i

Arnold Schwarzenegger

Governor

Performance, Cost and Environmental

Effects of Salt Water Cooling Towers

Prepared For:

California Energy Commission

Public Interest Energy Research Program

Prepared By:

John S. Maulbetsch, Consultant

Michael N. DiFilippo, Consultant

PIER

FINAL

PROJECT

REPORT

June, 2007

CEC-500-2007-XXX
ii
iii

Table of Contents

Preface ii

Abstract v

Executive Summary 1

1. Introduction

1.1 Motivation

1.2 Objective and scope

1.3 Organization of report

2. Cooling System Basics

2.1 Closed-cycle wet cooling systems

2.2 Mass and heat balances

2.2.1 Evaporation rate

2.2.2 Blowdown rate

2.3 Effect of water properties

3. Cooling Tower Performance

3.1 Effect of salinity on cooling tower performance

3.2 Seawater correction factor

3.3 Rules of thumb

3.4 Calculated capability with saline water make-up

4. Experience with Salt Water Towers

4.1 High salinity operation

4.2 Existing installations

4.3 Environmental effects

4.3.1 Blowdown

4.3.2 Drift

4.3.3 Baseline determinations

4.3.4 Cooling tower emissions impacts

4.3.5 Environmental studies

4.4 Operating experience

5. Salt Water Cooling System Costs

5.1 General cost factors

5.2 Case study

6. Summary and Conclusions

6.1 Effect on cooling tower thermal performance

6.2 Effect on cooling system cost

6.3 Effect on environmental impact

6.4 Effect on operation and maintenance

Appendices
iv

A Properties of Sea Water

B Derivation of Cooling Tower Performance Characteristics

C Site Visit Reports
v

List of Figures

2-1 Schematic of closed-cycle wet cooling system

2-2 Closed-cycle wet cooling system mass and heat balance

3-1 Salt water correction factor

3-2 Effect of salinity on correction curves

List of Tables

3-1 Base (fresh water) case study results

3-2 Comparative performance---fresh vs. salt water make-up

4-1 Salt water tower installations

5-1 Tower design configurations

5-2 Cost impact of thermal performance variation

5-3 Tower cost comparisons

5-4 Cooling system component cost comparisons
vi

Abstract

This report contains information on the design, use, cost, operation and environmental effects of

salt and brackish water cooling towers in comparison to fresh water towers as theya re used for

power plant cooling. The topics covered are

− a listing of salt water tower installations in the US and abroad

− an analysis of performance and cost differences between fresh and salt water towers

− a review of operating and maintenance experience on a few operating salt water

towers at US power plants

− a review of environmental effects at sites with operating salt water towers.

The use of high salinity make-up water typically imposes a 4 to 8% performance penalty and a

35 to 50% cost penalty in comparison to fresh water towers of comparable cooling capability.

Environmental effects of particular pertinence to salt /brackish water tower are primarily

related to drift emissions and there largely confined to the near-field effects on plant buildings

and equipment.

Keywords:

Salt water cooling towers

Brackish water cooling towers

Recirculating wet cooling

Cooling tower drift

Cooling tower performance

Environmental effects of cooling towers
7

Executive Summary
8

1

Introduction

1.1 Motivation

The California Energy Commission (CEC) is increasingly interested in the use of high

salinity water as make-up to power plant cooling towers for two reasons. First, the

growing demands for both electricity and water in the State have created pressure to

consider the use of non-fresh water for power plant cooling at new and existing plants

designed for and operating with closed-cycle cooling. Second, there is beginning to be

discussion of requiring some coastal plants, currently operating with once-through

cooling using ocean water, to convert to closed-cycle cooling, presumably using ocean

water as make-up.

Potential issues associated with the choice of saline make-up include effects on tower

thermal performance and on the choice of materials of construction, both of which may

affect the cost of the tower. In addition, operating and maintenance (O&M) problems

may arise as well as potentially exacerbated environmental problems particularly from

the salinity of the drift. In light of this, the CEC wishes to obtain a better understanding

of the effects on cost, performance, O&M requirements and environmental impacts

encountered in the use of salt water cooling towers for power plant cooling.

1.2 Objective and Scope

The objective of this study is provide information on the design, use, and operation of salt

water cooling towers which will assist the CEC in evaluating alternative cooling systems

for power projects in California.

The study consists of five tasks.

− A survey of salt water tower installations in the US and abroad

− An analysis of performance and cost differences between fresh and salt water

towers

− A review of operating and maintenance experience on a few existing salt

water towers at US power plants

− A review of environmental effects at sites with operating salt water towers

− The preparation of a summary report on the results of the study
9

1.3 Organization of Report

The remaining sections of the report are organized as follows. Section 2 provides a brief

review of cooling tower design, operation and nomenclature for convenience of

reference. Section 3 analyzes performance differences between cooling towers using

high salinity make-up water and those using fresh water. Some simple rules of thumb

from the existing literature are presented and evaluated. Some example calculations are

presented for conditions relevant to California. Section 4 provides a list of seawater and

brackish water cooling towers in operation in the US and abroad and summarizes the

experience that some power plants have had with salt water cooling towers both in

operation and maintenance and in environmental effects. Section 5 presents the effect of

using high salinity water in cooling tower on cooling system cost. Finally, Section 6

summarizes the conclusions of the study.

Three appendices contain properties of seawater (Appendix A), a derivation of the

performance characteristics for wet cooling towers (Appendix B) and detailed reports on

site visits and interviews with plants at which seawater or brackish water towers are

currently in operation.

A brief Executive Summary is provided at the beginning of the report.
10

2

Cooling System Basics

2.1 Closed-cycle Wet Cooling System

Closed-cycle, or re-circulated, wet cooling systems are usually the cooling system of

choice for modern thermal power plants. The system, as shown schematically in Figure

2-1, consists of two major components. These are the steam condenser, usually a shelland-tube surface condenser and a wet cooling tower, usually a mechanical-draft tower

(recently more commonly of the counter-flow type, although the schematic shows a

cross-flow configuration).

Figure 2-1: Closed-cycle wet cooling system (Mitchell, Robert D., 1989)

Steam from the low pressure turbine exhaust flows into the condenser, where it is

condensed on the outer surface of tubes with cooling water running through them. The

condensed liquid is returned to the boiler to complete the power cycle.
11

The warmed cooling water leaving the condenser is sent to the cooling tower where it is

cooled by heat rejection to the atmosphere, primarily by evaporation, and then recirculated to the condenser inlet. Figure 2-2 identifies the quantities involved in the

cooling and water use processes and defines the important nomenclature.

Figure 2-2: Closed-cycle cooling system mass and heat balance

The hot water from the condenser is introduced at the top of the tower and flows down

through a “fill” section where it is brought into intimate contact with ambient air flowing

counter to the direction of the falling water flow. Both sensible and latent heat transfer to

the air cools the bulk of the water, which is then collected in a basin and returned to the

condenser. The air leaves the tower, heated and humidified as an essentially saturated

exhaust plume.

The cooling is achieved by the evaporation of a small fraction (1 to 2%) of the

recirculating water flow. Therefore, once the system is filled, the only water withdrawn

from the environment is makeup water in amounts sufficient to replace that lost to

evaporation, blowdown,1and drift.2

1

 Blowdown is water discharged from the cooling system in order to control the buildup of dissolved and

suspended materials that concentrate in the system as a result of the evaporation.

CONDENSER

WET

COOLING

TOWER

W circ.

