Cooling towers remove heat from air conditioning systems and a range of other processes that generate excess heat. Although all of them use a closed loop system to constantly cycle water, they typically represent the largest use of water in manufacturing applications, which can be up to thirty percent or more of a facility’s total water usage. Cooling tower continuously pump warm water from heat sources to their top where it is sprayed or dripped through internal fill. The adhesive property of water molecules makes them adhere to the abundant surface area of the fill in a film-like fashion. As the water is pushed or pulled down the tower by fans, the heat energy is lost to evaporation, along with water.
Evaporation and drift – The best evaporation rate is achieved by mixing the air and water as thoroughly as possible to maximize surface space, allowing for the most water vapor pressure. Evaporative cooling releases just about 1,000 British thermal units (BTUs) for each pound of water, or about 8.34 gallons depending on temperature, atmospheric pressure, and dew point. The evaporation rate is approximately one percent of the circulating water’s flow rate for every 10° F reduction in water temperature. Water in the form of mist that is carried away from the cooling tower is known as drift losses. This water loss is near negligible since drift rates are generally 0.05 to 0.2% of the system’s circulation rate. However, an organization that employs a lot of cooling towers can justify the initiative to improve their water efficiency through using drift eliminators. These devices improve operating efficiency by preventing drift losses, which retains water as well as water treatment chemicals.
Blowdown and make-up – As evaporation occurs, the dissolved contaminants in the water build up since they don’t evaporate with the water vapor. If not addressed, it affects the thermal efficiency of the process and the life of the cooling tower. So to reduce contaminant build up, the water is flushed and replaced with fresh water. The flushed water is known as “blowdown” and the fresh water that replaces it is known as “make-up water”. Blowdowns may be controlled by valves triggered by timers of hydro-conductivity meters. The water quality in the cooling tower may be determined by the water composition, water treatment, and blowdown rate. In addition to the blowdown rate, the amount of make-up water a system needs, is dependent on the system’s evaporation and drift rates. To minimize the cooling tower’s blowdown rate, the system needs a suitable water treatment process to alleviate the blowdown frequency.
To optimize a cooling tower’s water efficiency, you need to pursue the highest concentration ratio possible, which is expressed below.
Minimizing the blowdown and drift rates will enable the system to make the most out of each make-up, which by relationship increases the system’s concentration ratio.
Cooling tower water balance – Apply the water balance method to a cooling tower system to gain an even better understanding. This technique includes the evaporation variable, which is dependent on water quality. An appropriate water balance is below.
A good water treatment process is important because maximizing the evaporation rate through better water quality will minimize amount the make-up required. The relationship between the concentration ratio and the water quality is expressed below.
Cooling tower water treatment – The objective of a water treatment process for a cooling tower is to provide an uncontaminated heat transfer surface for evaporation. A perfected evaporation rate will decrease water consumption and wastewater discharge. The factors of water quality include pH, alkalinity, conductivity, and hardness as well as microbial growth, biocide, scaling and corrosion inhibitor levels. Traditionally, automated feeders either on timed intervals or triggered by conductivity meters deposit treatment chemicals into the water circulation system. Modern technology allows more controlled chemical treatment in relation to quality requirements that may be monitored via internet anytime, anywhere.
Sulfuric acid treatment – Sulfuric acid controls scale accumulation by lowering the water’s pH to a level that will dissolve it. The harder and higher in alkalinity water is, the greater demand there is for this acid feed. This chemical needs to be administered with certain system precautions because its aggressive corrosion properties will wear away metal surfaces. Corrosion inhibitors alleviate this concern, but there is still concern for worker safety. Employees handling this chemical should be trained to do so and know how to response to possible accidents.
Side stream filtration – Side stream filtration alleviates solids accumulation concern by diverting a portion of the circulating water flow through a filtering system, typically using either rapid sand or cartridge filters. Changing or cleaning filters can be worthwhile if using make-up water that has high levels of suspended solids or if airborne particulates can settle in a system’s water. These filtration systems increase the systems overall efficiency by ensuring heat transfer and lowering blowdown rates since they reduce particle loading in the cooling tower.
Ozone treatment – Ozone may be injected into the circulating water to prevent problematic organics and control scale buildup. It controls scale by creating mineral oxides that precipitate to the basin of the cooling tower, usually in a separation tank or low-flow circulation area. Ozone treatment requires an air compressor, an ozone generator, a control system, and a diffuser or contractor. The precipitated solids, or sludge, must be removed periodically and capital costs are significant, but it can be cost effective if dealing with such contaminants because it will greatly increase the system’s efficiency.
Magnetic treatment – Unique magnetic particles produced for the purpose of water treatment have been reported to physically combat scale deposits on surfaces in the cooling tower system. Suppliers claim they do so by altering the surface charge of suspended particles in the water, which provides chemical-free treatment. The dislodged deposits settle in a low-flow circulation area and are removed from the system mechanically. Another reported alike treatment technology is an electrostatic field generator.