Analysis and optimization of a Puffer-type water heater

In this paper we present the design optimization of a Puffer-type water heater for sanitary water production in residential buildings. The optimization study is performed with the aim to fulfill the design requirements of the heater, i.e. the provided mass flow rate and the outlet water temperature, while minimizing the surface area of the coil characterizing the Puffer and thus reducing the overall cost of the device. The coil diameter and pitch and the diameter of the pipe composing the coil are analyzed in the optimization study. Starting from available correlations providing the heat transfer rate between the sanitary water and the heated water surrounding the coil, a simplified thermal resistance model of the heat transfer process within the heater is developed. The analysis show a significant impact on the overall heat transfer rate between the sanitary water and the heated water when the geometrical parameters of the coil are varied. Using an optimal value of the pitch to pipe diameter of the coil for a given coil diameter in particular, it is possible to maximize the heat transfer rate and thus minimize the size of the coil. As a first step, the optimization study is performed by introducing sizing constraints allowing for sharing the same coil among several types of water heaters. In this case, a reduction as high as 14% of the coil surface area can be obtained, whereas a reduction as high as 23% can be achieved if the optimization is performed free of such constraints.