MEMORANDUM

Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

phone	703-385-6000

fax	703-385-6007

TO:			Paul Shriner and Jan Matuszko, EPA

FROM:		John Sunda (SAIC) and Kelly Meadows

DATE: 		March 25, 2010

SUBJECT:		Water Balance, Flow Reduction, and Optimization of
Recirculating Wet Cooling Towers 

Introduction

Tetra Tech and SAIC were tasked with developing documentation to support
regulatory options that may require a facility to retrofit to
closed-cycle cooling.  This memo discusses the reduction in flow that
can be expected, ways to optimize those reductions, and estimated the
additional costs that will be incurred to optimize the cooling towers.

Estimation of Flow Components in Recirculating Wet Cooling Towers

Closed-cycle cooling systems significantly reduce the volume of cooling
water withdrawals.  However, there is still a need for some water to be
withdrawn; make-up water replaces losses to the system.  The water
balance in and out of a recirculating wet cooling tower is defined by
the following equation:

Make-up = Evaporation + Drift + Blowdown

The make-up flow volume is typically controlled by water level monitors
in the cooling tower basins which control the make-up water pumps to
ensure that a sufficient depth of water is maintained in the cooling
tower basin to allow for proper operation.  As such, the volume of the
make-up withdrawals is driven by the volume of the three components of
water exiting the recirculating water system (i.e., the right hand side
of the above equation).  Each of these is discussed in detail below.

Evaporation

The amount of evaporation is driven by the amount of heat that is
transferred to the cooling water in the exhaust steam condensers.  The
amount of this heat is a function of the power generation rate and the
efficiency of the boiler and the generating system.  In a typical
coal-fired steam boiler, 75% to 90% of the heat energy derived from the
fuel is converted to steam energy (Dbal et al 1995).  The component of
the steam energy that is not converted to electricity in the turbines
will exit the system through the cooling water system.  The heat exiting
through the cooling water system is transported to the towers through an
increase in the temperature of the cooling water passing through the
steam condenser.  For any given power plant output level, the
circulating cooling water flow rate will determine the temperature
difference between the condenser inlet and outlet. 

At the cooling tower end of this system, the heat that exits the system
leaves through a combination of evaporation of water and an increase in
the sensible temperature of the air passing through the system.  In
conventional wet cooling towers, the majority of the heat exits the
system through evaporation.  The evaporation rate can be estimated using
the following equation:

∆T ((F) x Evaporation Factor

Table 1 below shows evaporation factors provided by two technical
sources plus the value that is equivalent to a tower where 100% of the
heat exits the tower via evaporation.  While 100% evaporation is
possible, it would only occur when the ambient air has low humidity and
is as warm (or warmer) than the hot water entering the tower.  The
mechanical draft cooling towers engineering evaporation estimation
factors shown in Table 1 are equivalent to conditions where roughly 80%
of the heat exiting is via evaporation.  The actual proportion of heat
exiting the tower through evaporation at any given time will vary
depending on tower design, the amount of air flow, the water temperature
and meteorological conditions such as wet bulb temperature and dry bulb
temperature.  Differences in these factors will affect the evaporation
rate (and thus the make-up volume) by only a modest amount however,
since conventional towers are designed such that most of the heat exits
via evaporation.  Non-conventional designs such as hybrid plume
abatement towers will have a lower proportion of heat exiting through
evaporation and thus a lower make-up water volume.

Table 1

Wet Cooling Tower Evaporation Factors

Source	Evaporation Factor	Percent of Total Heat Exiting Tower As
Evaporation

Marley	0.0008	78%

NPCI	0.00085	82%

100% Evaporation	0.00103	100%



Drift

Drift (aka windage) is the small proportion of the recirculating cooling
water that leaves the tower as fine droplets entrained in the exiting
air flow.  Drift is typically estimated as a percentage of circulating
water flow and will vary with tower design.  Drift is different from
evaporation in that it is not responsible for removing heat from the
system, but can be responsible for removing some of the dissolved solids
that build up in the recirculating cooling water that otherwise would
exit through the blowdown.  A change in the drift rate at a tower
maintained at a specific cycle of concentration (see below) will affect
the blowdown volume but not the make-up water volume.

