Posted 2/19/07
Today's process and heating applications continue to be
powered by steam and hot water. The mainstay technology
for generating heating or process energy is the packaged
firetube boiler. The packaged firetube boiler has proven
to be highly efficient and cost effective in generating
energy for process and heating applications.
Conducting a thorough evaluation of boiler equipment requires
review of boiler type, feature and benefit comparison,
maintenance requirements and fuel usage requirements. Of
these evaluation criteria, a key factor is fuel usage or
boiler efficiency.
Boiler efficiency, in the simplest terms, represents the
difference between the energy input and energy output.
A typical boiler will consume many times the initial capital
expense in fuel usage annually. Consequently, a difference
of just a few percentage points in boiler efficiency between
units can translate into substantial savings. The efficiency
data used for comparison between boilers must be based
on proven performance to produce an accurate comparison
of fuel usage. However, over the years, efficiency has
been represented in confusing terms or in ways where the
efficiency value did not accurately represent proven fuel
usage values.
Sometimes the representation of "boiler
efficiency" does not truly represent the comparison
of energy input and energy output of the equipment.
Remember, the initial cost of a boiler is the lowest portion
of your boiler investment. Fuel costs and maintenance costs
represent the largest portion of your boiler equipment
investment. Not all boilers are created equal. Some basic
design differences can reveal variations in expected efficiency
performance levels. Evaluating these design differences
can provide insight into what efficiency value and resulting
operating costs you can expect.
However, every boiler operates under the same fundamental
thermodynamic principles. Therefore, a maximum theoretical
efficiency can be calculated for a given boiler design.
The maximum value represents the highest available efficiency
of the unit. If you are evaluating a boiler where the stated
efficiencies are higher than the theoretical efficiency
value, watch out! The efficiency value you are utilizing
may not truly represent the fuel usage of the unit.
In the end efficiency comes down to value. The value of
the boiler. The value of the burner. The value of the support
provided throughout the life of the equipment.
When you buy a boiler, you really are putting a down payment
on the purchase of steam or hot water. The payments to
generate the power are ongoing over the life of the equipment
and are driven by fuel-to-steam efficiency and maintenance
costs. Even with economical fuel costs, the selection of
a high efficiency boiler will result in substantial cost
savings. A boiler installation costing $75,000 can easily
consume over $400,000 in fuel every year it operates. Selection
of a boiler with "designed-in" low maintenance
costs and high efficiency can really provide savings and
maximize your boiler investment.
Efficiency is only useful if it is repeatable and sustainable
over the life of the equipment. Choosing the most efficient
boiler is more than just choosing the vendor who is willing
to meet a given efficiency value. The burner technology
must be proven to be capable of holding the air/fuel ratio
year in and year out. Make sure the burner design includes
reliable and repeatable features. How do you tell? Ask
any boiler technician who has worked on a variety of boiler/burner
designs. Burners with high pressure drop design, quality
fan and damper design, and simple linkage assemblies are
easy to tune and accurately hold the air-to-fuel ratios.
Burners with blade or louver damper designs and complex
linkage assemblies tend to be harder to set-up over the
firing range of the boiler and tend not to accurately hold
the air to fuel ratio as the boiler operates.
Why choose the most efficient boiler? Because the dividends
paid back each year far outweigh any initial cost savings
of a less efficient design. What is the most efficient
boiler? One that not only starts up efficiently but continues
to operate efficiently year in and year out.
The decision to purchase a new boiler is typically driven
by the needed replacement of an old boiler, an expansion
of an existing boiler room, or construction of a new boiler
room facility.
When considering the replacement of an old boiler, review
the following points to make sure you are performing a
comprehensive evaluation of your situation.
Review your maintenance costs carefully. The old unit is
costing you money in various ways, including emergency
maintenance, downtime, major maintenance requirements (past
and pending), difficult-to-find and expensive parts requirements,
operator time in keeping the unit on-line, and overall
vessel, burner, and refractory problems. Many of these
costs can be hidden within your overall maintenance budget.
You are paying the price for having outdated boiler room
equipment. But the costs need to be investigated and totaled.
New packaged firetube boilers have much higher performance
standards than older design units. Turndown, excess air,
automatic operation, accurate-repeatable air/fuel ratio
burner designs, computer linked combustion controls, low
emission technology, and high guaranteed efficiency all
are now available on premium designed packaged firetube
boilers. The result is low operating costs and automatic
power generation for your facility. All cost saving reasons
to consider a new packaged firetube boiler.
