3.3 Photograph
Nicmar Hyderabad. Page 22 Fig: Natural draft cooling tower
Fig: Forced draft cooling tower
Nicmar Hyderabad. Page 23
Fig: Induced Draft CT(Counter flow) Fig: Induced Draft CT with fill(Counter flow)
Fig: Induced Draft CT (Cross flow)
Fig: Induced Draft CT single-flow (Cross flow)
Nicmar Hyderabad. Page 24
3.4 CATEGORIZATION BY AIR-TO-WATER FLOW
Cross flow: Cross flow is a design in which the air flow is directed perpendicular to the water flow. 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 cross flow tower.
Gravity distributes the water through the nozzles uniformly across the fill material.
Counter flow: In a counter flow design the air flow is directly opposite to the water flow. 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 allows 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 cross flow and counter flow designs can be used in natural draft and mechanical draft cooling towers.
Counter flow & Cross Flow
Nicmar Hyderabad. Page 25
3.5 OPERATION PRINCIPLE OF NDCT
The cooling tower operation is based on evaporative condensation and exchange of sensible heat. Cooling effect is accomplished by transferring apportion of liquid into vapor state there by releasing latent heat of vaporization.
Hot water is pumped to a certain height and then distributed through piping system and discharged through nozzles to splash over a system called filling (or) stacking. The water splashes, drips and flows downwards through the filling.
The ascending air which enters to tower through the open space between the supporting columns rises through the open space between the supporting columns rises through the droplets of hot water through evaporation and consequently cools the water through evaporation and convection. The cooled water collected in a pond and re circulated by adding loss of water.
3.5.1 COMMONLY USED TERMS
Drift - Water droplets that are carried out of the cooling tower with the exhaust air. Drift droplets have the same concentration of impurities as the water entering the tower.
Blow-out - Water droplets blown out of the cooling tower by wind, generally at the air inlet openings. Water may also be lost, in the absence of wind, through splashing or misting
Plume - The stream of saturated exhaust air leaving the cooling tower. The plume is visible when water vapor it contains condenses in contact with cooler ambient air.
Blow-down - The portion of the circulating water flow that is removed in order to maintain the amount of dissolved solids and other impurities at an acceptable level.
Nicmar Hyderabad. Page 26
Leaching - The loss of wood preservative chemicals by the washing action of the water flowing through a wood structure cooling tower.
Noise - Sound energy emitted by a cooling tower and heard at a given distance and direction. The sound is generated by the impact of falling water, by the movement of air by fans, the fan blades moving in the structure, and the motors, gearboxes or drive belts.
Approach - The approach is the difference in temperature between the cooled-water temperature and the entering-air wet bulb temperature.
Range - The range is the temperature difference between the water inlet and water exit.
Fill - Inside the tower, fills are added to increase contact surface as well as contact time between air and water. Thus they provide better heat transfer. The efficiency of the tower also depends on them. There are two types of fills that may be used:
Film type fill (causes water to spread into a thin film)
Splash type fill (breaks up water and interrupts its vertical progress)
Nicmar Hyderabad. Page 27
3.6 TECHNICAL SPECIFICATIONS AND GENERAL PARAMETERS
General Parameters of NDCT
S.NO Description
1 Quantity 2 No‟s
2 Height 170m
3 Type of Tower Counter flow with P.V.C film fill
4 Water flow rate 75,000 Cum/hr
5 Cooling Range 10 Degree Celsius
6 Hot Water Temperature 43 Degree Celsius 7 Cooled Water temperature 33 Degree Celsius 8 Design Dry bulb temperature 39.78 Degree Celsius
9 Relative Humidity 40 ˚c
10 Design Capacity 18,750 cum/hr (flow through strainers)
11 Design Pressure 1 kg/cm2
12 Ratio of water to air weight 28 Degree Celsius
13 Location N 22˚ 06‟ 08” & E 76˚ 32‟ 45”
14 Coal Required 8.18 Million tons Annually
15 Heavy fuel & HDS Required 15,000 kilolitres 16 Proposed Plant zone Seismic Zone-III
17 Water Resource Indra Sagar Reservoir construction on Narmada river ( 6km away from site)
18 Construction method Jump Form method
19 Cooling Tower Shell Hyperbolic Paraboloid shape 20 Cooling Tower Manufacture M/S Gammon India Ltd
Nicmar Hyderabad. Page 28 TECHNICAL SPEICIFICATION OF NDCT
S.NO Description
1 Tower Capacity 75000 Cum/Hr 2 Cooling Range 10°C
3 Diameter at sill Level 132M
4
Design atmospheric Wet Bulb
Temperature 28°C
5 Relative Humidity 60%
6 Cooled water temperature 33oC 7 Hot water temperature 43oC
8 Drift Loss 0.005% (Maximum) 9 Excavation 49390X2 cum (Approx.) 10 PCC 3028X2 cum (Approx.) 11 RCC 28500X2cum (Approx.) 12 Reinforced Steel 3896x2 MT (Approx.) 13 Nozzles 11700x2 Nos (Approx.) 14 Shuttering Quantity 136605x2 Sqm (Approx.) 15 Bituminous painting 140510x2 Sqm (Approx.)
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3.7 CODES AND STANDARDS
IS:11504
(Re-Affirmed 1992)
Criteria For Structural Design of Reinforced Concrete Natural Draft Cooling Towers.
IS:1786
(Re-Affirmed 1990)
Specification For High Strength Deformed Steel Bars And Wires For Concrete Reinforcement.
IS:456-2000 Plain And Reinforced Concrete –Code of Practice.
IS:10262
(Re-Affirmed 1990) Recommended Guidelines For Concrete Mix Designs.
IS:383-1997
Specifications For Coarse And Fine Aggregates From Natural Sources For Concrete.
IS:2386
(Re-Affirmed 1997) Methods of Test For Aggregates For Concrete.
IS:1199
(Re-Affirmed 1999) Methods of Sampling And Analysis of Concrete
IS:12269 Specification For 53 Grade Ordinary Portland Cement.
IS:269 Specification for 33 grade ordinary Portland cement IS:8041 Specification for 43 grade ordinary Portland cement
IS:432
Specification for mild steel and medium tensile bars and hard drawn steel wire for concrete reinforcement.
IS:516 Methods of tests for strength of concrete.
IS:800 Code of practice for General Construction in steel.
IS:875 Code of practice for Design Loads for Buildings and Structures.
IS:650 Specification for standard sand for testing of cement.
IS:2210 Criteria for the design of RC shell structures and folded plates.
BS:4485 Specification for Water Cooling Towers (Part 1 to 4).
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3.8 DESIGN CRITERIA OF COOLING TOWER
The following loads shall be considered for the design of cooling tower which are as follows: - 1) Dead Loads: - For assessing the self weight of the structure, the specific weight of the
concrete shall be taken as 2500 Kg/sqm. All other loads shall be assessed in accordance with relevant codal provision.
2) Seismic/Earthquake load: - According to IS: 1893(part1&4) using the spectrum method and 5% damping shall be considered for analysis.
3) Thermal effect: - Due to this effect stresses are developed. It shall not exceed value given in IS: 4998(part1)-1975.