Process heaters are the largest consumers of energy in most
plants. A refinery, on average, burns
approximately 2 billion Btu/hr of fuel in fired heaters. The
total quantity of fuel burned (heat released) is so high that
any improvement will result in significant fuel savings.
Although there are many ways to improve heater performance,
including better design, operation and maintenance, excess air
is the No. 1 contributor to poor heater efficiency and must be
High energy costs and tighter emissions regulations require
increased understanding and control of excess air. Any
reduction in excess air will raise the efficiency of a heater
and reduce total emissions. NOX emissions are of the
highest concern in a fired heater, although excess air control
will also reduce refinery CO2 emissions
and boost heater efficiency.
Fuel efficiency in a fired heater is a function of heater
design, maintenance and operating
parameters. Heaters must be designed for optimum efficiency,
and providing an adequate heat-transfer area at the design
stage will ensure better efficiency. It is recommended that the
flue gas temperature approach (defined as flue gas temperature
leaving convection, minus process inlet temperature) be between
50°F and 100°F, depending on heater tube material and
the cost of fuel. However, heater efficiency may decline with
the degradation of heater components. The degree of degradation
is dependent on the quality of the maintenance program implemented at
Excess air is defined as the amount of air above the
stoichiometric air requirement that is needed to complete the
combustion process. Excess oxygen (O2) is the amount
of O2 in the incoming air not used during
In an operating plant, the airflow rate can be adjusted at a
fixed absorbed-heat duty (constant feed flowrate and
inlet/outlet conditions) until an optimum fuel-to-air ratio is
achieved. It is important to note that there is a limit on
minimum possible excess O2. Below this level,
combustibles can enter the flue gas, which poses a safety
hazard. Heater and burner manufacturers establish this minimum
limit during the design stage. Operators should also keep a
safe margin for upset conditions.
A frequently asked question is, Why do operators often
run heaters with higher excess air? The answer is that
additional excess air reduces flame temperature, shortens flame
length and decreases tube flame impingement, thereby making it
easier for workers to operate the heater without overheating
Excess O2 can be measured in flue gases, which
can be correlated with excess air. Fig. 1
provides a correlation between excess air and flue gas
O2 for a typical natural
gas. Also, the following equation can be used to calculate
the excess air based on flue gas O2:
EA = excess air, %
O2 = vol% of flue gas oxygen (dry).
Higher excess O2 helps achieve a stable flame in
the firebox. At the same time, it reduces the efficiency of the
heater. As a general rule, 3% O2 in flue gas is
equivalent to 15% excess air.
Flue gas quantity
Flue gas quantity increases with a rise in excess air, which
lowers heat and increases the fuel requirement. Fig.
2 provides a correlation between flue gas generated
during combustion, and excess air.
Fig. 1. Correlation between
excess air and flue
gas O2 for a typical natural gas.
Fig. 2. Correlation between
flue gas generated
during combustion, and excess air.
The following equation can be used to calculate approximate
flue gas quantity for natural gas:
qf = 1 + 0.167 3 (100 + EA)
EA = excess air, %
O2 = vol% of flue gas oxygen (dry)
qf = flue gas quantity in lb/lb of
As a general rule, flue gas quantity is approximately 20
times the fuel quantity at 15% excess air.
The net efficiency
The net eddiciency of a fired heater is equal to the total
heat absorbed, divided by the total heat input. The heat
absorbed is equal to the total heat input, minus the total
losses. The net thermal heater efficiency can be calculated
using the following equation:
η = Net thermal efficiency, %
LHV = Lower heating value of the fuel, Btu/lb of
Ha = Sensible heat of air, Btu/lb of
Hf = Sensible heat of fuel, Btu/lb of
Hm = Sensible heat of atomizing media,
Btu/lb of fuel
Hs = Stack heat losses, Btu/lb of
HL = Setting loss, Btu/lb of fuel.
For all practical purposes, we can assume
Ha and Hf to be
negligible. Hm is applicable for fuel oil
firing. Setting (casing) heat losses are in the range of
1.5%2.5%, depending on the capacity, design and size of
the heater. Given these assumptions, there are two parameters
for the estimation of efficiency: excess air/flue gas
O2 and stack temperature.
Fig. 3 depicts a graph for the estimation
of fired heater efficiency, based on flue gas O2 and
stack temperature for a typical natural
gas with a setting heat loss of 1.5%. For heat losses
higher than 1.5%, additional heat loss should be reduced from
the calculated efficiency.
For example, consider a heater operating at a stack
temperature of 400°F, with 4% O2 (dry), and a
1.5% setting loss. Using the graph in Fig. 3,
the efficiency can be estimated at 89%. For the same heater
with a higher setting loss of 2.5%, the efficiency is
Fig. 3. Estimation of fired
Knowledge of efficiency loss will clarify economic incentives
to lower the stack temperature or the percentage of excess
O2. As a general rule, every 35°F increase in
flue gas temperature reduces the heater efficiency by 1%.
Natural draft heaters
Natural draft heaters use the draft (buoyancy) effect of hot
flue gases to draw combustion air into the heater. The net
draft available is the draft created by the stack effect, minus
frictional and velocity losses. The net draft should be
sufficient to obtain a negative pressure along the heater flue
It is important to maintain a safe draft level in a fired
heater to achieve the best possible efficiency and operation.
The target draft of 0.1 inchWC is set at the heater arch. A
higher value of draft will result in ingress of tramp
air into the heater. Tramp air takes heat from the
combustion process and exits the stack, reducing heater
efficiency. The flue gas sample taken from the stack does not
represent the actual volume of O2 available for
combustion. It is the sum of unused O2 from the
firebox (actual excess O2) and O2 from
A positive draft value will result in the leakage of hot
flue gases through openings in the heater. This is a hazardous
operation that can overheat the steel structure, refractory and
heater supports, and, consequently, shorten heater life.
