Cooling towers are heat removal units used to transfer course of waste heat to the environment. Cooling towers might both use the evaporation of water to remove process heat and cool the working fluid to close to the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely solely on air to cool the working fluid to near the dry-bulb air temperature. Widespread applications embody cooling the circulating water used in oil refineries, chemical plants, energy stations and constructing cooling. The towers vary in size from small roof-prime models to very large hyperboloid buildings (as in Picture 1) that can be as much as 200 metres tall and one hundred metres in diameter, or rectangular buildings (as in Picture 2) that may be over 40 metres tall and 80 metres lengthy. Smaller towers are usually manufacturing unit-built, while larger ones are constructed on site. They are sometimes associated with nuclear energy plants in popular tradition, though cooling towers are constructed on many sorts of buildings.
A hyperboloid cooling tower was patented by Frederik van Iterson and Gerard Kuypers in 1918. The primary hyperboloid cooling towers have been constructed within the last 1920s in Liverpool, England to cool water used at a electrical power station that used coal.
Classification by use
An HVAC (heating, ventilating, and air conditioning) cooling tower is a subcategory rejecting heat from a chiller. Water-cooled chillers are usually extra power environment friendly than air-cooled chillers due to heat rejection to tower water at or close to wet-bulb temperatures. Air-cooled chillers must reject heat at the dry-bulb temperature, and thus have a lower common reverse-Carnot cycle effectiveness. Giant workplace buildings, hospitals, and colleges typically use a number of cooling towers as a part of their air conditioning programs. Typically, industrial cooling towers are much larger than HVAC towers.
HVAC use of a cooling tower pairs the cooling tower with a water-cooled chiller or water-cooled condenser. Aton of air-conditioning is the removal of 12,000 Btu/hour (3500 W). The equal ton on the cooling tower facet truly rejects about 15,000 Btu/hour (4400 W) as a result of heat-equal of the power wanted to drive the chiller’s compressor. This equal ton is outlined because the heat rejection in cooling 3 U.S. gallons/minute (1,500 pound/hour) of water 10 °F (6 °C), which amounts to 15,000 Btu/hour, or a chiller coefficient of efficiency (COP) of four.Zero. This COP is equivalent to an energy effectivity ratio (EER) of 14.
Cooling towers are additionally used in HVAC systems which have a number of water source heat pumps that share a typical piping water loop. In this kind of system, the water circulating inside the water loop removes heat from the condenser of the heat pumps whenever the heat pumps are working in the cooling mode, then the cooling tower is used to take away heat from the water loop and reject it to the environment. When the heat pumps are working in heating mode, the condensers draw heat out of the loop water and reject it into the house to be heated.
Industrial cooling towers
Industrial cooling towers can be used to take away heat from varied sources resembling machinery or heated course of material. The primary use of massive, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems utilized in power plants, petroleum refineries, petrochemical plants, pure fuel processing plants, meals processing plants, semi-conductor plants, and for different industrial facilities such as in condensers of distillation columns, for cooling liquid in crystallization, and many others. The circulation charge of cooling water in a typical seven-hundred MW coal-fired energy plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons per minute) and the circulating water requires a provide water make-up rate of maybe 5 percent (i.e., 3,600 cubic metres an hour).
If that very same plant had no cooling tower and used once-via cooling water, it might require about a hundred,000 cubic metres an hour and that amount of water would have to be constantly returned to the ocean, lake or river from which it was obtained and continuously re-equipped to the plant. Furthermore, discharging massive amounts of sizzling water could elevate the temperature of the receiving river or lake to an unacceptable stage for the native ecosystem. Elevated water temperatures can kill fish and different aquatic organisms (seethermal pollution). A cooling tower serves to dissipate the heat into the ambiance instead and wind and air diffusion spreads the heat over a much bigger space than scorching water can distribute heat in a physique of water. Some coal-fired and nuclear power plants located in coastal areas do make use of once-by means of ocean water. But even there, the offshore discharge water outlet requires very careful design to keep away from environmental problems.
