an experimental investigation of thermal characteristics of a mechanical draft wet cooling tower

M. Lemouari and M. Boumaza / 13th IAHR symposium on cooling towers, June 12-16, 2005, Poitiers, France

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AN EXPERIMENTAL INVESTIGATION OF THERMAL CHARACTERISTICS OF A MECHANICAL DRAFT WET COOLING TOWER

M. LEMOUARI 1, M. BOUMAZA 2 1 Department of Chemical Engineering

Faculty of Sciences and Engineering Sciences University of Bejaia, Bejaia, 06000, Algeria

2 Department of Chemical Engineering

College of Engineering –King Saud University Riyadh, Saudi Arabia

Abstract Usually, industrial processes produce a large quantity of heat which must be removed in order to maintain standard operating parameters. Cooling towers filled with packings are widely used to remove heat from these processes and from refrigeration and air-conditioning systems. The type of cooling tower packing plays an important role in this equipment, as it controls heat and mass transfer processes between water and air. This paper presents an experimental investigation of the thermal characteristics of a mechanical draft counter flow wet cooling tower filled with an “A. V. G.” type packing. This packing is 0.42m high and consists of four (04) galvanised sheets having a zigzag form, between which are disposed three (03) metallic vertical grids in parallel. The distance between each two grids is 0.05m (width of the cell). The cross sectional test area is, S = 0.15m X 0.148m. This study investigates the effect of the air and water flow rates on the cooling water range as well as the tower characteristic, KaV/L, for an inlet water temperature of 43°C. During the air and water contact, through the packing in the tower, two functioning regimes were observed: a first regime called pellicular regime (PR) and a second regime called regime of bubble and dispersion (RBD). These two regimes can determine the best way to promote the heat transfer. Indeed, the second regime seems to be more efficient than the first one, enabling to cool larger water flow rates. The comparison between the results and those found in the literature for other types of packing indicates that the cooling tower filled with the “A. V. G.” type packing has good thermal characteristics. Keywords: Wet cooling tower, Packing, flow, Tower characteristic, regimes Nomenclature a contact area between air and water, m2 / m3 A.V.G. Apparatus with Vertical Grid Cpw water specific heat at constant pressure, kJ/kg.°C d diameter of the particles, m G air mass flow rate, kg/h G’ air mass flux, kg/m2.h ℮ exponential H enthalpy of moist air (1: inlet; 2: outlet), kJ/kg H w enthalpy of saturated air at the bulk water temperature, kJ/kg K mass transfer coefficient, kg/m2.h


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KaV/L tower Characteristic, dimensionless L water mass flow rate, kg/h L’ water mass flux, kg/m2.h R cooling temperature range for water, °C S cross sectional test area, m2 T water temperature (1: inlet; 2: outlet), °C t moist air temperature (w: wet; d: dry; 1: inlet; 2: outlet), °C V volume of the exchange core (packing), m3 w specific humidity of moist air, kg/kg 1 Introduction Many manufacturing processes and chemical reactions produce large quantities of heat which must be permanently removed in order to maintain standard operating parameters. Cooling towers filled with packing are widely used to dissipate large heat loads from these processes and from refrigeration and air-conditioning systems to the atmosphere. Their principle is based on heat and mass transfer using direct contact between air and water through some type of packings. Indeed, the type of packings used in cooling tower plays an important role in the tower as it controls the heat and mass transfer processes between water and air. Several researchers have investigated this subject through experimental analysises of the heat and mass transfer processes in these equipments. Simpson and Sherwood [1] studied the performances of forced draft cooling towers. They used a tower having a cross-section of 1.057m X 0.606m, and a 1.05m packing height consisted of wood slats 6.35mm X 50.8mm X 66.675mm, bottom edge serrated, 15.875 horizontal center, 66.675-92.075mm vertical centers. While, Kelly and Swenson [2] studied the heat transfer and pressure drop characteristics of splash grid type cooling tower packings. The tests were conducted in a 3m2 experimental counter flow tower. The authors correlated the tower characteristic with the water/air mass flow ratio and mentioned that the factors influencing the value of the tower characteristic were found to be the water-to-air ratio, the packed height the deck geometry and, to a very small extent, the hot water temperature. They also mentioned that the tower characteristic at a given water-to-air ratio was found to be independent of wet bulb temperature and air loading, within the limits of air loading used in commercial cooling towers. Barile et al. [3] studied the performances of a turbulent bed cooling tower. They used in their experiments a 285.75 mm diameter and 1.63 m high plexiglass cylinder packed with 19.05 and 38.1 mm diameter hollow polypropylene spheres with bulk densities of 153.6 and 934 kg/m3, respectively; the packing heights used were 153.4, 304.8 and 457.2 mm. The water to be cooled enters at the top of the tower at the temperature of 41°C and undergoes a cooling of approximately 12°C. They correlated the tower characteristic with the water/air mass flow ratio. El- Dessourky [4] studied the thermal and hydraulic characteristics of a three-phase fluidized bed cooling rower. His experiments were carried out in a packed tower of 200mm diameter and 2.5m height. He used spongy rubber balls 12.7mm in diameter and with a density of 375 kg/m3 as a packing, and developed a correlation between the tower characteristic, hot water inlet temperature, static bed height, and the water/ air mass flux ratio. Bedekar et al. [5] studied experimentally the performance of a counter flow packed bed mechanical cooling tower, using a film type packing. Their results were presented in terms of tower characteristics, water outlet temperature and efficiency as functions of the water to air flow rate ratio, L/G. They concluded that the tower performance decrease with an increase in the L/G ratio, however they did not suggest any correlation in their work. Goshayshi and Missenden [6] also studied experimentally


