INTRODUCTION TO COOLING TOWERS

INTRODUCTION  TO COOLING TOWERS

What is a cooling tower? Cooling towers are a special type of heat exchanger that allows water and air to come in contact with each other to lower the temperature of the hot water. During the cooling tower working process, small volumes of water evaporate, lowering the temperature of the water that’s being circulated throughout the cooling tower.

In a short summary, the purpose of a cooling tower is to cool down water that gets heated up by industrial equipment and processes. Water comes in the cooling tower hot (from industrial process) and goes out of the cooling tower cold (back into the industrial process). Here we discover cooling tower functions and inner working of cooling towers for different applications.

cooling tower types :

1- cross flow  type   :    
                          natural draft                                                mechanical draft 

2- counter flow type :       
                                  natural draft                                                mechanical draft  

3- counter flow plume abated  :

cooling tower working principles

Hot water is coming at the inlet of the tower and pumped up to the header. The header contains nozzles and sprinklers which is used to spray water, and it will increase the surface area of water. After that, water comes to PVC filling; it used to reduce the speed of water. At the top the cooling tower, fans are used to lift air from bottom to the top. Because of slow speed and more contact area of water, it makes a good connection between air and hot water. The process will reduce the temperature of water by evaporation process and cooled water is collected at the bottom of the cooling tower, and this cooled water is used again in the boiler.

Cooling Tower Applications

Tradional HVAC heating and cooling systems are used in schools, large office buildings, and

hospital. On the other hand, Cooling towers are much larger than tradional HVAC systems

and are used to remove heat from cooling tower water systems in petroleum refineries,

plants, natural gas processing plants, petrochemical plants, and other industrial processes

and facilites.

What are the different parts of cooling towers ?

 A cooling tower may consist of

2-  a fan to intake the outside air

3-  a medium for heat transfer

4-  a liquid holding system or basin .

5-  a system for distributing the air .

6-  a outer structure or casing .

Types Of Cooling Tower Systems

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Cooling towers are usually designed for specific purposes. Not all cooling towers work for all applications or industrial processes. Here we help you understand the various types of cooling towers, there advantages/disadvantages and determine which cooling tower type is right for your industrial process. Check out the cooling tower list and parts list that provides an overview of cooling tower types to help you figure out which tower is right for your industrial application and what replacement parts you might need.

--Crossflow Cooling Towers

--Counterflow Cooling Towers

--Forced Draft & Induced Draft Cooling Towers Process

--Natural Draft & Fan Assisted Natural Draft Cooling Towers

--Factory Assembled Cooling Towers (FAP) Factory Assembled Product

--Field-Erected-Towers (FEP) Field Erected Product

Crossflow Cooling Towers Flow Diagram

In crossflow cooling tower systems the water vertically flows through the fill media while the

air horizontally flows across the falling water. That's why they call it "crossflow" because the

air and water cross paths or flows. Because of the crossing of flows, the air doesn't need to

pass through the distribution system. This permits the use of hot water flow via gravity and

distribution basins on the top of the tower right above the fill media. The basins are a

standard of crossflow cooling towers and are applied on all units.

Counterflow Cooling Tower Diagram

Difference between crossflow and counterflow cooling towers: In counterflow 

cooling tower system processes, the air vertically flows upwards, counter to the water flow

in the fill media. Due to the air flowing vertically, it's not possible to use the basin's gravityflow

like in crossflow towers. As a substitute, these towers use pressurized spray systems, 

usually pipe-type, to spray the water on top of the fill media. The pipes and cooling tower 

nozzles are usually spread farther apart so they will not restrict any air flow.

Forced Draft & Induced Draft Cooling Towers Process

Cooling tower fans are used on induced draft cooling towers to pull air up through the fill

media. On forced draft cooling towers, the air is pushed/forced by blowers at the bottom of

the air inlet louver.

Natural Draft & Fan Assisted Natural Draft Cooling Towers

Factory Assembled Cooling Towers (FAP) Factory Assembled Product

These factory-assembled cooling tower systems come somewhat disassembled and are 

shipped in a few sections, ready for final assembly or field erection. Although, small factoryassembled 

cooling towers can be shipped intact. FAP cooling towers can be induced draft, 

crossflow, forced draft or counterflow depending on the application its need for. TCIA 

cooling towers are widely used for light industrial applications and HVAC

Field-Erected-Towers (FEP) Field Erected Product

Field-erected cooling towers are usually constructed on the final destination site. The large FEP is usually prefabricated, marked by piece and shipped to the construction site for assembly. The cooling tower manufacturer usually handles all of the cooling tower construction process, final assembly, and labor involved. These type of towers can be counterflow or crossflow depending on the application. For heavy industrial applications or more power needed, field-erected cooling towers can be built to your exact specifications, structure, performance, plume abatement and drift.

cooling tower design considerations

The prevalent type of cooling tower (see Figs. 1 and 3) dissipates heat by the evaporation of some of the water sprayed into the air circulated through the tower. It is used where water is in limited supply,

where temperature pollution of natural water bodies is to be avoided, where water conservation is to be effected, or where otherwise polluted sources must be avoided. Figure 2 illustrates the functional cycle and the significant basic terms

Wet-bulb temperature, for design purposes, should not exceed the maximum expected value more than 5 percent of the time during summer

Approach is the difference in temperature between the cold water leaving the tower and the ambient wet bulb.

