In the condenser, the liquid refrigerant is cooled slightly below the saturation temperature to ensure that no flash gas is formed before the expansion valve. The level of sub-cooling achieved in the BPHE condenser is normally 0.5-4K. More sub-cooling further increases the available evaporator capacity by decreasing the amount of flash gas formed after the expansion valve (see Figure 10.11). The decreased vapor content also decreases the risk of maldistribution in the evaporator. This additional sub-cooling is best carried out in a separate sub-cooler, because a high level of condensate inside the condenser may disturb the stability and thus the performance of operation. The simplest sub-cooler uses a liquid stream, which can be the same as for the condenser (see Figure 10.12).