American Petroleum Institute

Characterizing 

Cooling Water Systems 

in Petroleum Refineries

Briefing Report

TABLE OF CONTENTS

	Page

	Executive Summary	v

  TOC \o "1-3"  I.	Introduction	  PAGEREF _Toc6125054 \h  1 

II.	Cooling Water Use Description	  PAGEREF _Toc6125055 \h  3 

II.1	What drives the need for cooling in petroleum refineries?	  PAGEREF
_Toc6125056 \h  3 

II.2	What types of equipment require cooling?	  PAGEREF _Toc6125057 \h 
5 

II.3	What are the types of cooling water systems?	  PAGEREF _Toc6125058
\h  7 

II.4	Are there specific requirements/limitations associated with cooling
water usage?	  PAGEREF _Toc6125059 \h  9 

II.5	How are these cooling needs and requirements/limitations met?	 
PAGEREF _Toc6125060 \h  11 

II.6	What dictates which cooling approach is used?	  PAGEREF _Toc6125061
\h  12 

II.7	What volumes of cooling water  are required for various approaches?
  PAGEREF _Toc6125062 \h  14 

III.	Recirculated Cooling Water Technology	  PAGEREF _Toc6125063 \h  17 

III.1	What are the components of a Recirculated cooling tower system?	 
PAGEREF _Toc6125064 \h  17 

III.2	What are the limitations of using recirculated systems?	  PAGEREF
_Toc6125065 \h  18 

III.3	What must be considered when increasing cycles of concentration in
cooling towers? ?	  PAGEREF _Toc6125066 \h  19 

III.4	What quality/quantity of make-up water is required and how is it
determined?	  PAGEREF _Toc6125067 \h  22 

III.5	Effects of brackish vs. fresh water?	  PAGEREF _Toc6125068 \h  24 

III.6	Blowdown flows, handling, treatment?	  PAGEREF _Toc6125069 \h  25 

III.7	Factors affecting drift and evaporation losses?	  PAGEREF
_Toc6125070 \h  26 

III.8	Factors affecting chemical usage and concentrations?	  PAGEREF
_Toc6125071 \h  27 

IV.	Converiting from Once-through to Cooling Towers	  PAGEREF
_Toc6125072 \h  28 

IV.1	What would be involved in changing from once-through cooling to
cooling towers?	  PAGEREF _Toc6125073 \h  28 

IV.2	Effects on wastewater discharge volumes and quality?	  PAGEREF
_Toc6125074 \h  30 

IV.3	Effects on metallurgy in piping and exchangers?	  PAGEREF
_Toc6125075 \h  31 

IV.4	Effects on refinery piping configurations?	  PAGEREF _Toc6125076 \h
 32 

IV.5	Effects on heat exchanger capacity?	  PAGEREF _Toc6125077 \h  33 

IV.6	Additional space requirements?	  PAGEREF _Toc6125078 \h  35 

IV.7	Changes in water treatment requirements?	  PAGEREF _Toc6125079 \h 
36 

IV.8	Changes in water pressure requirements?	  PAGEREF _Toc6125080 \h 
37 

V.	Changing to Dry Cooling	  PAGEREF _Toc6125081 \h  38 

V.1	What would be involved in changing to dry cooling?	  PAGEREF
_Toc6125082 \h  38 

VI.	Cooling Water Recycle and Reuse	  PAGEREF _Toc6125083 \h  39 

VI.1	What are the options for recycle/reuse of cooling water? 
Practicality?	  PAGEREF _Toc6125084 \h  39 

VII.	Cooling Water Intake Structures	  PAGEREF _Toc6125085 \h  40 

VII.1	What types of intake structures are used at refineries	  PAGEREF
_Toc6125086 \h  40 

VII.2	What is involved in retrofitting refinery intake structures (e.g.,
moving screens)?	  PAGEREF _Toc6125087 \h  41 

VII.3	Other information on cost-effectiveness of intake structure
controls?	  PAGEREF _Toc6125088 \h  42 

VIII.	Cost Implications	  PAGEREF _Toc6125089 \h  43 

VIII.1	Cursory review of the  EPA’s “Draft Initial Cost
Estimates”--  May 17, 2001?	  PAGEREF _Toc6125090 \h  43 

VIII.2	Estimate the cost of installing a 17,400 gpm cooling tower	 
PAGEREF _Toc6125091 \h  46 

IX.	References	  PAGEREF _Toc6125092 \h  51 

 

Appendix A	EPA Draft Initial Cost Estimate

Appendix B	Quote for Cooling Tower

Appendix C	Quote for Pumps

Appendix D	Quote for Clarifier

Appendix E	Quote for Traveling Water Screen

LIST OF FIGURES

	Page

Figure 1	Typical Distillation System	4

Figure 2	Once Through Cooling System	7

Figure 3	Closed Loop Cooling System	7

Figure 4	Evaporative Cooling System	8

Figure 5	Makeup & Blowdown vs. Cycles of Concentration	23

Figure 6	Cooling Tower System for Estimate	46

Executive Summary

Section 316(b) of the Clean Water Act requires EPA to ensure that the
location, design, construction and capacity of cooling water intake
structures reflect the best technology available for minimizing adverse
environmental impact.  EPA is developing national standards in three
phases:  Phase I for new facilities, Phase II for existing electric
generating plants that use large amounts of cooling water, and Phase III
for manufacturing plants and for electric generating plants using
smaller amounts of cooling water.  By June 2003, EPA must propose the
Phase III regulations addressing existing manufacturing facilities
(which would apply to existing petroleum refineries) and EPA must issue
final Phase III regulations by December 2004.  The proposed Phase II
regulations include a requirement to limit cooling water intake flow to
that commensurate with closed loop recirculation of cooling water.  This
requirement is an option EPA may consider for the Phase III regulations,
as well as an option to require air cooling in some cases as a means to
limit cooling water intake flow.

This report details the significant technical issues and costs
associated with implementing either closed-loop flow reduction or air
cooling at petroleum refineries.  The environmental benefit realized by
either of these options would not justify the high cost to the industry.
 EPA should take the information presented in this report into account
when drafting its Phase III rule, and should not require the petroleum
industry to install air cooling or reduce intake flows to that
commensurate with closed loop recirculation.  EPA should instead provide
the petroleum industry the option of minimizing adverse environmental
impact through the installation, operation, and monitoring of control
technologies effective at reducing impingement and entrainment at the
cooling water intake.   Depending on site-specific conditions and the
particular technology selected, control technologies have been shown to
be up to 90 -100 percent effective at reducing impingement and
entrainment.        

Technical and Economic Feasibility of Closed-Loop Recirculation and Air
Cooling

EPA may believe that the choice of cooling medium, and the switch from
once-through cooling to complete recirculation of cooling water with the
installation of cooling towers, is readily achieved at manufacturing
facilities, with little effect on process operations.  In practice,
however, the choice of once-through cooling vs. recirculation is a
complex decision based on numerous site-specific considerations.  In the
refining industry, a requirement mandating a particular cooling medium
(water vs. air) or configuration (once-through vs. recirculation) will
have significant technical and cost implications to process operations. 
EPA needs to understand the complex nature of cooling operations at
refineries and the significant potential impacts to the refining
industry associated with these regulations.  EPA should take the
following implications to the refining industry into account when
considering Phase III regulations that mandate cooling water
recirculation, cooling tower installation, and/or air cooling:

A recirculation system cannot provide cooling water at temperatures as
low as a once-through system, which provides cooling water at the
temperature of the water source.  This has significant process
implications at refineries.  For example, a key factor to refinery
profitability is to cut as deeply into the vacuum tower residue as
possible.  This is achieved by pulling as much of a vacuum as possible
in the vacuum column, but this vacuum requires low condensing
temperatures, which in turn requires cooler water.  

The recirculation temperature affects the design, sizing, and cost of
all water-cooled heat exchange equipment within the refinery; the higher
cooling temperature will require larger and more costly heat exchange
equipment to achieve the same cooling duty as a once-through system. 
Depending on specific space and pressure drop limitations, it may not be
possible to add incremental heat exchange; wholesale replacement may be
required.   

Installation of a cooling tower and related equipment (sumps, pumps,
piping, etc.) is dictated by the unit operations being conducted at the
particular refinery and by the space available.  While it might be
optimal to install one large cooling tower to replace an existing
once-through system, space and logistics will likely require building
multiple cooling towers.  This invariably leads to increased costs and
operational complexity.

Recirculation systems depend on evaporation to remove heat and thereby
increase the concentration of hardness and dissolved solids in the
cooling water system.  If the dissolved solids content of the water is
high it will limit the recirculation rate.  Unless the quality of the
intake water is very high, intake water pretreatment will be needed to
avoid scaling or corrosion in the recirculation system.  Such
pretreatment may entail precipitation, water softening, pH adjustment,
and suspended solids removal.  

