As we are well into the “Dog Days of Summer,” that period of time when Sirius, the Dog Star, rises at the same time as the sun, typically from early July to mid-August, thinking about how to keep cool becomes a priority.
For some, keeping cool is as simple as adjusting a thermostat, thanks to the invention of modern air conditioning.
July 17, 1902, is the date generally given as the invention of the modern air conditioner by Willis Carrier. But staying cool or staying in an optimum temperature range is as old as biology, and from a technology point of view, has been occurring since humans began using tools.
Understanding of heat and the flow of energy helps us to understand how one can cool their surroundings.
First heat moves from hot to cold. Add a hot stone into a pot of water, the heat from the stone moves to the liquid, until both the stone and the liquid are the same temperature. And, there is a flow of energy during a change in the state of matter. For a liquid like water to change into a gas, it takes 540 calories, or 0.540 food calories per gram of water at 212 degrees F or 100 degrees C to change from the liquid state to a gaseous state. This is called the heat of vaporization.
This means that if we are boiling water, we have to add heat to have the water change from the liquid to steam. Our bodies take advantage of this energy requirement, i.e., the water has to pull heat from its surroundings to go from a liquid to a vapor, to help maintain our body temperature. We perspire, i.e., sweat, and as the water moves from the liquid to a vapor state, energy is pulled from the surroundings, making us feel cooler.
Evaporative cooling works similarly.
An evaporative cooler, commonly referred to as swamp coolers, capitalize on this exchange of energy required to produce the phase change. In India, a company is producing larges terracotta tube structures that look like art but are passive evaporative cooling systems. Here the structure is soaked once or twice per day with water, and as a breeze passes over the structure the water is evaporating, cooling the surrounding area. This technique requires much less energy in the form of electricity to drive the system, as the only electrical input is that of pumping the water once or twice per day.
The modern air conditioner still utilizes the energy requirements of phase changes to move the heat from one location to another but also relies on a mechanical system to move the heat. To understand how this works, one looks at what is referred to as the refrigeration cycle. A refrigerant, a fluid used in the system is circulated through piping, on one side the cooling side, the fluid is in a gaseous state and is used to absorb heat from its surroundings. It then passes through a compressor, increasing the pressure, to the condenser, where the fluid now becomes a liquid. The phase change from a gas to a liquid gives off heat or sends heat to its surroundings. The liquid is pumped to an expansion valve, allowing for a change in pressure. Thus, the liquid is moving through the phase transition to a gas, and the cycle repeats. In the refrigeration cycle, there is a change in pressure to produce the phase of the fluid.
The modern air conditioning systems such as the Carrier process relies on a more extensive mechanical system, i.e., the compressor, fans, etc. There is a higher energy requirement in the form of electricity to get the system to function, which impacts our electric bill during the summer months. Evaporative or swamp coolers may also require electrical energy for fans to push the air through the evaporation unit as well as to drive the cooler air through ventilation systems, but typically are a bit less than that of modern air conditioners. But, evaporative systems have a drawback in that they do not work as well when the surrounding air is saturated with water, this is why swamp coolers are typically used in drier climates rather than in humid ones. Similarly, high humidity makes it harder for our bodies to cool using perspiration, and hence the importance of following the heat index.
By utilizing phase changes, and understanding of the flow of heat, you can evaluate your cooling options.
Move the air around with a fan; you have more opportunity for evaporation of the sweat on your skin.
You can understand why you can’t cool a room by opening the refrigerator door; it is because the heat is being redeposited in the same room. And, you can evaluate those personal cooling devices a bit better.
Do they work?
Possibly, but you need to know where the heat goes.
Editor’s note: This is a series of science-related articles by author Frankie Wood-Black, Ph.D., REM, MBA, to appear in Mid-Week section of the Ponca City News. The author currently runs her own environmental consulting firm based in Ponca City, Sophic Pursuits, Inc., and also serves as a Physics Instructor and the Director for Process Technology at Northern Oklahoma College.