International Journal of Modern Research in Engineering & Management (IJMREM) ||Volume|| 1||Issue|| 10 ||Pages|| 55-62 || November 2018|| ISSN: 2581-4540
Thermodynamic Analysis of Cooling Tower with Air to Air Heat Exchanger for Reducing Evaporative Losses 1,
Ans Ahmed Memon, 2, Muhammad waqas Chandio, 3, Saadat Ali khokhar 1,
Student M.E. (Energy Systems Engg.) PGS, Mehran UET Jamshoro; 2, Lab.Supervisor, Mechanical Engg. Mehran UET, Jamshoro 3, Student M.E. (Energy Systems Engg.), Mehran UET, Jamshoro
---------------------------------------------------ABSTRACT------------------------------------------------------In this paper a thermodynamic analysis of cooling tower with air-to-air heat exchanger is presented. During manual operation of conventional cooling tower, a good quantity of water is evaporated which requires equivalent amount of makeup water for their condensers. So, in this regard, the thermodynamic model of a cooling with an air to air heat exchanger is developed using engineering equation solver (EES) software and then simulated in different climatic conditions of two major cities of Pakistan namely Karachi and Jamshoro through the period of June & December 2017. The results show a significant variation in water vapor recovery with respect to atmospheric conditions mainly humidity and ambient air temperature. Results reveal that the when a cooling tower coupled with air to air heat exchanger maximum amount of water vapors are recovered at Karachi and ranges up to 67% and 62% in Jamshoro during the month of December.
KEYWORDS: Thermodynamic analysis, Makeup water consumption, wet cooling tower, water vapor recovery, effect of climate on wet cooling tower ----------------------------------------------------------------------------------------------------------------------------- --------Date of Submission: Date, 14 November 2018 Date of Accepted: 21 November 2018 ------------------------------------------------------------------------------------------------------------ -------------------------I. INTRODUCTION The world’s energy use is growing rapidly and increasing concern to the depletion of existing natural resources. The conservation of energy resources becomes a key issue in the world. Out of these resources, energy resources are consumed in power generation, industrial applications, commercial and residential buildings comfort conditioning. In current scenario, Pakistan is also facing water crisis along with energy crisis, most of thermal power generating systems, industrial hubs utilize plenty of water for its applications but as a water crisis concern, we have to focus how to minimize water losses in these sectors in order to subside these crises. Most power generating systems, refrigeration and industrial processes generates heat that must be removed and dissipated [1]. So, water is generally used as heat transfer medium to remove heat from condenser or industrial heat exchangers to sink(atmosphere) [2]. This is due to the fact that water has high specific heat than any other medium and gives higher heat transfer coefficient, an-other reason is that water is dense liquid and it enables in easiness in handling from one place to other. Cooling tower is a device that cools the warm water coming from systems i.e. engines, condensers etc. and recycle the water back into systems for economy and used as coolant once again [3]. It makes use of evaporation whereby some amount of water is evaporated by air stream moving in opposite direction to the water, and then subsequently discharged to atmosphere. The main source of energy loss in a cooling tower is the energy loss in terms of its water evaporation loss, pumping loss, leakage loss, inefficient equipment design, improper control on equipment’s. The water loss through evaporation can be compensated by addition of makeup water continuously into the water circuit. As a result of which remaining amount of water is cooled down significantly as shown Fig:1.1
Fig.1 Schematic diagram of conventional induced draft wet cooling water [4]
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Thermodynamic Analysis of Cooling Tower‌ According to nature of cooling, cooling towers are used in two configurations, i.e. wet and dry cooling towers. Mostly wet cooling towers are preferred in industrial application and power plants due to higher specific cooling capacity [5]. Wet cooling tower consumes the maximum amount of water in a power plant, it utilizes large amount of water in cooling down and creates loss in evaporation and waste greater amounts of water, so in order to proper functioning some amount of makeup water is added to compensate these losses. So, it is too important in industrial and power plants to minimize water losses in wet cooling tower. Following are the possible ways through which water losses in a cooling tower can be reduces. 1) By changing the wet cooling tower to dry or hybrid cooling tower. 