Torre De Enfriamineto

The investigation of cooling tower packing in various

arrangements

H.R. Goshayshi*, J.F. Missenden

School of Engineering Systems and Design, South Bank University, London, SE1 0AA, UK

Received 10 August 1998; accepted 24 January 1999

Abstract

The e€ect of form with corrugated packing on mass transfer and pressure drop characteristics in

atmospheric cooling towers has been studied experimentally. The results showed that the mass transfer

coecient decreased with increase in packing pitch and increase in the ratio of rib pitch to rib height.

Friction factors were expressed by a dimensional equation which included pitch and distance between

the packings, for both smooth and rough surfaces. From these results, the relationship between packing

mass transfer coecient and pressure drop was deduced. The correlations were veri®ed with additional

experimental data taken with 1.1 < P/D < 1.70 and 1Ep/eE5. This provides a useful semi-

experimental relation, in an area generally lacking in design and performance data. # 1999 Elsevier

Science Ltd. All rights reserved.

Keywords: Cooling towers; Packing; Pressure drop; Mass transfer

1. Introduction

Heat and mass transfer between a falling liquid ®lm along a vertical wall and upward

¯owing air contacting directly with the ®lm is an important and interesting phenomenon in

industrial apparatus such as cooling towers. While 96% of the cooling towers use PVC packing

with smooth and cross ribbing, no data on the ¯ow of liquid over a ¯at vertical wall with cross

ribbing have been published. Only some of the features of their operation in contact heat

exchangers have been investigated [1±5]. Major aspects that remain to be studied include: the

Applied Thermal Engineering 20 (2000) 69±80

1359-4311/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S1359-4311(99)00011-3

www.elsevier.com/locate/apthermeng

* Corresponding author. Tel.: +44-171-815-7639; fax: +44-171-815-7699.

geometry and layout of the main corrugation with and without the cross ribbings, the pattern

of ¯ow of the liquid ®lm and interaction between phases. In this paper the mass transfer and

pressure drop characteristics of many types of corrugated packing, including smooth and rough

surface corrugated packings, are investigated, and the relationship between packing mass

transfer coecients and pressure drops is discussed. Mass transfer performance of rough

corrugated packing is increased by 1.5 to 2.5 times the smooth packing values, but the pressure

drop of packings also increases with the increase in heat transfer performance.

2. Experimental apparatus and procedure

The experimental apparatus for the heat transfer experiments consisted of a counter¯ow

forced draft cooling tower, as shown in Fig. 1. Water stored in a tank at the base was pumped

into the spray nozzles. The supply water velocity was regulated by a valve. The cross sectional

test area was A= 0.15  0.15 m. Inlet and outlet air and water temperatures were measured

by mercury in glass thermometers with a range of 0±508C and an accuracy of 0.2 K. Packing

pressure drop was measured by an APM 2000 (0±2000 Pa) micromanometer with an accuracy

of21% FSD (i.e. maximum of 1.2 Pa error in our measurements). Measurements of mass

transfer and pressure drop were carried out in the steady state. The mass transfer coecients

Nomenclature

a pack density (surface area per unit volume) (m

ÿ1)

D distance between the cooling tower packing (rib) (mm)

e height of roughness element (mm)

G ¯ow rate (air) (kg s

ÿ1)

G' mass ¯ux (air) (kg m

ÿ2 s

ÿ1)

E height of corrugation (mm)

L ¯ow rate (water) (kg s

ÿ1)

L' mass ¯ux (water) (kg m

ÿ2 s

ÿ1)

k mass transfer coecient (kg m

ÿ2 s

ÿ1)

Nu Nusselt number

p distance between repeated ribs (mm)

P pitch of packing (see Figs. 3 and 4) (mm)

Pr Prandtl number

Z packed height (m)

Dp pressure drop (Pa)

Rew water Reynolds number=2L'D/mw

ra air density (kg m

ÿ3)

ua air velocity inside the packing (m s

ÿ1)

uw water velocity inside the packing (m s

ÿ1)

y angle of inclination of cross ribbing with the horizontal (8)

70 H.R. Goshayshi, J.F. Missenden / Applied Thermal Engineering 20 (2000) 69±80

and pressure drops were measured for a range of L/A (L') from 0.45 to 2.22 kg m

ÿ2 s

ÿ1 and

G/A (G') 0.20±1.50 kg m

ÿ2 s

ÿ1. A series of perimeter de¯ector plates was installed around the

inner perimeter of the column, made in the laboratory from clear polycarbonate plastic to

allow observation of the water ¯ow. These de¯ector plates removed the water ®lm from the

wall of the tower's column and redistributed the water in the packing zone. As a result of

de¯ection, most of the water was transferred to the packing surface from the outer wall,

forming descending thin ®lms, while air was blown vertically upward, counter current to the

water by a fan at the base.

The packings tested were of two types, smooth and ribbed, both of PVC. The smooth

packing had horizontal corrugations and the ribbed had horizontal corrugations with ribbing

Fig. 1. Outside view of forced draft cooling tower in transport phenomena laboratory.

H.R. Goshayshi, J.F. Missenden / Applied Thermal Engineering 20 (2000) 69±80 71

set at an angle to the main corrugations. The cross ribs were separated by distance p, ranging

from 2 to 10 mm, for the six sample packings, and the height e of the ribs ranged from 1 to

3 mm. The main corrugation pitch, P, ranged from 30 to 70 mm. The thickness of packing was

negligible. The forms of corrugated packings used in the experiments are listed in Table 1, and

typical shapes are shown in Figs. 2±4. The column packed height, Z, was 160 cm and the

water level in the sump was about 1.2 m below the top of the packing. Water inlet and outlet

temperatures were 37 and 278C respectively.

3. Experimental results

3.1. Heat transfer characteristics

Cooling tower packings typically have quite complex surface geometries, for which the mass

transfer co-ecient, k, cannot be analytically predicted. Because

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