United Trades Exclusive
By Dale Yelnich
Evaporators are one of the main reasons why refrigeration, and therefore air conditioning, became practical for use in both home and industrial cooling. Simply put, an evaporator allows a contained pressurized liquid to turn into a gas. While the liquid evaporates into a gas, it absorbs heat, and this heat absorption is the backbone to effective refrigeration.
The process of evaporation has been known for centuries. Egyptian kings would be cooled by hanging wet reeds from the ceiling, while servants moved air through them with fans. The resultant cooler air blowing through the reeds, due to water evaporation, would bring a welcome relief to the king, queen consorts and guests in the middle of the desert.
A Scotsman, Dr. William Cullen, began to study the evaporation of liquids inside of a vacuum in 1720. In 1748, he was the first person to demonstrate artificial refrigeration by allowing ether to boil inside of a container within a partial vacuum. As the ether boiled, the immediate area around the container absorbed heat and became cooler, and this cooling effect is the same used in evaporators to this day.
An American inventor named Oliver Evans, designed a refrigeration machine that actually used vapor instead of a liquid for cooling. It wasn’t until 1842 that a physician named John Gorrie, used Evans principles to cool patients in a Florida hospital. For the first time, the refrigeration machine that Gorrie invented used the same evaporative technique that all modern refrigeration and air conditioning units use.
Willis Haviland Carrier employed these basic principles of evaporative cooling to create air conditioning for commercial use in 1902, and by 1928, Carrier shrunk down the giant sized commercial units so that they would fit inside of a window frame. This was the first type of residential air conditioner for the home.
The evaporator made all of this possible, and in fact, it is considered the main part of any refrigeration system, which is precisely what an air conditioner is. It works like this. The partially liquid and partially gaseous refrigerant, at very low temperature, enters the evaporator where the substance to be cooled is kept. It is here where the refrigeration effect is produced. The refrigerant absorbs the heat from the substance to be cooled and gets converted into vapor state. This principle of evaporation used for cooling hasn’t changed much since Dr. William Cullen studied the phenomenon in 1720.
There are three types of evaporators used in the refrigeration process. The first two, bare tube evaporators and plate evaporators are generally used for chilling liquids or as refrigerator and freezer evaporators. They have a limited surface area, and although they do an effective job of removing heat, they are not very thermally efficient.
Finned evaporators are the third type, and these are commonly used for air conditioning, in both commercial and residential applications, including window mounted air conditioners and central air HVAC units. Their design makes them very effective and efficient at heat transfer through the evaporation process.
This arrangement uses a bare tube evaporator encased within cooling fins. The fins aid in heat removal of the evaporated liquid, so that when air is moved over the fins, the heat transfer becomes so effective that cold air is a byproduct. This is the hallmark evaporation system for residential and many commercial central air conditioners.
However, with all of this knowledge about modern evaporators, how they work and what they are used for, there is one piece of the puzzle that is missing. As was proven centuries before by the Egyptian kings, the most effective cooling from evaporation can only be done by the use of a fan. Without a fan, the evaporation cycle will not be viable for any cooling capacity.
Fan speed is absolutely essential for the evaporation process to occur. Heat exchange depends upon a contrasting temperature difference between the ambient air temperature and the refrigerant inside of the evaporator. If there is no fan to blow air over the evaporator coils, heat exchange will only occur as the heat rises of its own accord, which is not very effective. But a blowing fan removes heat quickly from the evaporator coils and thus speeds the heat transfer of the system. Every air conditioning evaporator needs a fan to keep it working effectively and efficiently during the course of its service life.
Although there are many parts of an air conditioning system, the advent of the pressurized and sealed evaporator make it all possible. In that context, it can be unequivocally stated that of all the components of a cooling system, from the compressor to the condenser, only the evaporator is capable of putting the real chill in air conditioning units.