¶ Two stage application
Two stage belongs to the application category since the result is not a single BPHE, but system of collaborative heat exchangers.
In the two stage system the primary flow entering the radiator heat exchanger is cooled down in two steps. Leaving the radiator, the flow is further cooled down in the pre-heater, which makes out one part of the tap water heat exchanger named the two stage heat exchanger. Here it is mixed with the primary flow from the after-heater, which makes out the second part. The main purpose of the pre-heater is to preheat the cold water before it enters the after-heater by utilizing the primary flow from the radiator heat exchanger which is further cooled down before returning to the heat source. The primary return temperature can be substantially lower compared to a parallel system provided recirculation of hot tap water is taken into consideration due to use of separate ports in the two stage system in contrast to a parallel system. If the primary flow from the radiator is much bigger than from the after-heater, there's a risk of an oversized heat exchanger for tap water production. The two stage configuration is suitable when the primary temperature after the radiator heat exchanger is higher than the primary temperature after the after-heater and tap water consumption is substantial.
The Two Stage window offers the same calculation functionality as the regular Single Phase window, one window each for Design, Performance and Rating calculation. But in contrast to the Single Phase window it´s possible to connect the Radiator heat exchanger and the Two Stage heat exchanger like in the description above. The calculation is made by in turn specifying the fluids, the radiator heat exchanger working conditions (winter conditions), the two stage heat exchanger working conditions (summer conditions) and possible connection between the heat exchangers (Include flow from radiator) (*). To enable flow optimization of the primary stream from the Radiator select "Auto Performance" (**). Performance mode works in a similar manner as Auto Performance for a given design.
The result includes number of plates in after heater followed by preheater. The after heater always have one more plate than the pre heater. Please note that the IC filter is not in operation why the IC plate counts are not reliable.
Fig. 1: Two stage application inputs and relative schema