The Boiling Process

10 Describe the process of boiling and the relationships among boiling point, vapor pressure, and surrounding pressure. 

When a liquid is heated in an open container, bubbles form, usually at the base of the container where heat is being applied. The first bubbles are often air, driven out of solution by an increase in temperature. Eventually, when a certain temperature is reached, vapor bubbles form throughout the liquid, rise to the surface, and break. When this happens we say the liquid is boiling. 

A liquid (in this case water) boils when its equilibrium vapor pressure equals the atmospheric pressure. 

Bubbles of vapor that form within the liquid consist of the same kind of molecules... 

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Most steam generating plants operate below the critical pressure of water, and the boiling process therefore involves two-phase, nucleate boiling within the boiler water. At its critical pressure of 3,208.2 pounds per square inch absolute (psia), however, the boiling point of water is 374.15 C (705.47 °F), the latent heat of vaporization declines to zero, and steam bubble formation stops (despite the continued application of heat), to be replaced by a smooth transition of water directly to single-phase gaseous steam. 

It is a matter speculation as to why such high values of heat flux are obtained with the boiling process. It was once thought that the bubbles themselves were carriers of latent heat which was added to the liquid by their movement. It has now been shown, by determining the numbers of bubbles, that this mechanism would result in the transfer of only a moderate part of the heat that is actually transferred. The current views are that the high flux arises... 

The occurrence of burnout is a serious inconvenience, for the boiling process must then come to a halt. Burnout during boiling is the result... 

Temperature difference between the condensing steam and the boiling process liquid is then (320°F — 240°F) = 80°F. This is called the temperature-difference driving force, or AT. 

Calculate the amount of ascorbic acid in each sample. Was any lost during the boiling process Explain. Could vitamin C be detected in the boiled water Typical values for vitamin C content are shown below. 

Covalent compounds, by contrast, are low-melting solids, liquids, or even gases. A sample of a covalent compound, such as hydrogen chloride, consists of discrete HCI molecules. The covalent bond within an individual molecule may be very strong, but the attractive forces between the different molecules are fairly weak. As a result, relatively little energy is required to overcome these forces and cause a covalent compound to melt or boil. We ll look at the nature of intermolecular forces and the boiling process in Chapter 10. 

Before specific relations for calculating boiling heat transfer are presented, it is suggested that the reader review the discussion of the last few pages and correlate it with some simple experimental observations of boiling. For this purpose a careful study of the boiling process in a pan of water on the kitchen stove can be quite enlightening. 

It may happen that the hot solution remains supersaturated, and crystallization does not take place, until the solution is cooled. The checkers obtained a poor yield when this occurred. They, therefore, seeded other runs during the boiling process in order to bring about crystallization. Posner1 had to concentrate the solution to about half its volume in order to bring about crystallization. 

In general, the investigation techniques used so far were constrained by the use of intrusive methods and the limitation in access to clear and non-intrusive visualization of the boiling process. 

The phrase boiling point must be used carefully for other reasons. Care must be exercised such that the boiling process is indeed defined. Two examples which illustrate this point are MgO and LiF. Magnesium oxide does not vaporize congruently (so that the total vapor pressure depends on the ambient oxygen potential), whereas LiF vaporizes congruently but monomer, dimer, and trimer species exist in the vapor. 

Fig. 10.6 Mental models concerning the boiling process of water [10]...

Recent studies have demonstrated that the boiling process is actually a convective one, with the vapor generated on the lower tubes creating a rising and growing two-phase flow across the upper tubes. The vapor and liquid separate at the top of the bundle and the clear liquid flows downward around the sides of the bundle to complete the circuit. Computer-based design methods employ this model, and the existing database is discussed by Collier and Thome [34]. 

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