The basic objective of the paper is to identify thedesired circulation ratio for the natural circulation of water tubeboilers in different operating conditions. This requires the basicstudy of heat flux and the mode of the boiling heat transfer, andthe phenomenon like a departure from nucleate boiling and tubeoverheating. The parameters, which need to be studied are heatflux, pressure, dryness fraction, void fraction, liquid velocity andtheir impact on the required circulation ratio. The present work isto develop a circulation analysis model for the natural circulationof the water tube boiler and to check the boiler design for differentfailure modes. For a natural circulation boiler, the circulationratio is one of the most important design parameters as the otherdesign parameter like critical heat flux and skin temperature aremainly derived from the circulation ratio. The requiredcirculation ratio can vary with the boiler pressure, liquid velocityand maximum heat flux. This study is intended to provide inputfor the safe and optimum design of a natural circulation boiler.