(lbm/hr)

Condensate

return

Steam

Wst(lbm/hr)

Hot water temp

Th(oF)

Cold water temp

Tc(oF)

Make-up

w m u (lbm/hr)

Blowdown

w bd (lbm/hr)

Inlet air

w air (lbm/hr)

Tw b (oF)

Plume w evap (lbm/hr)

w drift (lbm/hr)

pb(in Hga)

Tcond (oF)
12

2.2 Mass and Heat Balances

The elements of complete mass and heat balances are shown in Figure 2-2. The water

balance on the cooling tower is given by

wmu = wevap + wbd + wd (Eq. 2.1)

The drift rate, wd, is typically less than 0.002% of the circulating water rate (now usually

specified at 0.0005% or less) as compared to the evaporation rate of 1 to 2% and the

blowdown rate of 0.1 to 1%. Therefore, the drift rate is neglected in the following

calculations.

2.2.1 Evaporation Rate

The rate of evaporation of water from the tower is related to the heat load on the tower,

Qtower, which is equal to the heat load on the condenser, Qcond, given by

Qtower = Qcond = wcirc * cp * (Th – Tc) (Eq. 2.2)

with the evaporation rate given by

wevap = Qtower * flatent/hfg (Eq. 2.3)

where

hfg = latent heat of vaporization (Btu/lbm); ~1000 Btu/lbm

flatent = fraction of total heat rejected by latent heat transfer (0.9 is used here. It

can be lower depending on ambient conditions and design choice)

For an older steam plant, as might be considered for retro-fit to closed-cycle cooling, a

typical plant efficiency might be 35%. Of the 65% waste heat, perhaps 15% would be

dissipated in the stack gases and 50% in the condenser cooling water. The heat duty for

the cooling system would be approximately 5,000,000 Btu/hour per MW of electric

power generated. The values would differ slightly for the steam portion of combinedcycle plants, but these will be used for illustrative purposes. Therefore, the cooling

system heat duty per MW

Qtower = 5 x 106 Btu/hr per MW (Eq. 2-4)

wevap = 5 x 106x 0.9/1000 = 4,500 lbm/hr per MW or (Eq.2-4a)

wevap ~ 10 gpm/MW (Eq.2-4b)

2.2.2 Blowdown Rate

Blowdown rates are set to control scaling, fouling, and corrosion by limiting the buildup

of impurities in the circulating water. This criterion is normally expressed in terms of

maximum allowable cycles of concentration (n), defined as the ratio of the concentration

of conserved species in the circulating water (Ci circ) to that in the makeup water (Ci mu):

2

 Drift refers to liquid water droplets entrained in the tower exit plume and released to the atmosphere.
13

n = Ci circ/Ci mu (Eq. 2-5)

The mass balance of species i in the tower requires that

wmu × Ci mu = wbd × Ci circ (Eq. 2-6)

wbd = (wevap + wbd) × (Ci mu/Ci circ) = (wevap + wbd) x 1/n (Eq. 2-7)

Therefore,

wbd = wevap / (n ± 1) (Eq. 2-8)

Typical allowable cycles of concentration are from 3 to 6 (DiFilippo, M., 2003). For n

= 5 as a typical value, the required blowdown is

wbd = [1/(5 ± 1)] × wevap = 2.5 gpm/MWe (Eq. 2-9)

and the required make-up is

wmu = wevap + wbd = 12.5 gpm/MWe (Eq. 2-10)

Additionally, typical, consistent values of tower operating conditions are

 Circulating water flow rate, wcirc: ~ 500 gpm/MWe

 Condenser terminal temperature difference (TTD), Tcond – Th: 6 to 8oF

 Tower range, Th – Tc: 20 to 24oF

 Tower approach, Tc - Tamb. wet bulb: 8 to 12oF

Therefore, the achievable steam condensing temperature is given by

Tcond = Tamb. wet bulb + Approach + Range + TTD (Eq. 2-11)

For an ambient wet bulb temperature of 70oF, values in the typical ranges of TTD = 7oF,

Range = 22 oF, and Approach = 10 oF would provide a condensing temperature of 109oF,

corresponding to a turbine back pressure of 2.5 in. Hga. Tower approach temperature

depends on design ambient conditions as well as many other factors including tower type,

size, fill choice, and air flow. In general, warmer, more humid conditions lead to lower

approach temperatures, as in the southeastern U.S. and cooler, drier climates lead to

higher ones, as in the northern and western regions.

2.3 Effect of water properties

The properties of high salinity water differ from those of fresh water and those

differences lead to slightly altered cooling tower perforamnce. For purposes of this

report, high salinity water refers to seawater, concentrated sea water and to lower salinity

or brackish waters found in bays or estuaries or as groundwater. The chemical

composition of “normal seawater” is given in Table A-1 of Appendix A. Other tables in

the Appendix provide values of several thermo-physical properties important to the

performance of cooling towers for seawater of doubled concentration and of fresh water.

(Office of Saline Water, 1959)
14

The derivation in Appendix B provides an fundamental discussion to aid in the

understanding of the effect of these property differences on tower performance. The

effects of specific properties are summarized here.

Vapor pressure

The presence of salts in water reduces the vapor pressure at any given temperature as

seen in Figure A-2a and A-2b in Appendix A. This reduces the driving force for

evaporation and, in effect, lowers the “enthalpy of saturated air at the local water

temperature” which lowers the driving force. Therefore, a higher KaV/L will be required

to transfer the same amount of heat.

Surface tension

The surface tension for seawater is somewhat higher than for fresh water. (Figure A-3;

Appendix A) This tends to facilitate the breakup of the film, create smaller droplets and

slightly increase the interfacial area per unit of fill volume, a. This would slightly

enhance the performance with seawater compared to fresh water. The performance of

drift eliminators is also dependent on surface tension. In general, a reduced surface

tension will degrade the performance of drift eliminators.

Dynamic viscosity

The dynamic viscosity of seawater is slightly higher than for freshwater (Figure A-4;

Appendix A). This may result in slightly thicker films on the fill and a minor reduction

in performance compared to fresh water.

Thermal conductivity

The thermal conductivity of seawater is lower than that of fresh water for the temperature range of relevance to cooling tower

operation (Figure A-5 Appendix A). This may result in a lightly higher temperature drop through the liquid films which has been

assumed to be zero in the Merkel analysis. Therefore, a slight decrease in performance might result.

Density and specific heat

At a given temperature, the density of seawater is about 2 to 3% greater than fresh water,

while the specific heat is about 4% less. (Figures A-6 and A-7; Appendix A). Therefore,

a given volume of seawater has slightly less cooling capacity than fresh water. For a

given heat load, either the water flow or the range must be increased somewhat for a salt

water tower. These effects are all minor compared to the effect of vapor pressure.
15

Section 3 will provide the results of some tests, analyses and resulting rules of thumb for

adjusting the rating of freshwater cooling towers to account for the use of salt water as

the cooling medium.

Reference List

 1. DiFilippo, M., Use of Degraded Water Sources as Cooling Water in Power Plants,

EPRI and California Energy Commission, 1005359; 2003

 2. Mitchell, Robert D., Survey of Water-Conserving Heat Rejection Systems, EPRI,

EPRI GS-6252; 1989

 3. Office of Saline Water, "Normal" Seawater Chemistry, U. S. Department of the

Interior, 1959
16

3

Cooling Tower Performance

Effect of salinity on cooling tower performance

The thermal performance of a cooling tower on salt or brackish water will differ from

that of an identical tower under identical conditions cooling fresh water. As was

discussed in detail in Section 2, this results from variations in the thermodynamic and

transport properties of salt water as a function of salinity. Of these, the most important is

vapor pressure.

The following paragraphs review several approaches to accounting for the effect of

salinity on cooling tower performance. These include a simple correction factor and

some simple “rules of thumb” for adjusting the design specifications.

