Castair 3HP Rotary Screw Base Mount Compressor - R31BM50

Castair 3HP Rotary Screw Base Mount Compressor — R31BM50

The Castair R31BM50 is a 3-horsepower rotary screw compressor sold as a base mount unit — no tank included. This configuration is designed for shops that already have a receiver tank in place, want to upgrade an existing system's pump and motor, or need to mount the compressor remotely from the air storage. The rotary screw design means 100% duty cycle: this unit can run continuously without the piston heat buildup and valve wear that limits reciprocating compressors in high-demand applications.

Note: Manufacturer specifications for the R31BM50 were not available on Castair's website at time of writing. The specifications below are based on available model number data and Castair's standard rotary screw line. Contact us at 866-217-0063 for confirmed specs before ordering.

Why Rotary Screw vs. Piston for a 3HP Application?

• 100% duty cycle: A rotary screw at this horsepower can run continuously. A piston compressor at 3HP is typically rated for 50-75% duty cycle — in a busy shop, that matters.
• Quieter operation: Rotary screw compressors run at lower RPM than piston units of the same output, producing significantly less noise on the shop floor.
• Lower maintenance interval: No valves, pistons, or rings to wear out. Scheduled maintenance is oil, oil filter, air filter, and separator filter changes at manufacturer-specified intervals.
• Consistent pressure: Rotary screw compressors maintain steady output pressure without the pressure cycling of a piston/tank setup running to cut-out and recovering.

Base Mount Configuration — What This Means for Your Install

• No tank included: The R31BM50 ships as the compressor assembly only — air end, motor, belt drive, controls, and base frame. You supply the receiver tank.
• Flexible installation: Mount to an existing horizontal or vertical receiver, a wall-mounted tank, or a remote tank plumbed to your drop lines.
• Smaller footprint: Ideal when floor space is at a premium and a tank is already in service.
• System upgrade path: If you have an older piston compressor with a good tank, the base mount configuration lets you drop in a rotary screw head without replacing the entire system.

Castair Rotary Screw Build Quality

• All cast iron construction: Castair builds with cast iron throughout — not aluminum heads or pot metal fittings. Cast iron handles thermal cycling and wear better in shop environments.
• Industrial 1725 RPM motor: Heavy-duty motor with full 15% service factor. Lower RPM means longer bearing life compared to high-speed motors.
• Simple single-solenoid control: One solenoid operates the air end. No computers, no lockout codes, no proprietary diagnostic tools required. Any electrician or competent shop tech can troubleshoot and repair it.
• Pressure and temperature gauges standard: Easy-to-read analog gauges — no digging through menus on a control panel to check operating status.
• High pressure and dual high temperature shutdown: Built-in protection shuts down the unit before damage occurs. Protects the air end from overheating and the system from overpressure.
• Factory filled with synthetic lubricant: Ships with Cast-Lube synthetic oil good for 4,000 hours or one year. No break-in oil swap required.
• Off-shelf V-belts with adjustable belt tensioner: Standard V-belts available at any industrial supply. No proprietary belt sizes.
• Built-in air/oil separator with spin-on filter: Keeps oil carryover out of your air lines. Spin-on design makes replacement straightforward.
• Spin-on oil filter, paper element air filter: Automotive-style replaceable filters — no special tools, widely available replacements.
• Air/oil lines in off-shelf copper piping: No proprietary tubing or fittings to source. Standard copper fittings available at any plumbing or HVAC supply.
• Air-cooled aftercooler/oil cooler standard: Removable for cleaning. Keeps oil temperature in check and reduces moisture carryover in the compressed air.
• Hour meter standard: Tracks actual run hours for accurate maintenance scheduling.
• 10-minute shutdown timer: If no air is demanded for 10 minutes, the unit shuts down automatically, saving electricity during slow periods.
• Pressure ranges available: Standard 135 PSI; optionally ordered at 150 or 175 PSI to match your system requirements.
• 2-year warranty on air end, electric motor, and tank (where applicable)

Available Configurations — Castair Rotary Screw Lineup

The R31BM50 is Castair's entry-point base mount rotary screw, paired with a 50-gallon tank. For reference, the full single-phase rotary screw lineup with tanks:

• 5HP, 1-Phase, 80 Gal Vertical (R518VP75): 20 CFM @ 100 PSI — 480 lbs
• 5HP, 1-Phase, 80 Gal Horizontal (R518HP75): 20 CFM @ 100 PSI — 480 lbs
• 7.5HP, 1-Phase, 80 Gal Vertical (R718VP75): 31 CFM @ 100 PSI — 555 lbs
• 7.5HP, 1-Phase, 80 Gal Horizontal (R718HP75): 31 CFM @ 100 PSI — 555 lbs

Contact us to discuss which configuration fits your actual CFM demand and facility layout.

When Does a Shop Need This vs. Stepping Up or Down?

• Right for: Light commercial shop with an existing receiver tank, 1-2 lift bays, air tools running intermittently. Tire shop, small lube shop, or single-bay garage with existing air infrastructure.
• Step up if: You're running impact wrenches, air ratchets, and spray guns simultaneously. A 3HP rotary screw will keep up with one tech working steadily, but two techs running air tools at the same time will likely outrun it. Move to 5HP or 7.5HP with an 80-gallon tank.
• Step down if: You only need compressed air for occasional tire inflation or intermittent hand tools. A piston compressor at lower cost may be sufficient for truly light use.
• Base mount is right when: You have a functioning receiver tank already in place and just need a better head unit. Swapping in a rotary screw base mount on an existing tank is a cost-effective shop upgrade.

Technical Specifications

• Model: R31BM50
• Horsepower: 3 HP
• Phase: 1-Phase
• Configuration: Base mount (no tank)
• Suggested Receiver: 50 gallon
• Compressor Type: Rotary screw
• Duty Cycle: 100%
• Standard Pressure: 135 PSI (150 or 175 PSI available)
• Lubrication: Oil-injected, synthetic lubricant (4,000-hour / 1-year interval)
• Drive: Belt drive, standard V-belts
• Motor Speed: 1725 RPM industrial motor
• Controls: Single solenoid, magnetic starter (pre-wired), hour meter, 10-minute auto shutdown timer
• Warranty: 2-year on air end and electric motor
• Manufacturer: Castair (Spicer, Minnesota)

Detailed CFM, dimensions, and weight specifications are pending manufacturer confirmation. Contact us for the latest data sheet.

What's in the Box

• Castair rotary screw compressor assembly (air end, motor, belt drive, base frame)
• Pre-wired magnetic starter and controls
• Air/oil separator with spin-on filter
• Air-cooled aftercooler/oil cooler
• Hour meter
• Pre-filled with Cast-Lube synthetic oil

Tank not included. Receiver tank sold separately.

Why Buy From Air Compressor Factory Direct?

• Castair specialist: We know this line top to bottom — base mount configurations, tank sizing, voltage options, and maintenance kits.
• Free shipping on qualifying orders
• Free technical support: Call 866-217-0063 and talk to someone who understands rotary screw compressors and shop air systems
• Full manufacturer warranty honored and supported
• Honest guidance: If a different model fits your shop better, we'll tell you

Call 866-217-0063 for quick answers and help!