If your old unit is designed to fire low grade fuel oil,
or if you need to evaluate propane or any other different
fuel capability, review the conversion costs along with
existing maintenance, performance, and efficiency issues
to see if the time is right to consider a new boiler purchase.
Many times an investment is made in an old unit when the
costs associated with the next major maintenance requirement
will justify a new unit. The result is wasted money on
the old unit upgrade.
Your Wichita Burner representative can help check out the
efficiency of your old boiler with a simple stack analysis.
The data will give you a general idea of the difference
between the fuel cost of the existing boiler and a new
unit. Based on the results of the stack evaluation, a more
comprehensive evaluation of your boiler room requirements
should be performed. Boiler size, load characteristics,
turndown requirements, back-up requirements, fuel type,
control requirements, and emission requirements, all should
be evaluated. The result will be an accurate review of
the potential savings in fuel, maintenance, and boiler
room efficiency that can mean substantial cost improvement
for your facility.
All firetube boilers are the same? Not true! The fact is
there are key feature differences between firetube boilers.
The efficiency of a firetube boiler is not a mysterious
calculation. High efficiency is the result of tangible
design considerations incorporated into the boiler. Reviewing
some basic design differences from one boiler to another
can provide you with valuable insight on expected efficiency
performance. The following design issues should be considered
during your boiler evaluation.
Number of boiler passes
The number of boiler passes simply represents the number
of times the hot combustion gas travels across the boiler
(heat exchanger). A boiler with two passes provides two
opportunities for the hot gasses to exchange heat to the
water in the boiler. A 4-pass unit provides four opportunities
for heat transfer. Many comparisons have been made regarding
efficiency and number of boiler passes but, the facts are
clear and indisputable. The stack temperature of a 4-pass
boiler will be lower than the stack temperature of a similar
size 2- or 3-pass boiler operating under similar conditions.
The 4-pass will have higher efficiencies and lower fuel
costs. This is not an opinion. It is basic heat exchanger
physics. The 4-pass design yields higher heat-transfer
coefficients.
Many times the lower pass unit will include turbulators
or will be tested at less than capacity firing rates to
prove lower stack temperatures. Don't be fooled. Turbulators
may help pass an efficiency test but will cost you in maintenance
down the road. In fact, you would not need maintenance
intensive, boiler tube, add-on devices if the boiler was
designed for proper flue gas velocities in the first place.
Each boiler pass should be designed with a cross sectional
area providing proper flue gas velocity and heat transfer.
When it comes to efficiency, the proof is indeed in the
passes and in correct heat transfer design.
Burner / boiler compatibility
The term packaged boiler is sometimes used even if a burner
manufactured by one vendor is bolted on to a boiler manufactured
by a different vendor. Is bolting a "Buy-out" burner
on a vessel really a packaged boiler? And more importantly,
why does it matter? A true packaged boiler/burner design
includes a burner and boiler developed as a single unit,
accounting for furnace geometry, radiant and convection
heat transfer characteristics, and verified burner performance
in the specific boiler package. Development as a truly
packaged unit assures the performance of the unit is proven
and verified during development.
You can put an engine from one automobile into a different
automobile. The car will probably run. It will get you
from point "A" to point "B." But how
about performance? Will the car give fuel efficiency and
reliable performance for the life of the car? Would you
take a long trip where you had to depend on such a car?
And if you need service, who will take accountability to
repair and guarantee the car?
A boiler provides the same scenario. The buy-out burner
will fire the unit. But, will you have capacity, efficiency,
turndown, excess air performance and emission performance
too? And, who will make sure the unit gives you performance
after the initial start-up? Is there even a single accountable
manufacturer to make the unit perform in the first place?
Buy-out burner packaging can result in lower performance
levels and higher start-up and maintenance requirements.
It also can cost you money every time you have a problem
and the local service people cannot get factory support.
You may think you saved money with a buy-out burner package.
But did you really?
Repeatable air/ fuel control
The efficiency of the boiler depends on the ability of
the burner to provide the proper air to fuel mixture throughout
the firing rate, day in and day out, without the need for
complex set-up or adjustments. Many burner designs can
deliver the required air-to-fuel mix with enough time provided
to adjust the burner or for a single test period. The problem
is many of these complex linkage designs don't hold air
to fuel settings over time. And, often, they are adjusted
at high excess air levels to account for the inconsistency
in the burner performance. The fact is you pay for the
unit based on the actual ability to operate efficiently.