Seawater correction factor

A thorough discussion of the effect of salt water on cooling tower performance was

presented in a 1991 Marley publication. (Ting, Bing-Yuan and D. M. Suptic, 1991) The

approach recommended is to rate the cooling tower as if it were on fresh water and then

to apply a correction factor. Figure 6 of that reference is reproduced here as Figure 3-1

(with an estimated curve for a salinity of 50,000 ppm added) The correction factor is

plotted against L/G for a water salinity of level of two times seawater. The factor varies

from 7% at an L/G = 1 to just under 2.5% at an L/G = 5.5. The reason for the decrease in

the effect of salinity with increasing L/G can be seen by reference to Figure B-4 in

Appendix B. Towers for a given range and wet bulb temperature designed for high L/G’s

have much lower KaV/L and operate at much higher approach temperatures. The

magnitude of the driving force, (hsat @ Tw – hair), is correspondingly larger and the effect of

a given decrease on the vapor pressure at the water surface, hsat @ Tw, is also

correspondingly less.

Rules of thumb

Some rules of thumb have been put forward by the industry to estimate the reduction in

cooling tower capability (or alternatively, the increased tower size required to meet the

same heating load) incurred with salt water operation.
17

The simplest of these (Aull, R., 2005) suggests a 5% capability reduction for a salinity of

50,000 TDS. Similar estimates (Eftekharzadeh, S., M. Baasiri, and P. Lindahl, 2003)

suggest a performance loss of 5.4% at a salinity of 50,000 TDS. Reference is made in

that publication to a Fluor (Fluor R/D Division, 1957) paper which recommends

increasing the design wet bulb by 0.055 C (~ 0.1 F) for each 4,000 ppm of dissolved

solids. For sea water operating at 1.2 to 2. cycles of concentration, this corresponds to an

increase in the design wet bulb of 0.55 to 1.1 C ( ~ 1. to 2. F).

Figure 3-1: Salt Water Correction Factor (from (Ting, Bing-Yuan and D. M. Suptic, 1991)

Figure 3-1 gives the recommended correction for a 2x seawater salinity or approximately

70,000 ppm. Assuming a linear correction factor adjustment with salinity, the

corresponding line for 50,000 ppm would give a correction factor between 1.05 and 1.054

at approximately an L/G = 1 which seems appropriate for the conditions of interest to the

study. (Eftekharzadeh, S., M. Baasiri, and P. Lindahl, 2003)

Finally, using the rule of thumb suggested in the Fluor reference (Fluor R/D Division,

1957) of increasing the wet bulb temperature by 0.1 F for each 4,000 ppm of dissolved

solids, assume a 1.25 F increase for a TDS of 50,000 ppm.

Calculated capability with saline water make-up

Salt Water Correction Factor

(from Ting and Suptic)

CF = 0.0016(L/G)2 - 0.021(L/G) + 1.0894

1.020

1.030

1.040

1.050

1.060

1.070

1.080

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

L/G

Correction Factor

2x Seawater
18

The CTI Workbook (CTI, 2005) was used to calculate the capability correction implied

by the recommendation to increase the wet bulb temperature by 0.1 F for each 4,000 ppm

increase in circulating water salinity.

The calculation proceeds as follows. For a base (fresh water) case with design

specifications of

Wet Bulb Temperature: 75 F

Cold Water Temperature: 90 F

Hot Water Temperature: 120 F

the CTI Workbook, gives the following KaV/L’s and the corresponding fill coefficient,

C, for a range of L/G, as displayed in Table 3.1.

L/G KaV/L C

(n = -.7)

0.6 1.005 0.703

0.8 1.079 0.923

1 1.168 1.168

1.5 1.500 1.992

2 2.233 3.628

Table 3-1: Base (fresh water) case results

The fill is assumed to have a characteristic of

KaV/L = C * (L/G)-0.7 (Eq. 3-1)

The same computation is the performed for

§ an assumed wet bulb temperature of 76.25 F (increased

by 1.25 F per recommendation)

§ the same fill characteristics.

The L/G at which the same cold water temperature can be obtained is determined and

compared to the fresh water values in Table 3-1. The ratio of {(L/G)salt/(L/G)fresh} is the

corresponding capability correction factor. This approach neglects the minor affects of

variation in all thermo-physical properties with salinity and attempts to simulate the

effect of the reduced vapor pressure with an increased wet bulb temperature. The results

are tabulated in Table 3-2 and plotted on the same coordinates as Figure 3-1 for

comparison to the published correction factor and show reasonable agreement. This

suggests that for the usual range of circulating water salinities, operating temperature

ranges and design points, the correction curve and the recommended elevation of the wet

bulb temperature are consistent.
19

L/G (Fresh) C L/G (Saline) Ratio L/G

0.6 0.671 0.57 1.053

0.8 0.88 0.7625 1.049

1 1.11025 0.955 1.047

1.5 1.8725 1.442 1.040

2 3.32 1.936 1.033

Table 3-2: Comparative performance---fresh vs. salt water make-up

Figure 3-2: Effect of salinity on correction curve

Therefore, on the bases of literature recommendations and a simple computational correction, a capability

reduction of approximately 5% appears to be reasonable for typical conditions. This additional capability

can be provided in a variety of ways including

§ increased cell plan area (width or length)

§ additional cell or cells

§ increased air flow (fan power)

the effect on design, materials selection and cost will be presented in Section 5.

Reference List

 1. Aull, R., Personal Communication, 2005

Salt Water Correction Factor

1.010

1.020

1.030

1.040

1.050

1.060

1.070

1.080

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

L/G

Correction Factor

2x Seawater 50,000 ppm CTI Calculations for 50,000 ppm

 from Ting and Suptic

Linear correction from 2x

seawater curve
20

 2. CTI, CTI ToolKit, Version 3.0b, Cooling Technology Institute, 2005

 3. Eftekharzadeh, S., M. Baasiri, and P. Lindahl, Feasibility of Seawater Cooling

Towers for Large-Scale Petrochemical Development, Cooling Technology Institute,

TP03-17; 2003

 4. Fluor R/D Division, Impact of Salinity on Corrosion, Fluor Corporation, 1957

 5. Ting, Bing-Yuan and D. M. Suptic, The Use of Cooling Towers for Salt Water Heat

Rejection, The Marley Cooling Tower Company, 1991
21

4

Experience with Salt Water Towers

4.1 High salinity operation

The use of cooling towers on high salinity make-up water is not new. There are may such installations in

the U.S. and abroad, at both power plants and industrial facilities.

The make-up waters range from seawater with a nominal salinity of around 35,000 ppm (mg/kg) (See

Table A-1 in Appendix A) to brackish waters with salinities ranging from a few hundred (or a few

thousand) ppm in the case of groundwater or from a few thousand ppm to nearly full seawater

concentration in the case of bays or estuaries. Here the salinity can vary significantly on a time-scale

ranging from seasonal to hourly under the influence of rainfall, storms and tides.

The salinity of the circulating water in the cooling system is that of the make-up water times the cycles of

concentration, which is controlled by the rate of blowdown from the system. (See Eq. 2-5) Typically

towers with high salinity make-up (approaching seawater salt content) are operated at low cycles of

concentration in the range of x 1.5 to x 2. In the case of seawater this results in a circulating water

concentration of 50,000 to 70,000 ppm. While there have been some examples of operating a tower at

100,000 ppm, this is rare. Some inland towers, operating on limited supplies of brackish groundwater or

reclaimed water of perhaps 500 to 2,000 ppm at 5 to 10 cycles of concentration to conserve water and

reduce the cost of evaporation or crystallization of the blowdown results in circulating water concentrations

of 25,000 to 50,000 ppm.

4.2 Existing installations

The following table (Table 4-1) gives examples of existing towers operating on salt or brackish water. The

list is not complete but is based on information from two major tower vendors (GEA and SPX-Marley) plus

a variety of other sources and personal contacts.