2) Reducing in blowdown losses with increasing cycles of concentration 3) Utilizing an air-to-air heat exchanger. Modification in the design of steam turbine, condenser and related components is required for converting wet cooling tower to dry or hybrid cooling tower, which decreases the efficiency of the power plant. Reducing blowdown along with operating at higher (COC) cycles of concentration can increase the risk of scaling, fouling and corrosion except when some sort of treatment is applied. So evaporative lose could be minimized by utilizing an air to air heat exchanger. The work in [6] used a laboratory scale wet cooling tower with an air to air heat exchanger in order to study how much water consumption can be reduced. The air-air heat exchanger condenses the water vapor back into sump by inletting ambient air throughout the heat exchanger by an auxiliary fan. This laboratory model saves the evaporative water loss about 35%, depending on surrounding air conditions. According to [8] increases overall water efficiency of cooling tower by controlling blowdown and vapor losses. Ozone treatment reduces the use of chemicals, in this way reducing blowdown losses. A vapor recovery system that consists fiber filter of round shape, at the top of the cooling tower, absorbs the water vapor coming out of cooling tower and then condenses it. The results show that about 10% of the evaporated water can be recovered with this method. It is evident from the literature review presented above that evaporative water loss is crucial among all water losses in cooling tower. Most of the researchers have tried to reduce this evaporative loss with the applications of different techniques applied on cooling tower, some of them have utilized fiber filter to soak the evaporated water loss, increase in cycles of concentration (COC), using air to air heat exchangers to minimize evaporative loss and decrease in makeup water addition. So, in this regard the core purpose of this work is to minimize evaporative water loss by an air to air heat exchanger coupled at top of cooling tower.
II.
DESCRIPTION OF NEW DESIGN OF COOLING TOWER
The wet cooling tower is basically a heat exchanging device that cools the warm water coming from engines, condensers of power plants and dissipate this heat to environment by combination of sensible and latent heat into the ambient air. In wet type of cooling towers, hot water is carried at top of tower where it showered with the help of nozzles and stems the water on film media to ensures the proper maxing of water and ambient air in the tower. This outside ambient air is introduced in the tower with the help of louvers in the form of horizontal slats on both sides of cooling tower. These slats are inclined downwards to make sure water remains inside of tower. As the mixing between water and ambient air increases the heat transfer from water to air also increases which results in more decrease in water temperature. Now this cool water is sumped at basin, where some amount of water is removed in order to maintain required level of dissolved solids and impurities known as blowdown. Some amount of fresh makeup water is added in cooling tower basin to compensate water losses evaporative, blowdown and drift. When the cooling tower is equipped with a fan at its top location this is known as induced draft wet cooling tower, in this tower air flows throughout the tower with low to medium velocity via this fan. This fan also dissipates the air water vapor mixture to surrounding and guide into the proposed air to air heat exchanger. This fan has adjustable pitch blades for minimum consumption of electric power, its power consumption also depends on climatic conditions and cooling load. The humid warm air leaving the cooling tower from top lid passes throughout an air-to-air heat exchanger, where its temperature is lowered by an ambient air crossing through the heat exchanger simultaneously. So as a result of this, condensation of humid air which results in decrement of actual evaporative loss. This condensate water will decrease in makeup water addition subsequently. The design of new wet cooling tower equipped with an air-to-air heat exchanger is developed by using engineering equation solver software (EES).
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Thermodynamic Analysis of Cooling Tower‌
Fig.2 Schematic diagram of proposed induced draft wet cooling water
III. THERMODYNAMIC MODELLING DESCRIPTION The thermodynamic modelling of an induced draft wet cooling tower has been carried out by taking into consideration of fundamental equations of thermodynamics. Mass and energy expressions for cross flow wet cooling tower and other related equations are provided and furnished here as under. The main operating parameters considered for energy analysis of cooling tower are, inlet air temperature, pressure, relative humidity. In the similar fashion, the temperature of water is considered as the inlet of cooling tower. The energy balance and water balance equations are used to analyze thermal performance of cooling tower. A. ➢ ➢ ➢ ➢ ➢ ➢ ➢ ➢
Assumptions: The process is assumed to be steady. The mass flow rate of dry air remains constant during entire process. Dry air and water vapors is ideal gases. The kinetic and potential energy changes are negligible. The radiant heat transfer is assumed to be negligible. The mass flow rate of water is considered as constant. Heat transfer through the tower wall to the environment is negligible. Water loss by drift is negligible.
