Heat transfer correlations for low approach evaporative cooling systems in buildings

The experimental performance of an open industrial scale cooling tower, utilising small approach temperature differences (1-3 K), for rejection of heat at the low water temperatures (11-20 °C) typical of chilled ceilings and other sensible air-water heat dissipation systems in buildings, is examined. The study was carried out under temperate maritime climatic conditions (3-18 °C wet-bulb temperature range). Initially a theoretical analysis of the process at typical conditions for this climate was conducted, which indicated that a water to air (L/G) mass flow rate ratio of less than 1.0 was required for effective operation. Consequently for these low L/G ratios, the thermal performance of the experimental tower was measured and correlated. A new correlation is proposed which shows a significant increase in the NTU level achieved, for the required L/G ratios (0.3-0.9). As the cooling tower in this application is predominantly a mass transfer device under summer conditions, the evaluation of the total volumetric heat and mass transfer coefficient (kg_a s^-1 m^-3) is of particular relevance and is also determined.