Petroleum refineries also have very large cooling tower systems. A typical giant refinery processing 40,000 metric tonnes of crude oil per day (300,000 barrels (48,000 m3) per day) circulates about eighty,000 cubic metres of water per hour via its cooling tower system.
The world’s tallest cooling tower is the 200 metres tall cooling tower of Niederaussem Power Station.
Classification by construct
This sort of cooling towers are preassembled and could be merely transported on trucks as they are compact machines. The capability of package deal sort towers is proscribed and for that motive, they are often preferred by facilities with low heat rejection necessities akin to food processing plants, textile plants, buildings like hospitals, lodges, malls, chemical processing plants, automotive factories and so on.
Due to intensive use in domestic areas, sound level management is a relatively extra vital concern for package deal kind cooling towers.
Field Erected Kind
Such services as energy plants, steel processing plants, petroleum refineries, petrochemical plants desire field erected type cooling towers resulting from their giant requirements of heat rejection. These towers are rather a lot bigger in dimension in comparison with the bundle sort cooling towers. A typical discipline erected cooling tower has pultruded FRP structure, FRP cladding, a mechanical unit for air draft, drift eliminator and fill.
Heat switch strategies
With respect to the heat switch mechanism employed, the principle sorts are:
In a wet cooling tower (or open circuit cooling tower), the heat water could be cooled to a temperature decrease than the ambient air dry-bulb temperature, if the air is comparatively dry (seedew level andpsychrometrics). As ambient air is drawn previous a flow of water, a small portion of the water evaporate, the power required by that portion of the water to evaporate is taken from the remaining mass of water decreasing his temperature (approximately by 970 BTU for every pound of evaporated water). Evaporation leads to saturated air conditions, decreasing the temperature of the water course of by the tower to a worth near wet bulb air temperature, which is lower than the ambient dry bulb air temperature, the difference determined by the humidity of the ambient air.
To attain higher efficiency (extra cooling), a medium called fill is used to extend the floor area and the time of contact between the air and water flows. Splash fill consists of fabric positioned to interrupt the water circulation causing splashing. Movie fill is composed of thin sheets of material (often PVC) upon which the water flows. Each methods create elevated surface space and time of contact between the fluid (water) and the gas (air).
Air stream generation methods
With respect to drawing air by way of the tower, there are three forms of cooling towers:
Hyperboloid (sometimes incorrectly generally known as hyperbolic) cooling towers (Picture 1) have develop into the design commonplace for all natural-draft cooling towers because of their structural power and minimal usage of fabric. The hyperboloid shape also aids in accelerating the upward convective air circulation, bettering cooling efficiency. They are popularly associated with nuclear energy plants. Nevertheless, this association is misleading, as the identical sort of cooling towers are often used at large coal-fired energy plants as well. Equally, not all nuclear energy plants have cooling towers, instead cooling their heat exchangers with lake, river or ocean water.
Categorization by air-to-water flow
Crossflow is a design through which the air circulate is directed perpendicular to the water stream (see diagram beneath). Air move enters a number of vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) via the fill by gravity. The air continues through the fill and thus previous the water flow into an open plenum area. A distribution or scorching water basin consisting of a deep pan with holes or nozzles in the underside is utilized in a crossflow tower. Gravity distributes the water by way of the nozzles uniformly throughout the fill materials.
In a counterflow design the air flow is straight opposite to the water movement (see diagram below). Air movement first enters an open area beneath the fill media and is then drawn up vertically. The water is sprayed through pressurized nozzles and flows downward by way of the fill, opposite to the air move.
Frequent to both designs:
Each crossflow and counterflow designs will be used in natural draft and mechanical draft cooling towers.
Cooling tower as a flue gas stack
At some fashionable power stations, geared up with flue gas purification like the power Station Staudinger Grosskrotzenburg and the facility Station Rostock, the cooling tower is also used as a flue gas stack (industrial chimney). At plants with out flue gasoline purification, problems with corrosion could occur.
