Energy Saving of Ling Tower Using Propylene Filling Material - Report Example

ENERGY SAVING OF СООLING TOWER USING PROPYLENE FILLING MATERIAL Cooling tower is basically specialized heat exchange equipment used for dissipating low unusable process heat to atmosphere. Cooling tower is considered to be a secondary device in a process plant and thus less attention is paid on its performance and optimization. However with deeper insight research, optimization of a cooling tower performance can go a long way in increasing the efficiency of the power or plant as well as saving energy (Goshayshi , et al., 1998). A cooling water, therefore basically adopt the use of evaporation through which water carrying heat energy is evaporated into the moving stream of air and thereby dissipating the heat into the atmosphere. As a result the remaining water is consequently cooled down significantly. Cooling towers are therefore able to lower the temperature of process water more that devices which makes use of air only to dissipate heat, such as a radiator in a vehicle and there more energy efficient and cost- effective. Figure 1: Schematic Diagram of a Cooling Water system Energy in a typical cooling tower is mostly consumed in rotating the fan, a device crucial in the attainment of the proper air movement within the cooling tower. Another, equipment that consumes energy is the pump head of the particular cooling tower used in the operation of the condenser water pump. In saving energy, manipulation of either or all of the above energy consuming aspects either by adjusting the energy consuming aspects or altering the loads can be ultimately beneficial in saving the energy requirement of the cooling towers. Components of a Typical Cooling Tower A typical cooling tower includes the following main basic components Frame and casing structure, Coldwater basin, fill drift eliminators, nozzles, fans, air inlet and louvers. Ideally, they are designed to optimize water and air contact thereby offering as much evaporation as possible as a result of maximize surface are of water as it flows through the cooling tower structure. Figure 2: Draft Operation Diagram of an Induced Draft Cross Flow Cooling Tower From the figure above, it is evident that the hot water is distributed on the top of the cooling tower. This is basically a sprinkler kind of distribution decks. The distribution deck also apportions the falling hot water uniformly around the cooling tower circumference. As the water descends from the distribution deck, it the cooling tower surface area is increased by the fill section on the side walls. In the past, splash bars made of fibreglass, wood, and plastic that serve the purpose of breaking the falling water into small droplets. Recent cooling tower designs have been incorporated with ingenious labyrinth-like film or packing. This provides a closely packed film fill through which water flows through in thin films, thus boosting evaporation and thus thermal efficiency, hence boosting heat rejection. According to (Goshayshi , et al., 1998), a lot of attention is being paid on the designs of cooling tower as it is crucial in optimising energy conservation. (Goshayshi , et al., 1998), asserts that majority of the theoretical and experimental studies have dealt with the mass and heat transfer across smooth surfaces. However, majority of the cooling towers consists of rougher surfaces, created by turbulence promoters positioned to provide improve mass and heat transfer. The core of the cooling plant is the fill of packing, therefore is the cooling plant efficiency is to be increased, this part of the plant therefore receives more attention. This paper looks at the improve energy saving within the PVC filling. (Tezukas & Fusita, 1980) conducted an experiment to determine the factor of the performance of PVC fill packing. In the experiment they provided a number of fill arrangements in a cell and eventually tested them over a range of water and air rates, entering air conditions and heat loads. Figure 3: Typical Configuration of a Corrugated PVC Filling used in the Experimental Setup From their conclusion, (Tezukas & Fusita, 1980), suggested that the required fill depth is a function of temperature of the exiting water, the overall pressure drops and mass transfer coefficients of ribbed corrugated PVC pickings considerably increase as compared to the smooth packing. Also, from their configuration and shape, it was evident that the shape and configuration of the PVC panels and thus the height roughness projection is vital in determining the impact of the mass transfer effect and the Fanning friction factor. From their experiments, (Goshayshi , et al., 1998), determined that the surface area condition of the fillings, and the distribution of water on the PVC fillings for instance the water flow and air flow angle and the distance between the repeated ribs plays a crucial role in the efficiency of the each PVC filling pack, as a result of the impact of the different water distributions on both sides of the PVC filling sheets. Ideally, (Tezukas & Fusita, 1980), concluded that high increased air turbulence coupled with relatively low fluid velocity is more economical as compared to a highly straight and smooth combined with a high water velocity. Design of an Energy Saving PVC Filling of the Cooling Tower With respect to the filling section of the cooling tower, the most effective technique of overall plant energy efficiency is to increase the surface area of the fill area, leading to a reduction in the needed airflow and thus the related air pressure drop within the tower. This consequently decreases the fan motor size required to fan the same thermal task. According to (Morrison, 2014), the reductions of fan motor sizes of between 25% and 50% are practicable on majority projects. The additional initial costs on the tower as a result of the more PVC fill material and the