Session: 19-01 Applications 1

Paper Number: 123596

123596 - Design and Testing of a Compact, Reverse Brayton Cycle, Air (R729) Cooling Machine 

Due to their contribution to global warming, the European Union has decided to significantly restrict the use of fluorinated refrigerants. The goal is to reduce the emissions of these gases from 105 million metric tons to 35 million metric tons CO₂  equivalent. To achieve this target, a phase-down of hydrofluorocarbons (HFCs) restricts the maximum allowable quantities on the EU market in 2030 to 21% of the quantities in 2015. If there are feasible, climate-friendly alternatives for a certain application, the use of HCFs is not allowed any more. This legislation leads to efforts in the refrigeration and heat pump industry to replace fluorinated refrigerants with more environmentally friendly alternatives.

Most cooling machines are vapor compression refrigeration machines (VCR). In this type of refrigeration cycle the refrigerant undergoes a phase change. Established natural (non-fluorinated) refrigerants for these types of cooling machines are propane and ethane. Both refrigerants have a significantly lower global warming potential (GWP) than the HCFs which were previously used. However, their flammability does not make them widely applicable for cooling or heating tasks.

An alternative to vapor compression cooling machines are gas cooling machines which do not require a phase change of the refrigerant. The working principle of these machines is the reverse Brayton cycle. Gas is compressed, cooled to near ambient temperature, and expanded in a turbine. The expansion and work extraction by the turbine lead to a significant temperature reduction of the gas. Using the cold gas to pre-cool the gas at turbine inlet through an economizer heat exchanger significantly reduces the required pressure ratio. A major advantage of gas cooling machines is that they can operate with air (R729) as a refrigerant. Below cooling temperatures of -50°C (Ultra Low Temperature, ULT), air cooling machines are more efficient than vapor compression cooling machines. Above -50°C the efficiency can be significantly worse. However, even in non-ULT applications, where efficiency is not the most critical performance parameter, the benefits of air as a refrigerant can outweigh the efficiency penalty: Air is a free, non-hazardous (non-flammable, non-toxic), and environmentally friendly (no global warming potential, no ozone depletion potential) refrigerant. It can be used for direct cooling which has the potential to allow more effective and uniform cooling, quicker cool down, and a less complex cooling cycle. A common application for air cooling machines with turbo compressors and turbines are aircraft conditioning systems.

The HighSpeed drive platform of ebm-papst, which is currently under development, enables to drive air cooling machines with drive powers from 1 kWe to 45 kWe at rotational speeds of up to 300 krpm. This enables the use of turbo compressors and turbines even at comparably low cooling capacities. Together with the passive air bearing of the drive unit, these machine types enable oil-free and low-maintenance operation. Due to the high rotational speed, the units have a small installation space in relation to power and flow. The speed controllability of the drive allows to continuously control the cooling temperature and cooling. In this paper the design of a single- stage air cooling machine with a maximum cooling power of 300 W is presented. The design process (3D CFD) of the small (wheel diameter of less than 35 mm) centrifugal compressor and the radial turbine is discussed. Particular focus lies on matching the compressor and turbine operating map for the given application. The test rig and the test results for this air cooling machine are shown and discussed in detail.

Presenting Author: Sönke Teichel ebm-papst Mulfingen GmbH & Co. KG

Presenting Author Biography: Undergraduate study in Mechanical Engineering at the Leibniz University Hannover, Germany.
Graduate degree in Mechanical Engineering at University of Wisconsin-Madison, USA.
PhD in Mechanical Engineering focusing on the optimized design of mixed flow compressors at the Institute for Turbomachinery and Fluid Dynamics, Leibniz University Hannover, Germany.
Worked four years as a development engineer for turbo charger compressors at IHI Charging Systems International, Heidelberg, Germany.
Since 2021 development engineer for turbomachinery at ebm-papst, Mulfingen, Germany.

Authors:

Sönke Teichel ebm-papst Mulfingen GmbH & Co. KG
Renan Emre Karaefe ebm-papst Mulfingen GmbH & Co. KG
Ahmet Çokşen ebm-papst Mulfingen GmbH & Co. KG