BOILER OPERATION EFFICIENCY Insights and Tips - Lattner Boiler ...

BOILER OPERATION
EFFICIENCY
Insights and Tips
Debunking five common myths about boiler-system
Boiler systems have been operated for
decades with an eye to efficient and safe
operation. This article is intended
as a review of some of the basic
operating principles that most operators
and engineers already know, but
may tend to forget over the years. The
basic information presented in this article has been
gathered over a couple of years of testing boiler-operating
efficiency at ap-
proximately thirty industrial
plants with multiple
boilers each. The data illustrated
in the various
curves shown are actual
operating boilers at these
various locations across
the United States.
MYTH NO. 1: A BOILER
OPERATES MOST EFFI-
CIENTLY AT FULL LOAD.
This is not necessarily
the case as shown in the
following case study for a
boiler tested equipped
with a feedwater economizer
(similar results for
an air heater).
Figure 1 and its accom-
Combustion efficiency, percent
84.5
84.0
83.5
83.0
82.5
panying data illustrate two opposing principles at
work in a boiler: excess air and final flue gas.
In the lower boiler operating
ranges, the higher excess air is the
predominate operating inefficiency.
By midrange, the excess
has reduced to a steady minimum,
but the exit gas temperature keeps increasing
and becomes the predominate factor. For this
20 40 60 80
Boiler load, percent
Boiler Loading
20 percent
40 percent
60 percent
80 percent
operations
By GLENN SHOWERS, PE
Boiler and Burner Systems
Cincinnati, Ohio
Boiler Efficiency
83.5 percent
84 percent
83.5 percent
82.9 percent
Stack Temp.
312 F
326 F
350 F
373 F
Excess Air
36 percent
18 percent
15 percent
14 percent
Glenn Shower, PE, is the president of Boiler and Burner Systems, an engineering firm specializing in industrial
utility systems involving combustion processes and boiler plants. He has degrees in mechanical engineering and
environmental engineering, and has performed as a design, construction, and startup engineer for boiler and
other combustion systems. He can be reached at glennsh@boilersandburners.com.
FIGURE 1. Boiler
efficiency vs.
load. Boiler
efficiency is
influenced by
various factors
that create a
curve over the
boiler's
operating range.
HPAC Engineering • November 2002
53

Efficiency, percent
87
86
85
84
83
82
81
Boiler efficiency, percent
20 40 60 80
Boiler load, percent
Boiler Loading
20 percent
40 percent
60 percent
80 percent
Boiler Efficiency
81.4 percent
85.4 percent
84.9 percent
84.1 percent
54 November 2002 • HPAC Engineering
Combustion efficiency
Boiler efficiency
Combustion Efficiency
83.8 percent
86.6 percent
85.7 percent
84.7 percent
84
83
82
81
80
Group
79
78
77
76
Individual
75
20 40 60 80
Boiler load, percent
Boiler Loading
20 percent
40 percent
60 percent
80 percent
B O I L E R E F F I C I E N C Y M Y T H S
FIGURE 2. Radiation loss effects on efficiency. A boiler constantly radiates heat but the
effect on efficiency varies with boiler load.
Burners operating as one
146 percent excess air
82 percent excess air
49 percent excess air
15 percent excess air
Burners operating individually
70 percent excess air
50 percent excess air
15 percent excess air
15 percent excess air
FIGURE 3. Multiple burner operation effect on efficiency. How burners on a mutli-burner
boiler are operated has a strong impact on efficiency.
boiler, the mid-fire range is the most efficient
operation.
Myth No. 2: Radiation losses are so
minor they can be ignored.
As with the full-load assumption, it’s
also a myth that one can ignore “minor”
radiation losses. The radiation losses seen
in figure 2 are estimated based on a curve
prepared by the American Boiler Manufacturers’
Association. These ABMA
curves factor boiler size, partial load, and
whether or not the walls are water- or aircooled
into the estimate of the radiation
loss for any given boiler.
The basic principle in action here is
that the furnace is virtually operating at
the same temperature regardless of boiler
load, therefore the heat loss through the
walls to the ambient surroundings are almost
constant over the load range.
Therefore, at the lower boiler firing rates,
the heat lost to the surroundings will calculate
to a higher percentage of the heat
input to the boiler. Take, for example, a
boiler that has been “banked,” operating
just enough to keep it warm but not producing
steam. This boiler has an efficiency
of 0 percent and is giving up all its
heat as radiation losses to the space surrounding
the boiler.
Myth No. 3: For a multiple-burner
boiler, adding one burner at a time is
the preferred method.
While this may be the preferred
method, as with myth No. 1, this is not
the most efficient way to operate, as
shown in figure 3 for a boiler tested that
was equipped with three burners. The
simple principle at work in this boiler is
excess air, and the more there is, the lower
the efficiency, as can be seen from the
curve data for Figure 3.
For this test, airflow was reduced to its
lowest possible level as indicated by a
sudden build-up of carbon monoxide.
Excess air had to be run higher when the
burners were operated individually to
avoid the formation of undesired carbon
monoxide. In the lower boiler operating
ranges using the burners individually,
combustion air short-circuited around
the operational burner by going through
the out-of-service burner. Not until all