B. Applying Mass and Energy Balances on Cooling Tower ∑đ?‘šĚ‡đ?‘–đ?‘› = ∑đ?‘šĚ‡đ?‘œđ?‘˘đ?‘Ą (1) ∑đ?‘šĚ‡đ?‘–đ?‘› and ∑đ?‘šĚ‡đ?‘œđ?‘˘đ?‘Ą represents total mass flow rate of air and water at inlet and outlet of cooling tower. Mass balance of dry air for cooling tower: đ?‘šĚ‡đ?‘Ž,[1] = đ?‘šĚ‡đ?‘Ž,[2] = đ?‘šĚ‡đ?‘Žđ?‘–đ?‘&#x; (2) Mass balance for water and vapor content in air stream: đ?‘šĚ‡3 + đ?‘šĚ‡đ?‘Ž1 đ?œ”1 = đ?‘šĚ‡4 + đ?‘šĚ‡đ?‘Ž2 đ?œ”2 (3) Water evaporation rate: When air and water come in contact, some amount of water evaporates. The rate of evaporation can be calculated as below đ?‘šĚ‡đ?‘’đ?‘Łđ?‘Žđ?‘? = đ?‘šĚ‡đ?‘Ž1 (đ?œ”2 − đ?œ”1 ) (4) Energy balance on cooling tower: The energy balance equation for fluid flow inside of cooling tower revelers that “energy transferred from hot water (inlet circulating water) is gained by atmospheric air supplied to tower (outlet air) of cooling tower. ∑đ?‘–đ?‘› đ?‘šĚ‡â„Ž = ∑đ?‘œđ?‘˘đ?‘Ą đ?‘šĚ‡â„Ž (5) đ?‘šĚ‡đ?‘Ž1 â„Ž1 + đ?‘šĚ‡3 â„Ž3 = đ?‘šĚ‡đ?‘Ž2 â„Ž2 + đ?‘šĚ‡4 â„Ž4 (6) Solving for đ?‘šĚ‡đ?‘Ž gives
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Thermodynamic Analysis of Cooling Tower‌ �̇� = (ℎ
đ?‘šĚ‡3 (â„Ž3 −ℎ4 )
2 −ℎ1 )−(đ?œ”2 −đ?œ”1 )â„Ž4
(7)
Range of cooling tower: The temperature difference between water inlet and outlet of the cooling tower is known as range of a cooling tower. đ?‘… = ( Tw,1 – T w,2) (8) Approach of cooling tower: Approach = (T w,2 –Twet bulb) (9) Effectiveness: The effectiveness of a cooling tower is defined as the ration of range to ideal range i.e., (â„Ž2 −h1 ) ∈= (10) (hmax _2 −ℎ1 )
Alternatively, the effectiveness is the ratio of range to sum of range and approach. đ?‘… ∈= đ?‘…+đ??´đ?‘?đ?‘?đ?‘&#x;đ?‘œđ?‘Žđ?‘?â„Ž
(11)
C. Applying Mass and Energy Balances on Heat Exchanger ∑đ?‘šĚ‡đ?‘–đ?‘› = ∑đ?‘šĚ‡đ?‘œđ?‘˘đ?‘Ą (4.12) ∑đ?‘šĚ‡đ?‘–đ?‘› and ∑đ?‘šĚ‡đ?‘œđ?‘˘đ?‘Ą represents total mass flow rate of air and water at inlet and outlet of heat exchanger. Mass balance of dry air for heat exchanger: đ?‘šĚ‡đ?‘Ž,[7] = đ?‘šĚ‡đ?‘Ž,[8] = đ?‘šĚ‡đ?‘Žđ?‘–đ?‘&#x; (4.13) Mass balance for water and vapor content in air stream for heat exchanger: đ?‘šĚ‡9 đ?œ”9 + đ?‘šĚ‡đ?‘Ž7 đ?œ”7 = đ?‘šĚ‡10 đ?œ”10 + đ?‘šĚ‡đ?‘Ž8 đ?œ”8 +đ?‘šĚ‡[11]đ?‘?đ?‘œđ?‘›đ?