Reasonable temperature differences for each stage and heat transfer between air and water in multi-stage air treatment system

Compared to traditional air conditioning systems, the multi-stage air treatment system has a higher energy efficiency by using lower grade energy, but the reasonable temperature differences for each stage (∆tstage) and heat transfer between air and water (Δttrans) needed to ensure an efficient system design remain unclear. To obtain the reasonable ∆tstage and Δttrans values required to maximize the energy efficiency of the multi-stage air treatment system, mathematical models are established to calculate the system's energy consumption. Six representative climate zones, two operating modes, and several working conditions are selected to evaluate the effects of different ∆tstage and Δttrans values on the performance of the multi-stage air treatment system. Reasonable ∆tstage and Δttrans values are obtained by minimizing the energy consumption of the system. The results show that: (1) the difference in reasonable ∆tstage and Δttrans under the different conditions is small, and it is possible to have a unified reasonable ∆tstage and Δttrans. (2) In the cooling season, the reasonable ∆tstage and Δttrans values are 6–8 and 4–5 °C, respectively; in the heating season, these values are 10–12 and 5–7 °C, respectively. (3) Compared to the traditional system, the maximum and annual average energy-saving rates of multi-stage air treatment system in an office building in Beijing are 34.5% and 17.9%, respectively. This study's results provide a solid foundation for building multi-stage air treatment system.