urners are operating does the flame actually
use all the air supplied for combustion.
Therefore, to achieve the best possible
operating efficiency for this boiler, the
burners should be operated as a single
unit and not operated as individual burners.
Another option is for the dampers on
each burner to be tight shut-off to eliminate
the bypassing of combustion air, but
this is not always a practical solution.
Myth No. 4: Similar air and temperature
conditions mean similar efficiency,
regardless of fuel.
Again, as with myths 1 and 2, this is
not necessarily the case, as shown in Figure
4, which includes data for two identical
boilers, both equipped with air preheaters,
one burning light oil and the
other natural gas. The operating principles
at work in these boilers involve the
way each fuel burns.
The combustion of the hydrogen
component of any fuel forms water vapor,
which, when heated to the exhaust
gas exit temperature, is a major inefficiency
factor. Natural gas contains a high
amount of hydrogen (four hydrogen to
one carbon) as compared to fuel oil (one
hydrogen to six carbon); therefore, natural
gas has much higher water vapor inefficiency.
For this boiler, whenever fuel oil and
natural gas are at the same cost
($/MMBtu), the boiler should be operated
on fuel oil because it is about 4 percent
more efficient burning oil than gas.
Myth No.5. Boiler selection does not
affect the plant’s overall efficiency.
Like the first four, this is totally untrue.
The curves in figures 5a and 5b are intended
to demonstrate that a plant’s overall
operating efficiency is based, in large
part, on the selection of the boiler to place
in operation. In this example, the plant
has three boilers at different sizes and efficiencies,
which is summarized in the
curve table for figures 5a and b on page
56. As one can see, selecting the most efficient
combination of boilers and operating
each one at or near its most efficient
point will present the best overall operating
efficiency.
B O I L E R E F F I C I E N C Y M Y T H S
Boiler efficiency, percent
90
89
88
87
86
85
84
83
82
81
80
Boiler on oil
Boiler on gas
20 40 60 80
Boiler load, percent
Boiler Loading
20 percent
40 percent
60 percent
80 percent
Plant efficiency, percent
86
85
84
83
82
81
80
Burners on natural gas
80% excess air, 225 F exit temp
60% excess air, 245 F exit temp
55% excess air, 270 F exit temp
60% excess air, 310 F exit temp
Burners individually
125% excess air, 220 F exit temp
65% excess air, 260 F exit temp
65% excess air, 295 F exit temp
70% excess air, 305 F exit temp
FIGURE 4. Fuel selection effect on efficiency. Natural gas and fuel oil have different
effects on efficiency.
100
80
60
40
20
Boiler steam load, 1,000 pounds per hour 120
10 20 30 40 50 60 70 80 90 100 110 120 130
Total plant steam demand, 1,000 pounds per hr
Boiler 3
Boiler 2
Boiler 1
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190
Total plant steam demand, 1,000 pounds per hr
FIGURE 5a and b. Boiler selection vs. overall efficiency. The boiler plant's overall
efficiency is composed of the individual boiler efficiencies, so boiler selection greatly
influences overall plant efficiency. See page 56 for sizes and efficiencies of these boilers.
HPAC Engineering • November 2002
55

B O I L E R E F F I C I E N C Y M Y T H S
Boiler loading (lb/hr)
10
20
30
40
50
60
70
80
90
100
Boiler No. 1
83.9
84.2
84.4
84.5
84.65
84.7
56 November 2002 • HPAC Engineering
Boiler No.2
85.4
84.95
84.1
Boiler No. 3
83.8
84.3
84.6
84.81
84.7
84.25
83.3
82.6
81.6
80.8
FIGURE 5a. illustrates a plant’s overall efficiency and 5b shows multiple boiler load
management. This table shows sizes and percent load efficiencies of the boilers in
Figures 5a and b.
C O N T R O L F R E A K S
Continued from Page 11
Typically, when one evaluates the
capability of the “supervisory” network
interface device, the primary concern
is how many nodes the device can
manage. In addition to this, the engineer
and installing contractor need
to be cognizant of any limitation on
data traffic. This includes the transfer
of physical hardware point information,
software calculations, alarming,
trending, and, potentially, the data
transfer of additional parameter
details. Not all controllers have the
memory capacity required to handle
the input/output (I/O) data transfer
of these additional parameters. Some
controllers have the memory to handle
250 pieces of I/O, while others are
limited to 50. If a given controller has
16 points, and you require access to an
additional six, this won’t be a problem.
If you require the ability to put each
point into a test mode, to override each
point, and input an offset to some
of the analog inputs for calibration
purposes, you will quickly exceed these
memory limitations. Engineers need
to research these details or performance-specify
the criteria that dictate
acceptable performance.
Again, with DDC systems, there
are additional details that need
researching and specification as part
of the engineering process. These
additional details increase as we
mix products from different suppliers.
Unfortunately, some of these particulars
are not readily apparent. This is
not to say we shouldn’t mix systems,
but rather we need to do a little
more homework as we apply new
approaches in this quickly changing
field. It is much better to find out
system limitations before we build the
system.
Send comments and questions to
controlfreaks@penton.com. For
previous Control Freaks columns, visit
www.hpac.com.