It is necessary to treat recirculating cooling water with a variety of
chemicals to control scaling, deposition, biological growth and
corrosion.  Sulfuric acid is most often used to control pH.  Corrosion
inhibiters must be added to prevent corrosion.  Slimicides are used to
control biological growth.

Facilities will typically treat the blowdown from cooling tower systems
in the existing wastewater treatment plant and the extra loadings may
drive the need for increased capacity.  Cooling tower blowdown contains
high dissolved solids as well as those chemicals added to prevent
corrosion, scale, and biological growth.  The need to treat blowdown can
significantly reduce residence time and therefore overall efficiency in
the treatment system.

Direct air cooling used in refineries is dictated by the dry bulb
temperature at that site and conditions of the process streams such as
inlet temperature and required outlet temperature.  Air cooling can be
used in refineries only for specific applications.  Many process
requirements will still require the installation of cooling water
exchangers downstream of the air coolers.  Air-cooled exchangers tend to
be large, require significant plot space, and can be a source of noise. 

Replacement of once through cooling with air-cooled heat exchangers
would demand an economically infeasible number of changes to the
existing process equipment.  Furthermore, whereas cooling water
shell-and-tube exchangers can be stacked vertically in process units,
air coolers cannot be stacked (because lower units would send heated air
to higher units) and so it may be physically difficult or impossible due
to space constraints to replace water-cooled exchangers with air-cooled
exchangers.

Were conversion of a once-through cooling water system a refinery even
feasible, API’s estimate of the cost of such a conversion is much
higher than that estimated by EPA.  For a 17,400 gallon/minute system at
a refinery, API estimates an installed cost of $9.2 million, with an
annual operation and maintenance cost of $334,600.  EPA estimated an
installed cost of $1.34 million and an operation and maintenance cost of
$179,000 for such a facility.  This cost is an extremely burdensome cost
to achieve a limited environmental benefit.

 

 

       

Introduction

Section 316(b) of the Clean Water Act requires EPA to ensure that the
location, design, construction and capacity of cooling water intake
structures reflect the best technology available for minimizing adverse
environmental impact.  For many years, this provision has been
implemented without federal standards in place, on a resource intensive,
site-by-site basis.   EPA is developing national standards in three
phases:  Phase I for new facilities, Phase II for existing electric
generating plants that use large amounts of cooling water, and Phase III
for manufacturing plants and for electric generating plants using
smaller amounts of cooling water. EPA issued the Phase I regulations for
new utilities and manufacturing facilities on December 18, 2001.  In
March 2002, EPA plans to issue the Phase II proposed rules for existing
utilities, with the goal for making them final by August 2002.  By June
2003, EPA must propose the Phase III regulations addressing existing
manufacturing facilities (which would apply to existing petroleum
refineries) and EPA must issue final Phase III regulations by December
2004.  API expects the provision for the existing utilities (Phase II)
will be the basis for developing the Phase III rules.

Consequently, the Phase II regulations can have significant cost impacts
on petroleum refineries.  Some of the potential costs range from costs
for studies and restoration projects for offsetting perceived adverse
environmental impacts, to adding or retrofitting impingement reduction
equipment to the intake structure, relocating the entire intake
structure, reducing volumes by changing cooling tower operations (e.g.,
increasing cycles), reducing volumes from one-through cooling systems by
converting to recycle cooling towers, reducing volumes by reuse or
recycle of cooling water for process uses, reducing volumes by
installing dry cooling technology, and/or any combination of these
options.

EPA needs some additional information to better understand the nature of
cooling operations at refineries and to understand the significant
potential impacts to the refining industry associated with these
regulations.  EPA has invited API to meet and discuss these topics. 
This report will be the basis for the discussions at that meeting.

API Project Objectives

API senses EPA may have incorrect assumptions and/or incomplete
understanding about petroleum refinery cooling operations, including how
and why cooling water moves through a refinery, practical vs.
theoretical options for recycle/reuse, implications of increasing cycles
in existing cooling towers, the process consequences (e.g.,
thermodynamic effects, effects in stream composition, material and
equipment requirements, etc.) in changing from once through cooling to
cooling towers, and feasibility vs. practicality of dry cooling.

API commissioned the preparation of a briefing paper to address various
issues associated with changing from a once through cooling system to a
recirculating cooling tower system.  This briefing paper provides
answers to basic questions about refinery cooling operations  and
clarifies the  significant technical issues and extreme costs associated
with implementing either closed-loop flow reduction or air cooling at
petroleum refineries.  The environmental benefit realized by either of
these options would not justify the extremely high cost to the industry.
 

Cooling Water Use Description

What drives the need for cooling in petroleum refineries?

In petroleum refineries crude and refined oils are separated and
subdivided into various fractions based on boiling point.  This is
accomplished by fractional distillation.  The distillation is carried
out in distillation column where the oil is heated up and vaporized in a
fuel- (fuel oil, natural gas or refinery fuel gas) fired heater. 
Various fractions are separated by condensing and cooling products that
are withdrawn from the tower.  From an overall heat balance point of
view, the heat that is put into the system by burning fuel and/or the
introduction of steam has to be removed or “rejected”.  This is
accomplished in various ways such as:

Heat exchange with boiler feed water to generate steam

Heat exchange with other process streams

Rejection of heat using air coolers

Rejection of heat to cooling water

Figure 1 depicts a typical distillation system in a refinery.  In this
system three types of heat rejection systems are shown. The crude oil is
preheated by exchanging with another process stream and fed to a fired
heater.  The partially vaporized products are sent to the distillation
tower where different side streams are withdrawn based on the boiling
point range of the product.  The side streams are sent to strippers (a
type of distillation column) where the boiling point range of the
product is adjusted further by the addition of steam.  The bottoms
product from these strippers is cooled and sent to storage tanks or on
to further processing. The vapors from these side strippers are sent
back to the main tower.  The overhead vapors from the main tower are
condensed using an air-cooled exchanger and then further cooled using a
cooling water heat exchanger.  Three types of heat exchangers are shown
in this system:

Type 1 heat exchangers such as steam generators and process stream heat
exchangers

Type 2 heat exchangers which use cooling water

Air coolers

Figure 1

Typical Distillation System

What types of equipment require cooling?

Various types of heat exchangers in the refinery constitute the largest
requirement for cooling.  These are listed below:

Distillate Condensers: These are used to cool and condense the overhead
vapors from distillation towers.  Since this service involves
condensation of vapors, the heat duty of these exchanges tends to be
relatively large; and due to the fact that there are numerous
distillation towers in a refinery, distillate condensers represent the
largest use of cooling water in a refinery.  It is not uncommon to find
scores of distillate condensers in a refinery.

Steam Condensers: In these exchanges steam is condensed against cooling
water, and the heat duty of these exchangers also tends to be relatively
large since the service involves the condensation.  Refinery
distillation towers that run under vacuum conditions typically use steam
ejectors to generate the required vacuum.  The steam needs to be
condensed, and this is accomplished using cooling water exchangers.

Product Coolers:  Various refinery products need to be cooled before
they are sent to storage tanks. This cooling is accomplished using
cooling water exchangers, because of the low product outlet temperature
requirements.  It is not uncommon to find scores of product coolers in a
refinery.

Additionally, various process streams (liquid and vapor) are transported
by rotating equipment:

Compressors:  Compressors fall into three general categories:
centrifugal, rotary and reciprocating.  Compressors are usually equipped
with oil lubrication systems to dissipate some to the heat of
compression.  The heat that is picked up by the lubricating oil is
rejected from the system using air coolers or cooling water depending on
the amount of heat rejection required and the temperature requirements
of the circulating oil.  In addition, when multistage compressors are
used, the process stream is cooled between stages using intercoolers and
after coolers.

Vacuum Pumps:  Vacuum that is required in distillation processes is
generated by using vacuum pumps, steam ejectors or using a combination
of the two.  When vacuum pumps are used, the cooling requirements are
similar to compressors in that the heat picked up by the lubricating
system needs to be rejected using air coolers or cooling water
exchangers.  When ejectors are used to create the vacuum, all the steam
that is added needs to be condensed and cooled.  This is typically done
with cooling water exchangers.

Pumps: Pumps that are used in liquid service fall into two general
categories: reciprocating and centrifugal.  Pumps are also equipped with
lubricating system and can require cooling, depending on the operating
efficiency of the pump.



What are the types of cooling water systems?

There are three types of cooling water systems:

a.	Once Through Cooling Water System:  In this type of system where the
cooling capacity of the water is used only once without contacting the
fluid or vapor being cooled. These systems use of water withdrawn from a
surface water source such as a lake, river or estuary and typically are
returned to the same source.  Figure 2 shows a typical once-through
cooling water system.

Figure 2

Once Through Cooling Water System

b.	Closed Loop Cooling Water System:  In this system water is circulated
in a closed loop piping system and is subject to cooling and heating
without evaporation or air contact. Heat that is absorbed by the water
in a closed loop system is normally rejected using a heat exchanger to a
once through cooling system.  Figure 3 shows and example of a closed
loop system.

