x Compressor Flywheel Diameter/ Motor Speed

Suction and Discharge Pressures

•  Specific Volume increases on decrease in Suction Pressure (cubic ft/lb)

•  Volumetric Flow Rates should remain constant (CFM)

•  Mass Flow Rates decrease on decrease in Suction (lb/min)

Max Compression Ratio's (the higher the ratio the harder it is on the comp.)

Reciprocating

•  max 10 to 1 (Staging is recommended if this is exceeded) to 20 to 1

Rotary

•  5 to 1

Centrifugal

•  3 to 1

Screw

•  20 to 1

Scroll

•  20 to 1

Formula for finding Compression Ratio's

•  Absolute Discharge press. divided by Absolute Suction press.
•  "Absolute" Don't forget to add 14.7

Design Factors

•  piston/ comp. Displacement

•  suction/discharge valves (design)

•  clearance pocket (size)

•  number of cylinders

Piston and Compressor Displacement

Formula: for Volumetric Efficiency % of Compressor

Output (cfm) or Actual x 100% / Input (cfm) Theoretical

Example: a comp. Is designed to pump 10 cubic inch's each stroke (piston displacement). If it pumps 6 cubic inch each stroke. What is the Volumetric Efficiency of the compressor?

6/10 x 100 = 60% Volumetric Efficiency

Capacity Control

Centrifugal compressor

•  variable inlet guide vanes

•  number of impellers

•  butterfly dampers *limit amount of suction gas

Screw Compressors

•  suction slider

•  speed control

Reciprocating Compressors

•  on/off

•  speed control

•  multiple compressors

•  cylinder unloader (mechanical), suction bypass, suction cut-off

•  Hot gas Bypass * need quenching valve (to suction line, to evaporator inlet)