Experimental study of a fin-and-tube heat exchanger working as evaporator in subatmospheric conditions

Evaporation of water at sub-atmospheric pressures, close to its triple point, is gaining attention for the wide range of engineering applications. However, the mechanism and boiling regimes are still not completely clear and a correlation between operating conditions, geometric features of the evaporator and achievable heat transfer is still missing. In this context, the present paper aims at the study of a fin-and-tube heat exchanger working as an evaporator in the pressure range of 1-7 kPa. The experimental study analyses the effect of the main operating parameters (temperature of the heat transfer fluid, superheat, inclination of the heat exchanger) and attempts to find correlations suitable for the engineering design of evaporator under the investigated conditions. The results allowed the identification of the main factors influencing the heat transfer, i.e. the temperature of the heat transfer fluid and, secondarily, the logarithmic mean temperature difference, as well as the derivation of expressions that correlate the evaporation power and the heat transfer coefficients with operating conditions.