Figure 3

Closed Loop Cooling Water System

Closed loop cooling water systems in U.S. refineries are rare.  Since
closed loop systems are not typical, they are not discussed further in
this report.

Evaporative Recirculating Cooling Water System:  In this type of system,
the heat that is picked up by the recirculating cooling water is
rejected in a cooling tower by evaporation.  In the cooling tower the
hot water is sprayed against a rising stream of atmospheric air. The
heat in the cooling water is removed by heating the air as well as by
evaporation.  An example of an evaporative recirculating cooling tower
system is shown in Figure 4.

Figure 4

Evaporative Recirculation Cooling Water System

Are there specific requirements/limitations associated with cooling
water usage?

Cooling water must be of a quality that allows for control over scaling,
deposition of suspended solids and corrosion in the equipment through
which the cooling water passes.  Scale formation limits heat transfer
and is destructive to heat exchangers and other plant equipment;
therefore, scale prevention chemistry is extremely important in
recirculated cooling water system design.  Scale can be controlled by
optimization of chemical concentrations, pH and water temperature.

In a recirculated cooling water systems as shown in Figure 4, water is
evaporated which concentrates the dissolved solids contained in the feed
water. Make-up water is added to the cooling water system to replace the
water that is evaporated. The maximum cycles of concentration allowable
and still minimize scale formation is a function of water chemistry. 
Impurities that form scale will determine the maximum cycles of
concentration. Silica and Calcium concentrations are typically used to
determine the cycles of concentration.

Typical allowable concentrations of impurities in open recirculated
cooling water are as follows:

	ppm

Total Dissolved Solids	3,000

Total Suspended Solids	200

Oil and Grease	10

pH Range   	6.8 to 8.0

Aluminum (as Al)	1

Iron (as Fe)	1

Ammonia (as NH3)	10

Calcium (as CaCO3)	1000

Copper (as Cu)	0.2

Chlorides (as Cl)	1000

Silica (as SiO2)	150

The quality of the intake water is an important factor in the design of
cooling water systems.  In open recirculated systems, the intake water
can be pretreated to lower the concentration of specific ions in order
to achieve an optimized number of cycles of concentration. 

There can be limitations on the maximum return water temperature in both
closed loop and open recirculated systems.   In once-through systems
there is usually a limitation on the return temperature depending on
environmental considerations of the source of the water.  In open
recirculated water systems, the return temperature is often dictated by
the need to limit corrosion in the cooling water piping in the refinery.
 Cooling water return temperatures of 100oF to 110oF are typical.

Corrosion in the cooling water piping and heat exchanges is caused by
the dissolved oxygen contained in the recirculating water, dissolved
salt concentration and the temperature of the cooing water return.  This
is controlled by limiting the temperature of the return cooling water,
and by the addition of corrosion inhibitors.

How are these cooling needs and requirements/limitations met?

In once-through cooling water systems, the intake water can be clarified
to remove suspended solids and any limitation on the return water
temperature can be met by increasing the flow of water.

Makeup water to recirculated cooling water systems may be clarified to
remove suspended solids and/or softened  prior to being used as cooling
water make-up.  Frequently a surface water or well water source is found
that does meet minimum quality specifications without prior treatment.
The circulating cooling water is treated with a variety of chemicals to
control scaling, deposition, biological growth and corrosion.  Sulfuric
acid is most often used to control pH.

Softening and other methods of removing dissolved solids prior to using
the water for cooling tower makeup are usually not cost effective
because of the relatively large volume and low concentration of solids. 
Dissolved solids removal from a cooling tower with sidestream softening
is usually more cost effective because the impurities are more
concentrated.

What dictates which cooling approach is used?

When cooling systems are designed for use in refineries a series of
evaluations and decisions are made.  The following flowchart illustrates
such a decision tree.

In general, lower process outlet temperatures can be achieved using
water coolers than using air coolers.  This is because the dry bulb
temperature of the ambient air determines the lowest achievable
temperature in air coolers and the wet bulb temperature of ambient air
is the controlling factor for cooling water exchangers.

After going through the analysis as shown in the chart above, the heat
load that needs to be rejected to cooling water is determined. Then an
evaluation of once through cooling versus recirculating cooling water is
carried out.  The factors that go into this evaluation include:

Location of the refinery with respect to the water source.  In some
cases local regulations might preclude the use of once through cooling
water.

The required cooling water supply temperature based on the cooling needs
in the refinery.

Plot space considerations within the refinery.

The quality of the fresh water with respected to contaminants such as
total dissolved solids, chlorides etc. needs to be evaluated.  If the
dissolved solids content of the water is high it will lower the number
of cycles of concentration in the cooling tower.  Consequently, the
makeup water and blowdown treatment requirements will be higher.

What volumes of cooling water are required for various approaches?

Once-through cooling system:  In a once through system the volumetric
flow of the water required is dictated by the amount of heat that needs
to be rejected (Btu/hr) and the maximum allowable discharge temperature.


For example, if a refinery needs to reject 130,500,000 Btu/hr with an
intake temperature of 85oF and a maximum discharge temperature of 100oF,
the flowrate of water is calculated as follows:

 

	= 17,400 gpm

Even though this circulation rate is small compared to a mid sized
refinery, it is used for illustration purposes because EPA used this
size in the EPA study.  The next higher EPA case was 104,000 gpm, which
was considered too high to use as a representative case.

Open Evaporative cooling system: The cooling water required in this type
of system is the water required to makeup the losses due to:

Evaporation

Drift (entrainment)

Blowdown (% of the makeup rate to prevent buildup of ions in the system)

Makeup Requirement

The make up water rate (M) is the sum of the evaporative losses (E),
drift losses (D) and the blowdown (B).

 			(1)

The evaporative loss is a function of the heat rejected by the cooling
tower and the drift loss is a function the circulation rate. 

The amount of dissolved solids (TDS) coming into the system with the
makeup water has to go out of the system with the blowdown stream to
prevent TDS buildup in the circulating cooling water. Cycles of
concentration (n) is calculated by:

n = 	TDS concentration in circulating water (y)

 					(2)

and since the TDS concentration of the blowdown is the same as the
circulating water

 				(3)

Combining equations (2) and (3)

 		                (4)

 				(5)

Combining equations (1) and (3)

 				  (6)

and 

 					(7)

Where:

M	=	amount of makeup water

E	=	amount of evaporated water

D	=	amount of water loss by drift (specified by tower manufacturer)

B	=	amount of blowdown

n	=	number of cycles of concentration

x	=    mass fraction of TDS in the makeup water

The same equations apply when considering blowdown constituents other
than TDS (e.g, silica, hardness, etc.)The following example illustrates
the calculation of these parameters

 )	252 gpm

Drift (assumed to 0.1% of circulation)	17 gpm

Assumed cycles of concentration	5

Blowdown (by equation 4)	46 gpm

Makeup (by equation 3)	315 gpm

Recirculated Cooling Water Technology

What are the components of a recirculated cooling tower system?

Recirculated cooling tower systems can be divided into three parts:

Intake structure including the intake screens, pumps and piping 

Cooling Tower and cooling water circulation pumps, including any
pretreatment systems

Recirculation piping and heat exchangers

Intake Structure: Consists of the piping from the intake source
including screens to remove suspended solids and marine organisms,
intake pumps (number of pumps depends on the size and includes operating
and spare pumps), and the piping from the intake pumps to the refinery.

Cooling Tower:  The cooling tower consists of basin capable of providing
about 10 minutes of holdup based on the circulation rate of the system,
a tower equipped with fill (wood or fiberglass), cooling tower sump,
distribution piping and nozzles to maximize air/water contact, fans to
force atmospheric air through the tower and circulating pumps.  

Pretreatment: Depending on the quality of the intake water, a clarifier
is used to remove suspended solids.  If the dissolved solids content of
the intake water is too high, other pretreatment methods such as lime
softening, ion exchange and reverse osmosis are included to increase the
cycles of concentration allowable in the cooling tower. Pumps are
required to transfer the water from the clarifier to the cooling tower
and the recirculating cooling water.

Cooling Water Treatment:  Chemical treatment of the recirculating water
requires that chemical feed systems be installed.  This includes tanks
and pumps for each of the chemicals required.

Recirculation piping:  This consists of the cooling water supply and
return piping to the various process units in the refinery.  Also
includes all the sub-headers in the refinery to carry the cooling water
to heat exchangers.

Blowdown Treatment: The blowdown from the cooling tower system is
typically sent to the wastewater treatment plant.  This stream would
contain high dissolved solids and would contain hydrocarbons if heat
exchanger leaks have occurred.

What are the limitations of using recirculated systems?

The makeup water to a cooling tower may come from wells, reservoirs,
lakes, rivers, and even treated wastewater.  The impurities in these
makeup water sources may cause a number of water related problems in the
cooling system.  Corrosion, scale, biological fouling, sludge formation,
and chemical attack on the cooling tower components may be experienced
when the chemistry of the cooling water is not properly controlled.