Table 2 below presents drift estimation rates for different types of
cooling tower designs without drift eliminators and for those with drift
eliminators.  

Table 2 

Cooling Tower Drift Factors

Tower Type	Drift Estimation Factor*

Natural Draft	0.3 to 1.0%

Mechanical Draft	0.1 to 0.3%

Mechanical Draft Tower with Drift Eliminator	0.005%

Mechanical Draft Tower with High Efficiency Drift Eliminator	0.0005%

* Drift (gpm) = Recirculation (gpm) X Factor

Source (Engineering Tips 2009)

Blowdown

Of the three components that comprise the make-up water volume
requirements, blowdown is the one component that is completely under the
control of the cooling tower operator.  The blowdown is the component
used to control/prevent the build-up in the dissolved and suspended
solids in the recirculating water.  Theoretically, it can range from 0%
to 100% of the condenser cooling water flow (minus drift and
evaporation).  The latter condition (i.e., 100%) describes a cooling
tower operating in the helper mode with no recirculation.  

Since nearly all make-up water sources will contain some dissolved
solids, a tower operating with 0% blowdown would experience a continuous
build-up of dissolved solids in the recirculating water as dissolved
solids are left behind by the evaporating make-up water.  This build-up
would increase until the concentration becomes so high that the amount
of solids lost in the drift equals the amount coming in through the
make-up water.  Usually before that happens, the build-up of dissolved
solids in the system will reach saturation concentrations for certain
components, resulting in the precipitation of solids as scale, suspended
solids, and sludge.  These solids can create an array of operational
problems including the plugging of piping and tower media, a reduction
in the efficiency of heat exchange surfaces, and an increase in metal
corrosion due to anaerobic conditions beneath the deposited layer of
solids.  Corrosion may also increase due to higher salt content of the
water.  

There are no upper limits for the blowdown volume, and some systems may
operate at relatively high blowdown rates in order to completely avoid
dissolved solids-related problems or to increase turbine efficiency by
providing supplemental cooling by operating the system in essentially a
partial once-through mode.  This provides cooler water to the condensers
whenever the source water temperature is below the tower effluent
temperature, which is most often the case.

For any given make-up water quality and tower system design, cooling
tower manufacturers and treatment chemical vendors can evaluate what
should be the acceptable upper operating limit for the concentration of
the mix of dissolved solids present.  The acceptable upper limit is not
fixed and can be adjusted upward through the use of treatment chemicals.
 Rather than defining such upper limits in the form of a maximum
concentration, this limit is often expressed as a multiple of the
make-up water concentration.  The ratio of the dissolved solids
concentration of the more concentrated recirculating water to the
dissolved solids concentration of the make-up water is called the cycle
of concentration (COC).  

The recommended maximum cycle of concentration is directly related to
the quality of the make-up water and can be modified by use of water
treatment chemicals.  A make-up water source with high dissolved solids
(e.g., saltwater) or high hardness will tend to result in the need to
operate at a lower COC, even with the use of treatment chemicals. 
Towers that operate at a COC near the upper operating limit have a
greater potential for increased operating and maintenance costs due to
higher chemical costs or increased equipment maintenance costs due to
the higher potential for corrosion and scale buildup.  

In general, cooling tower treatment chemical vendors will evaluate the
water chemistry and associated costs for various COC levels and will
select an optimal operating COC and treatment chemical dosage.  As the
COC is increased, there will be an increase in chemical dosage and at
the same time a decrease in the amount of chemicals lost in the
blowdown.  This optimal COC will usually involve financial and operating
considerations that strike a balance between the variations in treatment
chemical dosage and cost, the limits of the ability of treatment
chemicals to prevent problems associated with scale formation, corrosion
and biofouling, and the benefits of using less water.  In cases where
there are significant costs associated with obtaining source water and
disposing of blowdown, these costs may provide additional incentive to
set the optimal COC to higher values and thus reduced make-up water
volumes.  A cooling tower treatment chemical vendor indicated that most
power plants practice minimal treatment chemical use, often operating
well below the optimal COC, primarily because of the relatively low cost
of obtaining make-up water and disposal of blowdown. 

