Introduction to Cooling Towers
Cooling towers are used in a variety of applications; from the 400 foot tall towers at nuclear power plants to small 4 foot cooling boxes used by neighborhood dry cleaners. The most common use is in large building central cooling systems, but also used for refrigeration, cold storage facilities, dry cleaning, medical equipment, manufacturing and industry. Cooling towers are generally the most efficient means to remove large amounts of heat from air and equipment. Unfortunately, cooling towers use large amounts of water when properly maintained, and can waste greater amounts of water when not maintained properly through wasteful practices, inefficient equipment and leaks.
Cooling systems transfer heat from one source or medium to another, often using water. In a cooling system with a cooling tower, cool water is pumped away from the cooling tower and is circulated through hot equipment (often chillers used to cool large buildings). The cool water (typically 85F (29.4 C)) absorbs heat from the equipment and becomes warmer. The warmed water (typically 100 F (37.8 C)) then returns back to the cooling tower. In the cooling tower the warmed water is sprayed downward, and air is blown upward with a fan. As the warm water droplets contact the air, some of the water droplets evaporate, and the air absorbs the heat released from this evaporation—thereby lowering the temperature of the remaining water. This cooling effect of the remaining water is called the latent heat of evaporation. During this process, some water is lost to the air from evaporation and some water is lost by the misting effect (called “drift”) into the air.
An outside source of water (usually from the local water utility), commonly referred to as “makeup water,” adds more water to the system to make up for evaporation and drift. Then, the water is re-circulated back to the heat exchanging equipment and the process is repeated.
Every water source has various levels of minerals, known as dissolved solids. When water evaporates from the system, these solids are left behind, causing the remaining water to become more concentrated in minerals. In order to maintain the same volume of water in the cooling system, more source water needs is added to the system. Again, this source water contains additional dissolved solids. Although the source water helps to somewhat dilute the concentration of minerals in the cooling system water, source water also contains some solids results in a net increase in concentration of solids in the system water. Therefore, as the system re-circulates the water in the cooling tower, some water evaporates leaving the minerals behind; the water’s impurities become more and more concentrated. As the system water increases in solids and minerals, the solids become more prone to attaching themselves to the pipe walls and other parts of the system. Concentrated solids can build up in the form of scale, causing blockages and corrosion to the cooling system materials. This scaling can cause catastrophic failure and damage to the system.
Most cooling towers purposefully remove some of the old water in the system and replace it with fresh water. To stay below this maximum acceptable concentration and to maintain the tower’s water balance, new water needs to be added to the cooling tower (called makeup water) and a portion of the concentrated cooling tower water needs to be discharged from the cooling tower (called blow-down or bleed). The amount of bleed needed depends on the system use and the quality of the fresh water supply. Often excessive bleed-off occurs due to improper adjustments made by the system operator.
Prescribed water levels must be maintained in the system to prevent catastrophic damage to the equipment; thus, automatic refill valves are installed to replace any water losses from evaporation, drift, bleed-off and leaks. Unfortunately, this allows leaks to often go unnoticed and continue for months or even years.
There are many measures that are needed to assure a cooling tower system is operating in a water efficient manner. At minimum the system should have: a) a dedicated water meter that is read daily by the local maintenance staff, and b) a TDS meter/controller to maintain proper bleed-off rates. In addition, acid treatment controllers and filtering equipment can greatly reduce water use while properly maintaining the equipment.