Ammonia as a refrigerant has been known for more than 150 years, but that doesn’t mean the technology can’t still be improved. Danish researchers have shown that, with a number of system design tweaks – notably reductions in recirculation rate and evaporator-tube diameter – a significant cut in refrigerant charge is possible for industrial systems, without compromising on cooling capacity.
The study, “Multi-objective optimisation of low charge liquid overfeed ammonia evaporators for industrial refrigeration,” was carried out by researchers from the Technical University of Denmark and the Danish Technological Institute. It was published in Proceedings of the 8th Conference on Ammonia and CO2 Refrigeration Technologies.
The research team of Martin R Kærn; Wiebke B Markussen; and Jóhannes Kristófersson looked at how to minimize the required refrigerant charge and the heat-transfer area in liquid-overfeed evaporators, without compromising the required cooling capacity. The heat-transfer area was used as a measurement of the evaporator cost.
The evaporator used in the study was a traditional flooded inline finned evaporator, operated with pumped liquid circulation and rated to a 22kW (6.3TR) cooling capacity. The experiments were carried out with a -30°C (-22°F) evaporation temperature.
To find out what has the largest influence on the required refrigerant charge, the researchers looked at the interaction between parameters like the refrigerant recirculation ratio, the heat-exchange area (represented by longitudinal tube pitch and transverse tube pitch), tube diameter, and the number of tube circuits. The recirculation ratio is defined as the ratio between the refrigerant mass-flow rate through the evaporator and the vaporized refrigerant mass-flow rate.
They found that the single biggest effect on the required refrigerant charge was a reduction of the recirculation rate, which reduced the charge significantly in all scenarios. They furthermore found that if the tube diameter is reduced, and the total number of tube circuits is increased at the same time, then the charge can be reduced even further. This is even possible with a decreased heat- exchanger size, thus saving cost.
In a previous study done by the same authors, they also found that the required charge could be reduced by a factor of two to three by reducing the recirculation ratio, the final reduction depending on the evaporation temperature. The researchers have also looked at a Danish cold-storage case study and found that the compressor power consumption can be reduced by 4.9% when the recirculation ratio is reduced from four to 1.5.
This article originally appeared in the November-December 2019 issue of Accelerate Magazine.