What determines the rate of evaporation of the liquid? Factors affecting the process

We all know well from the childhood one serious fact of life. In order to cool hot tea, it is necessary to pour it into a cold saucer and continue to blow over its surface for a long time. When you are six or seven years old, do not really think about the laws of physics, just take them as a given or, physically speaking, take them for an axiom. However, comprehending in time science, we find interesting similarities between axioms and successive proofs, smoothly translating our children's assumptions into adult theorems. It's the same with hot tea. None of us could have thought that such a method of cooling it directly related to the evaporation of the liquid.

The physics of the process

In order to answer the question of what the rate of evaporation of a liquid depends on, it is necessary to understand the physics of the process. Evaporation is the process of a phase transition of matter from the liquid aggregate state to the gaseous state. Any liquid substance, including very viscous substance, can evaporate. In appearance, you can not say that some jelly-like liquid can lose part of its mass due to evaporation, but under certain conditions this is exactly what happens. A solid body can also evaporate, only such a process is called sublimation.

How It Works

Having begun to understand what the rate of evaporation of a liquid depends on, one should start from the fact that this is an endothermic process, that is, a process that takes place with the absorption of heat. The heat of the phase transition (evaporation heat) transfers energy to the molecules of matter, increasing their velocity and increasing the probability of their detachment, while weakening the forces of molecular adhesion. Breaking away from the bulk of the substance, the fastest molecules break out beyond its boundaries, and the substance loses its mass. In this case, the emitted molecules of the liquid instantaneously effervesce, carrying out the phase transition process upon detachment, and their yield is already in the gaseous state.

Application

Understanding the reasons for the rate of evaporation of the liquid, it is possible to correctly regulate the technological processes that take place on their basis. For example, the operation of the air conditioner, in the heat exchanger-evaporator of which boils refrigerant, taking heat from the cooled room, or boiling water in the pipes of the industrial boiler, the heat of which is transferred to the needs of heating and hot water. Awareness of the conditions on which the rate of evaporation of the liquid depends, makes it possible to design and produce modern and technological equipment of compact dimensions and with an increased coefficient of heat transfer.

Temperature

Liquid aggregate state is extremely unstable. With our earthly n. Y. (The concept of "normal conditions", ie, suitable for human life), it periodically tends to go into a solid or gaseous phase. How does this happen? What determines the rate of evaporation of the liquid?

The primary criterion is, naturally, the temperature. The stronger we heat the liquid, the more energy we bring to the molecules of matter, the more molecular bonds we break, the faster the phase transition process takes place. Apotheosis is achieved with stable bubble boiling. Water boils at 100 ° C at atmospheric pressure. The surface of a pot or, for example, a kettle, where it boils, only at first glance is perfectly smooth. With a multiple increase in the picture, we will see endless sharp peaks, as in the mountains. Heat is pointwise applied to each of these peaks, and because of the small heat exchange surface, the water boils up instantly, forming a bubble of air that rises to the surface, where it collapses. That's why this boil is called bubble. The rate of evaporation of water is thus maximum.

Pressure

The second important parameter, on which the rate of evaporation of the liquid depends, is the pressure. When the pressure drops below atmospheric, water begins to boil at lower temperatures. This principle is based on the work of the famous pressure cookers - special pans, where the air was pumped out, and the water boiled already at 70-80 ° C. Increasing the pressure, on the contrary, increases the boiling point. This useful property is used when supplying superheated water from the CHPP to the TSC and ITP, where to keep the potential of the transferred heat, the water is heated to temperatures of 150-180 degrees, when it is necessary to exclude the possibility of its effervescence in the pipes.

Other factors

Intensive blasting of the surface of a liquid with a temperature higher than the temperature of the air jet is another factor on which the rate of evaporation of the liquid depends. Examples of this can be taken from everyday life. Wind blowing the surface of the lake or the example from which we started the story: blowing hot tea, poured into a saucer. It cools due to the fact that, breaking away from the bulk of the substance, the molecules take some of the energy with them, cooling it. Here you can see also the effect of the surface area. Saucer is wider than a mug, so with its quadrature, potentially a greater amount of water can escape.

The rate of evaporation is also affected by the type of the liquid itself: some liquids evaporate faster, others, on the contrary, are slower. An important influence on the process of evaporation is exerted by the state of the surrounding air. At a high absolute moisture content (very moist air, for example, near the sea) the evaporation process will go slower.