Cooling a Home: Swamp Coolers and Air Conditioning Units

In hot places, it becomes necessary to cool the inside of buildings. Modern buildings have refrigeration systems, commonly referred to as air conditioning or AC for short. An alternate system of cooling is a swamp cooler. Although it sounds primitive, a swamp cooler can be effective, and is almost always much cooler to run than an air conditioning unit.

Swamp coolers were developed, not in swampy areas, but in the desert. Rick Goettl, in the early 20th century, perfected home cooling in the southwestern desert of Arizona. Although the company would later move to become a major Phoenix air conditioning supplier, they started with swamp coolers. These worked well in the hot, dry air in ways that they would never have worked in the Midwest or Deep South.

Swamp coolers work by the principle of evaporation. When water evaporates it uses heat energy to change state from liquid to gas. Much like boiling water requires heat, evaporating require heat. This heat is drawn from the surrounding air. A swamp cooler is a large box with sponge-like pads lining three sides and the top. A large fan forms the fourth side. The floor of the cooler is a catch pan for water, which is introduced at the top of the pads. Water runs down the walls, keeping the pads saturated. Air flows through the walls of the cooler, through the wet pads, and out through the fan, into the room. On a dry day, the water in the pads evaporates quickly, keeping the pads and air quite cold. On a humid day, the water in the pads does not evaporate well, and the result is that the cooler only lowers the temperature slightly, while humidifying the room.

Air conditioning works on a completely different scientific principle, that of Boyle's gas law. Increasing the pressure of gas in a chamber will raise the temperature, if the volume of the chamber stays the same or decreases. Decreasing the pressure will decrease the temperature. This is why a child's balloon feels cold immediately after it is deflated, and why a bike pump will get warm if the tire is inflated quickly.

An air conditioning unit uses a fluid such as Freon that has a boiling point close to the ambient temperature of a room. A compressor pressurizes Freon gas, raising the temperature. The hot gas then runs through a condenser, which is like a radiator, and allows heat to escape into the surrounding air (this part of the unit is outside the building). As the pressurized gas cools, it condenses back to a liquid. Now, it is at high pressure, but room temperature. The liquid passes through an expansion valve, which lowers the pressure, resulting in a cold mixture of liquid and gas. This cold mixture runs through an evaporator, which is like a radiator, but working in reverse. As warm air from the room blows over the cold coils, the fluid in the coils turns to warm gas, and the air is cooled and returned to the room via a duct. The warm Freon then returns to the compressor, and the cycle starts over.

In addition to cooling the air in a room, an air conditioning unit also lowers the humidity. This is because as warm air from the room runs past the cold evaporator coils, moisture from the air condenses on the coils, much like water condenses on a cold glass on a hot day.

The Freon compressor requires a lot of power, in addition to the power necessary to run the fans that blow air over the evaporator and condenser. This means that an air conditioning unit is much more expensive to operate than a swamp cooler. But an air conditioning unit can operate and cool a room even when the air is humid, while a swamp cooler relies on dry air to operate effectively.