Heating, Cooling and HVAC

A cooling Tower is nothing but a machine which cools hot water by the method of evaporation( turning the water to vapor).Water always evaporates till the surrounding air is not 100% moist.Now, for evaporation the amount of latent heat it needs is taken out from water itself and it gets cooled.

The process of evaporation always takes place from the surface of water it is exposed to air. And that is why a glass of hot water takes much longer time to cool than when it is poured in a big plate.As the surface area of water in a plate exposed to air is much more than in a glass and so the rate of evaporation is more on the plate of water and it gets cool quicker.

In the cooling technology it is always tried to increase the surface area of hot water by different method of breaking the water into fine droplets and brining them in contact with as much and as many dry air possible to get the maximum cooling effect in minimum time period.

There are basically three types of cooling towers: 1. Natural draft. 2. Induced draft and 3. Forced draft.

1. Natural Draft Cooling Tower:-
A) Pond,lake or rivers…….Cold water is taken from the source and after use when it gets heated it is returned back to the source and gets automatically cooled by the method of evaporation from the huge surface of pond , lake or river water.In several industries the return hot water is sprayed back on the natural source.(which is called a spray pond)

Natural draft cooling towers:-(1) Spray Cooling Tower- used in Central Air conditioning projects and (2) Huge Hyperbolic - used in Power plants.

2.Induced Draft Cooling Tower:- Induced draft Cooling Towers are of two types (1) Cross Flow and (2) Counter flow.

Classification by use
Cooling towers can generally be classified by use into either HVAC (air-conditioning) or industrial duty.

HVAC
An HVAC cooling tower is a subcategory rejecting heat from a chiller. Water-cooled chillers are normally more energy efficient than air-cooled chillers due to heat rejection to tower water at near wet-bulb temperatures. Air-cooled chillers must reject heat to the dry-bulb temperature, and thus have a lower average reverse-Carnot cycle effectiveness. Large office buildings, hospitals, schools typically use one or more cooling towers as part of their air conditioning systems. Generally, 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. A ton of air-conditioning is the rejection of 12,000 Btu/hour (12,661 kJ/hour). The equivalent ton on the cooling tower side actually rejects about 15,000 Btu/hour (15,826 kJ/hour) due to the heat-equivalent of the energy needed to drive the chiller’s compressor. This equivalent ton is defined as the heat rejection in cooling 3 U.S. gallons/minute (1,500 pound/hour) of water 10°F, which amounts to 15,000 Btu/hour, or a chiller coefficient-of-performance (COP) of 4.0. This COP is equivalent to an energy efficiency ratio (EER) of 13.65.

Industrial
Industrial cooling towers can be used to reject heat from various sources such as machinery or heated process material. The primary use of large, industrial cooling towers is to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, semi-conductor plants, and other industrial facilities. The circulation rate of cooling water in a typical 700 MW coal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons per minute)[1] and the circulating water requires a supply water make-up rate of perhaps 5 percent (i.e., 3,600 cubic metres an hour).

If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres an hour [2] and that amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem. A cooling tower serves to dissipate the heat into the atmosphere instead and wind and air diffusion spreads the heat over a much larger area than hot water can distribute heat in a body of water.

Some coal-fired and nuclear power plants located in coastal areas do make use of once-through ocean water. But even there, the offshore discharge water outlet requires very careful design to avoid environmental problems.

Petroleum refineries also have very large cooling tower systems. A typical large refinery processing 40,000 metric tonnes of crude oil per day (300,000 barrels per day) circulates about 80,000 cubic metres of water per hour through its cooling tower system.

The world’s tallest cooling tower is the 200 metre tall cooling tower of Niederaussem Power Station.

Method Of Heat Transfer:
With respect to the heat transfer mechanism employed, the main types are:

wet cooling towers or simply cooling towers operate on the principle of evaporation.
dry cooling towers operate by heat transmission through a surface that divides the working fluid from ambient air. They thus rely mainly on convection heat transfer to reject heat from the working fluid, rather than evaporation.
hybrids are also available.
In a wet cooling tower, the warm water can be cooled to a temperature lower than the ambient air dry-bulb temperature, if the air is relatively dry. (see: dew point and psychrometrics). As air is drawn past a flow of water, the two flows attempt to equalize. The air, if not saturated, absorbs additional water vapor, leaving less heat in the remaining water flow.

To achieve better performance (more cooling), a media called fill is used to increase the surface area between the air and water flows. Splash fill consists of material placed to interrupt the water flow causing splashing. Film fill is composed of thin sheets of material upon which the water flows. Both methods create increased surface area.

Image-(1)

Image-(2)

Image-(3)

Image-(4)

Air flow generation methods
With respect to drawing air through the tower, there are three types of cooling towers:

Natural draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. Counterintuitively, more moist air is less dense than drier air at the same temperature and pressure (this is counterintuitive because intuition tells us that the O2 and N2 molecules of air should be lighter than the H2O molecule of water vapor). This moist air buoyancy produces a current of air through the tower.
Mechanical draft, which uses power driven fan motors to force or draw air through the tower.
Induced draft: A mechanical draft tower with a fan at the discharge which pulls air through tower. The fan induces hot moist air out the discharge. This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake. This fan/fill arrangement is also known as draw-through. (see Image 2, 3)
Forced draft: A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The forced draft benefit is its ability to work with high static pressure. They can be installed in more confined spaces and even in some indoor situations. This fan/fill geometry is also known as blow-through. (see Image 4)

Image 4: A forced draft cooling towerFan assisted natural draft. A hybrid type that appears like a natural draft though airflow is assisted by a fan.
Hyperboloid (aka hyperbolic) cooling towers (Image 1) have become the design standard for all natural-draft cooling towers because of their structural strength and minimum usage of material. The hyperbolic form is popularly associated with nuclear power plants, due to media coverage at Three Mile Island. However, this association is misleading, as hyperbolic natural-draft cooling towers are often used at large coal-fired power plants as well.