‘‘: (4.14) Energy balance on heat exchanger: The energy balance equation for heat exchanger revelers that “energy transferred from warm saturated vapors is gained by flowing atmospheric air throughout heat exchanger and remaining exhausted through heat exchanger. ∑đ?‘–đ?‘› đ?‘šĚ‡â„Ž = ∑đ?‘œđ?‘˘đ?‘Ą đ?‘šĚ‡â„Ž đ?‘šĚ‡đ?‘Ž9 â„Ž9 + đ?‘šĚ‡7 â„Ž7 = đ?‘šĚ‡đ?‘Ž10 â„Ž10 + đ?‘šĚ‡8 â„Ž8 + đ?‘šĚ‡[11]đ?‘?đ?‘œđ?‘›đ?‘‘: â„Žđ?‘“đ?‘” (4.15) Actual heat transfer rate in air to heat exchanger: đ?‘„̇đ?‘Žđ?‘?đ?‘Ąđ?‘˘đ?‘Žđ?‘™ = đ?œ€ đ?‘„̇đ?‘šđ?‘Žđ?‘Ľ (4.16) The condensing water (đ?‘šĚ‡[11]đ?‘?đ?‘œđ?‘›đ?‘‘: ) from the heat exchanger: ̇ đ?‘šĚ‡[11]đ?‘?đ?‘œđ?‘›đ?‘‘: = đ?‘šĚ‡đ?‘Ž9 (đ?œ”9 − đ?œ”10 ) (4.17) In order to compare the makeup water used in cooling tower including air to air heat exchanger with the condensing water to measure the recovery(saving) of the evaporative water loss. đ?‘šĚ‡ đ?‘…đ?‘’đ?‘?đ?‘œđ?‘Łđ?‘’đ?‘&#x;đ?‘Ś = ̇đ?‘?đ?‘œđ?‘›đ?‘‘đ?‘’đ?‘›đ?‘ đ?‘’ ∗ 100 (4.18) đ?‘šđ?‘šđ?‘Žđ?‘˜đ?‘’đ?‘˘đ?‘?
The thermodynamic model was developed by using EES software and then simulated for various operating parameters to ensure its validity. D. Validation: The simulated results of the model are compared with previously published work for the model validation. For proposed cooling tower (with heat exchanger), the values of simulated evaporated water recovery at different state point of input data taken from experimental work are matched with available values in the published work [6] and illustrated in figure.3 E.
Fig. 3 Results validation with Deziani.M[6]
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Thermodynamic Analysis of Cooling Tower… Table 1: Operating parameters for model validation [6] Dry bulb temperature of inlet ambient air State:1 29.4 0̇ C State:2 25.7 0̇ C Mass flow rate of water Hot water inlet temperature Cold water outlet temperature Cooling tower range
Wet bulb temperature of inlet ambient air 15.4 ̇0C 12.2 0̇ C 28500 m3/hr. 38.2 0̇ C 27.8 0̇ C 10.4 ̇0C
Table. 1 represents the operating parameters for cooling tower at two state points of climate data taken from Deziani.M[6] paper for model validation purpose. And fig. 3 shows experimental vs simulated amount of water vapors recovery at each sate point with its percent error.