The quality of the makeup water determines the amount and type of
internal chemical treatment required.  Chemicals are added to protect
the metal surfaces from the following problems:

Corrosion.

Scale deposits.

Biological fouling.

In a cooling tower, the recirculating water is saturated with
atmospheric air as it passes through the tower.  This condition,
combined with warm water temperatures and increasing dissolved salt
concentration, results in water that is very corrosive to carbon steel. 
Corrosion inhibitors are required for all recirculated cooling tower
systems.  Selection of the optimum inhibitor and dosage is based on the
analysis of the water, materials of construction, environmental
regulations, and other factors including cost of chemicals.  Typical
inhibitors are phosphates, molybdates, zinc and polyphosphates.

Several corrosion inhibitor chemical treatment programs have been
developed in the past fifteen (15) years.  The most successful of these
programs have been the stabilized phosphate and zinc phosphate
treatments, which incorporate the use of copolymer dispersants.

Scale is an adherent insoluble deposit laid down on surfaces of the
cooling water system during operation, causing impaired heat transfer. 
Calcium carbonate is the principal scale formed when calcium bicarbonate
breaks down and  becomes even more insoluble as the temperature rises
(reverse solubility).  Scale control can be accomplished through
limiting the cycles of concentration.

Refinery cooling towers also require continuous chlorination for control
of microbiological activity and usually acid  for pH control, which
prevents calcium carbonate scaling .

What must be considered when increasing cycles of concentration in
cooling towers? 

Cycles of concentration are limited by makeup water quality, the design
of the cooling tower itself, the design of the cooling system piping,
heat exchangers and by the type of pretreatment available for the makeup
water.

Variability of the makeup water quality must be considered: changes in
total suspended solids, iron, calcium to magnesium ratio, alkalinity,
sodium, silica, sulfate and chloride can lead to control problems,
corrosion and deposition.  Blowdown rates are difficult to control with
variable quality makeup water, since the blowdown rate is set or
adjusted based on laboratory tests.  Variations in makeup water quality
make setting this rate difficult without either exceeding chemistry
limitations or wasting water.  The only solution to this problem is
on-line analysis of some components of the circulating water quality,
such as conductivity, pH and phosphate.  These continuous analyses are
used to drive automatic blowdown valves, treatment chemical pumps and
sulfuric acid pumps via a PLC controller.  The cost for such automation
is about $100,000 installed for a  17,400 gpm system. The level of
reliability is questionable as the various probes become fouled with
deposits, requiring a preventative maintenance program to operate
dependably. Similarly, chemical dosages are set based on makeup water
characteristics and the cycled up components in the circulating water. 
Variable quality makeup water makes it difficult to control dosages
without either overshooting or undershooting the proper dosage.  As
stated above, the use of automated systems is the only means of
addressing the situation, adding both equipment and labor costs to the
operation of the cooling system.

Commonly used values (based on industry practice) exist for each of the
potential contaminants that may be found in the makeup water stream. 
These values are set for cycled up (concentrated) cooling tower water
and therefore directly limit the maximum cycles of concentration that
can be achieved before equipment fouling, equipment scaling or equipment
corrosion problems are encountered.  The various limits are set based on
many years of industry experience as well as extensive testing by
various suppliers of treatment products and by the end users themselves.
 Industrial water treatment has been practiced extensively at least
since the 1950’s so there is a large body of knowledge on the subject.

Calcium carbonate deposition is controlled by limiting the calcium
level, generally no higher than 1,000 parts per million, by the addition
of sulfuric acid to control pH within a  range of 6.8 to 8.0 and by the
addition of dispersants specific to calcium carbonate.  The general
limit of 1,000 ppm calcium as CaCO3 assumes the use of such dispersants.

Magnesium silicate limits are set to control scaling of surfaces
contacted by the recirculated cooling water.  The magnesium silicate
limit is an empirical value (product of the magnesium concentration x
silica concentration) set at 100,000 ppm, and can only be controlled by
limiting the levels of magnesium and silica in the recirculating water
via blowdown.

Because carbon dioxide is stripped from the water as it passes through a
cooling tower, the rise in pH (alkalinity) is controlled via the use of
sulfuric acid and/or blowdown.

Sodium and sodium chloride are limiting factors because of potential
damage to the wooden structures of cooling towers and due to the
corrosive potential for steel and stainless steel equipment components
in the cooling water system.  The general limit for chlorides is 1,000
ppm.

Silica itself must be limited to no more than 200 ppm as SiO2 to avoid
scaling.

Sulfates must be limited in cooling systems having concrete components,
such as the cooling tower basin, to avoid leaching of calcium from the
concrete matrix.  The general limit for water containing sulfates in
contact with concrete systems is 3,000 ppm as SO4.

Copper and iron in the makeup water can cause severe deposition and
corrosion problems at quite low levels.  Copper levels above 0.2 ppm and
iron levels above 1 ppm are unacceptable in the recirculated cooling
tower water.

Suspended solids in excess of 200 ppm in the cooling tower water are
generally not  tolerated due to fouling and plugging problems.

Typical design flow velocities for refinery heat exchangers range from 3
to 5 feet per second.  Older systems may have flow velocities as low as
1 to 2 feet per second.  A calcium limit of 1,000 ppm as CaCO3 would be
acceptable for the higher velocities; however, a limit of 500 ppm as
CaCO3 would be required for the lower flow velocities.

Examples of makeup water quality affecting cycles of concentration would
be as follows.  Given adequate flow velocities, one could assume an
upper calcium limit of 1,000 ppm as CaCO3.  A makeup water containing
100 ppm of calcium could theoretically be cycled 10 times, 200 ppm of
calcium in the makeup would limit cycles to 5 times, 300 ppm would limit
cycles to 3.3 times, and so forth.

Clarification is often used as a pretreatment of makeup water. 
Clarification of the makeup water can control the levels of suspended
solids in the makeup water; however, it will have no effect on the
concentration of dissolved solids contained in the cooling water.

Cold lime softening of cooling tower makeup can be used to lower the
calcium and silica levels of the makeup water.  Effective cold lime
softening can only reduce calcium levels to the 30 to 50 ppm range while
silica can generally be reduced to the 2 to 5 ppm range.  Cost for cold
lime softening is in the range of $50 per million gallons of water
treated.  In addition, the use of cold lime softening produces 200 to
300 pounds of sludge for each million gallons of water treated.  Sludge
disposal adds additional costs to the process.

What quality/quantity of make-up water is required and how is it
determined?

As illustrated in section II.7 makeup water is a function of the
evaporation rate and the number of cycles of concentration.  Equation
(6) and (7) in section II.7 are used to calculate the blowdown and
makeup rates

 

 

where

B is the blowdown rate

D is the drift rate

M is the makeup water rate

E is the evaporation rate

n is the number of cycles of concentration

In the example illustrated in section II.7 the makeup and blowdown rates
are as follow:

Makeup water – 315 gpm

Blowdown rate – 46 gpm

Using the equations shown above the following chart illustrates the
makeup and blowdown as a function of number of cycles of concentration.

Figure 5

Makeup & Blowdown vs. Cycles of Concentration

As can be seen from the chart, the rate of change of both makeup and
blowdown rates decreases rapidly until the cycles of concentration
reaches about 7 after which the rate of decline is small.  This is the
reason most cooling towers are designed with a cycle of concentration of
between 4 and 7.  Another consideration is the TDS content of the makeup
water and the allowable TDS content of the circulating water. 

Effects of brackish vs. fresh water?

Brackish water contains higher dissolved solids such as chlorides than
fresh water.  The chloride content of the brackish water could be as
high as 10,000 ppm.  This will impact on the following:

Cycles of concentration – The cycles of concentration could be reduced
to as low as 1.5 to 2 depending on the chloride content of the makeup
water.  This will require a larger volume of makeup water and also
result in a larger blowdown flow.

Cooling tower – Due to the higher chloride content of brackish water,
the cooling tower will be subject to greater corrosion and could require
an upgrade in materials of construction, resulting in higher capital
costs.

Treatment Chemicals – Depending on the TDS define content of the
circulating water, more treatment chemicals could be required to control
corrosion and biological growth.  This will result in higher operating
costs.

Circulation Pumps and Piping – Due to the higher chloride
concentration, the material of construction of the pumps and the
distribution piping will require an upgrade to corrosion-resistant alloy
material, which will result in significant capital expenditure.

Heat Exchangers – The material of construction of the heat exchangers
will require an upgrade to alloy materials and result in significant
capital costs. These costs could be minimized somewhat by passing the
cooling water on the tubeside of the exchanger.

Blowdown flows, handling, treatment?

In a cooling tower system, part of the circulating water is removed as
blowdown to prevent the buildup of TDS in the system. The example shown
in section II.7 illustrates in detail the calculation method.  In that
example, the blowdown is calculated to be 46 gpm. This is for system in
which the cooling tower makeup water has a calcium concentration of  ppm
as CaCO3 and is operated with five cycles of concentration.