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the mass transfer and the pressure drop characteristics of many types of corrugated packing, including smooth and rough surface corrugated packing in atmospheric cooling towers. Their experiments were conducted in a 0.15 m X 0.15 m counterflow sectional test area with 1.60 m packing height. From their experimental data, a correlation between the packing mass transfer coefficient and the pressure loss was proposed. Milosavijevic and Heikkila [7] carried out experimental measurements on two pilot-scale cooling towers in order to analyze the performance of different cooling tower filling materials. They tested seven types of counter flow film type fills and correlated their pressure drop data as well as the volumetric heat transfer coefficient with the water and air flow rates. More recently, Kloppers and Kröger [8] studied the loss coefficient for wet cooling tower fills. They tested trickle, splash and film type fills in a counter flow wet cooling tower with a cross sectional test area of 1.5m X 1.5m. They [8] proposed a new form of empirical equation that correlates fill loss coefficient data more effectively when compared to other forms of empirical equations commonly found in the literature. There exist several other mathematical models which can correlate heat and mass transfer processes occurring in wet cooling towers, such as the models proposed and discussed by Khan et al. [9] and Kloppers and Kröger [10].

The objective of this study is to investigate the thermal characteristics of a forced draft counter flow wet cooling tower filled with an “A. V. G.” type packing. This type of packing which was first proposed by Ignatenkov [11] for the mass transfer processes, has not been used in cooling water systems by direct contact between water and air. Recently, Lemouari [12] and Lemouari and Boumaza [13] used this packing in an evaporative cooling system to study its thermal and hydraulic performances. Therefore, this present study presents an experimental investigation of the thermal performances of cooling towers filled with the “A. V. G.” type packing. The results obtained which relate mainly the tower characteristic as well as the cooling water range variation with the air and water flow rates seem to be into perfect agreement with those for the works published in the literature. This suggests the validation of these results. 2 Experimental installation and procedure The experimental apparatus used in this study is illustrated in Fig. 1. It consists mainly of a cooling tower (1) which represents the main equipment of the transfer phenomena, a cold water basin (2), a load tank (3) which contains two electric heaters (12), a water pump (4), a flow meter device (5), a by-pass pipe (6), a water distributor (7), a fan (8), air distribution chamber (9), a separator of water drops (10), a thermostat (11). Auxiliaries items are also used such as temperatures and pressures measuring devices (13) (14) as well as the system of regulation of the water level (15) in the feed basin. The cooling tower (Fig. 2) [12] has a parallel form of dimensions 206 mm X 148 mm X 550 mm, and is made of plexiglass. It is filled with the “A. V. G.” type packing which occupies a cross-sectional area of 0.150 m X 0.148 m and has 420 mm height and consists of four (04) galvanised sheets having a zigzag form, between which are disposed three (03) metallic vertical grids in parallel. The distance between each two grids is 0.05m (width of the cell). The measurements which were taken consist of the raised temperatures (dry and wet) of the air at the entry and exit, as well as the inlet and outlet water temperatures.


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The measuring instruments of an existing experimental rig [14] are used in this study. The experimental procedure is as follows:

- Initiating the circulation of a water flow, and lighting the electrical heaters at the same time.


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- As soon as the temperature of feed water exceeds few degrees of the desired temperature, air is injected by operating the ventilator.

