The Castair D5112HC2-S is a 10 HP total (2x5 HP) two-stage duplex air compressor running on single-phase power with a 120-gallon ASME horizontal tank. This is a single-phase duplex — two independent pump-motor assemblies sharing one tank — which means you get 35 CFM at 175 PSI and built-in redundancy on standard single-phase electrical service. If one pump needs service, the other keeps your shop running. That is the core reason to choose a duplex over a single-unit machine.
The C2 pump configuration runs at 420 RPM (slower-speed C2 pump — quieter, longer service life). Both C1 and C2 configurations deliver the same 35 CFM at 175 PSI; the difference is pump speed, noise level, and long-term service life. The C2 pump, running slower, runs cooler and quieter and will generally outlast the C1 configuration in continuous-duty applications. The C1 is lighter and slightly lower cost.
Castair builds these in Spicer, Minnesota — family owned since 1991. The Duplex Series uses the same industrial pump design as their Industrial Series single-unit machines: all cast iron, tapered roller bearings, automotive-type rod bearings, and disc/spring valves.
Key Features
Pump Construction — C2 Configuration
- All Cast Iron Construction: Cast iron cylinders, heads, and crankcase — long service life and lower noise versus aluminum pump designs
- C2 Pump at 420 RPM: Slow-speed configuration — cooler running, quieter, and longer service life in continuous-duty applications
- Tapered Roller Bearings: Both ends of the crankshaft — superior load support versus ball bearings
- Precision Steel Connecting Rods: Automotive-type rod insert bearings at crankshaft and wrist pin — serviceable and built for long intervals
- Disc and Spring Valves: Higher efficiency and easier service than reed valves
- 4-Ring Pistons: Better sealing, less oil carry-over into discharge air
- Centrifugal Unloader: Loadless start every time — no fighting cylinder pressure at startup
Two-Stage Compression
- Two-Stage Design: Compresses to intermediate pressure, then to 175 PSI — more efficient and cooler than single-stage at this pressure
- 175 PSI Maximum: Covers all standard pneumatic tool requirements; higher than single-stage units
- 44 CFM @ 0 PSI / 35 CFM @ 175 PSI: Consistent output across the pressure range
Tank, Controls, and Electrical
- 120-gallon ASME Horizontal Tank: Powder-coated steel with channel base; ASME certified
- Dimensions (L x W x H): 85" x 28" x 57"
- ASME Safety Valves: Factory installed on both pump discharge lines
- Magnetic Starter — Mounted and Wired: Factory-installed for both 5 HP motors; includes overload protection
- Metal Belt Guard — Totally Enclosed: OSHA-compliant drive belt and pulley guarding
- Heavy Duty 1725 RPM Industrial Motors: Full 15% service factor — rated for shop duty cycles
Duplex Redundancy
- Two Independent Pump-Motor Assemblies: Either unit can carry shop air demand while the other is serviced
- No Shutdown for Routine Maintenance: Service one pump while the other runs
What's in the Box
- Castair D5112HC2-S duplex compressor (two 5 HP pump-motor assemblies, 120-gallon ASME tank, skid)
- Magnetic starter — factory mounted and wired
- ASME safety valves (factory installed)
- Pressure gauges and controls
- Belt guards (factory installed)
- Owner's manual
Ships on freight. This unit weighs 1,300 lbs — confirm delivery and unloading equipment before scheduling.
Available Factory-Mounted Options
- Low Oil Control (#6304x2M): Pump shutdown on low oil level
- Hour Meters (#HRM75x2M): Individual runtime tracking per pump
- Alternator (#ALT230M): Rotates lead pump each cycle for even wear — particularly useful on C2 units where manual alternation is otherwise required
- Electronic Auto Drain (#SDV03M): Timed automatic tank drain
- Aftercooler (#BGAC1x2M — 70 CFM rated): Reduces moisture in compressed air lines
- Belt Tensioner (#184Dx2M — 5 HP): Simplifies belt maintenance
- Intake Noise Silencer (#SA100x4M): Additional intake noise reduction
- Vibration Pads (#VP1004 — set of 4)
- Right From The Start Kit (#RFS001D — 10 HP)
- All Options C2 Package (#AOC2D)
Available Configurations — 10 HP Duplex Lineup
- D5112HC2-S (this unit) — 10 HP, 1-Phase, 120-gallon, 420 RPM C2 pump, $7,860.30: Current selection
- D5124HC2-S — 10 HP, 1-Phase, 240 gal, 420 RPM C2 pump: Larger tank for higher peak demand
- D5312HC1-S / D5312HC2-S — 10 HP, 3-Phase equivalents: 3-phase versions of the same system — preferred if 3-phase is available (more efficient motors, lower electrical stress)
- D7112HC1-S / D7112HC2-S — 15 HP (2x7.5), 1-Phase, 120-gallon: Step up to 49.6 CFM @ 175 PSI
When Does a Shop Need This vs. Stepping Up or Down?
The D5112HC2-S fits when:
- Your shop does not have 3-phase power and needs a duplex system — this is the only way to get duplex redundancy on single-phase
- Your sustained CFM demand is in the 25–35 CFM range
- You have multiple technicians but cannot justify the cost of a 15 or 20 HP system
- Downtime is a concern — the second pump keeps you running during service
Step up to 3-phase duplex (D5312/D5324 series) when: 3-phase power is available — 3-phase motors are more efficient and run cooler at equivalent HP, and the electrical installation is simpler at this power level.
Step up to 15 HP duplex when: Your shop consistently pulls more than 35 CFM or runs equipment that needs higher sustained output.
A single 10 HP compressor may be enough if: Redundancy is not a priority and your CFM demand fits a single-unit machine — the duplex adds cost and complexity that only pays off if you need the backup pump or if you cannot shut down for maintenance.
Technical Specifications
- Model Number: D5112HC2-S
- Series: Duplex Series
- Horsepower (each / total): 2x5 HP / 10 HP total
- Electrical: Single-Phase (1-Phase)
- Pump Configuration: C2
- Pump RPM: 420
- CFM @ 0 PSI: 44
- CFM @ 175 PSI: 35
- Maximum Pressure: 175 PSI
- Tank Capacity: 120 gallons
- Tank Style: ASME horizontal with channel base
- Dimensions (L x W x H): 85" x 28" x 57"
- Shipped Weight: 1,300 lbs
- Magnetic Starter: Included, factory mounted and wired
Why Buy From Air Compressor Factory Direct?
We have specialized in professional-grade shop compressors since 2016. If you are deciding between C1 and C2 pump configurations, between 120 and 240 gallons, or between single-phase and three-phase — call us. We will help you figure out what actually fits your shop's electrical service and CFM demand before you commit to a freight shipment of a 1,000+ lb compressor.
- Free Shipping on qualifying orders (freight; site requirements must be confirmed)
- Free Technical Support — 866-217-0063
- Full Manufacturer Warranty — Castair backs their duplex systems
- Honest Configuration Guidance — C1 vs. C2, tank sizing, electrical requirements
Call 866-217-0063 for quick answers and help!