Cooling tower blowdown is typically sent to wastewater treatment in
refineries via the sewer.  This is because in many cases the pressure on
the process side of heat exchangers is higher than the cooling water
pressure and any leaks in a heat exchanger would result in the
contamination of the cooling water with hydrocarbons.  This will impose
a hydraulic load on the existing wastewater treatment.  The full impact
on wastewater treatment needs to be evaluated on a case-by-case basis.

Factors affecting drift and evaporation losses?

In order to obtain efficient heat transfer in a cooling tower, the
return cooling water (hot) is sprayed on to the packing to increase heat
and mass transfer efficiency.  This results in small droplets of water
distributed throughout the tower.  The high flow of atmospheric air
passing through the tower results in the entrainment of some of these
droplets, which are released to the atmosphere and is referred to as
drift.  Cooling tower manufacturers have developed various methods such
as baffling, packing etc. to minimize drift.  The manufacturers often
consider these design methods and know-how proprietary.  The
manufacturers usually provide a guarantee on the maximum expected drift
from cooling towers.  Typical quantity of drift is <0.1% of the
circulation rate of the cooling tower.

The cooling tower operates on the principle of evaporative cooling by a
combination of heat and mass transfer. Circulating water is cooled when
some of it evaporates, removing the heat of vaporization.  The
evaporation loss in a cooling tower is a function of the total heat duty
that needs to be rejected.  The evaporation loss calculated by dividing
the heat duty by the latent heat of vaporization of water. In the
example illustrated in section II.7, the evaporation is 252 gpm for a
heat duty of 131 MM Btu/hr.

Factors affecting chemical usage and concentrations?

The factors affecting treatment chemical usage and concentration are
complex and inter-related.  Most  cooling tower vendors will utilize a
modeling program such as WaterCycle to determine the exact requirements
and limitations for a given makeup water.  In general, the higher the
concentration of a scaling or fouling component, the greater the
treatment requirement. 

In almost every instance, the use of sulfuric acid will be required in a
recirculating cooling system in order to control the pH of the water. 
The dosage is dependent upon makeup water alkalinity where the acid
requirement is based on alkalinity in the cooling tower; so as number of
cycles increases, and consequently the alkalinity increases,  the
sulfuric acid requirement also rises.

Dispersants to control calcium carbonate, magnesium silicate and silica
are dosed based on the levels of magnesium and/or silica contaminant in
the cooling water.  For example: 200 ppm of calcium would generally
require 20 ppm of dispersant while 1,000 ppm of calcium would require
100 ppm of dispersant.

Use of chlorine or bleach to control microbiological growth and fouling
is higher in cooling towers than in once through systems.  Continuous
chlorination is generally required for adequate control in refinery
cooling towers with the dosage in the range of 2 to 5 pounds per 1,000
gallons of recirculation capacity. Some operators of cooling towers find
that intermittent additions of chlorine are more effective.

Corrosion inhibitors are not generally required in once through systems
while recirculating systems will require inhibitors to control corrosion
of copper alloys and carbon steel piping and heat exchangers.  Again,
the higher the cycles of concentration, the higher the requirement for
corrosion inhibitors.

Converting from Once-through to Cooling Towers

What would be involved in changing from once-through cooling to cooling
towers?

The typical petroleum refinery is complex system of process components
that evolved over decades.  Designers provided heat removal systems
based upon the needs of the individual components and the available
cooling utilities.  Plot siting issues are major design problems at
refineries.  Changing once-through cooling systems to recirculating
systems will involve site-specific piping and equipment location issues
that may not be apparent to those more familiar with power plants.

Each refinery component may have several different heat transfer needs,
ranging from pump cooling to steam eduction-generated vacuum. If the
cooling tower can provide similar cooling capacity, then the process
heat exchangers will not need replacement.  The problem is that the
temperature difference provided by once-through cooling is not readily
replicated with cooling tower-based systems.  Consequently, the complex
nature of a petroleum refinery will demand site-specific cooling system
designs having multiple cooling towers (possibly placed far away from
the process unit), the replacement of pumps, and/or the replacement of
process heat exchangers (and possible associated piping, depending upon
system hydraulics).

There are practical aspects that need to be taken in account if changing
from a once-through system to a recirculating system is being
considered, such as:

A key factor to refinery profitability is to cut as deeply into the
vacuum tower residue as possible.  This in turn means pulling as low a
vacuum as possible in the vacuum column. Vacuum is generated using steam
jet ejectors, and lower column vacuum requires more steam. Lower vacuum
results in lower condensing temperatures, which in turn requires cooler
water. 

Installation of a cooling tower in an existing refinery is dictated to a
large extent by the space available and not by optimum location to
minimize pipe runs, access to utilities etc.  While it might be optimum
to install one cooling tower to replace the existing once through
system, space and logistics might require building multiple cooling
towers.  This invariably leads to increased costs.

A cooling tower requires a sump or basin for collection and storage of
the water recirculated over the tower and through the system.  Hydraulic
and pumping requirements set a minimum volume for the sump.  Generally,
the sump is sized to provide for at least 10 minutes of holdup based on
the recirculating rate.  

New recirculation pumps with enough head to overcome the hydraulic
losses of the supply piping, heat exchangers and return piping plus the
static head of the cooling tower will also be required.

New cooling water supply and return piping to and from the cooling tower
would need to be installed.

Since the flow of makeup water in the case of recirculation system is
significantly smaller  than a once through system, 315 gpm versus 17,400
gpm, (for the case illustrated in section II.7) new intake structure,
supply pumps and lines to the refinery may be required.

The thermal rating of heat exchangers in the refinery will need to be
reviewed to account for any differences in the cooling water supply and
return temperatures.  This could be a very significant cost issue.  The
example shown in section IV.5 illustrates a case where more than 20%
more heat exchanger area would be required.

Effects on wastewater discharge volumes and quality?

In a once through cooling water system there are no discharges of
wastewater from the system.  In an evaporative cooling water system, the
blowdown, which is a function of evaporative and drift losses and the
number of cycles of concentration (shown in the example in section II.7
to be 46 gpm) is usually sent to wastewater treatment.   The contaminant
in the blowdown is high dissolved solids, and possibly hydrocarbons, if
leaks from heat exchangers occur. Cooling tower blowdown is considered a
process wastewater and is subject to refinery effluent guidelines.

Effects on metallurgy in piping and exchangers?

In refineries that use fresh water as the source of once through cooling
water, the piping and heat exchangers (cooling water side) are usually
designed with of carbon steel but in some cases corrosion resistant
material are used.  If the system is changed to a recirculated cooling
water system the TDS content of the circulating water can be higher that
the once through water.  If the TDS is kept within reasonable limits (<
3000 ppm), carbon steel would be an acceptable material of construction.

If brackish water is used for the cooling tower makeup, higher alloys
such as admiralty, copper nickel alloys etc. would need to be considered
and would result in higher capital costs. As mentioned before, if the
pumps, distribution piping and exchangers in the refinery need to be
upgraded, it would require significant capital expenditures as well as
result in significant costs in order to reduce impacts on refinery
production capability.

In refineries that use salt water as the source of once through cooling
water, the piping and heat exchangers would most likely be made of alloy
material.  In these cases, changing to a recirculating cooling water
system would not require changes provided the dissolved solid
concentration in the recirculating water are kept within reasonable
limits (described previously in this report).

Effects on refinery piping configurations?

If the temperature difference between the cooling water supply and
return is the same for both once through and evaporative cooling water
systems, resulting in the same circulation flow, the impact on piping
configuration in the refinery should be minimal.  

If the temperature difference is substantially different resulting in
different circulation flows, the piping inside the refinery will need to
be evaluated to ensure that the flows are within the allowable pressure
drop and velocity limits.  Typical limits are as follows:

Pressure drop psi/100ft	Maximum velocity (ft/s)

1.0 – 2.0	10

The pressure drop limits are usually imposed in order to keep the
hydraulic losses in the system within reasonable limits.  Lower pressure
drops will result in lower velocities and could result in excessive
scaling and corrosion.  Higher pressure drops will require the
circulating pump differential head to be higher resulting in increased
capital and operating costs.

Effects on heat exchanger capacity?

In once through cooling water systems the supply temperature is  the
bulk temperature of the water source.  Any seasonal variations in the
cooling water supply temperature are also taken into account in the
design of heat exchangers in the refinery.

In evaporative cooling towers the local wet bulb temperature (1)
determines the cooling water supply temperature.  Typically, a 10oF
approach (2) between the wet bulb temperature and the cooling water
supply temperature is taken in the design of cooling towers – i.e.,
the cooling water supply temperature is 10oF above the design wet bulb
temperature.  Consequently, the cooling water supply temperature from
the cooling tower in a recirculating system is typically warmer than the
supply temperature for once through system. This will lower the log mean
temperature difference (3) in heat exchangers in the refinery and in
certain cases require significantly more heat exchanger area.  For
example:

	

 ) 	ft2	133	163

As illustrated by this example, 22% extra heat exchanger area would be
required if the system is changed from a once through to a cooling tower
system.  For the case illustrated in section II.7 this would mean that
an additional 3,900 ft2 of heat exchanger would be needed. But for
practical reasons, in some cases, new heat exchangers would need to be
installed instead of installing only the additional area required.  This
would lead to significant capital costs. A detailed analysis of all the
exchangers in the refinery should be carried out in order to understand
the impact on the whole refinery. 

