Towards long term, high accuracy difference thermometer sets for the purpose of measuring the efficiency of turbomachinery

Abstract

The performance of turbomachines can be principally defined by differential pressure, electrical power and hydraulic efficiency at various volumetric flow rates; often known as the pump curve. To date the measurement of hydraulic efficiency has proved inaccurate, time consuming and cost prohibitive using contemporary methods. The conventional method relies on the measurement of differential pressure, flow rate, electrical power and an estimation of motor efficiency, where in-situ measurement of volumetric flow rate is the single biggest source of uncertainty using this method. An alternative to the conventional technique is the thermodynamic method, which requires only the measurement of differential pressure and differential temperature across a turbomachine to compute the hydraulic efficiency. The disadvantage is that, due to the remarkably high specific heat capacity of water, the temperature difference is very small, requiring thermometers of milli-Kelvin accuracy. Simulations demonstrated that pump efficiency accuracy is some thousand times more sensitive to differential temperature than absolute prompting a design focus on difference thermometer sets. No differential thermometer sets existed with the needed accuracy and the ability to function in an industrial environment for extended durations, so this technology was designed and developed. There was little research into difference thermometer set calibration techniques, this being currently being undertaken by absolute means, so a protocol for this also had to be developed. A suite of hardware was developed and tested in a throttling calorimeter and, together with the developed calibration methodology, proved capable of achieving an accuracy of 0.29mK. Such accuracy means on-site, in-situ measurement of turbomachinery efficiency to an accuracy of 0.5% is now made possible. Industrial environment case studies demonstrated the identification of tangible engineering intervention cost-saving measures for turbomachinery, such as water pumps and ventilation fans, made possible through this technology. Keywords: turbomachinery, pumps, fans, energy efficiency, thermodynamic method, Poirson, difference thermometry, pump efficiency.