associated grillage, is compensated partly by the lower spec. fan used. Depending on the particular model adopted, the tower height may equally increase leading to higher pumping costs. However, this is usually a small factor as compared to the cost saving associated with the low running costs of the cooling fan. The proposed cooling tower will make use Polypropylene foils rather than the ordinary PVC material. The main advantage of polypropylene (PP material over the PVC material is the higher thermal coefficient thereby making it able to draw more heat energy from the falling hot water In the existing towers, most of the blockage within the fill material is as a result of weak foil thickness leading to pitting as a result of erosion realised due to constant admission of hot water. The proposed PP fill material will have extruded thicker sections than in other places as shown in figure 2 below. Figure 4: Optimized Polypropylene Fill foil thickness Distribution Since PVC fills foils being welded together, more resistance to the stresses induced by the hot water making them suffer severe deformation leading to clogging and chances of residual clogging. This consequently affects the general performance of the cooling tower. In order to achieve similar performance, a higher fan is required thereby increasing net energy consumption. The proposed PP fill foils are extruded directly from a molten mass so that they are free of any undue stresses. Therefore this kind of panels will basically retain their structure shaper and configuration, and thus performance during both the installation phase and operational life of the cooling tower. Most of the ordinary PVC fill material can barely withstand 600C, arising from a normal operating conditions, In some cases, malfunctions within the cooling circuitry leading to the release of hot water beyond the maximum temperature approaching in contact with the PVC fill packing . PP compound is can perform under extreme conditions of up-to 1200C as per the PP catalogue (2H Plastics (Australia). Among the sources of inefficiencies within cooling tower material is the build-up of scales on the surface of the PVC fill films. The scale deposits arise from minerals deposits such as magnesium, iron and silica within the hot water. When these minerals exceed the solubility point, they begin to precipitate out of water solution thereby clogging the PVC fill films. This consequently, impeding heat transfer from the hot water to the ascending air. Polypropylene material has a non-stick surface. This makes it impossible for calcium compounds from sticking and thus clogging the air pathways. This self- extinguishing cellular film is not only less fouling but easy to clean. Energy Saving by Optimized Drift Eliminators and Sprinklers As already noted in the previous sections, understanding how drift eliminators and sprinklers save energy requires a revisit to the cooling towers and the general composition. To begin with, as Taylor (2012) notes, cooling towers where there are drift eliminators and sprinklers consist of a vertical shell that are made of either plastic or metals. It manages to pass through the air that is made to flow from the bottom to the top using induced or forced draft fans. To this regard, principles of operation for analysis for drift eliminator and sprinkler should be incorporated. Principle of operation is the design on how they are going to save energy. As Copeland (2012) argues, the amount of energy that can be saved is often expressed in terms of approach and range with some models of sprinklers and drift eliminators. Beginning with cooling range, it is the temperature difference that can be expressed between the hot water coming from the cooling tower and that temperature of the cold water departing the tower as well as the surrounding air wet bulb temperature. This principle has been explained so as to help this report present the amount of energy that can saved by the drift eliminator and the sprinkler. Additionally, calculations that are related to the energy savings in drift eliminators and sprinklers always take into considerations of the generally accepted theory of the cooling tower heat-transfer process as it was developed by Merkel. In as much as understanding the theory and subsequent energy savings require terms as Blow-Down, Drift Loss, Make-Up and Evaporating Loss are essential this study focuses on Drift Loss in order to conceptualize research findings as it had been established in the earlier sections. Optimized Drift Eliminators Drift Loss Schwedler (2014) defines Drift Loss as the amount of evaporating water (this report will consider evaporating water in the calculating the exact amount of energy lost as evaporating loss). In order to get the exact amount of energy that drift eliminator saves the working rationale is that evaporating loss is very small when compared with the rate of flow of water, and as the air flow rate is essentially slightly less or equal to the water flow rate, it therefore means that both water flow rate and air flow rate can be constant so as to get the amount of energy that the drift eliminator and sprinkler can save at a given time. Taking the expression; Evaporation Loss (kg/s)/ Flow Rate of Water at Tower Inlet (kg/s), based on the expression above, it is essential to note that Drift Loss as it is a mechanic of drift eliminator is a given amount of un-evaporated water that can be lost from an atmospheric water-cooling apparatus in form of fine droplets. Drift is water loss which is independent of lost water through evaporation. Unlike evaporation, drift loos can be reduced by good practice and design and this is the point of departure in this report with regard to how drift eliminators can save and lose energy. How drift eliminators save energy According to Taylor (2011), the ability to save energy with drift eliminators is the performance indicators