STSD/STSP/STSP2 Series Product Overview

STSDSTSD (VSD)STSPSTSP (VSD)STSP2STSP2 (VSD)

The STSD, STSP, and STSP2 are directly-coupled oil-injected screw air compressors available in both fixed-speed (star-delta starter) and variable-speed drive (inverter/VSD) configurations. The STSP2 and STSP2 (VSD) are two-stage compressors — the only two-stage models in the South-Tek screw compressor lineup. All models use the AirMASTER FORM controller (UL certified).

Model Summary

Model	Stages	Starter	Drive	Fan Type
STSD	Single	Star-delta	Direct couple	Axial or Radial
STSD (VSD)	Single	Inverter	Direct couple	Axial or Radial
STSP	Single	Star-delta	Direct couple	Axial or Radial
STSP (VSD)	Single	Inverter	Direct couple	Axial or Radial
STSP2	Two-stage	Star-delta	Direct couple	Axial or Radial
STSP2 (VSD)	Two-stage	Inverter	Direct couple	Axial or Radial

Key Specifications

Specification	Value
Compressor Type	Oil-injected rotary screw (single or two-stage)
Drive	Direct couple (motor to airend — no V-belt)
Cooling	Air-cooled (axial or radial fan); water-cooled optional
Controller	AirMASTER FORM (UL certified)
Min. Ambient Temperature	+33.8°F (+1°C)
Max. Ambient Temperature	+104°F (+40°C)
Max. Operating Temperature	+230°F
Max. Altitude	3,280 ft above sea level
Max. Humidity	80%

System Components

Oil System

The oil system includes the screw assembly, separator tank, separator element, thermostatic valve, oil cooler, and oil filter. In the separator tank, the majority of oil is separated from the air/oil mixture by centrifugal force and accumulates at the bottom of the tank. Remaining oil passes through the separator element, collects at the element's base, and is returned to the screw assembly via the scavenge line (which includes a one-way check valve). Oil quantity to the screw assembly is controlled by an orifice at the airend inlet.

▲ Attention At high ambient temperatures (above +104°F / +40°C), oil entering the screw assembly should be at or above +122°F (+50°C). Cold oil causes condensation in the separator, leading to corrosion and oil breakdown that can seriously damage the screw assembly.

Cooling System — Axial and Radial Fan Models

STSD/STSP/STSP2 units may use either an axial fan or a radial fan, depending on the configuration.

Axial fan models: Cooling air is drawn through the cabinet and discharged through the cooling pads. Hot air must be ducted to the exterior. Duct resistance must not exceed 50 Pa (5 mm H₂O).

Radial fan models: Cooling air is drawn in from outside the cabinet and discharged to the exterior via the cabinet. The user must install intake and discharge hoods and air ducts. In cold weather when intake air is below +41°F (+5°C), a bypass line from the hot air outlet hood to the intake hood is required to prevent thermal shock to the air/oil coolers.

⚠ Warning — Radial Fan Safety Radial fans generate strong suction at the intake. Personnel wearing loose clothing, gowns, or similar must exercise extreme caution near the fan intake zone. Sudden suction can cause loss of balance. Install appropriate guarding around the intake area.

Suction Control Valve — JF Series (NA Solenoid)

These models use the JF series suction control valve with an NA (normally-open) solenoid valve and a venting valve — the same valve family as the STS series.

• Loading: NA solenoid energizes (closes); venting valve opens. Separator tank pressure discharges through the suction valve mouth, allowing the suction flap to open and air intake to begin.
• Unloading: NA solenoid de-energizes (opens); venting valve closes. Suction flap closes; air intake stops.
• Auto-stop: If the lower pressure setpoint is not reached during the idle timer period, the compressor stops and the separator tank vents through the suction valve mouth.

Direct Couple Drive — No V-Belt

Unlike the STS and STSH models, the STSD/STSP/STSP2 use a direct coupling between the motor and airend. There is no V-belt to tension or replace. V-belt maintenance procedures do not apply to these models. The coupling elastomer must be inspected and replaced at the intervals specified in the maintenance schedule (typically every 16,000 hours or 24 months).

Carbon Graphite (Coal) Brush — VSD Models 148 HP and Above

On VSD (inverter) models rated 148 HP (110 kW) or above, inverter-induced shaft currents can travel through motor bearings, causing premature bearing failure. A carbon graphite brush mounted to the motor shaft provides a low-resistance discharge path for these currents, protecting the bearings. The carbon brush must be inspected and replaced every 8,000 hours or 12 months. See the Electric Motor Carbon Brush Replacement article for the replacement procedure.

Optional Features

• Water-cooled heat exchanger: Installed on the oil circuit inside the cabinet. Water circuit must be active before first startup. Cooling water quality requirements apply — see section 2.3.1 of the STSD/STSP/STSP2 manual.
• Separator tank oil heater (125W): Warms oil before cold-weather startup. Controlled by the air-oil temperature sensor. At ambient temperatures below +32°F (0°C), the control system prevents startup automatically.
• Air filter clogging indicator: Installed on STSD/STSP/STSP2 models; must be replaced every 4,000 hours or 6 months alongside the air filter element.
• Oil filter clogging sensor: Monitors differential pressure across the oil filter; must be replaced every 8,000 hours or 12 months.

Related Articles

• STSD/STSP/STSP2 Maintenance Schedule
• Carbon Brush Replacement
• First Start-Up & Commissioning Checklist
• Safety Precautions & Symbols

For technical support or commissioning, contact South-Tek Systems: [phone] or [e-mail]