RefComp SP Screw Compressor Vi Modulation & Dual Casing

In heavy-duty industrial refrigeration systems and large-scale cold storage facilities, screw compressors need to accommodate wide fluctuations in condensing pressure caused by seasonal changes in ambient temperature. Operating with an mismatched compression ratio forces the motor to do extra compression work, or leads to gas backflow issues.

The RefComp SP series semi-hermetic screw refrigeration compressor solves these operational challenges with an adjustable built-in volume ratio (Vi) mechanism paired with a noise-dampening double-casing housing.

Variable Built-in Volume Ratio (Vi) Adjustment

The built-in volume ratio (Vi) refers to the volume ratio of the compression chamber before the exhaust port opens to the initial suction volume. When a fixed-Vi compressor runs under high condensing pressure, insufficient compression will occur, causing high-pressure exhaust gas to flow back into the rotor chamber.

RefComp SP compressors are equipped with a hydraulically driven sliding mechanism that adjusts the internal exhaust port geometry, with (Vi settings typically calibrated at 2.2, 3.2 and 4.2:

• Low Vi mode (2.2): Optimized for air-cooled chillers and high-evaporation temperature water cooling applications, avoiding over-compression energy loss.
• High Vi mode (4.2): Suitable for low-temperature freezing working conditions. When the condensing pressure rises to 1.8 MPa, the volumetric efficiency can still be maintained between 86% and 90%.

Adjusting Vi according to real-time pressure ratio reduces unnecessary shaft power consumption, cutting electricity use during partial-load operation.

Dual-Casing Acoustic and Mechanical Vibration Reduction

Gas pulsation and rotor meshing generate high-frequency mechanical vibration inside screw compressors. Conventional single-wall cast iron casings transmit these pressure waves directly to the piping in the surrounding equipment room.

RefComp adopts a high-density double-wall casing cast from HT250 gray cast iron with a tensile strength of no less than 250 MPa. An intermediate oil channel separates the inner rotor sleeve from the outer structural shell:

• Vibration damping: The continuous oil layer between the inner and outer casings absorbs high-frequency pressure pulses generated by the asymmetric 5:6 rotor profile.
• Noise reduction: Compared with single-casing products, the acoustic radiation under full-load conditions is reduced by 3 to 5 dBA. The noise level is kept well below the standard industrial limit without additional thick external thermal insulation layers.

Operating Parameters for Routine Compressor Maintenance

To maintain high volumetric efficiency throughout a long service life, strict fluid standards must be followed during preventive compressor maintenance:

• Differential oil pressure: Ensure that the net oil pressure is at least 0.18 MPa higher than the suction pressure, to guarantee the rapid response of the Vi hydraulic piston.
• Temperature limit: Keep the exhaust gas temperature below 110°C by checking the heat transfer efficiency of the oil cooler.
• Superheat control: Strictly maintain the suction superheat within 5K to 8K, to prevent synthetic polyol ester (POE) lubricant from being diluted by refrigerant.