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Cooling Tower principles :-

 

Hot  water is distributed over the filler through distribution systems by gravity. Atmospheric air is simultaneously blown across the filler causing a small portion of the water to evaporate. The heat is removed from the circulating water and transferred to the air stream  to be exhausted through the  stack. The cooled water falls into the tower sump where it is collected and returned to the heat source.

 
Introduction
A Cooling Tower is a direct contact heat exchanger, generally used to dissipate

heat in circulating water. The Heat is dissipated to the ambient air via. A process of heat and mass transfer from the circulating water. The water thus cooled is re-circulated through the process, heat exchanger or condenser.

The heat is transferred to the water either in a condenser like refrigeration or air-conditioning systems or chemical processes or heat exchangers in cooling process generating sets, engines, furnaces etc.

The cooling tower design has changed over the years to incorporate new materials, as and when available. The purpose being to make the Tower :-

1. Perform to required specification.
2. Be compact
3. Weight less
4. Consume least power
5. Be long lasting and withstand corrosion and have good finish/ looks.

With the introduction of fiberglass Reinforced plastics the above benefits are achieved in the counter flow, FRP Induced Draft Cooling Towers.

 
COMPONENTS
The various components of FRP Induced Draft Cooling Tower are as under:

Casing

The casing enclosing the PVC fills serves to isolate air stream, which passes over the fills. The casing is bottle shaped to reduce frictional resistance of air and aid airflow pattern. It is designed to withstand wind loads upto 75km/hr, and vibrations emanating from the motor and other equipment. FRP casing has a high impact resistance when laminated with Isopthalic Resin and even if damaged is easily repaired at site.

With proper surface treatment using neopenta glycol and imported wax release agents, it retains colours for long periods even when exposed to direct sunlight. The casing is in sections of easy to handle sizes and is assembled at site using bolting joints.  The bottle shape of casing is ideal with regard to cooling efficiency and space economy.

 
Sump Top

The basin serves the purposes of collecting the water descending from the fills and channeling it to the suction point.  Further the basin also acts as a reservoir of water.  The basin is also made of FRP and has similar characteristics as the casing.

 
Suction Tank/Auxiliary Tank

The suction tank is located below and in the Centre of the sump and has all connections for inlet/outlet, drain, filling and overflow.  The unit is at the lowest point and is always flooded and thus ensuring no cavitation on the pump suction.  The suction tank is fully molded in FRP to prevent corrosion and subsequent leakages.

 
Tower Structural

The structures of the tower support the casing, basin and motor mounting frame for sustaining and transmitting the loads to the foundations.  This are of MS and are hot dipped galvanized so as to resist corrosion.

 
Fills

The fills Section is designed to bring intimate contact of water and air so as to facilitate heat and mass transfer at the same time aiding in proper and even distribution of air and water over the cross sections, while maintaining minimum pressure drop.  The fills are of honeycomb section and are vacuum formed from Virgin PVC for excellent resistance to corrosion and give maximum area for wattage.

 
Sprinkler

The gravity die cast aluminium alloy sprinkler is used to distribute the water evenly over the cross section of tower.  An aluminium alloy rotary head with radial PVC  pipes having drilled holes serves this purpose. The rotation of the assembly is accomplished due to reaction of water jet being sprayed from the PVC Pipes. The sprinkles head is mounted on top of the central water supply pipe. To reduce frictional resistance and to ensure free rotation even at low flows the sprinkler has 2 sealed prelubricated ball bearings mounted on the central shaft. This system is followed up to 500 TR only, for the larger Tower the sprinkler becomes unwieldy and instead a stationary low pressure water distributor with nozzles located on an FRP grid pipe is used for equal distribution of water.

 
Fan

The Aluminium alloy fan is gravity die cast. It is an axial flow. Multiblade, version with aerofoil blade section and adjustable blade pitch. The fan is designed to deliver large volumes of air at low power consumption and low noise generation. The fans are dynamically balanced for smooth operation, longer bearing and more life including that of the supporting structure.

 
Fan Drive Motor
The fan drive motor is in IP55 weather proof design. The fan is directly driven by the special extended shaft motor made from EN8 steel with ‘F’Class Insulation  mounted facing downward on a mounting frame on the top of the Cooling Tower.
 
Grills

 Inlet grills and outlet grills in MS are provided to prevent contaminants  like leaves, birds etc. entering the cooling tower.

 
Drift Eliminators Top

Units with rotary sprinklers are designed to minimize carryover by judicious choice of air flow velocities. The air distribution is aided by using centrally located rotating eliminator section to avoid high velocity areas. Further the pipes are covered by aerofoil section eliminators which helps arrest small droplets from escaping and for evenly distributing the sprinkled water. For the towers using stationary sprinkler Nozzles a full width eliminator covering the full cross section is used. The shape of the eliminator is chosen to minimize pressure loss of air and to strip the maximum amount of entrained droplets.

Circulating Water Make-up

Loss of water from the cooling tower consists of evaporation of circulating water which removed the heat and carry-over, a minute water droplets scattered about as drift by the fan.

Continued evaporation of the circulating water results in condensation of water which may invite corrosion of the circulating system or may even cause the formation of algae or scale. For this reason a so-called blow-down becomes necessary which bleeds off a part of the circulating water.

1. Evaporation Loss (E)
 

Evaporation Loss (E) can be calculated with the following equation.