(1)Wet Bulb Temperature: The wet bulb temperature is a measure of the
amount of moisture, in the form of invisible water vapor contained in
the air. As the name implies it is measured by a standard thermometer
whose bulb is covered by a muslin sleeve and that has been moistened by
pure water.

The principle of the wet bulb thermometer is as follows; Water
evaporates from the muslin cover passing into the air in the form of
invisible water vapor. This is accomplished by absorbing heat from the
thermometer bulb and the mercury it contains. The thermometer therefore
indicates a lower temperature than that of the dry bulb thermometer.(4)
The difference between the readings of the dry and the wet thermometers
is called the depression of the wet bulb.

If the air contains nearly all the moisture it can possibly hold,
evaporation from the muslin will be slight and the depression of the wet
bulb will be small. However, if the air is very dry, containing little
moisture, evaporation will be quite rapid and the depression of the wet
bulb will be quite large. 

In hot dry desert climates wet bulb depressions of over 45°F have been
observed, but at sea the depression is seldom more than 10°F. If the
air contains all the moisture it can possibly hold, there is no
evaporation from the muslin, and the dry and wet bulb thermometers will
read the same. When this condition exists the air is said to be
saturated.

(2)Approach Temperature: The lowest possible temperature to which water
can be cooled in a cooling tower corresponds to a wet bulb temperature
of the air. This is not practical because at the wet bulb temperature,
the partial pressure of the water in the air is equal to the vapor
pressure of water resulting in a zero driving force.  The difference
between the water outlet temperature and the wet bulb temperature is
called the approach temperature in a cooling tower.  This is typically
about 10ºF.

(3)LMTD: Logarithmic Mean Temperature Difference (LMTD) is a measure of
the difference in temperature between the hot side and cold side of a
heat exchanger.  It is defined as

 

Where, (T1 and (T2 are the difference in temperature between hot and
cold side fluid at two ends of the exchanger. LMTD is an indication of
the driving force available for heat exchange.

(4)Dry Bulb Temperature: The dry bulb temperature is the atmospheric
temperature of air at a specific location and at a specific time. It is
measured by exposing a thermometer to the atmosphere. The temperature
reported by the meteorological stations is the dry bulb temperature.

Additional space requirements?

Installing a new cooling tower will require additional plot space in the
refinery.  For the cooling tower example used in this study (17,400 gpm
circulation rate), the plot space required for the cooling tower is 100
feet x 60 feet.  This includes the space required for the ancillary
equipment such as the circulation pumps, chemical feed systems etc.

In an existing refinery, the location of the cooling tower is often
dictated by the space available and not by optimum location to minimize
pipe runs, access to utilities etc.  While it might be optimal to
install one cooling tower to replace an existing once-through system,
space constraints and logistics might dictate the use of multiple
cooling towers resulting in increased capital expenditures.  Other
considerations include impact of cooling tower drift on roads, houses
and process units.

The routing of the interconnecting piping between the cooling tower
system and the existing cooling water distribution system needs to be
considered.

The cooling water intake structure for recirculation systems is
significantly smaller that a once through system (315 gpm vs. 17,400 gpm
in the example illustrated in section II.7) and, therefore, the space
required for the new intake structure would be significantly smaller.

Changes in water treatment requirements?

Once through cooling systems are in general not chemically treated. 
There may be a need to chlorinate the  once through cooling system
intermittently to control microbiological activity and fouling.  Some
once through systems use low levels of dispersant, 1 to 5 ppm, to
control calcium carbonate scaling and suspended solids fouling. 

The change to cooling towers would require the feed of several different
types of chemicals for adequate control and protection of the cooling
system. 

Sulfuric acid would be required to control alkalinity.

Chlorination  would be required to control microbiological fouling. 

The continuous feed of polymeric dispersants would be required to
control calcium carbonate scaling, magnesium silicate scaling and
suspended solids fouling. 

The continuous feed of corrosion inhibitors would be required to control
the corrosion of copper alloys and carbon steel components of the
system.  Corrosion is an electrochemical process; the higher the TDS of
the cooling water, the higher the rate of electrochemical reaction and
the greater the concern with corrosion issues for cooling towers over
once through systems.

There may be a requirement for clarification, to remove suspended solids
to produce makeup water suitable for cooling tower use.

There may be a requirement for cold lime softening, to produce makeup
water with a calcium concentration suitable for cooling tower use.

The following table lists the quantities of chemicals required for
treatment for a 17,400 gpm cooling tower.  As calculated in section II.7
the blowdown for this cooling tower would be 46 gpm and makeup water
would be 315 gpm.

Chemical	Concentration(ppm) in Circulating Water	

Usage (lbs/yr)*

Dispersant	100	27,600 

Corrosion Inhibitor	20	5,520 

Sulfuric Acid	150	41,400

Chlorine	150	41,400

*assumes operation 24 hours per day, 365 days per year

Changes in water pressure requirements?

The typical arrangement in a once through cooling system is to provide
enough pressure in the supply pumps to overcome the hydraulic losses in
the cooling water supply piping, heat exchangers and cooling water
return piping in the refinery.  As discussed, the volumetric capacity of
once through cooling water pumps is significantly lower for cooling
tower systems.  Therefore, new pumps and supply piping will need to be
installed to provide makeup water for the evaporative cooling system. 
New cooling water circulating pumps need to be installed inside the
refinery to carry the cooling water to all the exchangers and return it
to the cooling tower.   The table below gives a comparison of the
typical pressure requirement of the circulating pumps in a once through
system and a recirculating cooling water system.

Component	Differential Pressure  Allowance (psi)

	Recirculating Cooling Water System	Once Through Cooling Water System

Cooling Water Supply Main Header	15	15

Cooling Waster Supply Branches	5	5

Cooling Tower Static head (~45 ft.)	20	0

Cooling Water Return Branches	5	5

Cooling Water Return Main Header	15	15

Differential Head Required 	  =SUM(ABOVE)  60 	  =SUM(ABOVE)  40 

Differential Head Required	140 ft	93 ft



Changing to Dry Cooling

What would be involved in changing to dry cooling?

There are two types of dry cooling systems: direct dry cooling and
indirect dry cooling.  Direct dry cooling systems utilize air to
directly condense or cool the process stream while indirect dry cooling
systems utilize a closed cycle water cooling system to cool the process
stream and the heated water is then air cooled. Indirect air cooling is
used in power plants while direct cooling is used for certain
applications in refineries depending on process outlet temperature and
plot space considerations.  

Indirect air cooling is not used in refineries because the lowest
achievable cooling water supply temperature would be about 10oF higher
than the dry bulb temperature for that location.  Frequently, process
streams in refineries need to be cooled to a temperature lower than the
dry bulb temperature and, therefore, indirect dry cooling is not
technically feasible. 

Direct air cooling used in refineries is dictated by the dry bulb
temperature at that site and conditions of the process streams such as
inlet temperature and required outlet temperature.  As previously
discussed, air cooling can be used in refineries only for specific
applications and even in these applications cooling water exchangers are
installed downstream of the air coolers in order to meet the process
requirements. Additionally, since the heat transfer coefficient for air
coolers is relatively low, air cooled exchangers tend to be large,
require significant plot space, and can be a source of noise. 

Replacement of once through cooling with air-cooled heat exchangers
would demand an economically infeasible number of changes to the
existing process equipment.  Furthermore, whereas cooling water
shell-and-tube exchangers can be stacked vertically in process units,
air coolers cannot be stacked (because lower units would send heated air
to higher units) and so it may be physically difficult or impossible due
to space constraints to replace water-cooled exchangers with air-cooled
exchangers.

Cooling Water Recycle and Reuse

What are the options for recycle/reuse of cooling water?  Practicality?

Petroleum refineries in the U.S. practice water recycling where
possible.  For example, steam condensate is recycled back to the to
system since it has a low dissolved solids content.  Condensate that has
been in contact with process fluids is also recycled, such as
distillation overhead condensate and stripped sour water recycled to the
desalter.

In general, water needs to have a low dissolved solids content in order
to be considered as a candidate for recycle in a petroleum refinery. 
Cooling tower blowdown usually has a high dissolved solids content and
is sent to wastewater treatment. 

Cooling Water Intake Structures

What types of intake structures are used at refineries

Refinery water intake structures were built to accommodate design flow
rates and to screen out trash, aquatic life, and debris that could
damage pumps and heat exchange equipment. An intake structure typically
consists of a pipe submerged in the waterway or a concrete channel at
the shore, each with bar screens or trash racks.  More sophisticated
designs can include traveling screens, fixed screens, perforated pipe,
and velocity caps; however, it is believed that most facilities use
simple intake structure designs.