When it comes to choosing the burner, insist on a simple
linkage assembly and accessible burner design for true
efficiency and real savings.
An additional burner feature to look for is the fan design.
Squirrel cage type fans do not provide as reliable air
control as a reverse curve fan will provide. Aluminum cast
fan design also provides tight tolerances and maximum fan
life. Furthermore, register or blade type damper assemblies
tend to have limited control of air at low firing conditions
and tend to be much less repeatable than radial damper
designs. Control of combustion air is critical to burner
performance. If the burner cannot provide repeatable air
control, again the typical solution is to set the burner
up at high excess air levels, resulting in substantial
dollars wasted every time you fire the unit. The facts
are clear: Reverse fan and radial damper design result
in high efficiency and repeatable fuel savings, thus performance
paying dividends throughout the life of the boiler.
Heating surface
The heating surface in square feet per boiler horsepower
represents, in general terms, how hard the vessel is working.
The standard heating surface for a firetube boiler is five
square feet per boiler horsepower. How do we know this?
Cleaver-Brooks set the standard and provides five square
feet as a base design criteria for our firetube products.
Proper heating surface means longer boiler life and higher
efficiency. Five square feet is the standard.
Vessel design
Pressure vessel design is regulated by strict ASME code
requirements. However, there are many variations in vessel
design that will still meet the ASME codes. Water circulation,
low stress design and accessibility are key criteria for
proper pressure vessel design. Specific features to look
for include a single tubesheet design. Single tubesheet
design provides minimum weldments for low tube sheet stresses
and excellent water circulation. In addition to the single
tubesheet design, the boiler should include proper tube
spacing, cross sectional area sizing in each pass for proper
heat transfer, low furnace location, and proper inlet and
outlet location. Proper circulation must be incorporated
into the design for highest boiler efficiency and longevity.
Fully accessible front and rear tube sheets for ease of
inspection and low retubing costs are also key design criteria
to look for. You will inspect your boiler often, usually
every year. Single tube sheet design assures the longest
lasting tube sheet and longest tube life. Accessible front
and rear heads assure the lowest inspection and re-tubing
costs if they occur. Both result in the highest efficiency
and lowest possible maintenance costs for your boiler equipment.
Combustion Efficiency
Combustion efficiency is an indication of the burner's
ability to burn fuel. The amount of unburned fuel and excess
air in the exhaust are used to assess a burner's combustion
efficiency. Burners resulting in low levels of unburned
fuel while operating at low excess air levels are considered
efficient. Well designed burners firing gaseous and liquid
fuels operate at excess air levels of 15% and result in
negligible unburned fuel. By operating at only 15% excess
air, less heat from the combustion process is being used
to heat excess air, which increases the available heat
for the load. Combustion efficiency is not the same for
all fuels and, generally, gaseous and liquid fuels burn
more efficiently than solid fuels.
Thermal Efficiency
Thermal efficiency is a measure of the effectiveness of
the heat exchanger of the boiler. It measures the ability
of the exchanger to transfer heat from the combustion process
to the water or steam in the boiler. Because thermal efficiency
is solely a measurement of the effectiveness of the heat
exchanger of the boiler, it does not account for radiation
and convection losses due to the boiler's shell, water
column, or other components. Since thermal efficiency does
not account for radiation and convection losses, it is
not a true indication of the boilers fuel usage and should
not be used in economic evaluations.
Boiler Efficiency
The term "boiler efficiency" is often substituted
for thermal efficiency or fuel-to-steam efficiency. When
the term "boiler efficiency" is used, it is important
to know which type of efficiency is being represented.
Why? Because thermal efficiency, which does not account
for radiation and convection losses, is not an indication
of the true boiler efficiency. Fuel-to-steam efficiency,
which does account for radiation and convection losses,
is a true indication of overall boiler efficiency. The
term "boiler efficiency" should be defined by
the boiler manufacturer before it is used in any economic
evaluation.
Fuel-To-Steam Efficiency
Fuel-to-steam efficiency is a measure of the overall efficiency
of the boiler. It accounts for the effectiveness of the
heat exchanger as well as the radiation and convection
losses. It is an indication of the true boiler efficiency
and should be the efficiency used in economic evaluations.
As prescribed by the ASME Power Test Code, PTC 4.1, the
fuel-to-steam efficiency of a boiler can be determined
by two methods; the Input-Output Method and the Heat Loss
Method.
