Cooling Tower Fundamentals.ppt 5t512p

Cooling Tower Fundamentals • •

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The basic concept behind cooling tower is “evaporation”. Typically in a cooling tower 70 to 90% of cooling occurs due to evaporation and the balance through convective cooling. Evaporation is optimized by atomizing the water stream in to tiny droplets which exposes maximum surface area.To further facilitate the air water different types of fills are used. Dry air will saturate to equilibrium (wet bulb temperature) while the water is cooled below the ambient dry bulb temperature via the removal of evaporation heat. 1

Types of Cooling Towers • Cooling Towers are classified by their type of draft. – –

Mechanical (Forced / Induced draft) Unassisted (Natural draft)

• Fans are used in mechanical draft design to move air through the structure. • Mechanical draft tower is further categorized: – –

Forced draft – the fan located in the air stream entering the tower. Induced draft – the fan is located in the air stream leaving the tower.

• Induced draft is further categorized: – –

Cross flow – air enters tower from side and flows horizontally across the falling water stream. Counter flow - air enters tower from bottom and flows counter current to the falling water stream.

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Cooling Tower Design Criterion • While deg a seawater cooling tower the following design parameters are considered: – – – – – –

Makeup water availability and composition (cycles of concentration) Circulation rate Duty Design wet bulb temperature Approach temperature Drift requirements

• Selection of design wet bulb temperature is a critical design activity. Industry practice is to use a number that will not exceed more than 3 to 7% of the year. This infers that there will be certain peak wet bulb temperatures at which cooling tower will not be able to maintain design outlet temperatures. 3

United Seawater Cooling Tower • Jubail Industrial City seawater cooling system was designed as a once through supply and return canal network coolant system, which is part of the local infrastructure. • In case of United Petrochemical complex the allocation request far exceeded the availability of the seawater in the canal network. Thus there was an impetus to move to a seawater cooling tower. • The following are the three primary categories of cooling tower design activities: – Hydraulics – Thermodynamics – Mechanical design

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Hydraulics •

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Since seawater is supplied to United through a canal system, elevations were critical in the design to avoid unnecessary pumping requirements and to improve the reliability of the system. Since this tower was the first of its kind in Jubail area, it was decided to have a partial backup once through capability. The seawater intake basin level is controlled by seawater supply canal. Tower basin is elevated by 4 meters so that cooled seawater is returned to seawater intake basin by gravity, avoiding expensive booster pumps. 5

Thermodynamics •

Based on 5% not-to-exceed philosophy, the design wet bulb temperature was selected as 30°C.

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Due to the requirement of one of the process units – the maximum cooling water supply temperature was fixed at 38°C.

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As United cooling water system is a closed freshwater circulation system, it was necessary to select an economic plate and frame exchanger approach of 3°C, which has resulted in cold sea water temperature requirement of 35°C.

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Thermodynamics •

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The following slide # 9 showing meteorological data indicates that highest recorded wet bulb temperature is 35°C and occurring for few hours on some days of peak summer. United studied the possibility of deg the cooling tower for an approach of 3°C. However it was found that there would be significant capital cost with no appreciable facility performance improvement. The seawater exit temperature does not raise linearly with wet bulb temperature as tower efficiency increases at higher wet bulb temperatures. Additionally if the flow rate is reduced then the exit temperature will be reduced further as efficiency of the cooling tower will increase. Please see the following slide # 10. 7

Wet-Bulb Temperature Data Temp. Range

Year 1999

Year 2000

Year 2001

>30oC and <=31oC

120 Hrs 134 Hrs

119 Hrs

>31oC and <=32oC

115 Hrs 121 Hrs

46 Hrs

>32oC and <=33oC

70 Hrs

94 Hrs

8 Hrs

>33oC and <=34oC

43 Hrs

50 Hrs

0 Hrs

>34oC and <=35oC

3 Hrs

10 Hrs

0 Hrs

Note: One year is 8,760 Hours

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Performance Curves COLD WATER TEMPERATURE AS A FUNCTION OF WET BULB TEMPERATURE, AND WATERFLOW (% Design)

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COLD WATER TEMPERATURE

40 38 36 34 32 30 28 26 24 15

20

25

30

35

40

WETBULB TEMPERATURE DEGC

110.00%

100.00%

90.00%

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Design Conditions • •

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12°C differential across the tower (at a design flow rate of 70,000 M3/Hr). Design ambient wet bulb temperature is 30°C. – Corresponding to 96 – 97% not - to - exceed wet bulb temperature. The design wet bulb temperature considered as inlet to the tower is 31.1°C. The United cooling tower is designed for an approach of 5°C, with reference to ambient temperature without recirculation. Selection of design wet bulb temperature was considered by the design team as a critical activity. 10

United Cooling Tower Design Conditions Seawater 47 Degc

Cooling Water COOLING TOWER

12 0C Cooling Range

PLATE COOLERS

51 Degc

PLANT

13 0C Differential

35 Degc 38 Degc Tower Inlet Wet bulb temperature:

31 Degc

Ambient Wet bulb temperature:

30 Degc

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Mechanical Design – Various material selection available for seawater cooling tower: • • •

Fiberglass Wood Concrete

– Factors for selection of material: • • • • •

Cost Schedule Structural integrity Maintenance Material availability

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Mechanical Design – Due to high salt Gulf environment – chloride penetration leads to corrosion of reinforcing steel and concrete spalling. – Various preventive measures considered: • • • • •

Cathodic protection Epoxy coating Use of high density concrete Deep embedment of rebar in concrete External coating of concrete surfaces

– United seawater cooling tower utilizes a combination of high density concrete, deep embedment of rebar in the concrete and an external epoxy coating on concrete . 13

Mechanical Design – United seawater cooling tower has: • 20 cells (One spare cell for maintenance) • Cell dimensions – 16.6M X 15.9M X 18.2M (high)

– United seawater cooling tower structure consists of: • 1800 precast concrete beams • 170 precast concrete columns

– United seawater construction: • • • •

cooling

tower

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material

of

Tower – Concrete Siding / decking / fan stacks/ fan blades – Fiberglass Drift eliminators / internal distribution piping – PVC Splash fill – Polypropylene

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Chemical Treatment • Objectives of chemical treatment: – Bio Control: Control of Biological growth. Bio control is achieved by dosing halogens, typically Chlorine. Chlorine is stripped off in the tower, requiring continuous dosing. – Scale control: Higher operating cycles and high skin temperatures in downstream exchangers can lead to scaling in the exchangers. Scale control chemicals prevent scaling for a given operating cycle and skin temperature. 15

Conclusion • Cooling tower technology can be adapted to seawater cooling service as long as consideration is given to critical design parameters such as: – – – – –

Cycles of Concentration Design wet-bulb temperature Approach Materials of construction Chemical treatment 16

Conclusion • For plant expansions or even new projects, if cooling duty available from seawater is not sufficient to meet the plant cooling requirement – sea water cooling towers can be used to increase the apparent cooling duty available. • Cooling tower capital and operating costs can be much lower than the other alternatives. 17