Design of a Small 2-Stage Chiller Compressor

The customer, one of the world’s leading suppliers for HVAC industry, required the design of the compressor for an upgraded range of chillers using a new refrigerant R1234ZE. 

The new refrigerant has low Global Warming Potential (GWP ~ 6), and is used as a replacement for R134a. It is classified as A2L (low flammability and low toxicity) under ASHRAE classification. The chiller compressor takes a 2-stage back-to-back layout to achieve a good performance at both design and off-design conditions. An economized flow was injected in-between the two stages.

The design targets included four rating conditions (100%, 74%, 47% and 21% load) and the maximum lift condition. Both COP and IPLV were desired to be competitive for a small tonnage chiller. The maximum (choking) capacity had to be achieved within the maximum rotational speed provided by the customer.

 

Two stage compressor configurationFig. 1 - The two-stage compressor configuration.

 

INITIAL STAGE SIZING

TURBOdesign Pre was used to assess the performance potential of different stage pressure ratio split, different RPMs, shrouded or open impeller etc. A preliminary performance map prediction was also provided by TUROdesign Pre analysis. Based on these the design RPM was selected and the optimum compromise betewen stage performance and overall operational range was determined.

A shrouded blade was chosen over an open impeller for the advantage on efficiency, which was also compatible with the customer’s manufacturability. During the meanline analysis phase, it also appeared that a splitter blade design could be necessary to reach the maximum capacity requirement.

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Mehrdad Zangeneh

Mehrdad Zangeneh is Founder and Managing Director of Advanced Design Technology and professor of Thermofluids at University College London.

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