Input-Output Method
The Input-Output efficiency measurement method is based
on the ratio of the output-to-input of the boiler. It is
calculated by dividing the boiler output (in BTUs) by the
boiler input (in BTUs) and multiplying by 100. The actual
input and output of the boiler are determined though instrumentation
and the data is used in calculations that result in the
fuel-to-steam efficiency.
Heat Loss Method
The Heat Balance efficiency measurement method is based
on accounting for all the heat losses of the boiler. The
actual measurement method consists of subtracting from
100 percent the total percent stack, radiation, and convection
losses. The resulting value is the boiler's fuel-to-steam
efficiency. The heat balance method accounts for stack
losses and radiation and convection losses.
Stack Losses
Stack temperature is a measure of the heat carried away
by dry flue gases and the moisture loss. It is a good indicator
of boiler efficiency. The stack temperature is the temperature
of the combustion gases (dry and water vapor) leaving the
boiler and reflects the energy that did not transfer from
the fuel to the steam or hot water. The lower the stack
temperature, the more effective the heat exchanger design,
and the higher the fuel-to-steam efficiency.
Radiation and Convection Losses
All boilers have radiation and convection losses. The losses
represent heat radiating from the boiler (radiation losses)
and heat lost due to air flowing across the boiler (convection
losses). Radiation and convection losses, expressed in
Btu/hr, are essentially constant throughout the firing
range of a particular boiler, but vary between different
boiler types, sizes, and operating pressures.
Boiler efficiency, when calculated by the ASME heat balance
method, includes stack losses and radiation and convection
losses. But what factors have the most effect or "sensitivity" on
boiler efficiency? As discussed earlier, the basic boiler
design is the major factor. However, there is room for
interpretation when calculating efficiency. Indeed if desired,
you can make a boiler appear more efficient than it really
is by using a little creativity in the efficiency calculation.
The following are the key factors to understanding efficiency
calculations.
• Flue gas temperature (Stack temperature)
• Fuel specification
• Excess air
• Ambient air temperature
• Radiation and convection losses.
• Flue Gas Temperature
Flue gas temperature or "stack temperature" is
the temperature of the combustion gases as they exit the
boiler. The flue gas temperature must be a proven value
for the efficiency calculation to be reflective of the
true fuel usage of the boiler. A potential way to manipulate
an efficiency value is to utilize a lower-than-actual flue
gas temperature in the calculation. When reviewing an efficiency
guarantee or calculation, check the flue gas temperature.
Is it realistic? Is it near or less than the saturation
temperature of the fluid in the boiler? And can the vendor
of the equipment refer you to an existing jobsite where
these levels of flue gas temperatures exist? Jobsite conditions
will vary and have an effect on flue gas temperature. However,
if the efficiency value is accurate, the flue gas temperatures
should be confirmable in existing applications. Don't be
fooled by estimated stack temperatures. Make sure the stack
temperature is proven.
Fuel Specification
The fuel specification can also have a dramatic effect
on efficiency. In the case of gaseous fuels, the higher
the hydrogen content, the more water vapor is formed during
combustion. This water vapor uses energy as it changes
phase in the combustion process. Higher water vapor losses
when firing the fuel result in lower efficiency. This is
one reason why fuel oil fires at higher efficiency levels
than natural gas. To get an accurate efficiency calculation,
a fuel specification that represents the jobsite fuel to
be fired must be used. When reviewing an efficiency guarantee
or calculation, check the fuel specification. Is it representative
of the fuel you will use in the boiler? The representation
of efficiency using fuel with low hydrogen content will
not provide an accurate evaluation of your actual fuel
usage.
The Efficiency vs. H/C Ratio bar graph shows the degree
to which efficiency can be affected by fuel specification.
The graph indicates the effect of the hydrogen-to-carbon
ratio on efficiency for five different gaseous fuels. At
identical operating conditions, efficiencies can vary as
much as 2.5-3.0%, based solely on the hydrogen-to-carbon
ratio of the fuel. When evaluating boiler efficiency, knowing
the actual fuel specification is a must.
Excess Air
Excess air is the extra air supplied to the burner beyond
the air required for complete combustion. Excess air is
supplied to the burner because a boiler firing without
sufficient air or "fuel rich" is operating in
a potentially dangerous condition. Therefore, excess air
is supplied to the burner to provide a safety factor above
the actual air required for combustion.
However, excess air uses energy from combustion, thus taking
away potential energy for transfer to water in the boiler.