of an energy system that has one or several towers. Evidence based research shows that older towers have extra installed heavy drift eliminators which occasionally were prone to energy lose (Taylor, 2012). On the same regard, others have installed drift eliminators that have close spaced eliminator blades that range between 450-500 angles. At these angles, drift eliminators will naturally restrict air flow thus conserving energies. Furthermore, there are instances where users or manufacturers can replace such like drifts with staggered drift eliminators blades that have blades at a 600 instead of the 450-500 and in so doing, it will be realized that more air will be admitted in through the tower thus resulting in more capacity and efficiency when it comes to energy consumption. Therefore, as Babcock (2005) notes, one of the critical ways of ensuring that drift eliminators save energy is to replace less angled blades with more angled ones. Babcock (2005) has also noted that modifications to drift eliminator can save energy by up to 20%. These modifications can be done through replacements and as such, a counter flow tower will be working efficiently since on a 5 0C approach tower, there can be equivalence of about 0.5 0C colder tower thus saving the energy by 4 to 5 %. Though scholars such as Schwedler (2014) have shown that more energy can be saved when an extra pass of drift eliminator is installed so as to enable the change of gear or fan to draw maximum air flow rate, there has been questions raised especially with regard to such approach causing a drift loss problem. On mechanical equipment, there are some instances where additional capacity will be needed from the cooling tower so that extra energy can be saved. This is generally done where there is need for more air movement. It is however important to note that a number of installations operate at the maximum rated power. When this is the case then the procedure of saving energy in the drift eliminators is to take additional readings at the motor so as to determine how close the close the motor is running to nameplate amperage or full load. Additionally, if additional load is required then more energy will be saved by changing fan blade-angle. In as much, it has to be noted that since power varies as the cube of the airflow rate, it may be technically difficult to save more energy without a larger motor. Arguing on the basis of efficient drift eliminators, it can be noted, from comparative analyses done by researches and manufactures that drift eliminators can minimize loss of energy due to drift (drift is a liquid water that can be blown or splashed from the tower during normal operations). Going by recent studies from manufacturers and drift eliminator distributors, most drift eliminators control liquid water that can be blown or splashed from the tower during normal operations by 0.5% or more. As a matter of fact, modern practice for new tower installations should incorporate drift eliminators as the energy saved ranges up to 1%. Evapco, as a Company dealing with the supply of drift eliminators specifies drift eliminators that can limit energy losses to a maximum of about 0.8%. Due to such efficiencies, suggested code requires that drift eliminators should control drift losses to about 0.08 of the circulated water volume especially when dealing with counter flow towers as well as 1-2% energy saves for cross-flow towers on all retrofit and new towers. Mentioning counter flow, an illustration has been shown below. As shown in the figure below, it has to be noted that before water is passed into the atmosphere, the water-laden exhaust air is made to pass through a drift eliminator which get rid of water droplets as they pass along. Figure five: Scheme of Counter Flow and Cross Flow Cooling Towers Source: (Copeland, 2012) Looking at the position of drift eliminators in both figures (counter flow and cross flow) it is essential to mention that counter flow has cooling towers which in turn has a uniform exit air wet bulb temperature. Consequently, cross flow tower has a large variation of exit air wet bulb temperatures which ensure there is further evaporation minimization. The uniform exit air wet bulb temperature and a large variation of exit air wet bulb temperatures has been necessitated by the position of the drift eliminators. These principles ensure that energy is saved in the figure above. Consequently, the table 1 below has been incorporated to show a summary of the total average energy efficiency for all measures requirement combined (such as drift eliminator and flow-based controller) as tabulated by Copeland (2012). Table 1: water and Energy Savings by Tower and Drift Eliminator Source: (Copeland 2012) As indicated as ‘Electricity Savings’ in the table above, it represent energy savings or embedded energy that occur as consequent of savings done by sprinkler. Emission Effects and Energy Consumption Looking at the images above, it can be noted that the drift eliminators have been designed in such a way that they reduce drifts coming from cooling towers. Additionally, drift eliminators have been incorporated into the tower design and as such it can effectively remove as many droplets as practical from the air stream before such exit the tower. This essentially saves more energy. As Babcock (2005) also notes, modern drift eliminators that have been designed for cooling towers depend on inertial separation that are caused by direction changes especially when it pass through such eliminators. This essentially provides more impetus thus reducing the amount of energy that would have been consumed. This is has been confirmed by recent studies that has documented that drift eliminator configurations that have included blade-type (herringbone) cellular (honeycomb) and wave form have the potential of saving energy by up to 30% compare to the previous ones. Figure six: Mechanical Drift