Superheat required for boiling

TABLE 1A Calculated and Early Experimentally Measured Values of the Superheat Required for Boiling... 

Effect of Pressure. Results illustrating the effect of pressure on nucleate pool boiling are shown in Fig. 15.34 the superheat required for a given heat flux decreases with increasing pressure as shown. Results for subatmospheric conditions are presented by Schroder et al. [74] the trends shown in Fig. 15.34 are continued (increasing superheat for reducing pressure), but at the lowest pressure studied (0.1 bar), a new phenomenon is observed. Boiling starts with the creation... 

A few of the many contributors to the classical rate theory of boiling nucleation are Volmer (VI), Becker and Doring (B2), Frenkel (F7), Fisher (F3), and Bernath (B4). All agree that a prime requirement for nucleation to occur in a liquid is that the liquid must be superheated. The bubbles formed are cooler than the liquid therefore nucleation is strictly irreversible. Because of the superheat, a temperature driving force exists between liquid and bubble. However, because surface tension forces are immense for tiny bubbles, a collapsing tendency exists which may counteract the tendency of a bubble to grow by absorbing heat. One problem faced by any theory of nucleation is to explain the formation of a bubble which will not collapse. 

Microscopic vapor nuclei in the form of bubbles entrapped on the heat-transfer surface must exist in order for nucleate boiling to occur. Surface tension at the vapor-liquid interface of the bubbles exerts a pressure above that of the liquid. This excess pressure requires that the liquid be superheated in order for the bubble to exist and grow. The porous surface substantially reduces the superheat required to generate vapor. The entrances to the many nucleation sites are restricted in order to retain part of the vapor in the form of a bubble and to prevent flooding of the site when liquid replaces the escaping bubble. 

FIG. 20 A plot of experimental data from Skripov [6] for the homogeneous boiling of pentane. The part ACB is the normal superheat limit. As the temperature is increased, so the time required for explosive boiling of liquid drops rapidly decreases. In going from 145.6 to 146.0 C, the waiting time drops from 550 to about 0.5 sec. If the superheated liquid droplets are exposed to radiation, very much lower superheats are needed to cause bubble nucleation. Temperatures as low as 129.5 C can cause bubble formation. 

In many situations, similar violent boiling can take place with much smaller unstable superheated states than those required for homogeneous nucleate boiling, e.g. for LNG and LPG with superheats of only 1 or 2 K. These events are termed quasi-homogeneous nucleate (QHN) boiling events, and take place in the absence of solid surfaces with nucleation sites [5]. 

Example. The normal boiling point of saturated NaCl brine is 108.7°C. If we are to provide a temperature differential of 10°C, the steam must condense at 118.7°C, or at 190.3 kPa. Taking compression to be a constant-entropy process, we can follow its course on a Mollier diagram or in the steam tables. Increasing the pressure of the cogenerated steam from atmospheric requires an increase in enthalpy of 118 kJ kg and produces a vapor temperature of 196.5°C (78° superheat). We can perform similar calculations for... 

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