Categorization by air-to-water flow

Crossflow
Crossflow is a design in which the air flow is directed perpendicular to the water flow (see diagram below). Air flow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum area. A distribution or hot water basin consisting of a deep pan with holes or nozzles in the bottom is utilized in a crossflow tower. Gravity distributes the water through the nozzles uniformly across the fill material.

Counterflow
In a counterflow design the air flow is directly opposite of the water flow (see diagram below). Air flow 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 through the fill, opposite to the air flow.

Common to both designs:

The interaction of the air and water flow allow a partial equalization and evaporation of water.
The air, now saturated with water vapor, is discharged from the cooling tower.
A collection or cold water basin is used to contain the water after its interaction with the air flow.
Both crossflow and counterflow designs can be used in natural draft and mechanical draft cooling towers.

Cooling tower as a flue gas stack (Industrial chimney)
At some modern power stations, equipped with flue gas purification like the Power Station Staudinger Grosskrotzenburg and the Power Station Rostock, the cooling tower is also used as a flue gas stack (industrial chimney). At plants without flue gas purification, this causes problems with corrosion.

Wet cooling tower material balance
Main article: Cooling tower system
Quantitatively, the material balance around a wet, evaporative cooling tower system is governed by the operational variables of makeup flow rate, evaporation and windage losses, draw-off rate, and the concentration cycles:

M = Make-up water in m³/hr
C = Circulating water in m³/hr
D = Draw-off water in m³/hr
E = Evaporated water in m³/hr
W = Windage loss of water in m³/hr
X = Concentration in ppmw (of any completely soluble salts … usually chlorides)
XM = Concentration of chlorides in make-up water (M), in ppmw
XC = Concentration of chlorides in circulating water (C), in ppmw
Cycles = Cycles of concentration = XC / XM (dimensionless)
ppmw = parts per million by weight

In the above sketch, water pumped from the tower basin is the cooling water routed through the process coolers and condensers in an industrial facility. The cool water absorbs heat from the hot process streams which need to be cooled or condensed, and the absorbed heat warms the circulating water (C). The warm water returns to the top of the cooling tower and trickles downward over the fill material inside the tower. As it trickles down, it contacts ambient air rising up through the tower either by natural draft or by forced draft using large fans in the tower. That contact causes a small amount of the water to be lost as windage (W) and some of 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 raising the salt concentration in the circulating cooling water. To prevent the salt concentration of the water from becoming too high, a portion of the water is drawn off (D) for disposal. Fresh 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 balance around the entire system is:

M = E + D + W
Since the evaporated water (E) has no salts, a chloride balance around the system is:

M (XM) = D (XC) + W (XC) = XC (D + W)
and, therefore:

XC / XM = Cycles of concentration = M ÷ (D + W) = M ÷ (M – E) = 1 + [E ÷ (D + W)]
From a simplified heat balance around the cooling tower:

E = C · ΔT · cp ÷ HV
where:
HV = latent heat of vaporization of water = ca. 2260 kJ / kg
ΔT = water temperature difference from tower top to tower bottom, in °C
cp = specific heat of water = ca. 4.184 kJ / (kg°C)

Windage (or drift) losses (W) from large-scale industrial cooling towers, in the absence of manufacturer’s data, may be assumed to be:

W = 0.3 to 1.0 percent of C for a natural draft cooling tower without windage drift eliminators
W = 0.1 to 0.3 percent of C for an induced draft cooling tower without windage drift eliminators
W = about 0.005 percent of C (or less) if the cooling tower has windage drift eliminators
Cycles of concentration represents the accumulation of dissolved minerals in the recirculating cooling water. Draw-off (or blowdown) is used principally to control the buildup of these minerals.

The chemistry of the makeup water including the amount of dissolved minerals can vary widely. Makeup waters low in dissolved minerals such as those from surface water supplies (lakes, rivers etc.) tend to be aggressive to metals (corrosive). Makeup waters from ground water supplies (wells) are usually higher in minerals and tend to be scaling (deposit minerals). Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping however excessive levels of minerals can cause scaling problems.

As the cycles of concentration increase the water may not be able to hold the minerals in solution. When the solubility of these minerals have been exceeded they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. The temperatures of the recirculating water, piping and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water. Often a professional water treatment consultant will evaluate the makeup water and the operating conditions of the cooling tower and recommend an appropriate range for the cycles of concentration. The use of water treatment chemicals, pretreatment such as water softening, pH adjustment, and other techniques can affect the acceptable range of cycles of concentration.

Concentration cycles in the majority of cooling towers usually range from 3 to 7. In the United States the majority of water supplies are well waters and have significant levels of dissolved solids. On the other hand one of the largest water supplies, New York City, has a surface supply quite low in minerals and cooling towers in that city are often allowed to concentrate to 7 or more cycles of concentration.

Besides treating the circulating cooling water in large industrial cooling tower systems to minimize scaling and fouling, the water should be filtered and also be dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water. For closed loop evaporative towers, corrosion inhibitors may be used, but caution should be taken to meet local environmental regulations as some inhibitors use chromates.

Ambient conditions dictate the efficiency of any given tower due to the amount of water vapor the air is able to absorb and hold, as can be determined on a psychrometric chart.

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Source:-http://en.wikipedia.org/wiki/Cooling_towers