The cooling water system (CWS) in a refinery
consists of four loops that can be operated independently. Each loop is linked
to a cooling tower that is similar conceptually but has different capacities
(from two to five cells). Each of these cells is equipped with a fan that can
be manually switched on or off to optimize cooling performance. Every cooling
water (CW) loop has dedicated CW pumps (from two to four pumps) to maintain a
proper flowrate and manipulate the system pressure. No variable-speed drive is
applied on the CW pumps, so the only “control” for flowrate and pressure is to
switch the pumps on or off.
There are different interconnections
between the four CW loops that allow the CWS to operate flexibly, and which
ensure a high reliability of CW supply. During periods with reduced CW requirements
(winter season, plant shutdowns), it may be possible to switch off one CW cycle
completely. FIG. 1
shows an overview of the Izmit refinery's CWS. The different loops and supply
alternatives to units can be identified by the differently colored piping
system.
The main technical data of the cycles
are listed in TABLE 1.
The data show that the CW pumps and fans are substantial energy consumers, so it
is important to operate them in the most efficient way.
It was decided to replace cooling tower's fan blades with
hollow e-glass epoxy fiber-reinforced plastics (FRP) composite fan blades as
designed on the principles developed by a well-known institution and supplied
by a reputed Indian solution providera.
The function of a cooling system is to remove heat from a process
or equipment. Heat removed from one medium is transferred to another medium or
process fluid. Most often, the cooling medium is water in places where water
availability is not a constraint.The transfer of heat from process fluid or
equipment results in a rise in temperature in the CW. In an open
recirculating system, cooling is achieved through the evaporation of a fraction
of the water. An open recirculated system uses the same water repeatedly to
cool process equipment (FIG.
2). These systems are designed to cool water to a certain
temperature under a given set of conditions.
Counterflow induced draft
towers (FIG. 3)
are a type of tower in a refinery where the working principle of the fan is to
create vertical air movement up the tower and across the packing media in
opposition to the water flow. Therefore, the coldest water contacts the driest
air first. Considering construction and the selection of materials, mechanical
draft towers may be examined as components under the headings of water tank,
water distribution, casing, fan and packing.
Traditional design
guidelines dictate that fans should be designed to operate successfully in
cooling tower service for at least 5 yr. The fans should have a minimum of four
to six blades of aluminum alloy, stainless steel, monel or an equal blend.
Blades should be adjustable for degree of pitch and individually fastened to a
hub of welded steel or cast iron (stainless steel optional at extra cost).
Efficient modern fan design focuses on
the following factors:
Aerospace engineering scientists have been
involved in the aerodynamic design development of FRP fans and a strong body of
knowledge has been developed by the aerospace industry. Technical consultations
with scientists were involved with primary aeronautical designs. While fan
performance depends on material of construction, fan diameter and plenum
optimization, the number of blades and their efficiency can be demonstrated by
lower power consumption. Fan performance is dependent on airfoil section, blade
profile at different radii and staggering, chord length at different radii,
thickness, camber, blade twist, angle of attack and the smooth surface finish of the blades, among
many more parameters. The design of the hub can help minimize air cycling loss,
and selecting the number of blades after consideration of the above parameters determines
the setting of the required air volume and pressure.
While fan revolutions per minute (rpm) are
linked to tip speed, noise and vibration, most of a fan’s performance relies on
fan design and deterioration. Pressure distribution on the airfoil is one of
the main components of performance. Faster airflow causes a decrease in air
pressure. Air flow increases over the curved upper surface if the wing speed
increases.
Increased speed reduces the pressure above the wing and
produces the upward lifting force (Eq. 1):
ղ = Q × TP / KW (1)
where,
Q = Airflow, m3/sec
TP = Total pressure (Pascal)
ղ = Fan efficiency
Fan efficiency formulae. Many criteria factor into material selection: corrosion
resistance to acidic or alkaline water, higher efficiency due to
improvement, aerodynamics
characteristics, structural integrity, high strength-to-weight ratio, and general
adaptability to complex geometries are key parameters. The main types of
materials are epoxy resins and polyesters. Epoxy resins are prefered as they
provide higher mechanical strength, improved chemical resistance, negligible water
absorbption and superior heat resistance. Polyesters are a weak resin structure
and have high water absorption. E-glass composite is the ideal glass fabric of
an axial flow fan as it provides higher tensile, shear and cross-breaking
strength after being bonded with the epoxy resins.
