The Castair D7124HC2-S is a 15HP (2×7.5HP) single-phase duplex air compressor on a 240-gallon horizontal ASME tank with twin C2 cast iron pumps running at 585 RPM. This is the top configuration in Castair's single-phase 7.5HP duplex lineup: the largest tank, the heavier C2 pump, and the highest CFM output — 56.2 CFM at 175 PSI — in a machine that runs on standard single-phase electrical service.
If you are running a serious single-phase shop and you want the best combination of output, air storage, pump longevity, and backup redundancy that Castair builds in the 7.5HP class, this is that machine. The C2 pump moves more air per revolution than the C1 at the same horsepower by using a larger bore at a slower shaft speed. The result is more CFM, lower operating temperature, and a pump that handles high duty cycles more gracefully than a lighter, faster-spinning equivalent.
Who Needs the D7124HC2-S
Auto body shops, fleet maintenance operations, commercial tire centers, and industrial production facilities running single-phase power that need maximum sustained air output with genuine redundancy. At 56.2 CFM delivered at 175 PSI, this unit supports four or more technicians working simultaneously on air tools without significant pressure drop. The 240-gallon tank keeps cycle frequency low even during sustained peak demand.
Shops that run spray equipment, sandblasters, or other high-draw intermittent tools will particularly appreciate the 240-gallon reserve. The extra tank volume absorbs peak demand without immediately triggering the pumps, which reduces cycle frequency and heat accumulation in the pump internals over the course of a workday.
C2 Pump Features
- All cast iron construction: Cylinders, crankcase, valve plates — cast iron throughout. Not aluminum. Cast iron is dimensionally stable under thermal cycling, quieter under load, and longer-lived in continuous duty applications.
- Slow 585 RPM operating speed: The C2 achieves its output at 585 RPM by using a larger bore — not by spinning faster. Lower RPM equals lower operating temperature, lower wear rate, and longer maintenance intervals.
- Larger bore than C1: Same horsepower input, more air output. The C2 delivers 56.2 CFM versus the C1's 49.6 CFM at 175 PSI because it moves more volume per revolution at a slower speed. This is a more efficient use of the motor's power output.
- Tapered roller bearings: Both crankshaft ends run on tapered roller bearings — not ball bearings — for superior support under the combined radial and axial loads of reciprocating operation.
- Precision steel connecting rods: Automotive-type rod insert bearings at the crankshaft journal and at the wrist pin for long service life in continuous duty operation.
- Disc and spring valves: More durable and easier to service in the field than reed valves. Higher efficiency across the operating pressure range.
- 4-ring pistons: Four compression rings per piston for better sealing efficiency and lower oil carry-over into the compressed air stream.
- Centrifugal unloader: Every startup is unloaded — motor reaches speed before compression load engages, protecting both motor windings and pump internals at each startup cycle.
Tank, Motor, and Control Features
- 240-gallon horizontal ASME tank: Powder-coated with channel base. ASME certified — constructed and tested to meet pressure vessel safety standards, not just labeled as such.
- ASME safety valves: Certified pressure relief hardware, factory-original.
- Fully enclosed metal belt guard: Safety compliance and protection for drive components.
- Copper discharge and unloader tube or stainless-steel braided flex line: No plastic in the discharge path. Copper or stainless where heat and pressure demand it.
- Heavy-duty 1725 RPM industrial motors: Full 15% service factor or higher. These motors are rated for continuous commercial industrial use.
- Magnetic starter mounted and wired (-S models): Factory-installed and pre-wired magnetic motor starters. You connect your power supply to the panel — the internal wiring is complete.
Duplex Operation: The Logic Behind Two Pumps
A duplex compressor is two independent pump-and-motor assemblies sharing a common tank and control system. Each cycle, the control alternates which pump leads, distributing wear evenly over time. When pump A needs service — a valve, a belt, a motor bearing — pump B runs the shop. You schedule service on your timeline instead of the compressor's failure timeline.
For a shop billing 50/hour per technician bay, an unexpected compressor shutdown costs real money in hours. The duplex architecture eliminates that exposure. It is not a redundant purchase — it is a risk management purchase.