Eliminators for Emission Source: (Babcock2005) Sprinklers Cooling towers have sprinklers made of centrifugal fan units which are energy efficient and axial fan designs which according to Babcock (2005) have improved heat transfer surfaces that he terms as ‘fill’ (p. 105). These fills have the ability to save energy by up to 40% depending on the manufacturer. It needs to be noted that while the efficiency improvement of a given sprinkler have certainly lowered general energy consumption, even greater is seen to be possible with sprinklers that have recently been manufactured. Taking a practical example, there could be full load energy use in a 500 ton (approximately 1757kw) water-cooled chiller system, especially when based on Standard 90.1-2013 minimum energy efficiency. Working with this, it can be arbitrarily broken down as cooling tower – 8%, chiller – 77%, chilled water pump – 8% and con­denser pump – 7%. With chiller having the highest amount of energy consumed, it is has been seen that most of sprinklers, condenser and cooling tower tank are operated together so as to reduce energy consumption. Additionally, to lower chiller energy, the sprinkler is often operated at full speed and flow until the system gains ambient conditions that allow the minimum condenser water temperature to be attained. Sprinkler speed control This research finds that one of the best ways of saving energy with sprinklers of cooling towers is to control their speed. As required by Standard 90.1-2013, cooling tower sprinkler speed should have the ability to be controlled so as to be proportional to the leaving water/water temperature or condensing temperature (ANSI/ASHRAE/IES Standard 90.1-2013, Energy Standard). This standard requirement in itself saves energy for the user. To illustrate the possible energy savings we can take a case of a four cell cooling tower without and with variable speed sprinkler drives. When having a full water flow over all cells, operating the sprinklers in two cells at complete speed with the sprinkler in the other two cells idle gives essentially the same liquid/water temperature off the tower similar to when running the sprinklers in all four cells at averagely 55-60% sprinkler speed. In as much, when running all the sprinklers at averagely lower speed, the energy of the sprinkler is reduced by more than 60% when compared to step control. Related to this concept is the sprinkler motor and lower energy cooling tower. It has been found through comparative research that cooling tower sprinklers have a unique way of saving energy. One of such way is that they have been made in such a way that users can increase its amount of heat transfer surface thereby reducing the needed airflow and linked air pressure drop through the tower. When this is done, it also reduces the sprinkler’s motor size needed for the same thermal duty. Unlike previous cooling tower sprinklers, modern sprinklers have an open and flexible heads which have the ability to lower pump energy savings when compared to the ones that are achievable with closed loop systems like the chilled water piping or even the common closed condenser loop that are made to use closed circuit cooling tower (ANSI/ASHRAE/IES Standard 90.1-2013, Energy Standard). Though significant in terms of saving energy, these factors and concepts basically places sprinkler savings last in comparison to the cooling tower fan and the chiller. There the operator and designer should check on the possibility of energy savings of reducing the flow of water over the sprinkler with the operating risks to the system (ANSI/ASHRAE/IES Standard 90.1-2013, Energy Standard). Conclusively, it is worth arguing that drift eliminators and sprinklers have huge potential of saving energy, depending the applicability and the type of cooling tower. Essentially, water cooling systems save energy in the way they are designed and such is not depended on the drift eliminators and sprinklers. Therefore, the evaluation of systems and sub-systems of the cooling tower gives an opportunity to further enhance system energy consumption. Conclusion Cooling towers are hidden jackpots for both energy and cost savings. In majority cases, the limiting is the quality and quality of the released from the cooling tower. Energy management analysis is a very vital consideration in today’s escalating climate of energy and costs. However, in coming up with the optimum designs, it is necessary for through engineering inspections and evaluations by specialists are carried in order to arrive at the optimum operative conditions. References ANSI/ASHRAE/IES Standard 90.1-2013, Energy Standard for Buildings Except Low-Rise Residential Buildings, Paragraph 6.5.5.2. Babcock, G. 2005. “Because temperature matters: maintaining cooling towers.” ASHRAE Journal 47(3):51. Copeland, C.C. 2012. “Improving Energy Performance of NYC’s Existing Office Buildings.” ASHRAE Journal 54(8):38. Goshayshi , H. R., Missenden, R. Tozer, J. F. & Tozer, R., 1998. Cooling towerÐan energy conservation resource. Applied Thermal Engineering . Morrison, F., 2014. global strategy at Baltimore Aircoil Company ASHRAE Jounal. [Online] Available at: https://www.allwriting.net/control-panel/current/451618 [Accessed 19 September 2014]. Schwedler, M. 2014. “Effect of Heat Rejection Load and Wet Bulb on Cooling Tower Performance.” ASHRAE Journal 56(1):16. Taylor, S. 2011. “Optimizing Design and Control of Chilled Water Systems—Part 2: Condenser Water System Design” ASHRAE Journal 53(9):26. Taylor, S. 2012. “Optimizing Design and Control of Chilled Water Systems—Part 4: Chiller & Cooling Tower Selection.” ASHRAE Journal 54 (4):60. Taylor, S. 2012. “Optimizing Design and Control of Chilled Water Systems—Part 5: Optimized Control Sequences.” ASHRAE Journal 54(6):56. Tezukas, S. & Fusita, T., 1980. Performance of an induced draft cooling tower packed wit Parallel plates, in:. s.l., s.n. Read More