Cooling range is the difference in temperature between the hot water entering and the cold water leaving the tower.

Drift is the water lost as mist or droplets entrained by the circulating air and discharged to the atmosphere. It is in addition to the evaporative loss and is minimized by good design.

Makeup is the water required to replace total losses by evaporation, drift, blowdown, and small leaks.

The early, simple atmospheric towers have, because of reliance on natural air circulation, high pumping heads, excessive spray losses, and makeup, been largely superseded by three important types: (1) forceddraft,(2) induced-draft, and (3) hyperbolic (Fig. 3).

The mechanical forced-draft tower (Fig. 3a) gives (1) a controllable air supply from fans conveniently located for inspection and maintenance at ground level, and (2) reduced water- pumping head. Nonuniform distribution of air over the ground area of the tower cell, recirculation of vapor from the tower discharge to the tower inlet, with its deleterious effects and fan icing in cold weather, and limitations on the physical diameter of fans are all problems with forced-draft towers.

The induced-draft tower (Fig. 3b) is prevalent in U.S. practice. The fan is mounted on the top (discharge) of the cell, with consequent improved air distribution within the cell; drift eliminators reduce makeup requirements; spray nozzles, downspouts, splash plates, and splash bars ensure ample evaporative surface for the water, with maximum volumetric heat-transfer rates. In the counterflow design, air is introduced beneath the cell fill, but in the crossflow design (Fig. 3b), air is introduced at the sides of the fill.

The hyperbolic tower (Fig. 3c) utilizes the chimney effect (height, 3006 ft) for natural circulation. It has been favored in large European installations where the prevalent lower dry-bulb ambient

temperature gives a greater difference in density between entrance to and exit from the tower. The wide approach keeps the unit size within practical bounds, and the savings in fan power support the higher investment.

Recent installations in the United States attest to its attractiveness.

Precautions are necessary to avoid freezing troubles in cold weather, fire hazards with intermittent operations, corrosion, scale, and microbiological growth problems. 

Location must avoid recirculation: for tower lengths to 250 ft, the long-axis orientation should be parallel to the prevailing wind; towers longer than 250 ft should be arranged broadside to the prevailing wind. The tower should be isolated as much as possible; adjacent heat sources compel the specification of higher design wet-bulb temperatures

Performance Calculations

SEE FIG 1

Applicable equations are

       (1)

 WwB = WwA - (Wv2 - Wv1) Wa1 - Wa2                                                                    (2)

WwA(hfA - hfB) = Wa2ha2 + Wv2hv2 -(Wa1ha1 + Wv1hv1) - (Wv2 -Wv1)hfB

hfA - hfB = twA - twB             (3)

and

Wa ha + Wv hv      = total heat, from psychrometric chart     (4)

WwA(twA - twB) = total heat at 2 -  total heat at 1 - (Wv2 - Wv1)hfB  (5)

where W= flow, lb/h; a=  air; w = water; v = vapor; h= enthalpy, Btu/ lb; f = fluid; 

t = temperature, °F; 1, 2, A, B 5 locations.

Normally, a cooling tower is purchased for only one guarantee point. It is well, however, to have performance curves (Fig. 4) showing operation for various wet-bulb temperatures and cooling ranges. The investment for a cooling tower is essentially a matter of water flow and is

influenced by approach, range, and wet bulb (Fig. 5). The cost evaluation should include consideration of tower frame and fill, fans, motors, basin and pump pit, pump head, fan horsepower, freight, labor, and erection. In the choice of a tower for a power plant, there should be coordinated study and evaluation of the turbine and condenser for best overall economy.

The height of a field-erected induced-draft tower, from basin curb to fan deck, ranges from 8 to 50 ft; widths vary from 6 to 60 ft; lengths from 8 to 500 ft; fan-stack height between 2 and 15 ft.

Materials used are: frame—redwood (treated or untreated), Douglas fir (treated), steel (galvanized), concrete; fill—red wood, plastics (polyethylene, polypropylene), cement asbestos board (extruded);

casing—cement asbestos board (corrugated, slat), redwood, fiber glass, aluminum, concrete; fan blades—aluminum, glass-reinforced polyester, stainless steel, monel; fan hubs—cast iron, galvanized steel, stainless steel; fan stack—redwood, steel, masonite, drift eliminators— redwood, cement, asbestos board; spray nozzles—ceramic, bakelite; louvers—redwood, cement asbestos board.

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