Thermal Equilibrium and Wet Gas Compressor Performance
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- Institutt for elkraftteknikk 
The wet gas performance model established for this master s thesis is based on results achieved from a single stage, low pressure ratio, centrifugal compressor set up at NTNU. Water droplets are injected to the flow right before the inlet. The GMF varies from 0.9 to 0.65 throughout the experiments preformed. Compressor performance is achieved by simulations in HYSYS with atmospheric inlet conditions. The importance of accurate measurements is evident to achieve the correct performance. A literature study is performed on thermal equilibrium of wet gas fluid during compression. The process of reaching thermal equilibrium is determined using heat capacity, mass flow and temperature difference according to conservation of energy. The flow regime is a disperse and separated two-phase flow. Due to pressure and temperature increasing, water droplets will evaporate to vapor, to assure saturated moist air at the outlet. The evaporation process is highly dependent on the surface area of the droplets; hence smallest droplets will evaporate first. The liquid film will experience evaporation and droplet tear-off due to instabilities. The molecules on the surface of the liquid film have the highest kinetic energy and these molecules will easily entrain to the gas flow. It must be taken into account that some droplets might return to the liquid film. Depending on the outlet temperature and pressure the amount of evaporated water through the compressor has been calculated. It is assumed that the outlet moist air has 100% relative humidity and the multiphase flow is at thermal equilibrium. Due to evaporation the outlet flow will have a higher GMF than the inlet. Two new temperature sensors are installed, one at the inlet and one at the outlet. The accuracy of the PT100 SE012 temperature elements is +/-0.03oC and the performance achieved from these measurements is more reliable than previous experiments. A sensitivity analysis is performed for the discharge temperature from wet gas compression performance. This to illustrate the importance of accurate temperature measurements. The performance of the dry gas compression presents a reliable efficiency between 73 and 86%. For wet gas compression a reliable efficiency is not achieved. The importance of having a process simulation tool is evident. In HYSYS the challenge is to achieve the correct composition of the moist air entering the compressor. The model uses saturated air at the inlet and this will influence the performance. An optimal model would need a specified correct humidity and conditions of the flow entering the compressor.