IV. RESULTS AND DISCUSSION In this study thermodynamic model of cooling tower with an air-to-air heat exchanger is developed using engineering equation solver (EES) software and simulated to determine amount of water vapor recovery in different climatic conditions of two main cities of Pakistan namely Karachi and Jamshoro through JuneDecember-2017. The analysis includes effect of humidity and ambient air temperature on amount of water vapor expressed in terms of percent recovery. The weather data of above-mentioned cities of Pakistan at different seasons were recorded from official site of meteorological conditions [9-10]. The main parameters on account of data collection are temperature, pressure and humidity. A. Effect of humidity on Water Vapor Recovery in Karachi: Fig. 4 demonstrate the impact of humidity on amount of water vapor recovery of cooling tower with an air-to-air heat exchanger for a fixed ambient air temperature and pressure when operated in climate of Karachi in June and December 2017. The behavior of graph clearly indicates that as humidity increases the amount of water vapor recovery increases. While the recovery response in December is higher than that of in June, in December amount of water vapor recovery starts from 50% and reaches up to 67% while in case of June it starts from 15% and reaches up to 60%. This is due to the fact that when more humid air crossing cooling tower out of heat exchanger it gives out more its water vapor via condensation than that of less humid air. In winter atmospheric air becomes dry, so it has higher capacity to hold water vapors than compared to that of winter (June). As air leaving cooling tower in winter carry enough amount of water vapor due to evaporation inside of cooling tower and when crossing heat exchanger, it easily condensed out more amount of water vapor inters of recovery in winter than that of June.
Fig. 4 Effect of Relative humidity on Recovery in Karachi June-Dec-2017
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Thermodynamic Analysis of Cooling Tower‌ B. Effect of temperature on Water Vapor Recovery in Karachi : Fig. 5 illustrate the impact of ambient air temperature on amount of water vapor recovery of cooling tower with an air-to-air heat exchanger for a fixed relative humidity and atmospheric pressure when operated in climate of Karachi in June-December-2017. The response of graph clearly indicated that as ambient air temperature increases the amount of water vapor recovery decreases. This is due to the fact that, a cooling tower transfer heat to the environment by evaporation of water which will transfer heat to surrounding. As ambient air temperature increases its wet bulb temperature also increases which ultimately decreases the temperature difference of hot water with surrounding and decreases the heat transfer and amount of evaporation. Due to this lower amount of evaporation less amount of water vapor can be condensed out through heat exchanger. That’s why recovery response in Karachi June is in higher decreasing order than that in December. In December it starts decreasing from 62% and reaches up to 26%. While in June it starts decreasing at 65% and ends at 5%.
Fig. 5 Effect of temperature on Recovery in Karachi June-Dec-2017 C. Effect of humidity on Water Vapor Recovery in Jamshoro : Fig. 6 illustrate the effect of humidity on amount of water vapor recovery of cooling tower with an air-to-air heat exchanger for a fixed ambient air temperature and pressure when operated in climate of Jamshoro in June-December 2017. The behavior of graph clearly indicates that as humidity increases the amount of water vapor recovery also increases. While the recovery response in December is in higher order than that of in June, in December amount of water vapor recovery starts from 45% and reaches up to 66% while in case of June it starts from 17% and reaches up to 62%. This is due to the fact that when more humid air crossing cooling tower out of heat exchanger it gives out more its water vapor via condensation than that of less humid air. The above figures are little bit lower than that of Karachi due to more humidity caused by Arabian sea. In winter atmospheric air becomes dry, so it has higher capacity to hold water vapors than compared to that of winter (June). As air leaving cooling tower in winter carry enough amount of water vapor due to evaporation inside of cooling tower and when crossing heat exchanger, it easily condensed out more amount of water vapor inters of recovery in winter than that of summer (June)
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Thermodynamic Analysis of Cooling Tower‌
Fig. 6 Effect of Relative humidity on Recovery in Jamshoro June-Dec-2017 C. Effect of temperature on Water Vapor Recovery in Jamshoro : Fig. 7 illustrate the impact of ambient air temperature on amount of water vapor recovery of cooling tower with an air-to-air heat exchanger for a fixed relative humidity and atmospheric pressure when operated in climate of Jamshoro in June-December-2017. The behavior of graph clearly indicated that as ambient air temperature increases the amount of water vapor recovery starts decreasing. This is due to the fact that, a cooling tower transfer heat to the environment by evaporation of water which will transfer heat to surrounding. As ambient air temperature increases its wet bulb temperature also increases which ultimately decreases the temperature difference of hot water with surrounding and decreases the heat transfer and amount of evaporation. Due to this lower amount of evaporation less amount of water vapor can be condensed out through heat exchanger. That’s why recovery response in Jamshoro June is in higher decreasing order than that in December. In December it starts decreasing from 63% and reaches up to 38%. While in June it starts decreasing at 52% and ends at 10%. We achieve this higher amount of recovery in December due to lower wet bulb temperature in December than that in June.
