Two multi-zone models of the double-wall counter-current heat exchanger: gas cooler and condenser models were developed to design different heat exchanger configurations based on available existing heat transfer and friction correlations in the open literature. In addition, air-source heat pump water heater models for both subcritical and transcritical cycles were developed for predicting overall performance of the system, refrigerant and water mass flow rates, inlet condenser/gas cooler conditions, water pumping power etc. The refrigerant and water properties in the models were all calculated by using the in-built property function in EES and the models were also programmed and implemented by EES. For multi-zone calculations, water temperature difference in each zone was assumed to be constant and the optimal number of zone was investigated. The optimal number of zone was 90 zones for both condenser and gas cooler models.
The heat exchanger models were validated against available experimental data (only single-wall heat exchangers). It was found that the condenser model closely predicted the size of the condenser (difference of -0.8%) and the gas cooler model predicted the size of gas cooler with an acceptable deviation of -14%. The multi-zone model was significantly more accurate than one and three zone models.
Three double wall water heating heat exchanger configurations: the circular tube-in- tube with a small air gap (Configuration I), the flat tube-on-tube (Configuration II), and the twisted tube-in-tube with a small air gap (Configuration III) were investigated using the models. Two alternative refrigerants (R410A for condenser and R744 for gas cooler) were considered. Heating capacity, inlet and outlet water temperatures were assumed to be 3 kW, 15oC, and 60oC respectively. For the R410A, refrigerant condensation temperature was assumed to be 64oC and for the R744, refrigerant discharge pressure was assumed to be 11 MPa. Refrigerant was assumed to be sub- cooled to 25oC and refrigerant evaporation temperature was assumed to be 5oC for both R410A and R744. The criteria for comparison of different water heating heat exchanger configurations consisted of pressure drop and heat transfer coefficient for refrigerant and water, average temperature difference between refrigerant and water, heat exchanger length and weight, mean heat transfer surface area of the heat exchanger, water pumping power, and coefficient of performance of the system. The model was used to investigate the effect of key design parameters for each configuration. To investigate the optimal water flow channel size the refrigerant flow channel was fixed while the water flow channel was varied and to investigate the optimal refrigerant flow channel size the water flow channel was fixed while the refrigerant flow channel was varied. The predicted trends of the criteria by using the models showed that the configuration I case B (refrigerant flow in the annulus) performed better than case A (refrigerant flow in the inner tube). The optimal flow channels for configuration I case B with a 0.1 mm thick air gap were found to be di= 8 mm and Di −d2= 1.5 mm for R410A and di= 7 mm and Di−d2= 1.0 mm for R744.
The optimal flow channels for the configuration II with b1i= b2i= 9 mm were found to be
a
1i=a
2i= 1.5 mm for R410A anda
1i= 1 mm anda
2i= 1.5 mm for R744. The optimal flow channels for configuration III were found to be di= 7.94 mm and d1= 12.7 mm for R410A and di= 6.35 mm and d1= 9.53 mm for R744. At the optimal flow channel size for both the refrigerant and water sides for configuration I case Bfor R410A the heat exchanger had a length of 20 m, a weight of 16 kg, a mean HTA
of 0.60 m2, a pumping power of 0.2 W, and a COP of 3.3 while for R744 had a length of 30 m, a weigth of 21 kg, a mean HTA of 0.82 m2, a pumping power of 0.5 W, and a
COP of 3.3. The best configuration II heat exchanger for R410A had a length of 30.6 m, a weight of 10.5 kg, a mean HTA of 0.15 m2, a pumping power of 1.5 W, and a
COP of 3.3 while for R744 had a length of 19.6 m, a weight of 7.0 kg, a mean HTA
of 0.15 m2, a pumping power of 9.7 W, and a COP of 3.3. The best configuration III
heat exchanger with 94% contact for R410A had a length of 4.6 m, a mean HTA of 0.16 m2, a heat exchanger weight of 4.0 kg, a pumping power of 0.04 W, and a COP
of 3.3 while for R744 had a length of 6.9 m, a mean HTA of 0.19 m2, a heat exchanger weight of 4.5 kg, a pumping power of 0.3 W, and a COP of 3.3.
Among three double wall heat exchangers, the twisted tube-in-tube and the flat tube- on-tube were most promising to use in the air-source heat pump water heaters. The twisted tube-in-tube gave the smallest heat exchanger length, weight and water pumping power than the flat tube-on-tube. However, the investment cost for the twisted tube-in-tube might be higher than the flat tube-on-tube due to more difficult manufacture.
The models should be more fully validated against more experimental data particularly for double wall heat exchanger and a wider range of configurations because they were only validated with available data from the open literature for single wall heat exchanger for both condensing and gas cooling.