Rotary Screw Compressor Built-In Volume Ratio (Vi) Calculator
Calculate internal built-in volume ratio (Vi), internal pressure, power loss penalty from under-compression or over-compression, and optimal variable slide valve setting.
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Screw Rotor Built-In Geometry
Volume Ratio Mismatch & Power Impact
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Frequently Asked Questions
What is the built-in volume ratio (Vi) in a rotary screw compressor?
The built-in volume ratio (Vi) is the geometric ratio of the initial trapped gas pocket volume at the closing of the suction port to the final volume of that pocket just as the discharge port begins to open: Vi = V_suction / V_discharge.
What happens during under-compression in a screw compressor?
When the built-in volume ratio Vi is too low for the required pressure ratio, the gas pocket is opened to the discharge line before full pressure is reached. High-pressure discharge gas rushes backward into the lobe pocket, creating severe pressure pulsation, gas heating, and dramatic power consumption spikes.
How does a variable Vi slide valve eliminate efficiency losses?
A variable Vi slide valve dynamically shifts the physical axial location where the discharge port uncovers. As operating pressures fluctuate (e.g. changing seasonal ambient temperatures or refrigeration loads), the slide valve continuously modulates Vi to perfectly match system pressure ratio, eliminating all over- and under-compression losses.