The objective of the present work is to evaluate the performance of the evaporator tubes of a natural circulation boiler under different operating pressure and heat flux. A significant concern of the present work is assessing the dry-out/critical heat flux limit and the critical circulation ratio (CR) of an existing evaporator downcomer system of a natural circulation boiler. The analysis of these parameters is crucial from a safe operation perspective. To carry out the study, three two-phase flow models, namely homogeneous flow model (HFM), separated flow model (SFM), and drift-flux model (DFM), are developed. The sensitivity of the models on various thermo-hydraulic parameters, change of flow regime, and circulation ratio is assessed. A rigorous validation exercise is carried out for a host of experimental, numerical, and plant design data pertaining to two-phase flow thermosyphon. The results indicate that the safe circulation ratio at low (< 40 bar) and intermediate (40–80 bar) pressure predicted by HFM is relatively high and differs substantially from the two other models. However, beyond 80 bar pressure (high operating pressure), the CR predicted by different models is considerably less and in closer agreement with each other. The practical boiler performance studies with different capacities indicate that the maximum difference of critical heat flux predicted by different models for boilers of different capacities lies within a 15% margin. These data will help the engineers associated with boiler operation and the associated persons with the boiler design sector.