With larger systems, two independent refrigerant circuits are commonly used to follow the changes in cooling demand better and to increase system reliability. If one circuit malfunctions, the side remaining in operation may still provide sufficient cooling until help arrives.
The dual circuit system could be implemented by arranging two BPHEs in parallel, or mounting a "back-to-back" or "false dual" circuit BPHE. However, this would be at the expense of additional piping or a higher risk of freezing.
Part-load efficiency also decreases with these arrangements, because the flow arrangement means that only 50% of the secondary fluid undergoes heat exchange. The evaporation temperature at part load may therefore decrease, reducing system efficiency and increasing the risk of freezing. Instead, SWEP True Dual technology BPHEs have two independent refrigerant circuits combined with a common secondary fluid circuit. The patented plate technology ensures full counter-current flow and full symmetry between the refrigerant circuits. The True Dual models are available with or without the SWEP distribution device for evaporator or condenser duty. A True Dual heat exchanger is shown in Figure 10.1.
A True Dual BPHE running with both circuits active operates no differently from a high-efficiency single circuit evaporator with full contact between refrigerant and secondary fluid.
Even if one refrigerant circuit is closed, i.e. half-load operation, all secondary fluid channels remain in contact with the active refrigerant channel (see Figure 10.2). All the secondary fluid will still receive heat exchange, and the leaving water temperature will therefore be the same as for full-load operation provided the water flow is also halved. This allows the part-load evaporation temperature to remain at a high level, resulting in increased efficiency at part load. Because secondary fluid channels will surround the active refrigerant circuit, the evaporating process will also remain fully stable.
True Dual technology therefore results in higher operational efficiency than "back-to-back" or "false-dual" BPHEs, without additional risk of freezing at part load, and it requires less piping. For a schematic system sketch of a True Dual system, see Figure 10.3.