Ekg/h    =     G x (X2 – X1 )  =  Q     =   /\tXL
                                                   600       600
  .
.  . E (%)   =    /\t x   100                   600
Where
G:   Air flow in kg/h
Q:   Calories cooled kcal/h
T:    Temp. difference of water inlet and outlet C
L:   Circulating water flow kg/h
600:   Latent heat of evaporation in kcal/kg
X1, X2:  absolute humidity of air at inlet and outlet in kg/kg.

   
2. Carry over volume (C)
 

It is very small in volume, normally below 0.2% of the volume of the circulating water flow, though it slightly varies with tower structure.

   
3. Blow down volume (B)
 

In order to replace part of the circulating water periodically or continuously, if is effective to leave then drain valve slightly open during operation, or to let the water overflow constantly by raising the operating water level or to change the water periodically when the water basin is cleaned.

The blow down volume differs depending on the quality of the water or the degree of hardness. In case of air conditioning about 0.3% blow down is usually necessary in order to make the blow down more effective , it is best to use tap water instead of underground or river water.

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4. Make up water Volume (M)
  Makeup water volume (M) is obtained from the equation M=E+C+B

If the evaporation loss is (E)=0.83% carry over loss(C)=0.20% and blow down volume (B)=0.30% the make up water volume will be (M)=0.83+0.20+0.30=1.33% Hence the makeup water volume is estimated to be at about 1.5% of the circulating water flow.

   
5. Relationship between the Condensation Multiple(N) and water  Supply volume (M)
 

Blow down volume and Makeup Water Volume (to maintain the circulating water at specified Condensation Multiple during the operation) are obtained by the following equation.

 
1.

Condensation Multiple (N)

                                  N  =  E  +  C  +  B      =     M                   (1)    (M  =  E + C + B)
                                              C  +   B                C + B


2. Blow down volume (B)



                                  B   =    E       -  C                     (2)
                                           N  1
3. Make up Water volume (M)


                                  M  =     N    .  E
                                                N – 1
 

Where
       N:   Condensation Multiple                 
       E:    Evaporation Volume                      (% 1/min m3 /h)
       C:    Carry over Volume                        (% 1/min m3 /h)
       B:    Blow down Volume                       (% 1/min m3 /h)
       M:   Makeup Water Volume                  (% 1/min m3 /h)


(Calculation Example)

When given Conditions are Cooling Tower Model ACT-0720-80TR 
Inlet Water Temperature Tw1 = 36 C 
Outlet water temperature tw2 = 31 C
Ambient wet bulb temperature W.B.= 27 C
Circulating Water Volume L = 1060 l/min from items 1 & 2 given above
Evaporation Loss E = (36 31) x 1060 = 8.8(1/min) 600
Carry over volume C = 1060 x 0.001 = 1.1 (1/min) where condensation Multiple N = 3 from equation 2
Blow down volume B= _8.8_ - 1.1 = 3.3 (1/min) 3-1 From equation 3
Makeup water volume M = 3 x 8.8 = 13.2 (1/min) 3-1
   
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"Normal" Conditions of cooling tower water  
Water quality and environmental conditions on the vast majority of HVAC Cooling Tower applications permit acceptable service life from standard Cooling Tower construction using the materials previously described. Significant deviation from these “normal” conditions often demands alternate materials choices.

For most purposes, the following criteria define “normal” conditions:

Standard Tower design assumes a maximum of 120 F hot water to the tower, including system upset conditions. Temperatures over 120F ,even for short duration’s, may impose damaging effects on PVC fill, many thermoplastic components, galvanizing and plywood. Those rare applications demanding hot water in excess of 120F usually benefit from careful review with the tower manufacturer to assure that appropriate materials

Changes from the standard configuration are included in the initial purchase specification.

“Normal” circulating water chemistry falls within the following limits
(note the   distinction between circulating water and make-up water):

PH between 6.6 and 8.0 although PH down to 5.0 is acceptable if  no galvanized steel concrete and cement products, fiberglass and aluminum. High PH attacks wood, fiberglass and aluminum.

Chlorides (expressed as NaCl) below 750ppm.

Calcium (asCaCO3)below 1,200 PPM –except in arid climates  where the critical level for scale formation may be much lower

Sulfates below 5,000 PPM - if  calcium exceeds 1, 200 PPM,  sulfates  should  be limited to 800 PP(less in arid climates) to prevent    scale formation.

Sulfides below 1ppm.
Silica (as SiO2) below 150 PPM.
Iron below 3ppm
Manganese blow 0.1 PPM

Langelier saturation index between –0.5 and +0.5 –negative LSI indicates corrosion likely; positive indicates CaCO3 scaling likely.

Suspended solids below 150 PPM if slides are abrasive-avoid film – type fills, if solids are fibrous, greasy, fatty or tarry-wood, PVC, Polypropylene or ABS fills  can be used, but PVC usually is the material of choice.

Oil and grease below 10 ppm or loss of thermal performance will occur.
No organic solvents.
No organic nutrients, which could promote growth of algae or slime,

Chlorine (from water treatment)  below 1ppm free residual for intermittent treatment; below 0.4 ppm free residual for continuous Chlorination. These conditions define normal circulating water. Including the Chemical concentrating effects caused by re-circulating the water to some pre-determined number of concentration.

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