20. September 2016

Design, commissioning and testing of gas bearing turbo compressors with gases other than air

Oil-free turbo compressors with gas bearings are not only used for the compression of air with 1 bar ambient pressure, but also for other pressure levels or refrigerants.

Oil-free turbo compressors with gas bearings do not only have to be used for the compression of air with 1 bar at the compressor inlet (see user report). Other applications are possible with varying pressure levels (e.g. multi-stage compressors), use with refrigerants (see article) or in the process industry. The design of such oil-free compressors is based on basic physical equations, which are equally valid for air as well as other gases.

The commissioning can be easily carried out by the team at Celeroton thanks to their design and test knowhow and the infrastructure available at Celeroton. Besides determining the key parameters of the motor, the functionality and stability of the radial and axial gas bearings are permanently monitored. The stability depends on, for example, the actual fluid composition, speed, actual absolute pressure ratio and the temperature. More information on this topic can be found in the technology section on our website. The use of appropriate sensor technology for the monitoring of the rotor excursion and rotor stability creates a high degree of certainty during the first commissioning and additionally allows for comparison between the calculated and measured parameters. Celeroton uses its own self-developed and highly sensitive sensors. These sensors are based on the eddy current principle and measure extremely small displacements of the rotor in the stator case during operation. Figure 1 graphically shows the starting position of the rotor and the gas bearing stator case in a cross sectional view. The distances of the deflected rotor to the eddy current sensor are depicted with red lines, with the resulting eccentricity is shown as a yellow line. This sensor technology is used for the commissioning of gas bearing turbo compressors with and without air.

Figure 1: Drawing sketch of a gas bearing rotor and radial position sensors

Aerodynamic tests are then conducted in a closed-loop test bench. The test bench is equipped, besides the mandatory gas bearing sensors, with mass flow, pressure and temperature sensors. This enables the measurement of compressor maps for different inlet conditions (fluid, pressure, temperature, mass flow). A pressure range from approx. 0.01 to 10 bar and a temperature range from -10 to +70 °C are possible.

In addition, the thermal system management of an oil-free compressor under load and with defined cooling water temperature can be analysed. Some special applications require full system functionality with a cooling water temperature of +70°C or above.

Figure 2 shows a schematic diagram of Celeroton’s closed-loop test bench, and Figure 3 a photograph of the test bench. The test bench is modularly constructed in order to enable customer specific modifications based on the knowhow available at Celeroton. Therefore compressors for different gases and gas mixtures, i.e. helium, nitrogen, methane, etc. can be measured.

Figure 2: Schematic diagram of Celeroton's closed-loop test bench
Closed-loop test bench
Figure 3: Celeroton closed-loop test bench

Application example: Oil-free turbo compressor with gas bearings for a butane (R600) heat pump

As an example of a gas bearing turbo compressor without air, a compressor for a heat pump is presented that has a thermal performance between 9 and 22 kW. This compressor has been developed in close collaboration with the Swiss company BS2. The operating medium of the heat pump circuit is butane. The project goal was to integrate the conceptual efficiency advantages of an oil-free turbo compressor into a heat pump and thereby to further increase its COP. Figure 4 shows the realised and tested prototype turbo compressor. It generates a pressure ratio of >2.5 and is designed for a rated electrical power of 2 kW at 200,000 rpm. The cooling is carried out by the primed, gaseous butane. Further technical details on the compressor and a datasheet can be requested from BS2.

Figure 4: Gas bearing turbo compressor for butane

The challenge in the commissioning of this gas bearing compressor primarily lies in the handling of the flammable butane gas with the different inlet conditions (pressure, temperature and the proximity to the condensation barrier). These parameters are given to the compressor by the process within the heat pump and cannot be freely chosen. With measurements carried out with the closed-loop test bench, Celeroton was able to confirm the functionality of the gas bearing turbo compressor over a wide range of operating conditions in the heat pump. The commissioning process of the turbo compressor was done efficiently thanks to the centralised implementation of the design, construction, production and testing at Celeroton.