The table attempts to provide information on tower size, operating point, materials of construction and

source water. In many instances, this information was not available or easily obtained. The essential

message from the accumulated information, however, is that towers of a wide range of size, application,

materials of construction and make-up water quality have designed, installed and operated for several

decades.
22

Table 4-1: Salt Water Tower Installations
23

Table 4-1: Salt Water Tower Installations (cont.)
24

Table 4-1: Salt Water Tower Installations (cont.)
25

4.3 Environmental Effects

All closed cycle wet cooling systems with cooling towers have some environmental effects. Those

normally considered are

§ Blowdown

§ Drift

§ Noise

§ Visible plumes

Of these, noise and visible plumes from salt/brackish water towers are no different from those from fresh

water towers. As such, they will not be discussed further in this study.

The effects from blowdown and drift differ from those from fresh water towers to the extent that the

salinity of the circulating water is higher.

4.3.1 Blowdown

The environmental issues related to the discharge or treatment of blowdown from salt or brackish water

cooling towers appears to differ little from those for fresh water towers. However, it should be noted that

discharge options are limited.

i. High salinity (50,000 to 70,000 ppm) cannot be discharge to municipal water treatment

facilities.

ii. Treatment, volume reduction or on-site disposal of the blowdown steam would be

prohibitively expensive. For towers operating on seawater make-up at 2 cycles of

concentration, the blowdown rates are very high on the order of 10 gpm per MW or 5,000

gpm for a 500 MW steam plant. The cost of evaporation ponds, even if the plant were

located in an area with a high net annual evaporation rate, would be extremely high of the

order of a few hundred million dollars. The cost of evaporator/crystallizer systems,

frequently used on zero-discharge plants but at far lower input rates, would be equally

unacceptable.

iii. For plants discharging to the Nation’s waterways under an NPDES permit, the stringency of

the rules can vary from state to state and from site to site. For ocean discharges, at least

under the California Ocean Plan, the increased levels of dissolved solids do not appear to be

regulated with the exception of those specified toxic species of arsenic, copper, mercury,

silver and zinc. For power plants (and other high volume discharges) limitations on both

effluent concentrations and total mass discharges are imposed according to procedures and

formulas documented in the Plan.

A comparison with ocean desalination plants may be illuminating. Typical values for desalination plant

capacity range from 20 to 112,000 acre-feet per year and “recovery” (defined as gallons of fresh water per

gallon of seawater) from 15 to 50% depending on the technology used. Assuming a plant size of 100,000

acre-feet per year (~90 million gallons per day) and a recovery rate of 50%, the desalination plant would

have a discharge stream with a salt concentration of 70,000 ppm and a flow rate of about 620,000 gpm.

(California Coastal Commission, 1993)This would be roughly equivalent to the blowdown from seawater

cooling towers operating at two cycles of concentration at twelve 500 MW power plants operating at 2

cycles of concentration.

4.3.2 Drift

Elevated salinity in the circulating water could effect drift emissions and their subsequent environmental

effects in two ways.

First, if the modified properties of the water affected the performance of the drift eliminators themselves,

the amount of drift emitted could change. However, inquiries with a manufacturer of drift eliminators
26

revealed that “there should be no difference in the efficiency of drift eliminators, since the drift capture

efficiency of a drift eliminator section is a function of droplet size, air velocity and workmanship of

installation”.....” as long as the salt water concentration does not lower the water's surface tension

significantly”. Reference to Appendix A-5 indicates that the surface tension is slightly higher than for fresh

water, so for the same drift eliminators and the same quality of installation workmanship, the drift

emissions should be unchanged.

Second, the drift droplets have a higher salinity so the mass emission of salt increases while the emission of

the drift itself remains unchanged. This introduces two concerns. First, the amount of fine particulate

matter (PM10 and PM2.5)

3

 released into the atmosphere as the drift droplets evaporate will increase. Second,

the mass deposition of salt on neighboring soils, vegetation, buildings, vehicles and equipment will be

higher than for fresh water towers. The effect of near-field salt drift on equipment on the plant site will be

discussed in the following section.

4.3.2.1 PM10 emissions

Cooling tower drift is often discussed in the context of PM10 emissions. National ambient air quality

standards (NAAQS) limit PM10 concentrations to an annual arithmetic mean of 50 ȝg/m3(or 150 ȝg/m3in

any 24 hour period). (Environmental Protection Agency, 2006) In order to maintain these levels emissions

limits are placed on point sources; in this case, 100 tons per year. The agency has defined an “emissions

factor” for cooling tower drift emissions of 1.7 lb/103gallons of recirculating water flow. This is based on

the assumption that cooling tower drift eliminators limit drift to 0.02% of the recirculating water flow rate.

It is further assumed that all of the drift evaporates in the atmosphere and that all the dissolved and

suspended matter in the drift is released as PM10.

The simple application of this factor and that assumption to a wet cooling tower with seawater make-up

operating at 1.5 cycles of concentration (drift salinity of 52,500 ppm) and a circulating water flow of

250,000 gpm (typical for a 500 MW steam plant) gives an emission rate for PM10 of approximately 4,700

tons per year for an 80% capacity factor.

However, these assumptions have been challenged as unrealistically conservative in several analyses.

(Micheletti, Wayne, 2006;Reisman, J. and G. Frisbie, 2002)Even the EPA has characterized them as

“conservatively high”. (USEPA, 1995) Specifically,

i. Modern drift eliminators have drift rates of 0.002% to 0.0005%, a factor of x10 to x40

lower than the 0.002% assumed in the Agency’s “emission factor”;

ii. Calculations based on accepted drift drop size spectrum data (as for example in Figure

4-1) and plausible assumptions about the density and shape of the particles remaining

following evaporation of the drift droplets suggest that only a small fraction (less than

15%) of the residual particles will have an aerodynamic diameter of less than 10 ȝ.

(Micheletti, Wayne, 2006)

3

 Particles with a mean aerodynamic diameter less than or equal to 10 (or 2.5) microns
27

Figure 4-1: Drift droplet size spectrum (from (Micheletti, Wayne, 2006))

Using a value of 0.002% and a 15% PM10 fraction, the calculated PM10 emission rate for the 250,000 gpm

seawater tower is reduced to 70 tons per year. It should be recognized that these assumptions and resulting

calculations are uncertain. Drift rates are a function of the quality of installation and state of repair of drift

eliminators and can be higher in practice than they are under ideal conditions. In addition, the drop size

spectrum can change with different drift eliminator designs and with age and condition. Additionally, some

fraction of the drift will inevitably hit the ground or surrounding structures and the dissolved material in

those drops will not be released into the atmosphere. Only careful on-site measurements can give a

credible estimate of the PM10 emission rates from any cooling tower.

4.3.2.2 Salt deposition

The assessment of the impact of emissions from evaporative cooling systems must be done with an

understanding of the background conditions which exist at the site. For instance, in the case of a coastal

power plant, while cooling tower drift emissions will contribute to the ambient salt levels (i.e. deposition

and concentration), there is already natural salt in the atmosphere. It is also likely that native vegetation

has adapted to this environment and is more salt tolerant than natural species which are further inland.

Further, the presence of sea salt in the air may already be having some local effects on the surrounding

community, in the form of generalized corrosion or other effects.

Studies of ambient or baseline conditions associated with prospective cooling system operation and impacts

have been conducted in a number of situations. Some of these are cited in the table below.
28

Utility Site Proposed Cooling System

Atlantic City Electric B. L. England – Coal-Fired,

NJ (Wilber, K. R., 1975)

Natural Draft - Brackish Water

General Public Utilities Forked River Nuclear, NJ

(Shofner, F. M. et al., 1973)

Natural Draft – Brackish Water

Potomac Electric Chalk Point – Coal-Fired,

MD (Davis, E. A., 1979)

Natural Draft – Brackish Water

Pacific Gas & Electric Geysers Geothermal, CA

(PG&E, 1979)

Mechanical Draft – Boron and Salts

Oak Ridge National

Laboratories

K-25 Gaseous Diffusion

Plant, TN

Mechanical Draft - Chromates

Pacific Gas & Electric Pittsburg – Oil Fired, CA Mechanical Draft – Brackish Water

Florida Power Corp. Turkey Point Nuclear,

FL

Spray Systems, Mechanical Draft –

Sea Water

Table 4-??: Example Cooling Tower Environmental Impact Assessments

Beyond, cooling-system specific studies, a number of atmospheric or background assessments have been

conducted for the purpose of better understanding issues such as formation and role of condensation nuclei,

impacts on visibility, assessment of corrosion issues, etc. (Porter, et al., 2000; Wilber, K. R., 1974;

Woodcock, A. H., 1952; Woodcock, A. H., 1953; Boyce, S. G., 1954;Rossnecht, G. F. et al., 1973) Many,

but not all of the studies have focused on coastal environments, where naturally occurring salt is produced

from wind and wave action.

