Condensation is defined as the removal of heat from a system in such a manner that vapour is converted into liquid. This may happen when vapour is cooled sufficiently below the saturation temperature to induce the nucleation of vapour. This mode of heat transfer is often used in engineering because of possible high heat transfer coefficients. However, condensation heat transfer is degraded when non-condensable gases present in the condensing vapour. The presence of even a small amount of non-condensable gas in the condensing vapour has a profound influence on the resistance to heat transfer in the region of the liquid- vapour interface. The non-condensable gas is carried with the vapour towards the interface where it accumulates. The partial pressure of gas at the interface increases above that in the bulk of the mixture, producing a driving force for gas diffusion away from the surface. This motion is exactly counterbalanced by the motion of the gas-gas mixture towards the surface. Since the total pressure remains constant the partial pressure of gas at the interface is lower than that in the bulk mixture providing the driving force for gas diffusion towards the interface [1].

In-tube condensation of steam-air mixture is important for the passive thermal safety features of new advanced designs and therefore is subjected to experimental investigations at several research institutes. In addition to the studies of the University of California, Berkeley [2] and Massachusetts Institute of Technology [3], some detailed information and previous results of the experimental research being carried out at Middle East Technical University, Mechanical Engineering Department [4].

When the pure steam experiments are considered as the reference for comparison, a first indicator of the effect of air is a remarkable decrease in centreline and inner wall temperatures. Comparisons show that difference between saturation temperature, corresponding to the pure gas case, and measured centreline temperatures varies between 10 K and 50 K, depending on inlet air mass fraction. In other words, the temperature difference increases considerably as air mass fraction increases. It is found that there is a drastic decrease in the performance of the heat exchanger as the inlet air mass fraction increases. The inhibiting effect of air on condensation manifests itself as reduction in heat transfer coefficient. However, the inhibiting effect of air diminishes as system pressure and gas flow rate increase. The heat transfer coefficient can be based on either the measured centreline temperatures (Tc) or on the predicted one (Ts). The heat transfer coefficient considerably decreases when Ts is used since Ts is always greater than Tc. The ratio of the heat transfer coefficients computed from these two methods shows that increase in air mass fraction leads to larger deviation from the measured one calculated by Tc [4].

In the new advanced passive boiling water reactor design (SBWR and ESBWR), the main component of the passive containment cooling system (PCCS) is the isolation condenser (IC). The function of the IC is to provide the ultimate heat sink for the removal of the reactor coolant system sensible heat and core decay heat. In performing this function, the IC must have the capability to remove sufficient energy from the reactor containment to prevent it from exceeding its design pressure shortly following design basis events and to significantly reduce containment pressure in the longer run.

After a loss of coolant accident, the steam/air mixture from the reactor containment may flow to the IC which will then reject decay heat to a pool of water [5]. Similar advanced design features are also envisaged for AP-600. The researchers are focusing their attention on the AP — 600’s passive safety core cooling systems, whose major elements are a steel reactor containment shell surrounded by a concrete shield building, natural air conditioning between the containment and the shield building, and large volumes of gravity-fed water stored in tanks above the reactor itself. The passive systems performs the principle safety functions such as primary coolant inventory control, reactivity control and residual heat removal, but they rely on natural forces like condensation, evaporation and gravity rather than the mechanical equipment that is standard in traditional active designs.