Fig. 7 Effect of temperature on Recovery in Jamshoro June-Dec-2017
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Thermodynamic Analysis of Cooling Tower‌ V. CONCLUSION The thermodynamic analysis of the induced draft wet cooling tower with air to air heat exchanger is peroformed using Engineering Equation Solver (EES) to investigate the evaporative losses in different climatic conditions of two main cities of pakistan namely karachi, jamshoro through June and December 2017. The simulated results shows that: • The ambient temperature and humidity plays an effective role in recovering of evaporative loss, with the decrease in wet bulb temperature amount of water vapor recovery increases. • As relative humidity increases it ilso increases water vapor recovery. • The maximum amount of water vapors are recovered at Karachi and ranges up to 67% and 62% in Jamshoro during the month of December. • With the attachment of air to air heat exchanger to cooling tower it almost diminishes the drift losses.
VI. ACKNOWLEDGEMNTS Authors are thankful to Mehran University of Engineering & Technology Jamshoro for providing necessary resources for carrying this research work. Nomenclature T w M đ?‘šĚ‡ P đ?‘„̇ đ?‘… đ?‘šĚ‡đ?‘’đ?‘Łđ?‘Žđ?‘?
Greek letters Îľ Effectiveness (-) đ?‘Ł Vapor
Temperature (oC) humidity Ratio (-) Makeup water (kg/s) Mass flow rate (kg/s) Pressure (kPa) Heat (kW) Universal Gas constant Evaporation loss (kg/s)
Abbreviations COC Cycle of Concentration WCT Wet cooling tower
Subscripts WB Wet bulb temperature R Relative humidity max maximum
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Dehaghani, S. T., &Ahmadikia, H. (2017). Retrofit of a wet cooling tower in order to reduce water and fan power consumption using a wet/dry approach [Abstract]. Applied Thermal Engineering,125, 1002-1014.doi: 10.1016/j.applthermaleng.2017.07.069 [2] ASHRAE handbook (American Society of Heating, Refrigerating and Air-Conditioning Engineers) (2008) Ashrae Handbook: HVAC Systems and Equipment, I-P Edition edn.: American Society of Heating, Refrigerating & Air-Conditioning Engineers, Incorporated,2008. [3] Will Lintner, Brian Boyd (2011) 'Cooling Towers: Understanding Key Components of Cooling Towers and How to Improve Water Efficiency', U.S Department of Energy (Energy Efficiency & Renewable Energy), pp. [Online]. Available at: https://energy.gov/sites/prod/files/2013/10/f3/waterfs_coolingtowers.pdf [4] https://journal.uptimeinstitute.com/wp-content/uploads/2016/06/fig1.jpg (Accessed: 10th December 2017). [5] Rezaei, E., Shafiei, S., &Abdollahnezhad, A. (2010). Reducing water consumption of an industrial plant cooling unit using hybrid cooling tower. Energy Conversion and Management,51(2), 311-319. doi: 10.1016/j.enconman.2009.09.027 [6] Deziani, M., Rahmani, k., Roudaki, S. M., &Kordloo, M. (2017). Feasibility study for reduce water evaporative loss in a power plant cooling tower by using air to air heat exchanger with auxiliary Fan. Desalination, 406, 119-124. Doi: 10.1016/j.desal.2015.12.007 [7] U.S Patent “US4249391 A� for Cooling Tower vapor recovery Unit. [8] Melany Hunt (2008) Increasing Cooling Tower Water Efficiency Chang Sub Kim , Available at: https://www.sustainability.caltech.edu/documents/33 chang_sub_final_paper.pdf (Accessed: 15th december 2017). [9] https://www.timeanddate.com/weather/pakistan/karachi/historic?month=12&year=2017 (Accessed: Date: 5September-2018: Time: 11:44 AM) [10] https://www.timeanddate.com/weather/pakistan/jamshoro/historic?month=12&year=2017 (Accessed: Date: 5-September-2018: Time: 12:02 pM)
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