What is involved in retrofitting refinery intake structures (e.g.,
moving screens)?

Upgrading refinery intake structures involves a variety of
considerations, and will depend on the type of upgrade being
implemented.

Permitting.  Any construction at the shore of a waterway will require
permits from the Army Corps of Engineers under Section 404 of the Clean
Water Act.  Such permits can involve significant studies to evaluate the
impact of new construction on the waterway. In some cases, certain
technologies (e.g., velocity caps) may be difficult to permit because of
potential interference with navigation.

Redesign of intake structure to reduce intake velocity.  Meeting
potential maximum intake velocity requirements could mean extensive
redesign of the structure to make the intake area larger or to replace
pipes with intake channels.  

Redesign to accommodate new technology.  Retrofit of new technology
would likely require substantial disruption of the existing operation. 
For example, installation of traveling screens will likely require
increased intake channel width and depth.  If the existing intake is a
pipe, installation of traveling screens could require shutdown of the
cooling water flow  for days to weeks.  

Fish return system.  For traveling screens, a fish return system would
be needed to return fish and other aquatic life to the waterway.  Such a
system would require adequate distance from the intake to insure that
the organisms are not re-entrained.

Utility Access.  Traveling screens and some other technologies would
require electrical service to operate pumps, motors, lighting, etc.  In
many cases, the intake can be remotely located, and installation of
electrical service can be prohibitive.  For passive screens, an air
supply would be required to provide periodic pulse-cleaning
capabilities.

Tie-in/refinery down time.   Installation of new intake structures would
likely require that the cooling water flow be curtailed or stopped
completely while the tie-in is made.  Depending on the nature of the
tie-in, the activity could take days or even weeks to complete.  Since
the refinery would not be able to operate at capacity during this time,
the cost of lost production can be substantial.

Land availability.  In some cases, access to land at the water body may
be limited, and installation of larger equipment and associated
utilities may require the purchase of additional property.

Other information on cost-effectiveness of intake structure controls?

The cost-effectiveness of intake controls can be highly variable for the
population of refinery intakes.  In some cases, intake controls can be
very cost-effective; that is, the technology provides a significant
reduction in adverse environmental impact (e.g., ambient fish population
increases through reduced impingement and entrainment) of the cooling
water intake.  On the other hand, in some cases, even the best controls
will have little impact on the populations of aquatic species in the
water body.  Therefore, evaluation of cost-effectiveness of controls
should be done on a site-specific basis.  Some of the factors that will
impact cost-effectiveness include ambient species that would be
potentially affected, water body size/flow/tidal exchange, prevailing
currents, ambient water quality for supporting aquatic life, and
seasonal variations in all these factors.

In the Technical Development Document (EPA-821-R-01-036, November 2001)
for the 316(b) regulations for new facilities, EPA has provided
documentation of entrainment and impingement performance of various
control technologies at power plants.  The table below summarizes the
performance information for some of the technologies that may be
applicable to refinery intakes.  However, the TDD notes that, although
these technologies are promising, the performance is subject to
site-specific considerations, and that there are relatively few
successful examples of full-scale installations.  Therefore, information
on qualitative costs and technical feasibility considerations has been
included in this table to provide further insights on technology
selection.

Technology	Effectiveness	Relative cost	Technical Feasibility

	Impingement	Entrainment



Diversion/behavioral devices	Highly variable, depending on species	No
effect	Low	Generally feasible

Fabric Screen	100%	>90%	Low	Subject to damage under severe conditions

Velocity cap	Species dependent, up to 90% reported	No effect	Moderate
For offshore intakes

Wedge-wire screen	Reported to be very effective ("no impingement")
Variable, depending on slot size	Moderate	Prone to plugging, biofouling;
adequate ambient water current needed.

Traveling fine-mesh screen	Up to 90%	Variable, depending on mesh size,
over 90% reduction reported for certain species	Moderate	Prone to
plugging, biofouling. Little operating experience.

Traveling screen with modified fish return system	70-80% reduction over
conventional traveling screens	Generally not effective, depends on mesh
size	Moderate	Generally feasible



Cost Implications

Cursory review of the  EPA’s “Draft Initial Cost Estimates”--  May
17, 2001?

Section 316(b) of the Clean Water Act requires EPA to ensure that the
location, design, construction and capacity of cooling water intake
structures reflect the best technology available for minimizing adverse
environmental impact. EPA prepared a draft report dated May 17, 2001,
that contains the preliminary analyses to estimate the cost of cooling
water intake structures and cooling system technologies that are in
place at existing facilities.

The draft document was reviewed and we offer the following comments:

Chemical Addition Systems: Chemical addition system costs are not
addressed  in the draft report.  Typically, once through cooling systems
in refineries do not have chemical addition systems other than
intermittent chlorination.  The chemical use is cooling tower systems
contribute significantly to operating costs

Salvage Credit: In the EPA report, credit is taken for the salvage value
of equipment when changing over from once through to recirculating
cooling water system.  Credit for old equipment in the case of
refineries is location specific and depends to a large extent on the
condition of the old equipment.  Our experience shows that salvage
credit for equipment in good condition is no more than 10 cents on the
dollar.  The analysis of costs should be based on no credit for salvage
on old equipment and piping.

Installed Costs: An installation factor of 30% on equipment costs has
been used to determine installed costs.  Our experience with refineries,
supported by the literature, indicate that this factor should be 300 to
600% of equipment costs( depending on the location of the refinery and
the general standards (for piping and equipment) that are typically
applied to equipment and piping in refineries.

Dry Cooling: The costs associated with the Dry Cooling option are
incomplete.  They do not include the costs of replacement of turbines,
which would represent significant capital costs.  Switching entirely to
dry cooling for refineries is not a viable option since, as discussed
earlier in this report, the cooling needs frequently require the process
streams to be subcooled and it is not possible to achieve this using air
coolers. 

Economic & Engineering Analysis of Proposed 316(b) New Facility Rule,
August 2000

The EPA draft report stated that with certain exceptions, EPA developed
all of the draft initial cost estimates using the cost data presented in
the Economic and Engineering Analysis of Proposed 316(b) New Facility
Rule, August 2000 (EEA).

After review of the EEA, taking into account the referenced exceptions,
it appears that the cost estimates presented to convert once through
cooling to a recirculating system are based on a number of assumptions
that are not representative of conditions at petroleum refineries and do
not recognize the inherent differences between refinery and power plant
cooling systems. If these assumptions were changed and the differences
were recognized, the cost for conversion of once through to
recirculating systems (cooling towers) at refineries would be
significantly higher. Examples of these assumptions and factors are
briefly described below along with the reference in the EEA.

Differences between Refinery and Power Plant Cooling Systems: The cost
for conversion of once through cooling to cooling towers at refineries
would be significantly higher than for power plants. This is due to the
inherent differences between petroleum refineries and power plants.
Based on the greater number of refinery sources that require cooling
water and the typical geographic layouts of these various refinery
sources, petroleum refineries would tend to end up with more cooling
towers, of smaller size, than power plants. For equal intake cooling
tower flows, the cost for conversion at petroleum refineries would
therefore be significantly greater than for power plants. The reference
that discusses the cost of switching to a recirculation system is found
on page 6-4 of the EEA.

Differences in Refinery Cooling Tower Intake Flows: The average cooling
tower intake flow is presented for the category of "petroleum and coal
products", Appendix A, Table A-2. The number presented is 320 gpm, for
3,509 facilities. There are only approximately 150 refineries operating
in the United States, so this sample base of 3,509 facilities is not
representative of refineries. 

A limited survey of five refineries using once through water shows the
average intake flow to be twenty times higher than the EPA estimate.

Consequently, the actual average intake flow rate for refineries is
significantly greater than used in the EEA. This suggests that the cost
curves that were developed and included in Appendix A, which correlates
capital cost and flow may be inaccurate.

Effect of Additional Refinery Heat Exchanger Capacity. As described in
the response to question II-13, use of cooling tower water instead of
once through cooling water will result in a 22% increase in heat
exchanger area. Considering that refineries are more likely to have
significantly more sources that require cooling than power plants, the
costs for additional heat exchanger capacity, for the same cooling water
intake flow, will be greater at refineries than for power plants. This
factor is not included in the discussion on page 6-4 of the EEA on
switching to a recirculating system.

Estimate the cost of installing a 17,400 gpm cooling tower

Capital Costs:

For comparison purposes, the cost estimate for a 17,400 gpm (circulation
flow) cooling tower was developed. Even though this circulation rate is
small compared to a mid sized refinery, it is used for illustration
purposes because EPA used this size in the EPA study.  The next higher
EPA case was 104,000 gpm, which was considered too high to use as a
representative case. The cost estimate includes the cost of the raw
water intake system, a clarifier for suspended solids removal, induced
draft cooling tower, circulating pumps and interconnecting piping.  The
costs do not include any changes that might be required to the heat
exchangers in the refinery due to the change from once through to a
recirculating system.  The costs also do not include demolition costs
and does not take any credit for salvage of old equipment and piping.
Figure 6 shows a schematic of the system that illustrates the systems
included in the cost estimate.

