Surge boiling

Everyone knows what boiling is, and we discussed it at some length in Chapter 2, where we stated that boiling occurs when the vapor pressure of a liquid becomes equal to the external pressure exerted on it, and that the boiling point temperature therefore depended on what this pressure is. Now, we have to admit that reality is a bit more complex than that, and that the real situation is that when the vapor pressure of a liquid reaches the point where it's equal to the surrounding pressure, a bubble can form. Can - not will.

Before a bubble can form, a vast number of molecules must be "persuaded" to simultaneously move apart from each other to create the void we call "a bubble". Once the bubble is formed, evaporation into it is simple and the bubble can grow quite easily.

That first step, the formation of a bubble, is statistically an incredibly rare event! Without some external trigger, it's quite possible - indeed, common - for the temperature of a liquid to rise above its "normal" boiling point. The trigger that causes a bubble to form is called "nucleation", and may be one of many things. It may be a local disturbance caused by a crystal of salt or sugar dissolving, or dissolved air coming out of solution. It may be a tiny, localized hot-spot on a heater element, or even be a sharp edge on the surface of the container.

Pure water, which has been previously boiled to expel all dissolved air, is particularly prone to superheating if contained in something with a very smooth surface, like a glass or a glazed ceramic mug. Many people have been seriously burned when water in such a container was superheated in a microwave oven. The temperature of the undisturbed water can rise to alarming heights and, when disturbed in any way (even by a spoon tapping the side of the container) the superheated water can literally explode!

Fig. 8-11

Dr. Louis A. Bloomfield, Professor of Physics at The University of Virginia, has done remarkable experiments along these lines and published pictures and movies of the results on the website "howthingswork.virginia.edu" (select "microwave ovens" from the menu).

Surge boiling is a mild version of this explosive activity, and is usually encountered when heating a large volume of liquid with a thermostatically controlled heater. The initial boiling expels dissolved gases, removing a major nucleation source. When the thermostat turns the element off, the liquid cools and all bubbles collapse. During the next cycle of heating the bubbles that initially form appear only after the local temperature adjacent to the heating element has risen several degrees above the "normal" boiling point of the liquid. Since the element is usually placed at the bottom of the boiler, they are forming at higher than atmospheric pressure due to the depth of the fluid. These bubbles then trigger an avalanche of bubbles, which rapidly expand as they rise. The net result is a sudden surge of boiling each and every time the thermostat cycles, and large fluctuations in the rate at which vapor is formed. The end result can be like sitting your column on top of Old Faithful (which, in fact, operates in exactly the same way!)

This is why we advocate using a heat spreader with a hotplate, or an electronic (but compliant) power controller if using an immersed heater element. These methods allow you to maintain a steady simmer and a steady generation of vapor.

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  • kaiju
    What is surge boilint?
    6 months ago

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