Steam Boiling Curve – How to simply understand it?

The boiling point of a liquid or also known as the saturation temperature is the temperature at which the vapor pressure of the liquid is equal to the ambient pressure of the liquid. At this point the liquid will turn the phase into steam. The saturation temperature of pure water at atmospheric pressure is 100°C. At this point the water will turn the phase into steam by forming steam bubbles.

The saturation temperature becomes a unique function of pressure. The higher the pressure around the water, the higher the boiling point will. And also its vice versa. This is because the water pressure will affect the characteristics – such as the enthalpy of water, latent heat, and steam enthalpy – of steam formed at that pressure.

At a critical pressure condition of 3200 psi (22.1 MPa) for example, the latent heat required to form water vapor becomes zero, and in this condition there will be no bubbles of vapor during evaporation. So that the transition process changes the water phase into water vapor under these conditions will occur more smoothly. On the basis of this phenomenon, we know a boiler technology called super and ultra-super critical boiler. Those boilers works by circulating water on boiler pipes with a pressure above the critical pressure of 22.1 MPa (221 bar).

Let me introduce you a curve called boiling curve. This curve will explain to us the characteristics of the water boiling process. Research for this curve is done by dipping a hot metal into a certain amount of water. The rate of heat transfer per unit area or called the heat flux fills the Y axis of the curve. While the X axis is filled by the temperature differential between the metal surface with the surrounding water.

From point A to B, convection heat transfer will cool the metal so that the boiling process will be retained. As it passes slightly through point B, it is known as the early boiling process, where the water temperature will rapidly adjust to the metal surface temperature and closer to its saturation temperature. Water vapor bubbles begin to form on the metal surface. Periodically the bubbles will collapse (smaller) because it interacts with other water. This phenomenon is called a subcooled boiling, and is characterized by points B and S on the curve. In this process, the heat transfer rate is high enough, but there is still no amount of water vapor. From point S to C, the water temperature has reached the saturation temperature more evenly. The vapor bubbles no longer collapse and shrink, it gets bigger and more bubbles form. This area curve is commonly called nucleate boilling region, which has a fast heat transfer rate, and the metal surface temperature is slightly higher than the water saturation temperature.

Approaching point C, the evaporation surface will be wider. At this time the process of steam formation occurs very quickly, causing the vapor to form as blocking the water to approach the metal surface. The metal surface becomes isolated by a kind of layer made of water vapor, resulting a reduction in heat transfer rate. This process (C-D) is known as critical heat flux (CHF), where the process of heat transfer from metal to water becomes slow because of the film layer formed.

Further, as illustrated by point D to E, it is called the unstable film boilling process. At this process, the metal-fluid contact surface temperature does not increase. Consequently, the heat transfer performance per area and energy transfer process are decreasing. From point E passing D ‘to F, the vapor insulation layer on the metal surface becomes very effective. So that the heat transfer from the metal surface through the film layer occurs by way of radiation, conduction, and micro-convection to the water surface adjacent to the film layer. In this phase the evaporation process continues with the marked formation of water vapor bubbles. This phase is known as stable film boiling.

The process of forming water vapor in the water pipe boiler is theoretically referring also to the boiling curve. In more detail, the process of vapor formation you can see the process in the another curve above. What distinguishes from the usual process of moisture formation is, the process of vapor formation in a water pipe boiler occurs in the flow of water at a certain flow rate. This process is known as forced convection boiling, which is a more complex process involving two-phase fluid flow, gravity, material phenomena, and heat transfer mechanisms.

The second picture above is a process of boiling water on long pipe sections and heated evenly. As it approaches point (1), water enters the pipe and becomes convectionally a pipe cooling medium. Just at the point (1) begin to form water vapor bubbles, a sign that the initial boiling process begins. At point (2), more water vapor bubbles are formed and create bubbly flow.

Between points (2) and (3), more vapor bubbles are gathered to form larger bubbles. This flow known as intermediate flow has a phase called saturated nucleate boiling.

At point (3), the water temperature is higher and reaches its saturation temperature and reaches the nucleate boiling region phase. In this phase a mixture of water with vapor begins to form a bubbling stream, and forms an annular flow. This phenomenon as a result of the complex interaction between the surface tension force, the phenomenon of two surfaces, the drastic reduction of pressure, the water-vapor mass, and the momentum effect of the boiling process on the surface of the pipe.

The heat transfer process continues so that after passing through point (3) the annular flow is enlarged and the film water layer is formed on the pipe wall. Subsequent heat transfer occurs by conduction and convection by passing through the film layer, so that the evaporation process occurs at the water layer coat with water vapor. This heat transfer mechanism is called convection boiling, which also produces a high heat transfer rate.

At point (4) the heat transfer process reaches the CHF (Critical Heat Flux), where the film water layer on the pipe wall is replaced with a film layer of water vapor. In this phase there are several possible phenomenon risks:

1. Increase the metal temperature of the pipe so it can damage the pipe.
2. Heat transfer loss. And,
3. Temperature fluctuations are very likely to cause thermal fatigue failures.

From point (4) to (5) is called post-CHF heat transfer, which occurs very complex. After point (5), all water has been evaporated and turned phase into water vapor.