ALGOR HEAT TRANSFER ANALYSIS SOFTWARE AIDS IN DESIGN OF
TURF CONDITIONING SYSTEM TO KEEP THE CLEVELAND BROWNS FIELD
GREEN
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The new Cleveland Browns stadium, which just saw
the completion of its first football season, hosted
games late into December with the aid of a turf conditioning
system developed and installed by REHAU, Inc. North
America. The system enables healthy, green turf to grow
long after the normal growing season ends by distributing
heated fluid through an underground piping network.
The heat radiates through the soil to keep the grass
root zone at a constant temperature, preventing the
turf from freezing even in chilly lake-effect weather.
The above picture shows the exposed piping network on
a bed of gravel, before the turf was installed. |
Rising 12 stories above Lake Erie, the state-of-the-art
Cleveland Browns stadium boasts some impressive statistics,
including 72,500 seats, 148 luxury boxes, 103 permanent
and portable concessions stands, 948 toilets and 11.65 miles
of plumbing pipe. Also at the top of the notable list is
a turf conditioning system consisting of more than 40 miles
of PEX pipe installed beneath a 75-yard by 115-yard playing
field of natural, fully irrigated turf. Designed and installed
by Virginia-based REHAU, Inc. North America (REHAU), the
turf conditioning system enables healthy, green turf to
grow long after the normal growing season ends, which is
important to playing year-round outdoor football in Cleveland.
Rehau’s turf conditioning systems distribute heated fluid
from a series of boilers through an underground piping network.
The heat radiates through the soil to keep the grass root
zone at a constant temperature, preventing the field from
freezing even in chilly lake-effect weather. Without the
system, the field would deaden late in the football season,
resulting in severe damage since the turf would be unable
to repair itself after the wear and tear of a Sunday afternoon
game.
REHAU engineers used heat transfer analysis software from
Pittsburgh-based ALGOR, Inc. to determine the amount of
energy required to run the system and optimize overall system
performance under varying environmental conditions. In addition,
replays of ALGOR transient heat transfer analysis results
provided stadium developers with a visual explanation of
how REHAU’s existing turf conditioning technology would
work at the new stadium.
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Workers installed more than 40 miles of a cross-linked
polyethylene pipe called RAUPEX, a pipe developed by
REHAU that exhibits superior strength and flexibility.
REHAU engineers used steady-state heat transfer analysis
software from ALGOR, Inc. to determine the spacing needed
between the pipe as well as the fluid temperature required
to heat the turf root zone. |
Existing Design Technology, New Analysis Methodology
While ALGOR was new to the design of the Cleveland turf
conditioning system, the technology used to build the system
has been tried and tested for over 25 years, according to
Product Manager Patrick Sauer.
"The Cleveland turf conditioning system uses RAUPEX?/font> piping, which is made of high-density cross-linked
polyethylene," Sauer explains. "Individual polyethylene
molecular chains are linked into a 3-D network under high
temperature and pressure to provide superior strength and
flexibility."
REHAU’s cross-linking process, developed by German scientist
Thomas Engel and licensed by REHAU in 1967, significantly
enhances the temperature resistance, long-term strength,
impact strength, creep resistance and elastic behavior of
the polyethylene. The Cleveland turf conditioning system
is a closed-circuit system that uses RAUPEX piping with
a ?inch diameter arranged in rows beneath the surface of
the playing field, end zones and sidelines. The piping segments
are connected at approximately 2000 circuit locations using
REHAU’s patented EVERLOC?/font> fitting
system. As fluid circulates from the boilers, it flows through
several large headers, which then branch off into smaller
circuits to uniformly heat about 96,000 square feet of surface
area. The system is divided into four rectangular sections
that can be independently controlled so that if part of
the field is shaded, for example, it can be heated without
overheating the other areas of the field.
System designers used ALGOR’s steady-state heat transfer
analysis software to optimize the pipe spacing to ensure
uniform heating. If pipes are placed too far apart, brown
strips of grass may appear on the field surface where the
grass root zone is not heated adequately.
The flow of fluid through RAUPEX pipe causes it to expand.
The molecular structure of RAUPEX piping enables it to compress
to its original shape after expansion, causing significant
loading on fittings that connect individual piping segments.
According to Sauer, the EVERLOC fitting system takes advantage
of the pipe’s compressive properties.
"The pipe is expanded over an EVERLOC insert to provide
maximum strength without reducing the inside diameter of
the pipe," says Sauer. "Then a sleeve is pressed onto the
fitting to ensure the integrity of the connection. This
fitting system enabled us to design the conditioning system
without worrying about the accessibility of the joints after
installation."
