Experimental results of a tri-partite gas cooler (D4.3)
Creators: name: Alireza Zendehboudi
name: Pardiñas, Ángel Álvarez
Publisher: name: Zenodo
Year: 2020
Subjects: Heat pumps
transcritical cycle,
carbon dioxide
tri-partite gas cooler
experimental study

description: An experimental investigation on the performance of a tri-partite brazed plate gas cooler in a transcritical carbon<br> dioxide (CO2) heat pump water heater system is conducted. The gas cooler under investigation is partitioned to<br> separately serve the functions of domestic water preheating, space heating, and domestic water reheating. The<br> gas cooler is studied at a steady state for only space heating (SH) operation, only domestic hot water (DHW)<br> operation, and simultaneous space heating and domestic hot water (DHW+SH) operation. Measurements are<br> performed for counterow conguration with two different internal arrangements: (i) three individual single-pass<br> heat exchangers with a U conguration and (ii) a two-pass arrangement for preheating and reheating domestic hot<br> water. The heat pump unit is designed with a heat duty of 10 kW, 8 kW, and 10 kW under DHW, SH, and DHW+SH<br> modes to provide the tap water heating and space heating water up to 70 C and 35 C, respectively. The effects<br> of inlet pressure, CO2 and water mass ow rates, and water inlet temperature on the total heat transfer coefcient,<br> heat duty, pressure drop, supply temperature, and temperature approach are analyzed in detail. To present a<br> reference for brazed plate gas cooler optimization design in transcritical CO2 systems, the heat duty and supply<br> temperature correlations for both space heating and domestic hot water cycles are also developed and validated<br> with experimental data.<br> As shown by the experimental results, the gas cooler shows promising performance and is more compact than<br> previous designs. Under the design conditions, the heat duty meets the requirements to provide the supply temperatures<br> of 35 °C and 70 °C for the space heating and tap water heating applications. The pressure drops in the<br> tri-partite gas cooler are low under different operating modes and the temperature approaches for the reheating<br> gas cooler vary between 6.52 K and 8.24 K under design conditions in DHW+SH and DHW modes, depending on<br> the internal heat exchanger arrangement. This value is 0.65 K for the SH operating condition. This shows that CO2<br> liquid phase enters the throttling device, indicating good performance of the evaluated gas cooler.<br> It is found that the operating pressure has a considerable impact on the heat transfer coefcients in the near-critical<br> region. The maximum of heat transfer appears at the region in the vicinity of the pseudo-critical point and the peak<br> point of the heat transfer coefcient shifts to the high-temperature state when the operating gas cooler pressure<br> increases. Furthermore, no signicant and clear inuence of the water inlet temperature is demonstrated on the<br> heat transfer coefcients when CO2 is in a high-temperature state. The values of heat duty increases with the<br> increases of water and CO2 mass ow rates. The lower the water inlet temperature is, the higher the average heat<br> duty is. Increase of CO2 mean temperature owing through the gas cooler causes a decrease in the density and<br> viscosity of the refrigerant to maintain the ow continuity, resulting in a larger Reynolds number and then pressure<br> drop. The increase in the hot side uid pressure can cause an increase in the density and thus a decrease in<br> the velocity of CO2, so the CO2 side pressure drop decreases. A larger mass ow rate of refrigerant and a lower<br> water mass ow rate increases the water supply temperature. The higher water mass ow rate is recommended<br> to decrease the temperature approach values under DHW, SH, and DHW+SH operations. Ultimately, the values<br> of coefcient of determination for the reheating heat duty, space heating heat duty, preheating heat duty, DHW<br> supply temperature, and SH supply temperature are 0.88, 0.86, 0.94, 0.99, and 0.94, respectively. The results<br> show that the correlations perform very well in the prediction of heat duty and supply temperature. The basic data<br> for the characteristics of CO2 in the present experimental study are of signicance in designing and optimizing<br> the transcritical CO2 systems.