Cooling towers are mechanical pieces of equipment that function on the principle of evaporative cooling. Evaporative cooling is the process by which a liquid is cooled to a lower temperature by evaporating a small portion of the liquid into an airstream. Relatively dry air moves through a falling liquid and as the air moves it picks up water vapor from the liquid, thereby increasing the air’s moisture content. In order for the liquid to evaporate, the liquid needs a heat source to meet the latent heat of vaporization. This heat source is the sensible heat loss from the remaining liquid.
A cooling tower consists of two fluid flows, the air flow and the water flow. The water flow starts from the top of the cooling tower. Warm water is pumped to a series of nozzles. The nozzles’ purpose is to break up the water into tiny droplets to increase the surface area of the water that is in contact with the air stream. The droplets then fall through a fill material, which also serves to break up the droplets further to increase the surface area of the water. As the water moves downward it steadily decreases in temperature as heat is lost due to evaporation. Finally the water collects at the basin and is sucked out distributed to its required location.
The air flow starts at the bottom of the tower, where cold dry air is brought into the cooling tower where it comes into contact with the water droplets. As the air moves upward through the tower it picks up water vapor and slightly increases in temperature. Prior to exiting the cooling tower, the air must travel through the drift eliminators which is a series of baffles. The purpose of the drift eliminators is to catch any suspended water droplets in the air stream and return them to the fill.
CHARACTERIZING COOLING TOWERS
The following section provides information on the different types of cooling towers used in the HVAC and Refrigeration field. This information is merely provided to give the engineer additional background on cooling towers.
Mechanical vs. Natural Draft Cooling Towers:
There are two main categories of cooling towers: (1) Mechanical draft and (2) Natural draft cooling towers. Natural draft cooling towers move air based on the difference in buoyancy of the airstream inside and outside of the cooling tower. Mechanical draft cooling towers move air through the cooling tower by means of a mechanical fan. In the HVAC and Refrigeration field, mechanical draft cooling towers are the primary type of cooling tower.
Induced vs. Forced Draft Cooling Towers:
Induced and forced draft cooling towers are both mechanical draft type fans and differ by the location of their fan. Forced draft fans blow air into the cooling tower and are located at the airstream entrance into the cooling tower. Induced draft cooling towers on the other hand, have the fans located at the exit of the airstream for the cooling tower and suck air into the cooling tower.
Counter-flow vs. Cross-flow Cooling Towers:
Counter-flow and cross-flow cooling towers are characterized by the relationship between the air flow and water flow. In a counter flow tower the air and water flow are at 90 degrees to each other. The water is falling downwards and the air is moving across from either left to right or right to left. In a cross-flow tower, the air and water flows have directly opposing directions. The water is falling downwards and the air is moving upwards.
The following figure is a schematic of a forced mechanical draft, counter flow cooling tower. The fans are located at the air inlets, near the bottom of the cooling tower. Also the air flow counters the water as the water drops downward through the fill material.
The following figure is a schematic of a forced mechanical draft, cross flow cooling tower. Since this cooling tower is forced draft, the fans are again located at the inlet of the cooling tower near the bottom. The air flows counter or perpendicular to the water as the water falls downward through the fill.
The following figure is a schematic of an induced mechanical draft, counter flow cooling tower. The fan is located at the exit of the cooling tower and air is sucked or induced through the cooling tower. This cooling tower is also a counter flow type, where air flows upward through the fill and counters the downward moving water droplets.
The following figure is a schematic of an induced mechanical draft, cross flow cooling tower. Again the fan is located at the exit of the cooling tower. This cooling tower is a cross flow cooling tower, where air flows perpendicular through the fill as it crosses the falling water droplets.