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Sullair LS-20 Rotary Screw Air Compressor

Operation and maintenance manual for Sullair LS-20 series 75-hp rotary screw air compressor. Covers fluid sampling, separator maintenance, and electrical troubleshooting.

Sullair LS-20-75

Safety Warnings

  • •Do not service while unit is running or pressurized
  • •Compressor fluid is hot during operation – burn hazard
  • •High voltage motor starter – LOTO before service
  • •Release all air pressure before opening any piping

Fluid Maintenance

Sullair AWF Fluid Change Intervals

  • Initial fill: Change at 500 hours
  • Subsequent: Change every 2000 hours or annually
  • Fluid analysis: Recommend every 500 hours for extended drain intervals
  • Capacity: 13.5 gallons

Fluid/Separator Filter Replacement

  1. Run compressor to normal operating temperature
  2. Shut down and allow to cool slightly
  3. Depressurize via unloader valve
  4. Drain fluid at drain valve
  5. Remove and replace fluid filter
  6. Remove separator tank cover and replace separator element
  7. Refill with correct fluid type and quantity
  8. Run 30 minutes – check for leaks

Separator Condition Test

  • Measure differential pressure across separator
  • New: 2-4 psi
  • Replace when: > 10 psi or annually
  • Carryover test: Oil in discharge air > 2-3 ppm = replace separator

V-Belt Tension

  • Deflection method: 1/64" per inch of span
  • Example: 16" span = 1/4" deflection with 4 lbs force
  • Replace belts in matched sets only

Temperature Limits

| Location | Normal | Shutdown | |----------|--------|----------| | Discharge fluid | 180-200°F | 230°F | | Ambient (inlet) | up to 100°F | 104°F | | Motor winding | up to 180°F | - |

Structured Troubleshooting

Common problems, likely causes, diagnostic steps, and safe resolution paths.

Sullair LS-20 compressor not building pressure

High confidence
Likely Causes
  • Inlet valve not opening fully — inlet valve solenoid or piston failure
  • Minimum pressure/check valve stuck open — system pressure bleeding back
  • Air/oil separator element clogged — excessive differential pressure across separator
  • Compressor unloading circuit fault — unit unloading immediately on load
Diagnostic Steps
  1. Check inlet valve operation — verify solenoid energizes on load command
  2. Listen for air returning through inlet during unload — indicates min pressure valve leak
  3. Measure differential pressure across separator element (normal <10 PSI; replace at 15 PSI)
  4. Check load solenoid valve operation and inlet valve position feedback
Resolution Paths
  • Replace inlet valve solenoid or piston if not operating
  • Replace minimum pressure valve if leaking back
  • Replace air/oil separator element per Sullair service interval
  • Repair or replace load solenoid valve

High oil temperature shutdown

High confidence
Likely Causes
  • Oil cooler fouled — fins clogged with dirt or debris
  • Low oil level — insufficient oil to absorb compressor heat
  • Thermostatic bypass valve stuck in bypass position — oil not flowing through cooler
  • Ambient temperature above compressor design rating
Diagnostic Steps
  1. Inspect oil cooler exterior — clean fins with low-pressure compressed air or water wash
  2. Check oil level in sight glass — should be at midpoint when running
  3. Verify thermostatic valve operation — oil should circulate through cooler above 180°F
  4. Compare ambient temperature to compressor rated maximum ambient (typically 104°F/40°C)
Resolution Paths
  • Clean oil cooler and verify airflow through cooler cabinet
  • Add oil to midpoint of sight glass (use Sullair approved synthetic lubricant)
  • Replace thermostatic bypass valve if stuck in bypass
  • Install additional ventilation if ambient consistently exceeds rating

Excessive oil carryover into compressed air

High confidence
Likely Causes
  • Air/oil separator element saturated or damaged
  • Oil level too high — oil above maximum line
  • Scavenge orifice plugged — separator bowl oil not returning to sump
  • System pressure too low — separator not functioning at design differential
Diagnostic Steps
  1. Inspect downstream filter/separator for oil — brown color indicates carryover
  2. Check oil level when running — reduce if above MAX line
  3. Verify scavenge tube and orifice are clear — remove and clean scavenge assembly
  4. Measure operating pressure — below minimum pressure valve setpoint (typically 65 PSI) reduces separation efficiency
Resolution Paths
  • Replace separator element on schedule (typically 8,000 hours or annually)
  • Drain oil to proper level
  • Clean scavenge orifice (0.060 inch diameter typical)
  • Correct system pressure to maintain above minimum valve setpoint
compressorsullairrotary screwLS-20compressed air75hp