A general rule of thumb for most towers is that calcium hardness in the
recirculating water should be limited to 350 to 450 ppm unless treatment
chemicals are used.  Other limiting factors, such as silica, phosphate,
and process contaminants should be considered as well (Harfst, 2008). 
Saltwater can be concentrated to about 55,000 ppm TDS in cooling towers
with no serious scaling problems without pH adjustment (Nelson, 1986).
The ratio of this 55,000 ppm to a typical seawater concentration of
35,000 ppm results in a calculated COC of 1.57, and so a COC for full
strength seawater of 1.5 is a reasonable value to use for systems
requiring minimal use of treatment chemicals.  Estuarine and tidal
waterbodies that contain some combination of saltwater and freshwater or
brackish water will be able to operate at somewhat higher COC values. 
The recommended COC values for saltwater systems range from 1.5 to 2.0.

∆T) ranging from 10 to 30 (F.  The percent flow reduction values
represent the intake water flow reduction that would occur if this
recirculating system replaced a once-through cooling system with the
same condenser cooling water flow rate.  Table 3 below shows the percent
flow reduction values for the different values of COC and ∆T based on
the data in Appendix A. 

Table 3

Figure 1 presents a graph of the data in Table 3 for ∆T = 10, 20, and
30 (F.  This data shows that the change in percent reduction in make-up
water volume diminishes rapidly as the COC exceeds a value of about 5.0
and becomes nearly asymptotic at a value of 10.0.  The COC and ∆T are
primary factors driving the percent flow reduction of retrofit cooling
towers.

  

Chemical Requirements Costs of Operating at Higher Cycles of
Concentration

A major component of cooling tower and cooling system maintenance
involves measures taken to control biological growth, corrosion, and the
build-up of deposits on metal surfaces and tower media.  These deposits
can consist of biological growth, solids contained in the make-up water,
solids cleaned from the air, and solids that precipitate out of
solution.  The most damaging metal corrosion is the result of localized
pitting which occurs under surface deposits.  Additionally, deposits on
heat transfer surfaces result in a reduction of overall system power
generating efficiency and the growth of certain microorganisms such as
Legionella can create a human health hazard. 

Control of suspended solids in recirculating cooling water systems can
include pretreatment of make-up water as well as side-stream filtration
of a portion of the recirculating water.  However, such measures are
often cost prohibitive for large cooling systems.  In some locations,
dust entrained from the air can be a major source of solids in the
system.  Certain BMP provisions to control dust, such as better control
of ash piles and suppression of dust from roads and coal piles, may be
helpful.

Chemical treatment of cooling water generally serves the purposes of
prevention and control of one or more of the following:

Biological fouling

Scale formation

Solids deposition

Corrosion

Treatment for control of biological fouling is almost universally
practiced in both once-through and recirculating systems.  Biocides are
often injected on an intermittent basis, and so the amount used is not
as dependent on the cooling water system input and output flows as are
other chemicals that must be maintained at a constant dosage in the
recirculating system.  Thus, while some change in the amount or type of
biocide used may occur when converting from once-through to
recirculating cooling towers or when changing the cycles of
concentration of a cooling tower, the change should not significantly
alter treatment costs for biocides use.  For many power plants,
including those that currently use cooling towers, chemical treatment
with biocides such as chlorine (including possible dechlorination of the
effluent) is the only chemical addition that is associated with cooling
water use.  For many cooling towers, the operators generally operate the
towers at a COC low enough to avoid serious solids- and scale-related
problems.  Problems not related to biofouling are dealt with using a
non-chemical maintenance approach such as periodic system cleaning to
deal with solids and cathodic protection, and ultimately equipment
replacement to deal with corrosion. 