The material used in this project offers
more ultimate tensile and compressive strength than even steel. The blade
material is defined as e-glass fiber-reinforced polymer with a high-grade epoxy
resin. A higher lift-to-drag ratio and aerodynamic design are districtive
chacteristics. The chosen blades are uniform in weight, shape and balance,
provide for manual adjustment of the pitch angle, and are easily removable. They
provide uniform air velocity from hub to tip with low noise and vibration. Each
fan blade is statically balanced and the hub dynamically balanced at the factory
for quieter and more efficient operation.
Process staff and related professionals carried
out the testing in accordance with international standards and code for a field
performance test for cooling tower fans. The international standard measurement
method specifies that measurement points should be measured on the hood with
the anemometer used to measure the air flow.
This kind of test requires mild wind conditions, as the standard
dictates that wind velocity should be less than the average fan outlet velocity.
Air flowrates before and after the execution
of the replacement were kept constant by adjusting the blade angles. Air flowrates
were measured while recording the energy consumptions. As discussed above, an approved
anemometer was used. Linear velocity was measured with this instrument and the
air flow was calculated by multiplying that velocity by the area. All air velocity
measurements were taken from the lowest point of the fan hood with the help of
the pier.
Data was collected from 20 points for some
20 min. After the fans were replaced with new, high-efficient FRP fans, all
measurements were repeated. Scaffolding was required around the hood to make
flow measurements.
Good
test results cannot be obtained in greater wind velocities. It is also not
recommended to perform the test if wind conditions exceed 9 mph–10 mph. İf the wind shifts
direction during the time of flow traverse, the test should be paused. It is
also known that winds higher than 14 mph–15 mph above average cause inconsistent test
measurement values. The delivered air volume can be calculated from the
anemometer velocity reading and is multiplied by the area of the fan stack
discharge. FIG. 4
summarizes the manner of anemometer test positions. An acceptable reading
number is around 20.
An electronic anemometer can directly
read velocity with high accuracy and moisture resistance. It can display
velocity and—for
each measurement quarter—calculate
the average of measurements. Due to the considerable amount of time required, “yaw”
(horizontal and vertical velocities) angle measurements may not be performed as
in this project.
The fan supplier company guaranteed a minimum
18% (± 3%) increase
in the air flow performance at the same or lower power consumption of the existing
operating cooling tower fan after the replacement with a high-efficiency
aerodynamic design e-glass epoxy FRP Fan. The company also guaranteed at least
a 25% (± 5%)
reduction in the power consumption performance at the same operating airflow of the existing
operating cooling tower fan assembly after the replacement with the high-efficiency
aerodynamic-designed e-glass epoxy FRP fan assembly.
The current conventional fan blades were
replaced with new fan blades in four cells in the 9E-501 tower, in five cells
in the 9E-601 tower, in three cells in the 10E-103 tower and in two cells in
the 17E-201 tower. In total, 14 sets of existing fan blades were replaced with a
one-blade addition for each set. The project was implemented from March 2021–August 2021.
Sample details of calculations and
measurements. TABLE 2 summarizes the energy consumption results of the Tower 1 fan blade
change. TABLE 3 shows the
energy measurement values before and after the fan blade replacement.
To calculate the fan area, the following
values were taken for the diameter of the point where the speed measurement was
made and the diameter of the hub. Smaller hub diameters create additional
advantage for the air passage rate:
TABLE 4 shows the energy consumption data as a result
of the Tower 2 fan blade change. TABLE 5 shows the energy consumption data as a result of the
Tower 3 fan blade change. TABLE
6 shows the energy consumption data as a result of the Tower 4 fan
blade change. TABLE 7
shows the air flowrates before and after replacement (m3/hr), and TABLE 8 shows the average
fan outlet air velocity rates (m/sec). FIG. 5 shows the Cooling Tower 9E-501 hot water
return from process (red) and cold water supply to process (blue) temperatures.
No unexpected issues were shown for 2 yr.
Takeaway. After the project’s
implementation, extensive field tests confirmed that the energy savings—for equal performance—averaged 25%–30%. No process issues
have been observed after the installations, and the fans have been working as
intended.
Energy savings as well as improved equipment safety and
control can be expected by imlementing this technology in process plants. Distillation
column overhead fin fan cooler condensers are an ideal place to implement this
technology to improve heat duty, improve mass transfer efficiency, reduce
flaring and significantly improve gross refinery margins. HP
NOTES
a Encon Group
ERHAN OGUS is a Process Superintendent with Tüpras at the İzmut refinery. He has worked in various refinery processes such as crude distillation, power and utilities, and energy management. He has taken part in many energy efficiency projects and is currently focused on water treatment technologies, such as cooling water and boioler feddwater treatments. He earned a BS degree in chemical engineering from Middle East Technical University in Ankara, Turkey.