4.3.2.3 Cooling Tower Emissions Impacts

Given these levels of naturally-occurring salts in coastal environments, it is important to

document, on a site-specific basis, these background levels and contrast them against expected

contributions from cooling systems. Background characterization would include:

§ ambient concentration and deposition of salts that are expected in the cooling tower

circulating waters,

§ concurrent wind speed, wind direction and humidity history (Note that humidity is

important relative to the size and fate of hydroscopic aerosols like sea salts and

resultant condensation nuclei.)

Cooling tower operating profiles (continuous, intermittent, etc.) and cooling tower emissions (droplet size,

drift emissions rate, etc.) determined from source term measurements can be made.

Published studies indicate that background salt concentrations vary considerably from, for example, 100

μg/m3 with horizontal mass fluxes ranging from 500-1000 μg m-2 sec-1 in the Hawaiian Islands (Porter, et

al., 2000)to ambient salt concentration in near coastal environments in the southeastern United States of 2-

50 μg/m3 [5]. Those measurements were conducted under lighter winds (2-4 m/s) and shorter fetches than

typically occur around the Hawaii Islands

These values are contrasted against vertical fluxes (i.e. deposition rates) of salts modeled from mechanical

draft cooling towers of 50-500 μg m-2 sec-1 within 100 meters of the source. (Wilber, K. R. and R. O. Webb,

1983) Therefore, it is seen that drift from cooling systems can be more or less significant depending on

background circumstances.

4.3.2.4 Environmental effects

A number of environmental effects studies have been conducted by comparing deposition rates on

surrounding land before and after the installation and operation of salt or brackish water cooling towers.
29

Appendix B-1, which documents the site visit to St. Johns River Power Park, contains a brief description of

the results of pre- and post-operation field monitoring of deposition and uptake at plots in the vicinity of the

towers. The conclusions, summarized there, are repeated here.

” The conclusions, documented in detail in permitting reports are summarized as follows :

− Some increased NaCl concentration was found in deposition samples after Unit #2

began operation.

− No significant increases were seen in soil or vegetation samples.

− Vegetation at the site with highest deposition was apparently unaffected.

− No injury symptoms related to NaCl were observed on pasture grass or other

vegetation on or in the vicinity of the Power Park.”

These results are consistent with those reported elsewhere in numerous studies. The most extensive study

was the Chalk Point Cooling Tower Project conducted in the late 1070’s by the Maryland Power Plant

Research Program. (Davis, E. A., 1979)The results, summarized in both (EPA, 2000) and (Eftekharzadeh,

S., M. Baasiri, and P. Lindahl, 2003), found “no measurable increase in soil salts concentration in the

tobacco fields near the facility”

and no instances [including Chalk Point] where cooling tower operation has resulted in measurable

productivity losses in agricultural crops or measurable damage to ornamental vegetation.”

4.4 Operating Experience

Information on performance and on operating and maintenance experience was solicited through visits and

telephone interviews with the staff of plants equipped with seawater or brackish water towers. These

included:

 Plants visited

§ St. Johns River Power Park, Jacksonville, FL

§ Plant Smith, Lynn Haven, FL

§ Plant Crist, Pensacola, FL

§ Plant Watson, Gulfport, MS

Telephone interviews

§ Pittsburg Power Plant, Pittsburg, CA

§ Palo Verde Nuclear Generating Station, Palo Verde, AZ

§ GEA Integrated Cooling Technologies, Lakewood, CO

Complete visit and interview reports are found in Appendix B. The major conclusions are summarized

below.

1. Recent installations of mechanical cooling towers have used fiberglass

reinforced plastic (FRP) structure, PVC fill and corrosion-resistant fittings (high

grade stainless steel, silicon bronze (coated), titanium). Operating experience

has been excellent but long term (> 15 years) is lacking.

2. Nearly all plants with high salinity cooling towers, both natural and mechanical

draft, have encountered accelerated corrosion on unprotected metal surfaces on

buildings and equipment on the plant site near the towers.

3. Both mechanical and natural-draft tower structures and basins constructed of

concrete have experienced varying degrees of deterioration from exposure to salt
30

water. In some cases, very extensive repairs have been required. Chloride

resistant concrete and rebar is recommended for high salinity applications.

4. In some cases, the high salinity water was also high in suspended solid at certain

times. This often led to plugging of high efficiency full and deteriorated

performance. This effect is unrelated to the salinity levels.

5. It must be recognized that some high TDS waters may result from operation

with moderate TDS make-up and high cycles of concentration. This usually

results in water chemistry that is very different from sea or brackish waters and

may be high is scaling constituents. In these cases, more elaborate water

treatments such as scale inhibition or softening maybe required.

Reference List

 1. Boyce, S. G., The Salt Spray Community, 1954

 2. California Coastal Commission, Seawater Desalination in California--Key

Desalination Facts, 1993

 3. Davis, E. A., Environmental Assessment of Chalk Point Cooling Tower Drift and

Vapor Emissions, Johns Hopkins University, PPSP-CCCTP-28; 1979

 4. Eftekharzadeh, S., M. Baasiri, and P. Lindahl, Feasibility of Seawater Cooling

Towers for Large-Scale Petrochemical Development, Cooling Technology Institute,

TP03-17; 2003

 5. Environmental Protection Agency, National Ambient Air Quality Standards

(NAAQS), 2006

 6. EPA, 316(b) Phase II Technical Devlopment Document, Chapter 6, Environemtal

Protection Agency, 2000

 7. Micheletti, Wayne, Atmospheric Emissions from Power Plant Cooling Towers,

2006

 8. PG&E, Assessment of vegetation stress and damage near the Geysers Goethermal

Power Plant Units, Pacific Gas & Electric Co., 1979

 9. Porter, et al., Vertical and Horizontal Aerosol Scattering Fields over Bellows

Beach, Oahu during SEAS Experiment, School of Ocean and Earth Science,

Universtity of Hawaii, 2000

 10. Reisman, J. and G. Frisbie, Calculating Realistic PM10 Emissions from Cooling

Towers, 2002

 11. Rossnecht, G. F. et al., The Size Distribution and Inland Penetration of Sea Salt

Particles, 1973
31

 12. Shofner, F. M. et al., Ambient Sea Salt Measurements in the Forked River, NJ

Environs, 1973

 13. USEPA, Compilation of Air Pollutant Emission Factors (AP-42; Section 13.4 --Wet

Cooling Towers), Environmental Protection Agency, 1995

 14. Wilber, K. R., An Experimental Approach to the Evaluation of the Collection

Efficiencies of Meshes Using the Mechanism of Inertial Impaction, University of

Tennessee, 1974

 15. Wilber, K. R., Ambient salt concentration study for Atlantic City Electric

Company, B. L. England Station, 1975

 16. Wilber, K. R. and R. O. Webb, Environmental Effects of Cooling Towers, Atomic

Industrial Forum, 1983

 17. Woodcock, A. H., Atmospheric Salt Particles and Raindrops, 1952

 18. Woodcock, A. H., Salt Nuclei in the Marine Air as a Function of Altitude and Wind

Force, 1953
5

Salt Water Cooling System Costs

In comparison to cooling systems designed to operate on fresh water, salt or brackish water systems are

more expensive. This is the result of several factors.

