Why does a scratched surface act better boiling surface in a Heat Exchanger as compared to a smooth surface?
The first detailed boiling study was conducted by Nukiyama in 1934 when he plotted the boiling curve showing the relationship between heat and temperature. The curve is known as the Nukiyama boiling curve. Figure 1 shows the nucleate boiling portion of the curve.
Boiling starts with the initiation of bubbles on a heated surface. As the heat increases the bubbling activity increases too. The physics behind the boiling phenomenon is complex; however, in simple words the more the bubbles generated on a surface the more will be the heat transfer, as each bubble acts as a “postman”. Scratches on the surface function as initiation seeds for boiling. These are known as nucleation sites. The more such sites, the more carrier points. Nukiyama curve is a plot of heat flux versus temperature difference between the wall and the saturation temperature of the boiling fluid. This difference is known as wall superheat! This wall superheat increases with heat flux. If there are more seeds on the surface, the heat will dissipate faster which will result in less overheating of the surface and therefore a drop in the wall super heat. This drop in layman’s language means power savings. Figure 1 shows this difference in the form of two curves. Curve A is for a smooth surface and B for the same surface however with scratches all over it. For the same heat flux, the wall superheat for B surface will be less than A surface. This aspect helps in designing either a smaller Heat Exchanger for the same operating condition or resulting in a lower approach temperature for the same size Heat Exchanger. A smaller approach temperature means higher saturation temperature which results in power saving.
