Bitzer OSK7441-K Open Screw Compressor

BITZER OSK7441-K Open-Drive Screw Compressor

The BITZER OSK7441-K is an open-drive rotary twin-screw compressor engineered specifically for medium-temperature refrigeration, marine cargo preservation, process cooling, and critical air-conditioning applications. Built to deliver sustained volumetric efficiency across harsh, continuous-duty operating conditions, the OSK7441-K couples directly via flexible clutch or belt drive to standardized IEC or NEMA electric motors, explosion-proof prime movers, or marine auxiliary diesel power take-offs.

The external open-drive configuration isolates the refrigerant loop completely from motor electrical windings, eliminating motor burnout system contamination and making it the compressor of choice for marine shipboard installations subjected to volatile supply power and challenging ambient engine-room environments.

Model Details:

• Model Series: OSK (Open-Drive Screw Compressor)
• Full Model Code: OSK7441-K
• Manufacturer Part Number (MPN): OSK7441-K / 7441-K-01
• Displacement @ 50 Hz (2,900 RPM): 165 m³/h (~97.1 CFM)
• Displacement @ 60 Hz (3,500 RPM): 199 m³/h (~117.1 CFM)
• Permissible Speed Operating Range: 1,450 to 4,000 RPM (Variable-frequency / direct-coupled compatible)
• Direction of Rotation: Counter-clockwise (facing compressor shaft end)
• Net Weight: 176 kg (388 lbs)

Technical Details:

Equipment Compatibility

• Electric Motors: Directly compatible with standard 4-pole or 2-pole IEC B3/B5 motors (Frame sizes 200L, 225S/M, 250M depending on running kW load) and NEMA 324T–405T frames via modular bell housing adaptors.
• Oil Separators: Designed for integration with external horizontal/vertical high-efficiency coalescence separators (BITZER OA4088, OA9011, or OAM series).
• Oil Coolers: Water-cooled shell-and-tube heat exchangers, sea-water cooled cupro-nickel coolers, or air-cooled remote radiator modules sized for high heat rejection under hot-gas conditions.
• Control Units: BITZER SE-B2, SE-B3, and IQ Module series; Allen-Bradley, Siemens S7, and Schneider Modicon marine automation PLC packages.

Frequently Asked Questions

What is the primary difference between the BITZER OSK7441-K and OSN7441-K models?

The OSK model is engineered with an internal volume ratio optimized for air-conditioning and medium-temperature refrigeration (evaporating temperatures between -15°C and +12.5°C). The OSN model features a higher internal volume ratio specifically designed for low-temperature freezing applications (-45°C to -10°C) to prevent internal under-compression and maximize part-load electrical efficiency.

What lubricating oil must be used with the OSK7441-K on HFC systems?

For systems operating on R404A, R507A, R134a, or modern low-GWP HFC/HFO replacements (such as R448A/R449A), BITZER BSE170 synthetic polyolester (POE) oil is specified. When running on legacy HCFC R22 systems, BITZER B150SH or B100 synthetic alkylbenzene oil must be selected.

Can the compressor be operated with an external Variable Frequency Drive (VFD)?

Yes. The OSK7441-K has an allowable operating speed range of 1,450 to 4,000 RPM. When using a VFD, verify that the minimum shaft speed maintains sufficient discharge-to-suction differential pressure to supply the required oil mass flow through the internal bearings and hydraulic capacity slider mechanisms.

Does the compressor come with an internal oil separator?

No. The OSK7441-K is an open-drive bare-shaft screw block that requires an external oil separation and lubrication circuit. This circuit consists of an external oil separator, an oil cooler (shell-and-tube or air-cooled), a high-capacity fine filter (5 to 10 micron), and an oil flow / differential monitoring switch.

How do I identify if the mechanical shaft seal needs replacement?

Shaft seal degradation is typically detected by inspecting the seal drain port (Port 7b/7c). Minor wetting is normal, but visible liquid oil dripping (> 50 ml per 24 hours), direct gas hissing, or bubbling during an electronic refrigerant leak detector sweep indicates that the carbon-silicon faces or elastomeric lip seals require replacement.