§ The properties of sea water lead to reduced thermal capability for a given tower.

§ The higher levels of dissolved and suspended solids in salt or brackish water sources

may require the use of lower efficiency, “low-clog” fill.

§ The aggressive nature of the higher salinity water normally requires the use of more

corrosion resistant, and more expensive, materials for cooling system components

and hardware.

§ Under some circumstances, deterioration of tower or basin materials may occur

resulting in costly maintenance or repair.

§ The effects of salt drift deposition may result in higher costs for cleaning, protection

or repair of equipment, structures and surfaces in the vicinity of the tower.

Many of these costs are highly variable, site-specific and hence difficult to generalize. The following

sections will briefly review some available information on the relative cost of fresh vs. saline water cooling

systems.

5.1 General cost factors

In late 2001, Washington Group International conducted a brief study (WGI, 2001) to estimate the cost of

retrofitting plants with once-through cooling to closed-cycle cooling with mechanical draft cooling towers.

Both fresh and saline water installations were considered.

The cost estimates developed in this study included the entire cooling system with the exception of the

condenser which was assumed to be retained in the retrofit. The system components for which a cost

differential was identified were

§ Cooling tower

§ Circulating water pumps

§ Make-up water pump

§ Make-up “system”

As noted above, the use of saline make-up results in a larger tower, made more so by the use of low clog

fill, which is also constructed of more costly materials. In the WGI study, the precise composition of the

saline make-up water was not specified. However, it was stated that the saline water towers were operated

at 2 cycles of concentration (as opposed to 5 cycles of concentration for the fresh water towers). Assuming

standard sea water as make-up, this gives a circulating water salinity of approximately 70,000 mgl.

The analysis began by setting the freshwater tower conditions at WBT = 80, a 1 F recirculation allowance

giving an inlet wet bulb of 81 F, an approach of 7 F, a range of 12 F and a condenser terminal temperature

difference of 8 F. This corresponds to a condensing temperature of 108 F and a turbine exhaust pressure of

2.5 in Hga. It was not specified how the size and cost of the baseline freshwater tower designed to meet

this performance was determined other than to say that
− “structure costs assume a fiberglass mechanical draft tower with standard noise and drift

abatement and no plume abatement” and

− “costs were estimated by scaling cooling tower costs provided by cooling tower vendors for

fresh water make-up cooling towers”.

It was not stated whether the baseline costs were consistent with “low first cost” designs or “total evaluated

cost” designs. However, as will be seen in the case study discussed below the cost ratio is essentially

identical for both assumptions.

The salt water tower costs were adjusted in two ways: first, by adjusting the size of the tower to account

for the performance reduction caused by the circulating water properties using the saltwater correction

factor discussed in Section 3 (Figure 3.1); second, by increasing the cost of the tower to account for the

more costly materials of construction

The adjustment in tower size was set at 1.07 which, for seawater at 2 cycles of concentration, is consistent

with the “2x seawater” curve in Figure 3-2 for an L/G = 1. Although WGI did not specify the L/G, this

would be in a reasonable design range as discussed in Chapter 2.

The adjustment in tower materials cost was set at x 1.4. This gives an overall cost ratio between saline and

fresh water towers of x 1.5 (~ 1.07 x 1.4). (NB: It should be noted that while this procedure and the values

used were clearly described in the text of (WGI, 2001), there was an apparent error in the report tables

which caused the reported costs to differ in some cases from those that this procedure would yield.)

Case study

As part of the analysis for this study, former and current representatives of a cooling tower vendor (Wilber,

K., 2004)

were asked to estimate cooling tower costs for a given set of design specifications. These were:

§ Water flow rate, L: 100,000 gpm

§ Hot water temperature, Th: 120 F

§ Cold water temperature, Tc: 90 F

§ Wet bulb temperature, Twb: 75 F

This corresponds approximately to the heat load of a 200 MW steam plant.

A comparison was provided between a fresh water tower and a tower operating with seawater as make-up

and at 1.5 cycles of concentration, resulting in a circulating water salinity of 54,000 ppm. The two cases

considered were a “low first cost tower” and an “evaluated tower”. The evaluated tower is larger with a

higher capital cost but uses less pump and fan power. The capital costs and the operating costs are

balanced to arrive at an optimized lifetime cost. In this example, the current value of power was set at

$100/HP for the pump and fan power. Table 4-1 shows the design configuration for towers of both low

first cost and evaluated cost designs sized to account for the performance difference between fresh and

brackish water at 54,000 ppm.
Table 5-1: Tower Design Configurations

Again, the cost increase consisted of two parts---an adjustment to account for the reduced thermal

performance and another for the more costly materials of construction. Table 5.2 shows the cost

differentials attributable to the reduced performance.

Low First Cost Evaluated Cost Make-up Water Cost ($1,000) Impact (%) Cost ($1,000) Impact (%)

Fresh water 1,100 Base 1,400 Base

Brackish (54,000 ppm) 1,149.5 4.5% 1,498 7.0%

Table 5-2: Cost Impact of Thermal Performance Reduction

Reference to Figure 3-2 indicates that a correction factor of 1.045 would be consistent with the Marley

estimates for L/G’s in the range of 1. to 1.5. The higher correction factor for the evaluated cost tower

suggests that larger plan area per cell and an additional cell (in the case of the evaluated cost, brackish

water tower) resulted in a higher air flow and hence a lower L/G in spite of the presumably lower fan/pump

power for the evaluated cost design. While this cannot be confirmed with the information provided, a

correction factor of 1.07 is consistent with an L/G of 0.5 or less.

The additional cost for the more costly materials to provide the corrosion resistance required for salt water

operation is shown in Table 4-3. The comparison is between a “conventional” fresh water tower

constructed of Douglas fir with galvanized fittings and an FRP tower with silicon bronze hardware and

epoxy coating.

Low First Cost Evaluated Cost Item Douglas Fir FRP Douglas Fir FRP

“Base Tower” 1,100 1,287 1,400 1638

Increase for

salinity

-- 58 -- 115

Silicon Bronze

fittings

-- 112 -- 120

Epoxy coatings -- 28 -- 30

Total 1,100 1,485 1,903

% increase -- 35% 36%

Table 5-3: Tower Cost Comparisons

Several points are noteworthy.

1. The case study and the generalized cost factor analysis are roughly consistent and suggest a range

of 35% to 50% increase in cost for salt or brackish water towers compared to fresh water towers.

2. The difference in the two estimates is primarily in the differential costs of materials rather than the

cost differences attributable to performance reduction. This may be due, in part, to the fact that

No. of Cells Cell Width Cell Length Fill Depth Inlet Air Height Fan HP Fan Dia

# ft. ft. ft. ft. HP ft.

No. of Cells Cell Width Cell Length Fill Depth Inlet Air Height Fan HP Fan Dia

# ft. ft. ft. ft. HP ft.

Brackish Water Tower, 54000 ppm 9 42 54 4 13 200 30

5.5 9 100 28 Fresh Water Tower

Douglas fir, galvanized hardware 8 42 54

12 200 28

8 42 42 4 13 200 28

8 36 42 5

Fresh Water Tower

Douglas fir, galvanized hardware

Brackish Water Tower, 54000 ppm

Low Cost Design

Evaluated Cost Design
the two estimates were done at different times; 2001, in the case of the WGI report and 2004 for

the case study. Significant variations in the costs of lumber, plastics, and high grade metal parts

have occurred over the past few years which may have altered the cost ratios. In any case, the

determination of a 35% to 50% range is considered adequate for a general survey of this type.

3. The cost analyses discussed above included only the cooling tower itself. Other components of

wet recirculating systems are also affected by make-up water salinity as noted in Section 5.1.