Wet cooling tower material balance
Quantitatively, the fabric stability around a wet, evaporative cooling tower system is governed by the operational variables of make-up stream price, evaporation and windage losses, draw-off rate, and the focus cycles:
Within the above sketch, water pumped from the tower basin is the cooling water routed by means of the process coolers and condensers in an industrial facility. The heat water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. Because it trickles down, it contacts ambient air rising up by means of the tower both by natural draft or by forced draft using massive fans within the tower. That contact causes a small amount of the water to be lost as windage (W) and among the water (E) to evaporate. The heat required to evaporate the water is derived from the water itself, which cools the water back to the original basin water temperature and the water is then ready to recirculate. The evaporated water leaves its dissolved salts behind in the bulk of the water which has not been evaporated, thus elevating the salt concentration in the circulating cooling water. To stop the salt concentration of the water from turning into too excessive, a portion of the water is drawn off (D) for disposal. Recent water makeup (M) is supplied to the tower basin to compensate for the loss of evaporated water, the windage loss water and the draw-off water.
A water stability around the entire system is:
Since the evaporated water (E) has no salts, a chloride steadiness across the system is:
From a simplified heat stability round the cooling tower:
Windage (or drift) losses (W) from large-scale industrial cooling towers, in the absence of manufacturer’s information, may be assumed to be:
Cycles of focus represents the accumulation of dissolved minerals within the recirculating cooling water. Draw-off (or blowdown) is used principally to control the buildup of these minerals.
The chemistry of the make-up water together with the quantity of dissolved minerals can differ broadly. Makeup waters low in dissolved minerals comparable to these from surface water provides (lakes, rivers and many others.) are usually aggressive to metals (corrosive). Makeup waters from ground water provides (wells) are normally larger in minerals and are usually scaling (deposit minerals). Increasing the amount of minerals current within the water by cycling could make water much less aggressive to piping however excessive levels of minerals could cause scaling problems.
As the cycles of concentration improve the water is probably not able to hold the minerals in solution. When the solubility of those minerals have been exceeded they’ll precipitate out as mineral solids and trigger fouling and heat alternate issues in the cooling tower or the heat exchangers. The temperatures of the recirculating water, piping and heat alternate surfaces decide if and where minerals will precipitate from the recirculating water. Typically knowledgeable water therapy guide will consider the make-up water and the operating circumstances of the cooling tower and recommend an appropriate vary for the cycles of concentration. Using water remedy chemicals, pretreatment reminiscent of water softening, pH adjustment, and other methods can have an effect on the acceptable range of cycles of focus.
Concentration cycles in the majority of cooling towers often range from three to 7. Within the United States the majority of water provides are effectively waters and have vital levels of dissolved solids. However one in all the largest water supplies, New York City, has a floor supply fairly low in minerals and cooling towers in that metropolis are sometimes allowed to focus to 7 or extra cycles of focus.
Apart from treating the circulating cooling water in giant industrial cooling tower systems to minimize scaling and fouling, the water must be filtered and also be dosed with biocides and algaecides to prevent growths that would interfere with the steady move of the water. For closed loop evaporative towers, corrosion inhibitors may be used, but caution ought to be taken to meet local environmental rules as some inhibitors use chromates.
Ambient situations dictate the effectivity of any given tower resulting from the amount of water vapor the air is ready to absorb and hold, as will be determined on a psychrometric chart.
Cooling towers and Legionnaires’ disease
Another essential reason for using biocides in cooling towers is to stop the expansion ofLegionella, together with species that cause legionellosis orLegionnaires’ disease, most notably L. pneumophila, or Mycobacterium avium. The various Legionella species are the reason for Legionnaires’ illness in people and transmission is through publicity to aerosols—the inhalation of mist droplets containing the bacteria. Common sources of Legionella include cooling towers used in open recirculating evaporative cooling water systems, home sizzling water methods, fountains, and related disseminators that faucet right into a public water provide. Pure sources embrace freshwater ponds and creeks.
