Evaporator, New refrigerants, Refrigerant charge, Falling film, Hybrid film
The impact of the refrigerant charge is growing more and more due to the high cost of the new synthetic refrigerants (HFO) and the increasing cost of the older refrigerants. In the field of the air conditioning, for the centrifugal systems using flooded evaporators it is useful to find alternative heat exchangers with lower refrigerant charge but the same performance. Namely, it is desired a low temperature approach between the water outlet and the evaporating refrigerant, both for increasing the COP of the machine and for reaching low pressure ratios, typically required in the centrifugal compressors. Nowadays, falling film or spray evaporators start to be considered and used. In general they require a complicated distribution system for the liquid inlet and for the flash gas dissipation, and a big volume of the shell in order to avoid liquid flowing to the compressor. Moreover, the performance at partial loads is problematic, as in general it is difficult to maintain wet all the tubes when varying the operating conditions. The present work describes a shell-and-tube evaporator with a completely new design, named “Hybrid Film Evaporator”, where the refrigerant evaporates on the external of the tubes, and the water is cooled flowing inside the tubes. In it there is a combination of a falling down motion of the liquid to be evaporated and a flooding of all the rows of tubes. The liquid is automatically redistributed on each row, and all the surface is always involved in the heat transfer. Two prototypes are investigated, one with four passes and one with six passes on the water side, in a oil-free centrifugal chiller. Both are 1 m long, and their shell diameter is 610 mm. The experimental analysis consists on the measurement of the capacity (ranging from 250 to 450 kW) in conjunction with the evaporating temperature (ranging from 5.5 to 6.5 °C) with R134a as refrigerant fluid. The water outlet temperature is kept at a constant value of 7 °C, and two different sets of experimental data are obtained, keeping constant either the water inlet temperature (at 12 °C) or the water flowrate (at 50 or 65 m³/h). The heat transfer coefficients (both global and external) are then studied. In addition, a flooded evaporator with the same overall geometry is experimentally studied and all the performance parameters are compared to the above ones. The conclusion is that the hybrid film evaporator reaches the same average heat transfer coefficient, and then the same temperature approach at a given heat flux, of a flooded evaporator with the same surface area, saving the refrigerant charge of a quantity between 30 and 40 %.