Variable Lead Helical Rotor Screw Machines

Twin screw compressors for air compression and refrigeration have been used since late 1950’s. The efforts of designers in all these years focused on optimization of rotor profiles keeping the uniform rotor lead mostly due to manufacturing constraints. However it is well known that by varying the pitch and/or cross section profile the performance of a compressor could be improved. Variable lead rotors have been extensively used in vacuum pumps and more recently in multiphase pumps and have proven beneficial although difficult to make.

With advancements in manufacturing and measurement technology and with the new manufacturing methods there are signs that screw machines with variable rotor profiles and variable rotor lead might be used more often in future.

The prerequisite for design of such machines is ability to design them, analyse their performance and optimise the. SCORG^TM is today the only grid generator for screw machines which can produce numerical mesh for performance analysis of such machines.

This example shows use of SCORG^TM to generate numerical mesh for three different screw compressors:

1. Constant rotor profile and constant rotor lead
2. Constant rotor profile and variable rotor lead
3. Variable rotor profile and constant rotor lead

The main advantages of varying the lead on screw rotors are:

• Steeper internal pressure increase,
• Lower throttling losses due to increased discharge port area for a given pressure ratio,
• Decreased leakage losses due to reduction in sealing line length towards the high compression zones.

More information about grid generation of variable geometry rotors could be found in the following publications:

1. Rane, A. Kovacevic et all, CFD grid generation and analysis of screw compressor with variable geometry rotors, 2013
2. Rane, S., Kovacevic, A., Stosic, N. & Kethidi, M. (2014). Deforming grid generation and CFD analysis of variable geometry screw compressors. Computers and Fluids, 99, pp. 124-141. doi: 1016/j.compfluid.2014.04.024

The capability of SCORG^TM to generate numerical grid for rotors with constant and variable lead as well as variable profile is demonstrated in the following video

Watch Video: Variable lead rotors