Design of a Chiller Compressor for Domestic CHP (R1233zd(E))

The scope of this project is the detailed design of a centrifugal compressor stage for a small chiller. The chiller will have to operate over a wide range of conditions, resulting in a challenging requirement on the compressor to have a very wide operating map and high efficiency.

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Introduction

The customer, a leading manufacturer of high-speed rotating machines, asked ADT to carry out the complete aerodynamic design of a brandnew centrifugal chiller compressor using TURBOdesign Suite’s 3D Inverse Design method.

Performance targets (pressure ratios and efficiencies) and operating points were set by the customer before the start of the project. Tight requirements on off-design and mechanical performance, as well as the overall size of the stage were also set by the customer.

Real gas effects are modelled directly in TURBOdesign Suite and CFD, with gas properties data included via .rgp files produced using NIST REFPROP data.

 

Fig. 1. 3D streamlines the optimized impeller

All of the targets on stage operating range were met and targets in stage efficiency were met or exceeded on all conditions.

Initial Stage Sizing

The initial meridional shapes and main dimensions for the compressor stage were produced using 1D preliminary design software TURBOdesign Pre. Starting from the basic stage requirements of mass-flow rate, pressure ratio and rpm, the code provides the overall stage sizing and detailed component performance and characteristic across the operating range.

 

Fig. 2. Contours of relative velocity for the impeller, vaneless diffuser and volute

 

3D Impeller and Vaneless Diffuser Design

Using 3D Inverse Design software TURBOdesign1, the blade loading distribution was optimized in order to achieve the target pressure ratio at the design mass flow and obtain good compressor impeller efficiency.

The compressor needed good efficiency over a very wide operating range, and so a vaneless diffuser design was produced. Vaneless diffuser pinch width and length were optimized as part of the design process.

Designs produced were tested by means of commercial CFD codes. Designs were analyzed both at design point and the most challenging off-design points. The final design was also analyzed across the entire operating map, considering different compressor speed lines between 40% and 120% of the design rotational speed.


CFD analysis showed that the final design reached all target pressure ratios and mass flows, whilst meeting all target efficiences and exceeding the design point target efficiency (including the volute) by 2%. Finite Element Analysis (FEA) was carried out on the final design, showing an acceptable stress level in the optimized blade, which allowed simple aluminium alloy to be used by the customer for manufacturing of the blades. In addition, blade natural frequencies were assessed and showed significant margin to requirements.

Optimized design geometries of all the components of the compressor stage, composed of the compressor impeller, a vaneless/vaned diffuser and an outlet volute.

Low Solidity Vaned Diffuser Design

For some end users with high pressure and efficiency requirements, a vaned diffuser was initially developed but it results in the characteristic drop of performances off-design, so ADT designed a low-solidity diffuser which maintained the higher pressure and efficiency of a vaned diffuser configuration but also maintained the off-design performances of a vaneless design. The low-solidity diffuser, interchangeable with a cassette, resulted in +2% efficiency gains.

 

Fig. 3. Contours of relative velocity for the impeller and low solidity vaned diffuser

 

Volute Design

The volute for the stage was designed using TURBOdesign Volute. A large minimum volute height relative to the volute size was specified for ease of manufacturability. The code then smoothed the area distribution around the circumference to ensure good performance was maintained, showing good performance across the opeating range and was a major contributor to the good overall stage performance.

 

Conclusion

TURBOdesign Suite was used to design and optimize a compact chiller compressor for domestic CHP using the new refrigerant R1233zd(E). The final compressor stage included both vaneless and low-solidity vaned diffuser options to adapt to the user requirements and need for extra boost/performance. The stage was manufactured and tested, confirming CFD and FEA predictions.

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