- After a few moments, the temperature of water decreases and passes again by its initial value (set point) which corresponds to the measurements, of the dry and wet temperatures of the air at the entry and the exit of the tower and the inlet and outlet water temperature.

3 Cooling tower thermal characteristics Evaluation In this study, two different parameters were used in determining the thermal characteristics of the cooling tower, the cooling water range (R), which is defined as the difference between the inlet water and the outlet water temperature,

R = T1- T2 (1)

and the tower characteristic, KaV/L, defined by the following equation [15]:

∫ −=

1

2

T

THH

dTCL

KaVw

Pw (2)

Equation (2) is solved numerically to evaluate the tower characteristic for different experimental conditions. The following equations were used for the numerical integration:

Hw = α ℮ λT (3)

where: α = 20.231, λ = 0.05314 for 17°C ≤ T ≤ 44°C

H = H1 + Cpw (L/G) (T1 -T) (4)

where: H1 = (1.005 + 1.884w) t1d + 2502.3w (5)

Equation (3) was obtained by approximating the enthalpy of the air in saturation, Hw, by using the values tabulated in the literature [14, 16]. 4 Results and discussions

Two operating regimes were observed during the air and water contact, through the “A. V. G.” type packing in the tower: - A first regime, called pellicular regime (PR), exists with low water flow rates. - A second regime, called regime of bubble and dispersion (RBD), appears with relatively larger water flow rates, as reported by Lemouari [12].

Figure, 4, shows the variation of the tower characteristic, KaV/L, with the water/air mass flow ratio, L/G, for an inlet water temperature of 43°C (in log-log co-ordinates). The tower characteristic decreases with an increase of L/G. This decrease becomes less pronounced as L/G increases, for the case in the regime of bubble and dispersion (RBD). The effect of L/G on the tower characteristic, as explained by El-Dessouky [4], can be attributed to the decrease in the fraction of water that evaporates per unit of inlet water. It has also been observed during the experiments, that an increase in water flow rate is accompanied by an increase in the water hold-


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up in the cells of the packing, and this might cause a decrease in the evaporation rates of water into the air stream.

Figure 5, shows the variation of the cooling water range, R, with the air flow rates, G, for several values of the water flow rates, L, carried at the inlet water temperature of 43°C. For each value of L, the cooling water range increases gradually with the increase in the air flow rate. Such evolution can be attributed to an increase in the water mass that evaporates per unit of inlet air stream. This increase becomes more pronounced in the bubble and dispersion regime, and can be explained mainly by the increase in the interfacial area between air and water which becomes important in such case. On the other side, a decrease in the cooling water range is observed while increasing, L. The highest variation is reached with the lowest water flow rates corresponding to lower values of L/G. Such evolution is primarily explained by the development of a resistance to the heat transfer at the water side which increases as the water flow rates increase, and therefore decreases the transfer rate. 5 Correlation of experimental results and comparison with published works As already mentioned, the tower characteristic, KaV/L, is influenced by the air and water flow rates. Therefore in order to derive an equation characterizing the heat exchange through the “A. V. G.” type packing, results of the three tests are gathered, and the following correlations were proposed:

- Pellicular regime (RP): KaV/L = 0.79 (L/G) -0.48 (6.a)

- Regime of bubble and dispersion (RBD): KaV/L = 1.83 (L/G) -0.81 (6.b) The results obtained were then compared with those obtained by using the Barile et al’s correlations [3] proposed for a packing consisted of hollow polypropylene particles (0.46m packing height). These correlations are as follows:

- Diameter of the particles, d = 19 mm: KaV/L = 1.118 (L/G) -0.514 (7.a)

- Diameter of the particles, d = 38 mm: KaV/L = 1.147 (L/G) -0.430 (7.b)

where: (L/G) = 0.7 - 3 Figure 4 shows clearly the efficiency of the heat exchange through the “A. V. G.” type packing despite its low height. It is also observed that the cooling water range obtained by the authors (12°C), is weak compared to that in using the “A. V. G.” type packing and which can reach 26°C, showing good thermal characteristics of the cooling tower filled with the “A. V. G.” type packing.


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6 Conclusion In this study, thermal characteristics of a mechanical draft counterflow wet cooling tower filled with an “A. V. G.” type packing have been investigated for a wide range of experimental flow


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rates of air and water. The packing used is 0.42m high. Results were obtained for an inlet water temperature of 43°C and water and air flow rates varying between 1638-8527 kg/m².h and 1621.6-7297 kg/m².h respectively. These results can be summarized as follows: - During the air and water contact through the packing in the tower, Two functioning regimes

of the tower were observed: a pellicular regime: existing with low water flow rates, and a regime of bubble and dispersion: appearing for relatively larger water flow rates. These two regimes can determine the best way to promote the heat transfer.