In this way, excess air reduces boiler efficiency. A quality
burner design will allow firing at minimum excess air levels
of 15% (3% as O2). O2 represents percent oxygen in the
flue gas. Excess air is measured by sampling the O2 in
the flue gas. If 15% excess air exists, the oxygen analyzer
would measure the O2 in the excess air and show a 3% measurement.
Seasonal changes in temperature and barometric pressure
can cause the excess air in a boiler to fluctuate 5% -
10%. Furthermore, firing at low excess air levels can result
in high CO and boiler sooting, specifically if the burner
has complex linkage and lacks proper fan design. The fact
is even burners theoretically capable of running at less
than 15% excess air levels rarely are left at these settings
in actual practice. A realistic excess air level for a
boiler in operation is 15% if an appropriate safety factor
is to be maintained.
When reviewing an efficiency guarantee or calculation,
check the excess air levels. If 15% excess air is being
used to calculate the efficiency, the burner should be
of a very high quality design with repeatable damper and
linkage features. Without these features, your boiler will
not be operating at the low excess air values being used
for the calculation, at least not for long. If less than
15% excess air is being used for the calculation you are
probably basing your fuel usage on a higher efficiency
than will be achieved in your day to day operation. You
should ask the vendor to recalculate the efficiency at
realistic excess air values.
Ambient Temperature
Ambient temperature can have a dramatic effect on boiler
efficiency. A 40 degree variation in ambient temperature
can effect efficiency by 1% or more. Most boiler rooms
are relatively warm. Therefore, most efficiency calculations
are based on 80 deg. F ambient temperatures. When reviewing
an efficiency guarantee or calculation, check the ambient
air conditions utilized. If a higher than 80¡ F value
was utilized, it is not consistent with standard engineering
practice. And, if the boiler is going to be outside, the
actual efficiency will be lower due to lower ambient air
temperatures regardless of the boiler design. To determine
your actual fuel usage, ask for the efficiency to be calculated
at the lower ambient conditions.
Radiation and Convection losses
Radiation and convection losses represent the heat losses
radiating from the boiler vessel. Boilers are insulated
to minimize these losses. However, every boiler has radiation
and convection losses. Some times efficiency is represented
without any radiation and convection losses.
This is not a true reflection of fuel usage of the boiler.
The boiler design also can have an effect on radiation
and convection losses. For example, a waterback design
boiler tends to have much higher rear skin temperatures
than a dryback design. This is easy to prove. Just go to
the back of a quality dryback boiler and touch the rear
door. Cool rear temperatures are the result of low radiation
and convection losses in the rear of the boiler. Boilers
operating with high rear temperatures are wasting energy
every time the unit is fired.
Radiation and convection losses also are a function of
air velocity across the boiler. A typical boiler room does
not have high wind velocities. Boilers operating outside,
however, will have higher radiation and convection losses.
Summary
Selection of a boiler with "designed-in" low
maintenance costs and high efficiency can really pay off
by providing ongoing savings and maximizing your boiler
investment. Remember, first cost is a relatively small
portion of your boiler investment.
High boiler efficiency is the result of specific design
criteria, including:
• Number of boiler passes
• Burner / boiler compatibility
• Repeatable air/fuel control
• Heating surface
• Pressure vessel design
Boiler efficiency calculations that are accurate and representative
of actual boiler fuel usage require the use of proven and
verified data, including:
• Proven stack temperature
• Accurate fuel specification
• Actual operating excess air levels
• Proper ambient air temperature
• Proper radiation & convection losses
When evaluating your boiler purchase, ask your boiler vendor
to go through the efficiency calculation to verify it is
realistic and proven. Also review the type of boiler /
burner being utilized to check if the unit's performance
will be consistent and repeatable. You will pay for the
fuel actually used, not the estimated fuel based on the
efficiency calculation. Once the boiler is installed, you
can't go back and change the design efficiency of the unit.
The facts regarding boiler efficiency are clear: optimal
high efficiency boiler designs are available. You will
get superior performance with these premium designs. And
efficiency calculations can be verified and proven. Make
sure the efficiency data you are using for your boiler
evaluation is guaranteed and is accurate and repeatable
over the life of the equipment.
Make sure your actual fuel usage requirements of the boiler
are understood before you buy.
In the end, the time spent evaluating efficiency will be
well worth the effort. Insisting on a high efficiency,
repeatable design firetube boiler will pay off every time
your new boiler is fired, for the entire life of the equipment.
Source: http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Primer/Boiler_Efficiency.asp
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