French researchers found that Legionella micro organism travelled up to 6 kilometres by means of the air from a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais, France. That outbreak killed 21 of the 86 individuals who had a laboratory-confirmed infection.
Drift (or windage) is the time period for water droplets of the process flow allowed to escape in the cooling tower discharge. Drift eliminators are used in order to hold drift rates sometimes to 0.001%-zero.005% of the circulating flow price. A typical drift eliminator supplies a number of directional changes of airflow whereas stopping the escape of water droplets. A nicely-designed and nicely-fitted drift eliminator can vastly cut back water loss and potential for Legionella or different chemical publicity.
Many governmental companies, cooling tower manufacturers and industrial trade organizations have developed design and upkeep tips for preventing or controlling the expansion of Legionella in cooling towers. Under is an inventory of sources for such pointers:
Cooling tower fog
Under certain ambient circumstances, plumes of water vapor (fog) will be seen rising out of the discharge from a cooling tower, and could be mistaken as smoke from a hearth. If the outside air is at or close to saturation, and the tower adds more water to the air, saturated air with liquid water droplets can be discharged—which is seen as fog. This phenomenon usually occurs on cool, humid days, however is uncommon in many climates.
This phenomenon will be prevented by reducing the relative humidity of the saturated discharge air. For that purpose, in hybrid towers, saturated discharge air is mixed with heated low relative humidity air. Some air enters the tower above drift eliminator stage, passing by heat exchangers. The relative humidity of the dry air is even more decreased instantly as being heated while coming into the tower. The discharged mixture has a comparatively lower relative humidity and the fog is invisible.
Cooling tower operation in freezing weather
Cooling towers with malfunctions can freeze throughout very cold weather. Usually, freezing begins at the corners of a cooling tower with a reduced or absent heat load. Increased freezing situations can create rising volumes of ice, leading to elevated structural hundreds. In the course of the winter, some sites repeatedly operate cooling towers with 40 °F (4 °C) water leaving the tower. Basin heaters, tower draindown, and other freeze protection strategies are often employed in cold climates.
Some commonly used phrases in the cooling tower industry
Correctly sizing a facet-stream filtration system is important to acquire satisfactory filter efficiency. There is some debate over tips on how to properly size the aspect-stream system. Many engineers dimension the system to constantly filter the cooling tower basin water at a rate equal to 10% of the total circulation circulate charge. For instance, if the whole circulation of a system is 1,200 gal/min (a four hundred-ton system), a 120 gal/min side-stream system is specified.
Cooling towers that are constructed in complete or in part of combustible supplies can help propagating inside fires. The ensuing injury can be sufficiently severe to require the replacement of the whole cell or tower structure. For that reason, some codes and requirements[eleven] advocate combustible cooling towers be supplied with an automatic fireplace sprinkler system. Fires can propagate internally throughout the tower construction throughout upkeep when the cell is not in operation (resembling for maintenance or construction), and even when the tower is in operation, especially these of the induced-draft sort due to the existence of comparatively dry areas within the towers.
Being very large buildings, they’re susceptible to wind injury, and several other spectacular failures have occurred prior to now. At Ferrybridge energy station on 1 November 1965, the station was the site of a major structural failure, when three of the cooling towers collapsed owing to vibrations in 85 mph (137 km/h) winds. Though the structures had been built to withstand increased wind speeds, the form of the cooling towers meant that westerly winds have been funnelled into the towers themselves, creating a vortex. Three out of the unique eight cooling towers had been destroyed and the remaining five have been severely damaged. The towers had been rebuilt and all eight cooling towers had been strengthened to tolerate hostile weather conditions. Building codes have been modified to incorporate improved structural support, and wind tunnel tests introduced to examine tower buildings and configuration.