However, majority of the cooling towers consists of rougher surfaces, created by turbulence promoters positioned to provide improve mass and heat transfer. The core of the cooling plant is the fill of packing, therefore is the cooling plant efficiency is to be increased, this part of the plant therefore receives more attention. This paper looks at the improve energy saving within the PVC filling. (Tezukas & Fusita, 1980) conducted an experiment to determine the factor of the performance of PVC fill packing.

In the experiment they provided a number of fill arrangements in a cell and eventually tested them over a range of water and air rates, entering air conditions and heat loads. Figure 3: Typical Configuration of a Corrugated PVC Filling used in the Experimental Setup From their conclusion, (Tezukas & Fusita, 1980), suggested that the required fill depth is a function of temperature of the exiting water, the overall pressure drops and mass transfer coefficients of ribbed corrugated PVC pickings considerably increase as compared to the smooth packing.

Also, from their configuration and shape, it was evident that the shape and configuration of the PVC panels and thus the height roughness projection is vital in determining the impact of the mass transfer effect and the Fanning friction factor. From their experiments, (Goshayshi , et al., 1998), determined that the surface area condition of the fillings, and the distribution of water on the PVC fillings for instance the water flow and air flow angle and the distance between the repeated ribs plays a crucial role in the efficiency of the each PVC filling pack, as a result of the impact of the different water distributions on both sides of the PVC filling sheets.

Ideally, (Tezukas & Fusita, 1980), concluded that high increased air turbulence coupled with relatively low fluid velocity is more economical as compared to a highly straight and smooth combined with a high water velocity. Design of an Energy Saving PVC Filling of the Cooling Tower With respect to the filling section of the cooling tower, the most effective technique of overall plant energy efficiency is to increase the surface area of the fill area, leading to a reduction in the needed airflow and thus the related air pressure drop within the tower.

This consequently decreases the fan motor size required to fan the same thermal task. According to (Morrison, 2014), the reductions of fan motor sizes of between 25% and 50% are practicable on majority projects. The additional initial costs on the tower as a result of the more PVC fill material and the associated grillage, is compensated partly by the lower spec. fan used. Depending on the particular model adopted, the tower height may equally increase leading to higher pumping costs. However, this is usually a small factor as compared to the cost saving associated with the low running costs of the cooling fan.

The proposed cooling tower will make use Polypropylene foils rather than the ordinary PVC material. The main advantage of polypropylene (PP material over the PVC material is the higher thermal coefficient thereby making it able to draw more heat energy from the falling hot water In the existing towers, most of the blockage within the fill material is as a result of weak foil thickness leading to pitting as a result of erosion realised due to constant admission of hot water. The proposed PP fill material will have extruded thicker sections than in other places as shown in figure 2 below.

Figure 4: Optimized Polypropylene Fill foil thickness Distribution Since PVC fills foils being welded together, more resistance to the stresses induced by the hot water making them suffer severe deformation leading to clogging and chances of residual clogging. This consequently affects the general performance of the cooling tower. In order to achieve similar performance, a higher fan is required thereby increasing net energy consumption. The proposed PP fill foils are extruded directly from a molten mass so that they are free of any undue stresses.

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