Table 5-4 lists the cost ratios for other major components used in the WGI study. While the

cooing tower is typically the largest cost component of the system, these differences can be sizable

for large installations.

Table 5-4: Cooling System Component Cost Comparisons (from (WGI, 2001))

Reference List

 1. WGI, Estimated Cost of Compliance with EPA Proposed Rule 316(b), Washington

Group International, Project 25258; 2001

 2. Wilber, K., Personal Communication, 2004

Item Fresh water Salt water Cost ratio

Cooling tower $24/TU $33.6/TU 1.4

Circ. water pump $130 - $260/BHP $210 - $416/BHP 1.6

Make-up water pump $337/BHP $539/BHP 1.6

Make-up system $150/gpm $200/gpm 1.3
6

Summary and Conclusions

The use of seawater, brackish estuarine or ground water or high salinity reclaimed water

for cooling tower make-up is an attractive alternative to the use of fresh water in some

locations. Table 2-1 lists over 50 seawater or brackish water towers currently installed in

the United States and over 90 worldwide. This report reviews the effect of using nonfresh water as cooling tower make-up on thermal performance, cost, environmental

impact and operation and maintenance.

6.1 Effect on thermal performance

Several of the thermo-physical properties of high salinity water differ from those of fresh

water. These include

- density

- specific heat

- surface tension

- thermal conductivity

- vapor pressure

- viscosity.

Of these the most important in its effect on cooling tower performance is vapor pressure.

The volumetric heat capacity (the product of density times specific heat) can affect the

optimum design and operating point. Thermal conductivity, viscosity and surface tension

play minor roles in the thermal performance of cooling towers; surface tension can affect

the effectiveness of drift eliminators.

The presence of salts in water reduces the vapor pressure at any given temperature as

seen in Figure A.2 in Appendix A. This reduces the driving force for evaporation and

the accompanying latent heat transport. Therefore, a higher fill volume or fill transfer

coefficient will be required to transfer the same amount of heat, requiring a larger and

more costly cooling tower. Alternatively, a tower of the same size and fill configuration

will cool less water to the desired cold water temperature or deliver a higher cold water

temperature.

For circulating water salinities, given by the salinity of the make-up water multiplied by

the tower cycles of concentration, of x 1.5 to x 2 that of seawater (~50,000 to 70,000

ppm) the thermal capability of a high salinity tower will be reduced by 4 to 8% below

that of a fresh water tower at the same operating conditions. A recommended rule-of-
thumb for specifying towers is to increase the design ambient wet bulb temperature by

0.055 C (0.1 F) for each 4,000 ppm of circulating water salinity.

6.2 Effect on cooling system cost

In comparison to cooling systems designed to operate on fresh water, salt or brackish

water systems are more expensive. This is the result of several factors.

§ The properties of sea water lead to reduced thermal capability requiring a

larger and more expensive tower.

§ The higher levels of dissolved and suspended solids in salt or brackish water

sources may require the use of lower efficiency, “low-clog” fill again

requiring a larger tower.

§ The aggressive nature of the higher salinity water normally requires the use of

more corrosion resistant, and more expensive, materials for cooling system

components and hardware.

§ Under some circumstances, deterioration of tower or basin materials may

occur resulting in costly maintenance or repair.

§ The effects of salt drift deposition may result in higher costs for cleaning,

protection or repair of equipment, structures and surfaces in the vicinity of the

tower.

Cost estimates were obtained from a study by an experienced A&E firm using

generalized cost adjustment factors and from a case study for a particular set of design

specifications.

4. The case study and the generalized cost factor analysis are roughly consistent and

suggest a range of 35% to 50% increase in cost for salt or brackish water towers

compared to fresh water towers.

5. The difference in the two estimates is primarily in the differential costs of

materials rather than the cost differences attributable to performance reduction.

This may be due, in part, to the fact that the two estimates were done at different

times nearly three years apart.

6. The differential cost estimates apply only to the cooling tower itself. Other

components of wet recirculating systems are also affected by make-up water

salinity.

6.3 Effect on environmental impact

A number of environmental effects studies have been conducted by comparing deposition

rates on surrounding land before and after the installation and operation of salt or

brackish water cooling towers.

All studies reach essentially similar conclusions that no significant increase in salt

concentrations in soils or vegetation nor any symptoms of environmental injury were

observed in the vicinity of the plants.
6.4 Effect on operation and maintenance

Plant visits were made to four plants and telephone interviews were held with two

additional plants and one cooling tower vendor.

The major conclusions obtained from these visits and interviews are:

1. Recent installations of mechanical-draft cooling towers operating

on high salinity make-up water using fiberglass reinforced plastic

(FRP) structure, PVC fill and corrosion-resistant fittings (high

grade stainless steel, silicon bronze (coated), titanium) have

operated satisfactorily with no extraordinary O&M problems.

2. Operating experience has been excellent but long term (> 15 years)

is lacking.

3. Nearly all plants with high salinity cooling towers, both natural

and mechanical draft, have encountered accelerated corrosion on

unprotected metal surfaces on buildings and equipment on the

plant site near the towers.

4. Both mechanical and natural-draft tower structures and basins

constructed of concrete have experienced varying degrees of

deterioration from exposure to salt water.
Appendix A

Properties of Sea Water

A-1: Composition

A-2: Vapor Pressure

A-3: Density

A-4: Specific Heat

A-5: Surface Tension

A-6: Viscosity

A-7: Thermal Conductivity
Table A-1: Composition of “Normal“ Seawater

Na 10,556 mg/kg (2)

K 380 mg/kg

Ca 400 mg/kg

Mg 1,272 mg/kg

HCO3 140 mg/kg

Cl 18,980 mg/kg

Br 65 mg/kg

SO4 2,649 mg/kg

SiO2 <10 mg/kg

Other 31 mg/kg

Total 34,483 mg/kg

Notes.....

1. Only major ions shown.

2. These are true "ppm" units and are approximately the same as mg/l.

"Normal" Seawater Chemistry (1)

US Department of Interior, Office of Saline Water, July 1959
Vapor Pressure

Correlating Equations for Vapor Pressure

Fresh water

p = 8.0439E-09T4 – 6.5750E-07T3 + 8.5848E-05T2 – 1.0248E-03T + 4.6622E-02

Seawater (TDS ~ 35,000)

p = 1.0511E-08 T4 – 1.7569E-06 T3 + 2.3539E-04 T2 – 9.1406E-03T + 1.9585E-01

2x Seawater (TDS ~ 70,000)

p = 3.9773E-09 T4 – 2.1465E-08 T3 + 6.6345E-05 T2 – 2.1540E-03T + 8.8876E-02
Figure A-2a: Vapor pressure (80 to 100 F)

Vapor Pressure Comparison - 80F to 100F

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

80 90 100

Temperature, deg F

Vapor Pressure, psi

Fresh water

Seawater

2x Seawater
Figure A-2b: Vapor pressure (100 to 140 F)

Surface Tension

Vapor Pressure Comparison - 100F to 140F

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

100 110 120 130 140

Temperature, deg F

Vapor Pressure, psi

Fresh Water

Seawater

2x Seawater
Correlating Equations for Surface Tension

Fresh water

ı = -6.2500E-05T2 - 7.7607E-02T + 7.8297E+01

Seawater (TDS ~ 35,000)

ı = -8.0357E-05T2 - 7.4107E-02T + 7.8914E+01

2x Seawater (TDS ~ 70,000)

ı = -4.4643E-05T2 - 8.0679E-02T + 7.9991E+01

Surface Tension - 40F to 140F

64.00

66.00

68.00

70.00

72.00

74.00

76.00

78.00

20 40 60 80 100 120 140 160

Temperature, F

Surface Tension, dynes/cm

Pure Water

1x Seaw ater

2x Seaw ater

Figure A-3: Surface tension (40 to 140 F)
Viscosity

Correlating Equations for Viscosity

Fresh water

ȝ = 1.3063E-04T2 - 4.4363E-02T + 4.7808

Seawater (TDS ~ 35,000)