Factory-Mounted Options Available
- Low Oil Control (#6304x2M): Shuts down both units before oil starvation can score pump cylinders or wipe bearings.
- Hour Meter (#HRM75x2M for 230V): Track actual operating hours per pump unit for interval-based maintenance scheduling.
- Alternator (#ALT230M): Automatic cycle-by-cycle alternation between pump units to equalize long-term wear.
- Electronic Auto Drain (#SDV03M): Timed condensate purge. Tanks not drained daily accumulate moisture and corrode internally over time. This option automates that maintenance task.
- Dual Control (#DC2x2M for C2 pumps): Enables continuous unloaded operation during sustained demand periods instead of cycling on and off repeatedly. Motor inrush at startup is 4 to 5 times running amperage. Reducing start frequency reduces electrical cost and motor stress proportionally.
- Intake Noise Silencers (#SA100x4M for C2 duplex): Inlet silencers on each pump head reduce intake noise without restricting airflow.
- Belt Tensioner (#215Dx2M for 7.5-10HP): Spring-loaded belt tensioner for consistent drive geometry as belts stretch.
- Vibration Pads (#VP1004): Four isolation pads to reduce floor vibration from the unit.
- Belt Guard Aftercooler (#BGAC1x2M, 70 CFM for 10-20HP units): Reduces air temperature and moisture content before the compressed air enters the tank.
- All Options Bundle (#AOC2D): Order all asterisk options together for 10% off the options package total.
Available Configurations — 7.5HP Single-Phase Duplex
- D7112HC1-S: 2×7.5HP, single-phase, C1 pump (895 RPM), 120-gal, 49.6 CFM@175 PSI, 1,240 lbs.
- D7112HC2-S: 2×7.5HP, single-phase, C2 pump (585 RPM), 120-gal, 56.2 CFM@175 PSI, 1,430 lbs.
- D7124HC1-S: 2×7.5HP, single-phase, C1 pump (895 RPM), 240-gal, 49.6 CFM@175 PSI, 2,140 lbs.
- D7124HC2-S (this unit): 2×7.5HP, single-phase, C2 pump (585 RPM), 240-gal, 56.2 CFM@175 PSI, 1,780 lbs. — maximum output and storage in single-phase.
When Does a Shop Need This vs. Stepping Up or Down
- Step down (D7124HC1-S): If 49.6 CFM at 175 PSI covers your typical tool load and you want to keep weight and cost lower, the C1 version on the 240-gallon tank gives you the same storage with a lighter pump and slightly lower CFM. Note that the C1 240-gallon unit actually weighs more (2,140 lbs.) than this C2 240-gallon unit (1,780 lbs.) because of frame differences.
- This unit (D7124HC2-S): Single-phase power, maximum CFM demand, need for redundancy, shop runs the compressor hard through the full workday. The definitive single-phase 7.5HP duplex configuration.
- Step up (three-phase or 10HP): If your facility has three-phase power, the D7324HC2-S delivers equivalent performance at lower single-phase amperage draw. If you need more than 56 CFM from a duplex, the next step is the 10HP duplex series.
Technical Specifications
- Model: D7124HC2-S
- Configuration: Duplex (2 pumps, 2 motors)
- Motor: 2×7.5HP, 15HP total
- Phase: Single-phase
- Pump Series: C2 (cast iron)
- Pump RPM: 585
- CFM @ 0 PSI: 70
- CFM @ 175 PSI: 56.2
- Tank: 240-gallon horizontal ASME
- Dimensions (L×W×H): 85 × 30 × 64 inches
- Weight: 1,780 lbs.
- Magnetic Starter: Included, factory-mounted and wired
- Max Pressure: 175 PSI
Why Buy From Air Compressor Factory Direct?
We have been selling and supporting professional-grade air compressors since 2016. We carry the full Castair lineup across every HP class, pump series, and tank configuration. Free shipping on all orders. Free tech support at 866-217-0063 — if you want help comparing single-phase amperage requirements for this unit versus the three-phase alternatives, or working out whether 56 CFM is the right number for your shop, call us. Full manufacturer warranty on all units. Honest guidance — if a different configuration better fits your shop, we will tell you before you order, not after.
Call 866-217-0063 for quick answers and help!
