More competition in the form of solar and wind energy has put more emphasis on lower cost of production. Further, higher plant efficiencies mean lower carbon footprint. Plant efficiencies are directly affected by cooling system efficiencies. A two-degree higher cold water can have an effect of Rs. 5 crores or more on a 500 MW plant. Higher efficient cooling system also improves its availability by lowering the downtime, maintenance and operation cost.
There are quite a few developments that needs to be implemented for promoting water management at thermal plants. They are:
• More use of sea water as the cooling media
• Higher operating COC
• Reduce drift by using upgraded technology
• Use a combination of dry and wet cooling
• Use of hybrid cooling technologies
Cooling tower has three major zones of mass transfer: the nozzle, fill and the rain zone. The fill zone usually contributes about 75 per cent cooling in the tower.
Thermal performance upgrade (what to achieve)
Thermal performance upgrade has become one of the important aspects in overall plant efficiency. An important factor is to provide improved cold water outlet in the cooling tower. A thermal performance upgrade is necessary to improve cold water (provide guaranteed improvement in cold water at tower outlet), reduce drift loss, improve fill and condition of other deteriorated components, possible reductions in power consumption, and reduce maintenance cost and equipment down time.
Major factors affecting cooling tower performance
• Fill performance may be lesser than anticipated
• Fan power provided may be lesser than required
• Distribution system may not be optimum
• Fills may be clogged, damaged or missing
• Fan may be working at lower efficiencies or at higher pressure and hence lower air flows
• Obstruction at tower inlet
• Low air inlet heights causing high inlet velocities thereby causing areas of poor air flow
Cooling tower performance audit and analysis
By identifying the extent of performance being lost due to individual components and other factors, we can decide on the most economical way of optimum upgrade. Because cooling tower efficiency is a combined efficiency of all components, such a method would involve assessment of major components of the tower.
A complete analysis of the cooling tower includes simultaneous collection of cooling tower data with automated data logging equipment such as wet bulb, hot water, cold water, wind velocity, water flow etc. Additionally, fan pressure measurement would provide data to ascertain the static and total pressure along with the actual fan air flow. Simultaneously, exit air measurement would help understand the mass transfer coefficients, which would define the actual efficiency of the operating fill. Velocity measurement of air at inlet and exit gives an indication of maldistribution of air and pressure losses. Further, a complete study of the actual water quality in operation is made so that a decision on upgrading to advanced nozzles and fills can be taken.Once the analysis of the tower is completed then some of the solutions listed below may have to be undertaken.
Upgrade of fill media
Three types of fills exist in cooling towers: splash fills (bars/slats), film pack, and trickle packs.
Upgrade for existing splash type fills
• Upgrades from splash to higher efficiency splash fills or denser splash fill (percentage performance improvement is low to medium)
• Upgrades from splash to trickle (subject to water quality) – medium to high
• Upgrades from splash to film pack (subject to water quality) – high
Upgrade of towers for existing film type fills
• Upgrade from film to high efficiency film
• Upgrade from film to log clog film
• Upgrade from film to combination of straight and cross fluted fills
• Upgrade from film to high efficiency trickle packs
Upgrade of other components and factors
Distribution system: It consists of ducts, pipes and nozzles. Improper distribution system causes loss of efficiency in the fill. The distribution system itself could contribute 5-20 per cent of the total cooling and depends upon the uniformity of flow provided by distribution system.
Pipes: They may need to be changed in case of improper size or clogged pipes. Changes in ducts are envisaged if the sizing is not correct.
Nozzle: Nozzle type and selection is critical to cooling tower performance. Providing nozzles suitable to the water quality and existing grid of pipes and nozzles take care of the expected coverage. Well designed nozzles can utilise the capability of the fill with fewer dry areas. The nozzle operating point is the key. Laboratory tested nozzles with selection curves provide important data for selecting right nozzle for the operating point.
Cooling tower fan: Low fan efficiencies have been observed to be a frequent and major cause of non-performance in cooling towers. Fan operating duty points may vary the actual efficiency of the fan. Higher pressures move the operating point towards the stalling zone. Designed point of fan operation may not always be correct. Operating static and total pressures along with fan air flow, fan efficiency can be found using the pitot tube and manometer.
Drift eliminators: Drift eliminator losses are calculated based on manufacturers curves. Slight variation in profile can cause higher pressure drops.Lab tested drift eliminators could provide more stable and reliable data.
High tower air inlet velocities: Cooling tower performance is best when the uniformity of air flow through the fills are the highest. However, this also depends strongly on the air velocity through the inlet and the ratio of inlet to fill velocities. High inlet velocities tend to create unequal air loadings at the fill apart from higher bend pressure drops. In case the air inlets cannot be restored to higher levels, it would be possible to provide some guide vanes to improve the performance of the tower.
Low plenums and fan coverage ratios: It affects the cooling tower performance and need to be addressed if existing.
Very high fan inlet velocities: They create huge pressure drop in the system and need to be identified
External obstruction at tower air inlet: It can cause distortion and lowering of air flow due to higher pressure drops. These need to be identified and sorted out if possible.
• Testing is the heart of assessing the tower performance. It is generally carried out as per CTI CODE ATC 105, performance curve method, or demand curve method.
• Since a 1-degree difference in cold water amounts to about 10 per cent capability in performance, sensitivity of testing and accuracy of testing methods and instruments becomes extremely critical. Changes in environment conditions with respect to design and daily operation also demands that a proper performance curve be available so that parameters of performance may be marked at any given point during the year.
• Since the atmospheric conditions may vary and recirculation of exit air varies with wind speed and direction, it is recommended to carry out testing with automatic data logging equipment so that sensitivity of readings are maintained as the frequency of readings can be taken every minute to collect the data.
• Water flow can be ascertained using pitot tube or using UFM. However, upstream and downstream straight lengths are to be taken care for better accuracies in results.
• Evaluation of the collected data using the analysis method as mentioned in the code would provide reliable data for taking corrective measures
• By comprehensive replacement of all major components, the cooling tower can be upgraded thus, optimising the plant efficiency
• Reasonable performance improvement can be achieved be analysing the various components of the tower, and replacement of non performing components.
• How and how much to upgrade depends on system requirement, economics and present performance
• A good analysis may save the time and money by providing the feasibility of upgradation or need to add cells
• Reliability of the upgrade can be increased using lab tested components
• Referential assessment of improvement by changing components is possible
Authored article by:
Suresh Sarma, Proprietor
SS Cooling Tower Consultants