Recirculating systems operating at higher cycles of concentration will
develop correspondingly higher concentrations of the dissolved solids,
hardness, and alkalinity in the make-up water, which can result in
problems with respect to scale formation.  The formation of calcium
carbonate scale is the result of a reaction of calcium hardness with
alkalinity.  One of the simplest treatment methods for control of
calcium carbonate scale is the addition of acid (such as sulfuric or
hydrochloric acid) to the make-up water, which reduces the alkalinity. 
In this treatment method, enough acid is injected into the make-up water
to reduce the total alkalinity to 50 to 100 ppm and maintain the pH of
the cooling water within the range of 6.8 to 7.5 (Harfst, 2008). 
However, treatment with acid must be carefully controlled to prevent an
excessively low pH, which could lead to corrosion problems and involves
the risk of onsite storage of large quantities of a hazardous material.

Another common treatment chemical useful for preventing scale formation
is phosphonate.  Phosphonate controls scale by adsorption onto crystal
surfaces disrupting the crystalline structure of calcium carbonate.  A
typical dose of phosphonate for a cooling tower operating with a COC in
the range of 3-6 would be 2-4 ppm (Keister 2009).  The term “dose”
as used here is the concentration that must be maintained in the
recirculating water.  As such, the rate of chemical addition associated
with any dosage will be dependent on the rate of depletion, such as loss
through blowdown, drift, and degradation.

A common treatment chemical useful in controlling solids deposition is
polyacrylate. Polyacrylate acts as a dispersant and is very effective at
controlling solids with particle sizes of 2 microns or less.  For
cooling towers operating with a COC in the range of 3-6, a typical dose
of polyacrylate would be 4-5 ppm (Keister 2009)

As a cooling tower’s water efficiency is increased by increasing the
cycles of concentration, a point will be reached where treatment
chemicals may become necessary, and the dosage will increase as the COC
increases beyond that point.  This may also result in an increase in
costs for more frequent monitoring of tower water quality and control of
blowdown rates.  However, the associated reduction in blowdown volume
will also result in a reduction in the rate of chemical lost from the
system.  Treatment chemical vendors are able to evaluate the variation
in chemical costs associated with these tradeoffs and recommend the
optimum level that minimizes chemical and other operating costs while
also minimizing risks.  One treatment chemical vendor said that the
optimum level for most industrial cooling towers is at a COC ranging
between 3 and 6, with the lower end being applicable to systems with
high hardness make-up water (Keister 2009). 

Estimated Costs for Modifications to Operate Cooling Towers at Higher
Cycles of Concentration

Operating at higher COCs in the range of 3 to 6 or higher for freshwater
systems and 1.5 to 2.0 for saltwater systems requires more precise
control and monitoring of the blowdown rate and use of additional
treatment chemical to control solids and scale.  The following added
costs may be necessary:

Capital costs for a make-up flow meter and control system;

Capital costs for chemical feed systems for up to two chemicals such as
polyacrylate and phosphonate;

O&M costs for additional chemical treatment; and

O&M costs for additional labor for system maintenance, monitoring, and
laboratory analysis.

To illustrate these additional costs and to estimate scalable cost
factors for the increased costs of operating at a higher COC, an example
cooling tower configuration was developed.  In this example, chemical
treatment is assumed to include a scale inhibitor such as phosphonate
for both saltwater and freshwater systems, plus a solids control
chemical such as polyacrylate for freshwater systems.  While these
chemicals serve as examples for cost estimation purposes, other
chemicals may be substituted where appropriate with little change in
costs (Keister 2009B).  

All costs are assumed to be linear and are expressed in dollars/gpm of
recirculating flow.  The costs are based on a nominal system with the
following design:

A recirculating flow rate of 100,000 gpm

∆T of 15 (F

A COC of 4.0 for freshwater systems and 1.5 for saltwater systems

Table 4 below provides the corresponding design data for a 100,000 gpm
recirculating flow cooling tower.

