Screw - Compressors- Mathematical Modelling And Performance Calculation

The includes mechanical losses (bearings, oil shear, rotor windage): ( W_{shaft} = W_{ind} + W_{mech} ).

The fundamental governing equation is the for a control volume with mass flow: [ \frac{dU}{d\theta} = \dot{m} {in}h {in} - \dot{m} {out}h {out} + \dot{Q} - \dot{W} ] where ( U ) is internal energy, ( \theta ) is the rotation angle, ( \dot{m} ) are mass flow rates (suction, discharge, and crucially, leakage), ( h ) is specific enthalpy, ( \dot{Q} ) is heat transfer to the casing/rotors, and ( \dot{W} ) is shaft work. The includes mechanical losses (bearings, oil shear, rotor

The (( \eta_{ind} )) compares this to isentropic compression work: [ \eta_{ind} = \frac{W_{is}}{W_{ind}} ] Unlike reciprocating compressors that rely on pistons, or

Introduction Screw compressors, particularly the twin-screw variant, are the workhorses of modern industrial refrigeration, air compression, and gas processing. Unlike reciprocating compressors that rely on pistons, or centrifugal compressors that depend on high-speed impellers, the screw compressor operates on a principle of positive displacement through intermeshing helical rotors. Its popularity stems from a unique combination of high efficiency, reliability, and the ability to handle a wide range of flow rates and pressure ratios. The performance (leakage

While modern CFD offers a glimpse into the complex three-dimensional flow, the core of practical design and optimization still relies on validated 1D chamber models. Understanding these mathematical foundations allows engineers to predict performance, diagnose losses (e.g., under-compression, blow-hole leakage), and optimize rotor profiles for specific applications—from energy-efficient air compressors to high-pressure natural gas injection systems. The screw compressor, therefore, is not just a mechanical assembly; it is a physical manifestation of carefully balanced mathematical relationships.

However, the very geometry that grants these advantages—the complex, three-dimensional helical lobes—makes performance prediction a formidable challenge. A screw compressor cannot be designed by intuition alone. This essay provides a helpful overview of the mathematical modelling techniques used to describe screw compressor geometry and the thermodynamic and fluid-dynamic calculations essential for predicting their performance. The first and most critical step in modelling a screw compressor is defining the rotor profiles. The performance (leakage, friction, and built-in volume ratio) is almost entirely determined by the shape of the lobes. Typically, one rotor is convex (male) and the other concave (female).