A study on the mechanism of dropwise condensation. 1st report Measurement of heat-transfer coefficient of steam at low pressures.

The heat transfer coefficient was measured very carefully and repeatedly for dropwise condensation of steam in a pressure range from atmospheric pressure down to 1 kPa using five condenser surfaces. The experimental heat-transfer coefficient at 1 kPa was between 30 and 35 kW/m2K, being about six times smaller than that at atmospheric pressure. A gold-plated copper block, an ultra-finished gold disk, a silicon disk vapor-deposited with gold, and two chromium-plated copper blocks were employed as the condenser surface. The magnitude of the surface roughness of the gold and the silicon disks was smaller than the radius of the smaller droplet that can exist thermodynamically on the condenser surface under the experimental conditions. Contrary to expectation, the nucleation-site density was not directly related to the surface roughness, but seemed to be affected by subtle physico-chemical conditions of the surface.