Table 4

Chemical Cost Design Assumptions for a 100,000 gpm Cooling Tower

	COC = 4.0	COC = 1.5

Make-up (gpm)	1,700	3,825

Blowdown (gpm)	425	2,550

Polyacrylate Dosage (ppm)	3.0	3.0

Phosphonate Dosage (ppm)	4.5	4.5

Additional Labor (hrs/day)	1.0	1.0



Capital Costs

Capital costs for the make-up control system and chemical feed system in
the example cooling tower are based on the following costs:

Flow meter for 1270 gpm make-up piping costs $4,000 or $2.80/gpm
(Keister 2009);

Chemical feed system costs $3,000 per chemical;

Installation costs are 100% of total equipment costs; and

Other indirect and direct costs including controls, electrical,
engineering, site preparation, contingency, and allowance are 100% of
total installed costs.

Based on the above assumptions, the capital costs for upgrading a
recirculating cooling tower system to operate at higher COCs is $43,000
for freshwater and $55,000 for saltwater for a 100,000 gpm system or
$0.43/gpm and $0.55/gpm, respectively.  This is a relatively small
one-time cost item representing about 0.2% of the capital cost of an
average difficulty cooling tower system.

O&M Costs

O&M costs consist primarily of the cost of treatment chemicals and added
labor for monitoring and maintaining the system and conducting
laboratory analysis of cooling water samples.  It is assumed that the
power plants have an onsite wet chemistry laboratory and maintenance
staff and that these labor cost will only require an incremental
increase in labor hours.  O&M costs are based on the following
assumptions:

Cost of biocide treatment will not change significantly;

Amount of treatment chemical added is equal to dosage times the volume
lost to blowdown plus 10% to account for degradation;

Average unit cost of treatment chemicals is $4.00/lb of active
ingredient;

Total added labor is 1.0 hour/day per 100,000 gpm of recirculating water
(based on BPJ); and

Labor rate is $54/hr based on the $41.1/hr labor rate used in the Phase
II cost modules adjusted for inflation from 2002 to 2009 dollars.

Table 5 shows the resulting costs for a 100,000 gpm cooling tower.

Table 5

Estimated Increase in O&M Costs for Optimized Operation of Cooling
Towers

	COC = 4.0	COC = 1.5

Chemical Costs	$61,000	$148,000

Labor	$20,000	$20,000

Total	$81,000	$168,000

Unit Costs/gpm	$0.81	$1.68



Based on the above methodology, these costs can be considered as
representative of the added costs to operate at higher COCs on a broad
scale, but actual costs will vary based on site-specific water quality
conditions.  The average of these two values is $1.25/gpm and is nearly
equal to the non-power-related net increase in O&M costs derived from
the EPRI model that was used in calculating cooling tower costs for the
revised Phase II rule development.  The exact details of EPRI’s cost
methodology are not known (i.e., which cost elements are included and
what assumptions were used, but given that they are intended to be
representative of past experience, it is reasonable to assume that the
EPRI costs are based on the current practice of operating at lower COC
values with minimal treatment using biocides.  Thus, the above costs
represent the incremental increase in O&M costs that should be added to
cooling tower retrofit costs as well as for existing plants required to
operate at higher values of COC.  The COC that can be achieved with such
a chemical treatment scheme will be dependent on the quality of the
make-up water, but at a minimum should be in the range of 3-6 for
freshwater systems and 1.5 to 2.0 for saltwater systems.

Cooling Tower Media

Cooling tower media serves the purpose of increasing the interaction of
water and air by slowing down the rate at which water drops through the
tower and by increasing the surface area of the air/water interface. 
There are generally two kinds of media:  splash fill and film fill. 
Splash fill consists of material that intercepts water droplets and
redirects (splashes) them as they move down through the tower.  Film
fill is composed of thin corrugated plastic sheets upon which the water
flows as a thin film.  Film fill tends to produce a more efficient heat
transfer, but is more susceptible to plugging by solids and requires the
initial spray system to be capable of providing a more uniform
distribution of water droplets.  Thus, while film fill may be more
efficient it is also more likely to experience operational problems
requiring maintenance. 

