A means of controlling the temperature, moisture, movement and purity of air. Changes in these affect human comfort. They also affect some industrial processes.
High temperatures and moisture levels are extremely uncomfortable. Invariably they make us feel hot and sticky. They also make it difficult for the body to control its temperature. Usually sweat evaporates and cools the body. If humidity is high, sweat cannot evaporate.
Man has experimented for centuries with ways of controlling air conditions. There were many ideas, one of the simplest being to hang wet mats over doorways. This cooled the incoming air. But it also moisturized it and that is not always a desirable thing.
None of the many other systems really did the job. It was not until refrigeration techniques were developed that modern air conditioning standards could be achieved.
Temperature and moisture also affect materials like cloth, paper and certain metals. When printing on paper, or weaving cloth, these air conditions must be kept constant. In the manufacture of delicate instruments, such as those used in spacecraft, conditions must be controlled to protect fine metal parts from rusting and corrosion.
Air conditioning can be used either to reduce or to increase heat and moisture. However, it is usually concerned with cooling and drying. That is mainly what we will consider in this article.
In most air conditioning equipment, a refrigeration system is used. A cooling chemical, such as the gas Freon, begins at room temperature. It is condensed by pressure, and this makes it hot. This excess heat is blown away by a fan and the Freon turns into a liquid at room temperature. It is then fed through a fine nozzle into an evaporator where it returns to its gas form. In the course of evaporating it absorbs heat. This process is repeated continually.
A fan blows air over the evaporator chamber. The air loses heat, which it gives up to the evaporating Freon. It also loses moisture, which condenses on the surface of the chamber. The cooler and drier air is then filtered to remove impurities, such as dust, pollen or smoke, and pumped into the room that is to be air conditioned.
Most air conditioners can be adjusted to make air hotter and more moist if required.
The first effective air conditioning machine was invented by Willis H Carrier in 1902. He used it to solve problems that were occurring at a colour printing plant in New York. Paper stretches when it is moist and shrinks when dry. This meant that a sheet could change size between one application of ink and another of a different colour. Thus air conditioning became a necessity rather than a convenience.
Since then air conditioning has become common in cinemas, public buildings, private homes and even in automobiles.
