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Publication Title | On Thermal Performance of Seawater Cooling Towers

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Mostafa H. Sharqawy John H. Lienhard V1

e-mail: lienhard@mit.edu

Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307

Syed M. Zubair

Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

1 Introduction

Cooling towers are used in many applications to reject heat to the atmosphere. Heat rejection is accomplished within the tower by heat and mass transfer between the hot water droplets and ambient air. Seawater cooling towers have been used since the 1970s in facilities on the coast, as there is a potential to reduce freshwater consumption in power plants and other industries. In addition, the use of once-through cooling systems where hot water is rejected back into the sea caused many environmental prob- lems. Therefore, seawater cooling towers have been found to be a competitive alternative in which seawater is recycled in a closed- loop cooling system 1 . The salts in the water create a number of engineering challenges, including salt deposition, packing block- age, corrosion, potentially rising salt concentration, and salt emis- sions drift . Moreover, the salts in seawater change the thermo- physical properties with respect to freshwater, which in turn change the thermal performance of cooling towers.

The corrosion problems in seawater cooling towers can be avoided by appropriate selection of construction material and equipment. The use of plastic and asbestos for packing, pipes and water distribution system provided a practical and predictable so- lution for most of the corrosion problems. The use of exposed ferrous metal must be avoided, and if it is necessary to use metal for specific requirements, Monel or stainless steel should be se- lected. Coatings such as epoxy may also be used to cover special metal construction joints, or sometimes galvanized rebar is used in critical areas. More details and material selection for seawater cooling towers can be found in Walston 2 . Obviously, all of these special materials add to the capital cost of the tower, which is beyond the scope of this paper.

The thermal design and performance of cooling towers have been abundantly discussed in the literature. The first cooling tower theory was developed by Merkel 3 , and it included many ap- proximations. The major assumptions in Merkel’s model are the following: the water loss by evaporation is neglected; the Lewis factor is assumed to be unity; and the exit air is assumed to be

1Corresponding author.

Contributed by the Power Division of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received April 26, 2010; final manuscript received June 11, 2010; published online November 22, 2010. Editor: Dilip R. Ballal.

saturated. Sutherland 4 found that using the Merkel model can result in undersizing the tower between 5% and 15%. A more accurate model was developed by Poppe and Rögener 5 without using any of Merkel’s approximations. The cooling tower charac- teristics or Merkel number determined by Poppe’s approach is approximately 10% higher than the Merkel number determined by the Merkel model 6 . Knowing that the effect of seawater prop- erties on the cooling tower thermal performance may be small at lower salinities, it is intended in this paper to use an accurate cooling tower model that does not make any of the Merkel ap- proximations.

The thermal performance of seawater cooling towers has not been studied carefully in the literature. The available data are mostly in technical reports, feasibility studies, or design guidance 7,8 . A general discussion about the effect of seawater properties on the thermal performance was given by Nelson 9 and Warner 10 . However, no detailed performance calculation was made. As a rule of thumb, cooling tower vendors recommend degrading the tower performance by approximately 1% for every 10,000 ppm of salts in the cooling water. In practice, most engineering contrac- tors specify a 0.55–1.1°C margin on the wet bulb temperature to account for salts in the cooling water 8 . The objective of this paper is to investigate the thermal performance of seawater cool- ing towers by using a detailed model and to provide a correction factor CF that relates the performance of the seawater to that of freshwater cooling tower that has the same size and operating conditions.

On Thermal Performance of

Seawater Cooling Towers

Seawater cooling towers have been used since the 1970s in power generation and other industries, so as to reduce the consumption of freshwater. The salts in seawater are known to create a number of operational problems, including salt deposition, packing blockage, corrosion, and certain environmental impacts from salt drift and blowdown return. In addition, the salinity of seawater affects the thermophysical properties that govern the thermal performance of cooling towers, including vapor pressure, density, specific heat, viscosity, thermal conductivity, and surface tension. In this paper, the ther- mal performance of seawater cooling towers is investigated using a detailed model of a counterflow wet cooling tower. The model takes into consideration the coupled heat and mass transfer processes and does not make any of the conventional Merkel approxima- tions. In addition, the model incorporates the most up-to-date seawater properties in the literature. The model governing equations are solved numerically, and its validity is checked against the available data in the literature. Based on the results of the model, a correction factor that characterizes the degradation of the cooling tower effectiveness as a function of seawater salinity and temperature approach is presented for performance evaluation purposes. DOI: 10.1115/1.4002159

2

Seawater Properties

The thermophysical properties of seawater are different from those of freshwater. This difference is sufficient to affect the heat and mass transfer processes in cooling towers. The literature con- tains many data for the properties of seawater, but only a few sources provide full coverage for all relevant thermophysical properties. A recent review and assessment of seawater properties was given by Sharqawy et al. 11 . The properties that most strongly affect the thermal performance of cooling tower are va- por pressure, density, and specific heat capacity. In addition, ther- mal conductivity, viscosity, and surface tension affect the heat and mass transfer coefficients within the packing. In this section, cor-

APRIL 2011, Vol. 133 / 043001-1 Downloaded 15 Dec 2010 to 212.26.1.29. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm

Journal of Engineering for Gas Turbines and Power

Copyright © 2011 by ASME

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