Typically, a travelling screen would not be installed for a small intake
flow (315 gpm) as is the case for this tower system.  However, since the
purpose of the cost estimate is to compare it to the estimate in the EPA
draft document, a travelling screen has been included in the estimate.

The costs are based on vendor written and oral quotes for major
equipment and ENSR’s database for other items.

Basis:

Cooling Water Circulation (gpm)	17,400

Design Temperature Difference (oF)	15

Design Dry Bulb Temperature (oF)	91

Design Wet Bulb temperature (oF)	80

Cooling Water Supply Temperature (oF)	90



Figure 6

Cooling Tower System for Estimate

The following table provides a summary of the cost estimate.

1	Process Equipment Items (Note 1)	Quantity	Unit Cost	Total Cost

2	Intake System



	3

Travelling Water Screens	2	70,000	$140,000

4

Intake Pump (315 gpm x 1.25 (SF) = 400 gpm, 120 Ft)	2	10,700	$21,400

5

Clarifier Mechanism (40' Diameter and 16')	1	180,000	$180,000

6

Clarifier Tank (40' D; 16' L Field Erected)	1	235,000	$235,000

7

Feed Pump (315 gpm x 1.25 (SF) = 400 gpm, 30 Ft)	2	8,200	$16,400

8	Cooling Tower



	9

Circulation Pump (20,000 gpm, 160 ft)	2	279,000	$558,000

10

Basin (36' x 72')	3	100,000	$300,000

11	Treatment System



	12

Dispersant Tank - 2000 gallon	1	3,000	$3,000

13

           Feed Pump and Controls	2	2,000	$4,000

14

Corrosion Inhibitor Tank - 500 gallon	1	1,000	$1,000

15

           Feed Pumps and Controls	2	2,000	$4,000

16

Sulfuric Acid Tank - 2000 gallon	1	7,000	$7,000

17

           Feed Pump and Controls	2	19,000	$38,000

18

Bleach Tank - 5000 gallon	1	8,000	$8,000

19

           Feed Pumps and Controls	2	19,000	$38,000

20	Total Process Equipment (T.P.E.)

	$1,553,800

21	Piping & Mechanical	[20]T.P.E. x	0.80	$1,243,000

22	Civil – Site Work, Structures, Foundations & Earthwork	[20]  T.P.E.
x	0.90	$1,398,000

23	Electrical	[20]  T.P.E. x	0.30	$466,000

24	Instrumentation	[20]  T.P.E. x	0.15	$233,000

25	Fire Protection	[20]  T.P.E. x	0.01	$16,000



26	Insulation	[20]  T.P.E. x	0.02	$31,000

27	Painting	[20]  T.P.E. x	0.1	$155,000

28

Sub Total (Equipment and Installation T.E & I.C)

	$5,096,000

29	Project Management & Engineering



	30	Home Office Engineering	[28]  x	0.15	$764,000

31	Construction Management	[28]  x	0.05	$255,000

32	Line Item (Note 1)	3	$1,034,000

	33	Cooling Tower (9000 gpm 2 operating, one spare) 

	$3,102,000

34	(Field erected, marked up for engineering and civil works)



	35	Total Installed Cost (T.I.C.)

	$9,217,000

36	Rounded up Cost

	$9,200,000

Cooling tower cost obtained from vendor has been marked up:

20% for engineering

90% for civil, sitework and foundation

Process Equipment Cost: Costs of all process equipment are based on
vendor quotes and ENSR database. It does not include the cost of any
spare parts, taxes, freight and future escalation. It also does not
include buyer's indirect costs.

Cost estimate is based on using ANSI pumps.  If API pumps are required
the cost of the pumps will increase by a factor of three.

Piping & Mechanical: The cost of installing pipes interconnecting the
process equipment. It includes all the valves, fittings and
installation.

Civil, Site Work, Structures, Foundation and Earthwork: Includes the
concrete pad for installation of process equipment. It does not include
construction of any buildings.  Does not include demolition and removal
of any equipment. It also does not include installation of any sewer
system. 

Electrical: Includes the cost of connecting the new equipment to
existing motor control centers.  Does not include the cost of upgrade of
the MCC.  Does not include the cost of bringing additional power or
upgrade of transformers.

Instrumentation: Connecting all the necessary process equipment into a
new control loop and connecting it to an existing control room.

Fire Protection: Installation of fire protection.

Insulation: The cost of installing insulation on pipes and equipment. It
does not include any heat tracing of pipes and equipment.

Painting: The cost of painting process equipment.

Home Office and Engineering: Includes process engineering and detailed
engineering for the cooling tower systems.

The following is the list of items that are not included in the cost
estimate.  This list is not intended to be a comprehensive list of
exclusions but highlights important items:

Does not include the cost of any spare parts, taxes, freight and future
escalation. It also does not include buyer's indirect costs.

Cost estimate is based on using ANSI pumps. If API pumps are required
the cost of the pumps will increase by a factor of three. 

Does not include construction of any buildings.  Does not include
demolition and removal of any equipment. It also does not include
installation of any sewer system. 

Does not include the cost of upgrade of the MCC if required.  

Does not include the cost of bringing additional power to the site or
upgrade of transformers.

Does not include the cost of engineering analysis to upgrade heat
exchangers in the refinery.

Does not include the cost of replacing the heat exchangers in the
refinery.

Does not include the cost of handling and treatment of sludge from the
makeup water system.

Does not include the cost of handling and treatment of the blowdown
stream.

Does not include the cost land acquisition (if necessary).

Operating Costs

The following table defines the operating and maintenance costs for the
cooling tower system.  The operating costs are based on 8,000 hrs/yr of
operation.

Category	Cooling Tower ($/yr)	Once Through ($/yr)	Net Costs ($/yr)

Pumps and Blowers (@ 8,000 hrs/yr, 0.07 $/kwh)



	Cooling Tower Fan	156,700

156,700

Intake Pump	6,800

6,800

Feed Pump	2,300

2,300

Recirculating Pump	452,600	452,600	_

Chemical Addition Pumps	2,800

2,800

Chemicals (@ 8,000 hrs/yr)



	Dispersant	42,000

42,000

Corrosion Inhibitor	9,600

9,600

Sulfuric Acid	5,000

5,000

Chlorine	27,600

27,600

Spare Parts (2% of Total Purchased equipment -$2,963,800)	59,300



59,300



Labor (Operating and Maintenance (1 @ $30 ea, 8760 hrs)	262,800	262,800
_

Total Operating Cost per Year (rounded)	1,050,000	715,400	334,600

Cost Comparison

The following table compares the costs included in the EPA May 17 2001,
draft document to the costs developed by ENSR for refineries:

17,400 gpm CT w/travelling screen	ENSR estimate	EPA estimate

Capital Costs (installed)	$ 9,200,000	$ 1,340,000

Incremental Operating Costs	$ 334,600	$ 179,000

References

Gary, James H., & Handwerk, Glenn E., Petroleum Refining Technology and
Economics, Marcel Decker Inc, New York, 1975

Kern, Donald Q., Process Heat Transfer, McGraw-Hill Book Company, 1950

Rase, Howard F., & Barrow, M.H., Project Engineering for Process Plants,
John Wiley & Sons Inc., 1957

Evans, Frank L., Equipment Design Handbook for Refineries and Chemical
Plants, Gulf Publishing, 1974

Nelson, W.L., Petroleum Refinery Engineering, McGraw Hill Company, 1958

Perry, John H., Chemical Engineers Handbook, Fourth Edition, McGraw Hill
Book Company 1963

Peters, Max S. and Klaus D. Timmerhaus, Plant Design and Economics for
Chemical Engineers, Second Edition, McGraw-Hill, New York, 1968.

( Installed cost should reflect the total fixed capital investment,
including not only equipment installation, but costs of piping,
utilities, instrumentation, yard improvements, service facilities,
buildings, contractor’s fees, and contingency.  Fixed capital
investment typically ranges from 371% to 501% of the purchased equipment
cost; or up 566% under inflationary economic conditions.  From Plant
Design and Economics for Chemical Engineers, Second Edition, Peters, Max
S. and Klaus D. Timmerhaus, McGraw-Hill, New York, 1968, p.105.

 

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No

Yes

Yes

No

No

Yes

Yes

No

Determine

temperature to

which process

stream needs to

be cooled

Is stream hot

enough for steam

generation

Is stream hot

enough for  heat

recovery

Design steam

generator and

determine process

outlet temperature

Is air cooling

feasible

Use cooling water

exchanger

Design heat

exchange with

other process

streams

Design air cooler

and determine

outlet temperature

Is further cooling

required

Done