ȝ = 1.3688E-04 T2 - 4.6918E-02T + 5.1116

2x Seawater (TDS ~ 70,000)

ȝ = 1.4313E-04 T2 - 4.9283E-02T + 5.4285

Figure A-4: Dynamic viscosity (40 to 140 F)

Viscosity - 40F to 140F

1.00

1.50

2.00

2.50

3.00

3.50

4.00

20 40 60 80 100 120 140 160

Temperature, F

Viscosity, lb/ft-hr

Pure Water

1X Seaw ater

2X Seaw ater
Thermal Conductivity
Figure A-5: Thermal conductivity
Density

Correlating Equations for Density

Fresh water

ȡ = -2.1661E-08T3 - 7.7583E-06T2 + 6.7946E-04T + 8.3283

Seawater (TDS ~ 35,000)

ȡ = -4.4870E-08 T3 + 1.1936E-06 T2 - 4.8336E-04T + 8.5780

2x Seawater (TDS ~ 70,000)

ȡ = -4.6417E-08 T3 + 3.9388E-06 T2 - 1.1106E-03T + 8.8145

Density - 40F to 140F

8.10

8.20

8.30

8.40

8.50

8.60

8.70

8.80

8.90

20 40 60 80 100 120 140 160

Temperature, F

Density, #/gal

Pure Water

1x Seawater

2x Seawater

Figure A-6: Density (40 to 140F)
Specific Heat

Correlating Equations for Specific Heat

Fresh water

cp = 1.6927E-10T4 - 7.2627E-08T3 + 1.2276E-05T2 - 9.5425E-04T + 1.0269

Seawater (TDS ~ 35,000)

cp = 1.3021E-11 T4 - 4.5718E-09 T3 + 6.5104E-07T2+ 1.5847E-05T + 9.5287E-01

2x Seawater (TDS ~ 70,000)

cp = 3.7037E-09 T3 - 1.4196E-06 T2 + 2.7834E-04T + 9.0270E-01
Heat Capacity - 40F to 140F

0.90

0.92

0.94

0.96

0.98

1.00

1.02

20 40 60 80 100 120 140 160

Temperature, F

Heat Capacity, BTU/#-F

Pure Water

1x Seawater

2x Seawater

Figure A-7: Comparison of Specific Heat (from Perry¶s [???]
Appendix B

Derivation of Cooling Tower Performance

Characteristics
Tower Performance Characteristics

The cooling of the water is achieved through the combined processes of heat and mass

transfer from the hot water to the air as they pass through the tower fill. A brief

description of the process will assist in understanding how the properties of the water,

and in particular how the differences between the thermo-physical properties of salt or

brackish water and those of fresh water affect cooling tower performance.

As indicated in Figure B-1, the hot water enters the tower at the top and falls under

gravity down through the tower fill and into the cold water basin. The air is drawn into

the tower at the bottom and flows upward, counter to the downflow of the water, and

exits through the fan to the atmosphere.

Figure B-1: Closed-cycle cooling system mass and heat balance

The purpose of the fill is both to slow the water as it descends through the fill and to

create a large interfacial area between the water and the air, providing more time and a

larger area for the transfer processes to take place. Fills are of two general types; film fill

and splash fill. The film fills cause the water to flow in narrow films on the fill surfaces.

The fill geometry is designed to and by geometric disrupt and mix the films by causing

them to turn or reform as they flow downwards. Splash fills create small droplets as the

falling water impacts the fill surfaces creating fresh surface for heat and mass transfer.

The determination of the tower performance characteristics required for a given cooling

load proceeds as follows:

The heat load which must be rejected by the cooling tower to cool a given water flow, L,

from the hot water temperature, Th, to the desired cold water temperature, Tc, is given by

CONDENSER

WET

COOLING

TOWER

W circ.(lbm/hr)

Condensate

return

Steam

Wst(lbm/hr)

Hot water temp

Th(oF)

Cold water temp

Tc(oF)

Make-up

w m u (lbm/hr)

Blowdown

w bd (lbm/hr)

Inlet air

w air (lbm/hr)

Tw b (oF)

Plume w evap (lbm/hr)

w drift (lbm/hr)

pb(in Hga)

Tcond (oF)
Qw = L * cp * (Th - Tc) (Eq. B-1)

The heat transferred to the air equals that from the water and is given by

Qa = G * (hairex – hairamb) (Eq. B-2)

where

G = air flow rate (lb/hr)

h = enthalpy of air stream per lb of dry air (Btu/lb)

The transfer of heat from the water to the air stream at each point on the air-water

interface within the tower is made up of

i) a sensible heat component, driven by the difference between the local water

surface temperature and the local free stream air temperature and

ii) a latent heat component, carried by a mass transfer of evaporating water

driven by the difference between the vapor pressure of water at the local water

temperature and the vapor pressure of the water vapor in the air stream.

Several critical assumptions simplify the analysis. These include

1. the water film is well mixed so the water surface temperature is the same as

the bulk water temperature

2. the air at the surface of the film is saturated at the water temperature

3. The physics of the heat and mass diffusion processes are essentially the same;

i.e., the Lewis number is equal to 1.

With these assumptions the total heat transfer per unit interfacial area can be accurately

represented with a combined transfer coefficient, K, and the difference between the

enthalpy of air saturated at the local water temperature and the enthalpy of the bulk air

stream.

Figure B-2: Incremental Heat Transfer Analysis

Tw hair + dhair

dQ

 L G dV

Tw - dTw hair
From Eq. (B-1) and Figure B-2,

KadV(hsat @ Tw – hair) = LdTw (Eq. B-3)

Integrating across the fill volume from 0 to V, yields

KaV/L = ∫

−

h

c

w

T

T

sat T air

w

h h

dT

@

 (Eq. B-4)

The integration can be understood graphically in Figure B-3.

Figure B-3: Cooling tower operating curves

The driving force for the combined heat transfer is the distance from the line A-B which

represents the water operating line between Th and Tc and also the enthalpy of saturated

moist air. The line C-D represents the air operating line running from an inlet enthalpy

consistent with the ambient wet bulb temperature and increasing linearly with a slope of

L/G to an exit enthalpy consistent with the overall heat balance.

Since the enthalpy of moist air is essentially a function only of the wet bulb temperature

independent of relative humidity, it is not possible from the foregoing analysis to

determine the exit state of the air. A further assumption by Merkel (Ref. ??) states that

the exit air is saturated at the prescribed exit enthalpy and corresponds to some

temperature intermediate between the water hot and cold temperatures.

hex = hamb + L/G * (Th – Tc) Eq. (B-5)

E

n

t

h

Temperature
This suffices to specify the complete state of the exit air and to determine the evaporation

rate from the cooling tower from

wevap = G * (Wex – Wamb) Eq. (B-6)

where

W = specific humidity in lb moisture/lb of dry air

Based on the foregoing analysis, knowing the ambient wet bulb, the inlet and outlet water

temperature, determines the required tower characteristic (KaV/L). This value is readily

generated by programs such as the CTI Workbook. (Ref. ???) A sample set of curves for

a given wet bulb and a given range is shown in Figure B-4 for a number of approach

temperatures as KaV/L vs. L/G.

The performance characteristics of many fills can be approximated by a polynomial

expression of the form

 KaV/L = C * (L/G)n(Eq. B-7)

Figure B-4 shows such a fill characteristic with nominal values of n = -0.7 and C = 1.5

superimposed on the performance curves of a tower with a 32.4 F range at an 80 F wet

bulb. The intersections of the characteristic line with the performance curves gives the

approach temperature which can be obtained for the given range and wet bulb

temperature for any L/G.
Figure B-4: Example Performance Curves and Fill Characteristic (from CTI Workbook

for specified range and wet bulb temperature)