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Optimisation of screw machines based on a variable rotor pitch

Title: Optimisation of screw machines based on a variable rotor pitch
Subject: Construction of an optimised rotor geometry of screw machines (compressors and expanders) regarding the energetic efficiency.

 

Screw machines are used as compressors and expanders in several industrial applications. The rotors of screw machines are usually build with a constant rotor pitch, while a non-uniform distribution of the pitch is already used in the field of screw vacuum pumps and leads to enhanced efficiency.

The purpose of this project is to reveal the potential of screw machines with variable rotor pitch. An example of the machines can be seen in Fig. 1. As a first step, the variable pitched rotor is equiped with two different pitches.

 

rotors
Fig. 1: Screw machines with constant and variable pitched rotors

 

The approach can be seen in Fig. 2. The goal is the optimisation of  effective machine power while the boundary conditions mass flow, inlet and outlet pressure remain constant.

The first step is the building of a chamber model:The investigated machine is abstracted to a zero-dimensional model that contains all chamber volumes and inlet ports ("capacities") as well as clearances and opening areas ("connections"). Afterwards the chamber model is used for thermodynamic calculations performed by KaSim. The program is based on mass and energy conservation and determines the states of the chambers iteratively until a steady state is reached. Subsequent the machine is scaled geometrically similar to ensure the boundary condition of a constant mass flow. The result of the simulation is the effective power, that is compared with those of other machine versions and will be analysed in order to identify advantageous parameters. Before the cycle starts again, geometric parameters of the screw machine are adjusted.

approach2

Fig. 2: Approach of the optimisation process

 

First results for screw expanders:

The optimisation process was performed for an examplarily Organic Rankine cycle. The ORC fluid was set to ethanol with a mass flow of 0.6 kg/s and an expansion from 4 MPa at the inlet to 0.15 MPa at the outlet.

 

It turned out, that machines' efficiency can be improved when the rotor is equiped with a low pitch at the high pressure side and a high pitch on the low pressure side. This causes a moderate increase in chamber volume at the beginning of the working cycle, what improves the chamber filling. Afterwards, the high rotor pitch maintains a rapid increase in chamber volume and following this a quick expansion with reduced time for gap mass flows. The optimised machines for constant and variable rotor pitch can be seen in Fig. 3. While the constant pitched rotor never reaches the inlet conditions of 4 MPa because of throttling effects, the machine with variable pitch shows a working cycle on a higher pressure level. In this exemplarily case, inner power could be raised about 10 percent.

pV
Fig. 3: Indicator diagrams for optimised machines with constant and variable pitched rotors

 

 



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Contact

Univ. Prof. Dr.-Ing. Andreas Brümmer
Tel.: 0231 755-5720
Matthias Utri, M.Sc.
Tel.: 0231 755-5724