The type of fill can affect the air pressure drop across the media,
which in turn affects the air flow rate through the tower.  Within the
different types of media are numerous design modifications that could
result in tradeoffs between system efficiency, maintenance costs and the
potential for operational problems.  For example, a film fill with
smaller channels may be more efficient than one with larger channels
because of the greater water surface area, but may also be more prone to
plugging with solids, which could result in lowered efficiency and
higher maintenance costs.  Decisions regarding selection of fill type
and design must take into consideration the expected tower efficiency,
water quality, spray system design requirements, the control measures to
be employed both chemical and physical, and expected maintenance
requirements.  Use of film fill in towers where solids control is not
adequate may result in a requirement to periodically clean the towers to
remove the solids, and under worst-case situations the tower media may
become so plugged that the media must be completely replaced.

References

Carney, Barbara. Feeley, Thomas J. McNemar, Andrea. NETL Power Plant
water research Program. EPRI Advanced Cooling Technology workshop. July
9, 2008.

Daniels, David. M&M Engineering Associates, Inc.  Put a Lid on Rising
Chemical Costs.  POWER Magazine. September 15, 2008.  (  HYPERLINK
"http://www.powermag.com/issues/features/1347.html" 
http://www.powermag.com/issues/features/1347.html 

DOE EIA. Electric Power Industry Overview- Electric Power Generation.
Accessed on June 17, 2009 at website:   HYPERLINK
"http://www.eia.doe.gov/cneaf/electricity/page/prim2/chapter3.html" 
http://www.eia.doe.gov/cneaf/electricity/page/prim2/chapter3.html 

Drbal , Lawrence F. Boston, Patricia G. Westra, Kayla L. Black & Veatch.
Power Plant Engineering. 1995.

Engineering Tips. Chemical Process Engineering FAQ – Cooling Tower –
How to calculate cycles, blowdown, evaporation, make-up.  Accessed on
October 20, 2009 at website:   HYPERLINK
"http://www.eng-tips.com/faqs.cfm?fid=1152" 
http://www.eng-tips.com/faqs.cfm?fid=1152  

Harfst, William F. Enhanced Cooling Tower Maintenance Saves Water.
Maintenance Technology. October 2008.

  HYPERLINK
"http://www.mt-online.com/article/1008-Enhanced-Cooling-Tower-Maintenanc
e-Saves-Water" 
http://www.mt-online.com/article/1008-Enhanced-Cooling-Tower-Maintenance
-Saves-Water 

Keister, Timothy. ProChemTech International Inc. Log of telephone
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7National Productivity Council of India (NPCI).  Guide Book 3 – Energy
Efficiency in Electric Utilities.  Chapter 7 – Cooling Towers.  No
Date.  Accessed on March 25, 2009 at website:    HYPERLINK
"http://www.em-ea.org/Guide%20Books/book-3/Chapter%203.7%20Cooling%20Tow
er.pdf" 
http://www.em-ea.org/Guide%20Books/book-3/Chapter%203.7%20Cooling%20Towe
r.pdf . 

Nelson, John A. Marley Cooling Towers. Cooling Towers and Saltwater.
November 5, 1986.

  HYPERLINK "http://spxcooling.com/pdf/CTs-and-Salt-Water.pdf" 
http://spxcooling.com/pdf/CTs-and-Salt-Water.pdf 

Appendix A

 Note that engineering costs for the revised Phase II rule included
drift eliminators for all cooling tower installations.

 In other words, a facility could only recirculate a portion of the
heated circulating water through a cooling tower and back to the
condenser; the remaining portion would be discharged as if it were
once-through cooling water.

 Keister, Timothy. ProChemTech International Inc.

 Using the EPRI cooling tower design and costing model, which assigns
costs on various factors, including an assessment of the difficulty
(easy, average, or difficult) for the tower retrofit.

 Based on conversation with Timothy Keister, ProChemTech International
Inc.

 Approximate average based on estimated chemical cost of $3.44/lb and
$4.25/lb active ingredient for phosphonate and polyacrylate,
respectively.  These costs are adjusted upward using a 40% markup and 5%
tax from purchase costs reported by a chemical vendor of $2.34 and
$2.89, respectively (Keister 2009).

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