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The pipe network lies on a bed of gravel, as shown
in this cross section, to enable drainage away from
the pipe. High levels of moisture in the soil can cause
increased thermal conductivity, which results in the
rapid loss of heat. Soil composition affects how well
soil retains moisture. REHAU engineers considered both
composition and moisture level when specifying material
properties for the ALGOR steady-state and transient
heat transfer analyses. |
The pipe network lies on a bed of gravel beneath a sandy
soil mixture. The composition and moisture level of the
soil can affect the conductivity of heat from the pipes
through the soil to the root zone inches above. Sandy loam
releases fluid at a faster rate than dense clay soil, while
wet soil releases more energy in the form of heat into the
air than dry soil does, causing faster decreases in field
temperature. On the other hand, drier soil releases less
heat from the field, but requires longer periods of time
to heat the root zone. According to Sauer, the herringbone
construction of the gravel drainage system beneath the pipe
network helps to prevent downward heat loss, optimizing
the amount of heat that reaches the root zone above.
REHAU engineers also needed to consider external variables,
including weather conditions and the length of the grass,
in the ALGOR heat transfer analyses. The wind speed, which
helps to determine the heat transfer coefficient in the
analysis calculations, and the ambient temperature will
affect the rate of heat loss from the surface. In addition,
longer grass will retain more energy than will shorter grass.
According to Project Analyst Scott Posey, these varying
factors required new considerations when performing the
heat transfer analyses.
"This application was different from other heat transfer
analyses I’ve performed. In this case, the heat is transferred
to another substance entirely, not just across a uniform
surface having one material type," says Posey. "The heat
transfer analyses also helped us to better understand how
external environmental conditions can affect thermal conductivity."
Based on the steady-state results, Posey also developed
transient heat transfer analyses to determine how the system
would respond to changes in external environmental conditions
over time.
Steady-State Analyses Help Determine Energy Requirements
To be awarded the Cleveland turf conditioning system contract,
REHAU needed to demonstrate both installation and operational
costs, including the quantities of pipe needed, energy required
to operate the system (which determines the number of boilers
needed) and the temperature at which a tarp would be required
to protect the field. The required fluid temperature within
the system was the key to determining all of these factors.
Posey started with a steady-state heat transfer analysis
to find the optimal fluid temperature needed to keep the
root zone at 72?/font> F when the ambient
temperature is 5?/font> F, the criteria
specified by the stadium developers. He created a 2-D isotropic
model of a typical cross section, which consisted of two
parallel pipes within layers of gravel, soil and turf, using
Superdraw III, ALGOR’s single user interface for FEA and
precision finite element model-building tool. Posey used
hand-meshing techniques to make a uniform mesh across the
model and then refined the mesh around the pipes.
Once he had built the model, Posey used individual groups
with representative colors (i.e., group 8, identified as
gray, was used for gravel) to specify the different material
properties for gravel, turf, soil and polyethylene. The
1993 Ashrae Fundamentals Handbook provided material properties
for gravel and turf, including convection coefficients for
turf under varying wind speeds and at no wind speed, which
Posey used in this case. In addition, Posey used the Ashrae
handbook to find the appropriate material properties for
the soil.
"We consulted with the field contractor to determine the
approximate soil composition and moisture level to use in
the analysis and then sourced the handbook to get the properties,"
says Posey. "We assumed a conservative moisture level of
42 percent to ensure the system would be effective through
highly conductive soil."
The material properties for polyethylene were taken from
REHAU’s past extensive materials research and testing. Using
Algor’s Material Library Manager, Posey added the material
properties to a customized library so the properties will
be available for future analyses.
Finally, Posey added a temperature boundary of 58?/font> F, the ground temperature at that depth, to
the bottom edge of the model and a temperature boundary
of 128?/font> F at the internal circumference of the pipes.
According to Posey, he chose a temperature of 128?/font> F based on Sauer’s extensive design experience
with previous conditioning systems. In addition, the maximum
allowable temperature for RAUPEX piping is 140?/font> F.
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The steady-state heat transfer analysis results
showed that an optimal fluid temperature of 128?F is
needed to maintain a root zone temperature of 72?F when
the ambient temperature is 5?F with no wind. REHAU engineers
used these results to determine how much energy would
be required to operate the turf conditioning system. |
With Sauer’s knowledge of the systems as a guide, Posey
conducted about five analysis iterations, in which he varied
the fluid temperature and pipe spacing, to verify the optimal
pipe spacing and fluid temperature needed to properly heat
the field surface area. Posey found ALGOR’s total flow option,
which automatically calculates heat flow over a surface,
to be especially useful in determining the conditioning
system’s heating requirements.
"In the past, I have manually calculated the total heat
flow by averaging heat flux values at individual nodes.
This is a linear approach to an often nonlinear problem,"
says Posey. "With the total flow calculation, the software
saves me time by automatically calculating these values
and it provides a more accurate result."
Posey examined the temperature distribution and heat flux
results using ALGOR’s built-in visualization capabilities
and determined that a fluid temperature of 128?/font>
F would sustain the minimum root zone temperature. Factoring
in this temperature value, Sauer and Posey calculated the
energy required to operate the system in BTUs per hour per
square foot. The calculation indicated that the Cleveland
turf conditioning system would require a maximum of nine
boilers under severe conditions; however, the engineers
estimated that half that number would be needed under normal
conditions.