- The tower characteristic, KaV/L, decreases with an increase of the water/air mass flow ratio, L/G. This decrease is less pronounced for the bubble and dispersion regime.

- The cooling water range, R, increases with an increase in the air flow rates, whereas it decreases with an increase in the water flow rates. The highest values of, R, are reached for lower values of L/G.

- The cooling tower filled with the “A. V. G.” type packing, despite of its low height, compared to systems filled with other types of packings possesses very interesting thermal characteristics.

It is recommended to extend the range of variation of the air and water flow rates for relatively higher inlet water temperatures, in towers of higher size, in order to complete the conclusions of this study. References [1] Simpson W M., and Sherwood T K, “Performance of small mechanical draft cooling

towers”, Am. Soc. Refrig. Eng., 52 (1946), pp.535-543 and 574-576 [2] Kelly N. W. and Swenson L. K., “Comparative performance of cooling tower packing

arrangements”, Chem. Engng. Prog., 52 (1956), pp.263-268 [3] Barile R. G., Dengler J. L., Hertwig T. A., “Performance and design of a turbulent bed

cooling tower”, AIChE Symp. Ser., 70 (1974), pp.154-162 [4] EL-Dessouky H., “Thermal and hydraulic performance of a three-phase fluidized bed

cooling tower”, Experimental Thermal and Fluid Science 6 (1993), pp.417-426 [5] Bedekar S. V., Nithiarasu P., Seethatamu K. N., “Experimental investigation of the

performance of a counter flow packed bed mechanical cooling tower”, Energy 23 (1998), pp.943-947

[6] Goshayshi H. R. and Missenden J. F., “The investigation of cooling tower packing in various arrangements”, Appl. Therm. Engng. 20 (2000), pp.69-80

[7] Milosavijevic N. and Heikkila P., “A comprehensive approach to cooling tower design”, Appl. Therm. Engng. 21 (2001), pp.899-915

[8] Kloppers J. C. and Kröger D. G., “Loss coefficient correlation for wet cooling tower fills”, Appl. Therm. Engng. 23 (2003), pp.2201-2211

[9] Khan J. R., Qureshi B. A., Zubair S. M., “A comprehensive design and performance evaluation study of counter flow wet cooling towers”, Int. J. Refrigeration 27 (2004) pp.914-923

[10] Kloppers J. C. and Kröger D. G., “A critical investigation into the heat and mass transfer analysis of counterflow wet-cooling towers”, Int. J. Heat and Mass Transfer 48 (2005) pp.765-777

[11] Ignatenkov Y. I., “Study and elaboration of a method for calculating optimum parameters of mass exchange apparatus with vertical grids”, Doctorate Thesis, LENSSOVET Institute of Technology, Leningrad, (1979) (in russian)


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[12] Lemouari M., “Experimental study of the air/water heat transfer by direct contact in a column packed with vertical grids - application to the water cooling”, Magister thesis, University of Bejaia, Algeria, September (2001) (in French)

[13] Lemouari M. and Boumaza M., “Experimental study of the air/water heat transfer by direct contact in a column packed with vertical grids - application to the water cooling”, Proceeding 11th International Meeting on Heat Transfer (JITH2003), Vol. 2, pp.457-464 (2003) (in French)

[14] “Instruction manual: BASIC water cooling tower”, Engineering Teaching and Research Equipment, ARMFIELD, England, (1993).

[15] Perry R. H., Green D. W., Maloney J. O., Chemical Engineers Hand Book, 6th ed., McGraw-Hill, sec. 12, pp.13-24, (1987).

[16] Treybal R. E., “Mass-Transfer operations”, 3rd ed., McGraw-Hill, (1980).

Messaoud Lemouari holds a Bachelor and a Master degree in Chemical Enginering, obtained at Bejaia University, Algeria, and currently is a lecturer at this university. He is also pursuing a PhD research project in the field of cooling towers. Mourad Boumaza holds a Bachelor, a Master and a PhD degree in Chemical Engineering, from Bradford University, G.B. He is currently a full professor in the department of Chemical Engineering, at King Saud University, Riyad, Saudi Arabia. He is the author of several papers related to Refrigeration, Cryogenics, heat and Mass transfer operations. He is also a member of the IChemE (G.B) and the international Institute of Refrigeration (France).

an experimental investigation of thermal characteristics of a mechanical draft wet cooling tower

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