The need to regulate the number of boilers in use and to
adjust the fluid temperature to accommodate environmental
changes led Posey to explore how quickly the system could
respond to falling air temperatures. He was able to do this
using ALGOR’s transient heat transfer analysis software.
Transient Analyses Yield More Than Just Results
Using the same model geometry and material properties,
Posey set up the transient heat transfer analysis to simulate
a drop in ambient temperature, similar to what would occur
over the course of a sunset, and a corresponding rise in
fluid temperature to keep the root zone at a constant 72?/font>
F. He adjusted the temperature boundary conditions, defined
load curves and specified the duration of the event for
the transient analysis.
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ALGOR’s transient heat transfer analysis software
was used to simulate a drop in ambient temperature,
similar to what would occur over the course of a sunset,
and a corresponding rise in fluid temperature to keep
the root zone at a constant 72?F. REHAU engineers specified
two load curves in Superdraw III, ALGOR’s single interface
for FEA and precision finite-element model building
tool. The plots helped the engineers to ensure their
data was correct before performing the analysis. |
Posey first ran a steady-state heat transfer analysis to
determine the fluid temperature needed (105?/font> F) to maintain the root zone temperature with
an ambient temperature of 35?/font>
F. Then for the transient heat transfer analysis, he set
the duration of the event. Posey specified 384 timesteps
(or every 15 minutes for four days) and then defined a capture
rate of one, so that he could review the results for each
timestep. Posey also redefined the applied temperature boundaries
at the inside pipe circumference to correspond with the
first load curve, which described changes in the system
fluid temperature. Then he placed temperature boundaries
at the top of the model (at the turf surface) to correspond
with the second load curve, which described the changes
in ambient temperatures.
The first load curve caused the system fluid temperature
to ramp up to the previously analyzed temperature of 105?/font> F and then establish a steady-state situation.
Then Posey increased the temperature again to 128?/font> F to correspond with the second load curve,
which simulates a drop in ambient temperature from 35?/font> F to 5?/font> F over a
period of about three hours. Posey ran several analysis
iterations to determine when and how much to increase the
fluid temperature as the ambient temperature drops.
"The load curve plots displayed in Superdraw’s data entry
screens let me visualize load curve data and ensure that
it was correct before running the analysis," says Posey.
"Conducting the transient analyses required a lot of trial
and error on my part," continues Posey, who had no prior
experience with this analysis type. "I was able to successfully
perform the analyses with the help of the documentation
provided through ALGOR’s DocuTech system."
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Using ALGOR’s Monitor utility, the engineers superimposed
temperature changes vs. time plots for the fluid, soil,
root zone and turf, which were determined in the transient
heat transfer analysis. The engineers concluded that
the system fluid temperature should be increased slightly
before the anticipated drop in ambient temperature to
keep the root zone temperature constant. |
The REHAU engineers used ALGOR’s Monitor utility to superimpose
temperature changes vs. time plots for the fluid, soil,
root zone and turf. Having all of the data on one screen,
Posey was able to conclude that the system fluid temperature
should be increased slightly before the anticipated drop
in ambient temperature to keep the root zone temperature
constant. Posey also generated analysis replays in AVI format
to animate the changes in temperature distribution over
time.
"ALGOR’s bitmap to AVI converter enabled me to add in text
at important points throughout the event. This helped to
explain what happens in the analysis to others who may not
be familiar with our turf conditioning systems or finite
element analysis," Posey says.
ALGOR’s transient heat transfer analysis software illustrated
system performance under falling environmental temperatures,
showing that the system is capable of adjusting to changing
conditions without dramatic temperature changes at the root
zone. The transient heat transfer analysis also verified
the performance capabilities for an automatically controlled
heating system. In addition, replays of the transient results
provided an invaluable visual tool that aided REHAU in earning
the Cleveland turf conditioning system contract, according
to Sauer.
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Analysis replays of the ALGOR transient heat transfer
analysis show the temperature distribution throughout
the cross section of the system. The first temperature
plot (left) shows a steady-state situation, in which
the fluid temperature is at 105?F, the temperature needed
for a constant root zone of 72?F with an ambient temperature
of 35?F. The second plot (center) shows the fluid system
temperature increasing as the ambient temperature decreases.
The third plot (right) indicates a fluid temperature
of 128?F with an ambient temperature of 5?F. |
"While the technology used in the Cleveland system is not
new for REHAU, the use of ALGOR heat transfer analysis software
was new," Sauer said. "Being able to illustrate how the
system works using analysis replays was an important factor
in the contract discussions for the Cleveland project. The
analysis replays also have been an integral part in securing
additional contracts with other NFL stadiums."
REHAU is already planning future transient heat transfer
analyses using ALGOR. The engineers plan to study how the
turf conditioning systems can be used as cooling mechanisms
to keep turf from scorching during the humid summer months
or in year-round hot climates. REHAU is currently developing
turf conditioning systems for other NFL teams.
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