Fig. 4. Value of draft
generated in the heater
for flue gas and ambient air temperatures.
Fig. 4 provides the value of draft
generated in the heater for flue gas temperature and ambient
air temperature. It should be noted that stack effect decreases
with an increase in site altitude. The calculated draft should
be amended using the correction factor for site altitude. The
following equation can be used to calculate the draft generated
in a heater:
H = Height, ft
Patm = Atmospheric pressure, psia
Tamb = Ambient air temperature,
Tfg = Flue gas temperature, °R.
As a general rule, for every 10 ft of firebox height, the
draft increases by 0.1 inchWC:
Draft at burner (inchWC) ≈ 0.1 +
Hfb ÷ 100 (5)
Hfb = Firebox height, ft.
As an example, for a 50-ft-tall firebox, the draft at burner
is 0.6 inchWC (0.1 inchWC at arch, plus 0.5 inchWC as a stack
effect). Generally, the stack effect decreases with an increase
in site elevation. For example, a taller stack will be needed
for a heater operating in Wyoming (altitude ~ 5,000 ft) than
for one operating along the Texas Gulf Coast (altitude ~ 0
The typical combustion air preheater
The typical combustion air preheater (APH) will increase the
heater efficiency by approximately 10%. Fuel gas generally
contains H2S or sulfur, which convert into
SO2 and then into SO3. The APHs
heat-transfer surface is subject to cold-end corrosion caused
by condensation of sulfur trioxide (SO3), which
results in APH leakage. Air preheater leakage is one of the
most common APH operating problems, and any such leakage
results in a reduction in the overall efficiency of the
In APH operation, the flue gas is generally at negative
pressure, and the air is at positive pressure. Therefore,
leakage occurs from air to the flue gas side. This reduces the
quantity of air available for combustion, and it increases the
quantity of flue gas leaving the APH.
This leakage can be detected by measuring the flue gas
O2 content at the APH inlet and outlet. Any leakage
will result in higher flue gas O2 at the APH exit,
compared to the APH inlet. Generally, the APH is not equipped
with a flue gas O2 analyzer at the inlet and the
outlet; however, the inclusion of 2-in. connections at the
APHs inlet and outlet will enable operators to measure
O2 levels using a portable analyzer.
Another method of measuring leakage involves heat balance.
The flue gas/air heat balance across the APH can be described
(m 3 Cp ∆T)flue
gas = (m 3 Cp
For a typical fuel gas at 15% excess air:
mflue gas ≈ 1.05 3
Cpflue gas ≈ 1.15 3
∆Tair ≈ 1.2 3
m = Flowrate
Cp = Specific heat
∆T = Temperature difference across the APH.
Any leakage in the APH will reduce the ratio of
∆Tair to ∆Tflue
gas. For example, for a 10% leakage in the APH, the
ratio of temperature difference will be around 1.1.
Fig. 5 indicates the percentage of air
leakage based on the ratio of ∆Tair
to ∆Tflue gas for a typical natural
Fig. 5. Percentage of air
leakage for a typical
natural gas firing.
Air leakage through openings
A fired heater is not a 100% sealed unit; there are always
openings through which air ingress (tramp air) can move. The
volume of tramp air depends on the opening size and the draft
at the location of the opening.
After the draft at the opening location is estimated, the
following equation can be used to estimate the air leakage
through an opening:
∆P = C 3 0.003 3 ρ 3
This equation can be simplified for the leakage calculation
purpose based on the following data:
- Molecular weight (MW) of air = 28.96
- Atmospheric pressure (psia) = 14.7
- Velocity head (C) = taken as 1
The simplified equation reads:
ΔP = Draft at opening location, inchWC
ρ = Density of air at ambient temperature,
v = Velocity of air through opening, ft/s
C = Velocity head
ql = Air leakage, lb per
T = Ambient air temperature, °R.
Fig. 6 provides the quantity of air leakage
per ft2 of opening size. This figure is based on an
ambient air temperature of 60°F. Once the opening size is
known, the amount of air leakage can be estimated. The
estimated air can be translated into the additional firing rate
Fig. 6. Quantity of air
leakage per ft2 of opening
A commonly asked question in heater discussions is,
How much fuel can be saved if excess air is
optimized? The efficiency chart in Fig.
3, which shows operating O2 and target
O2, helps calculate the savings.
However, there is a drawback. The absorbed heat duty of the
fired heater is constant. Any increase in the O2
level will reduce the efficiency, resulting in a higher firing
rate. This increase in the firing rate will lead to a rise in
stack temperature, which results in another reduction in
efficiency. This reduction, in turn, demands a further increase
in the firing rate.
For example, a 100-MM-Btu/hr fired heater is designed for
operating at a stack temperature of 600°F, with 84%
efficiency at 3% O2. The operating efficiency at 6%
O2 is around 80% (and not 82%, as shown in the
The method of efficiency calculation for off-design
operating conditions presented in API-560 Appendix G can be
used to estimate the stack temperature when excess air is
present. This method can be simplified for excess air as
Ts = Flue gas stack temperature,
EA = Excess air, %
Tf = Feed inlet temperature, °R
(Tf1 = Tf2)
Φ = Excess air correction factor (subscripts 1 and 2 refer
to design and operating conditions, respectively).
Once the new flue gas stack temperature at excess air is
known, then the heater efficiency can be estimated.
Fig. 7 shows the estimated fuel savings for a
reduction in the O2 level to 3%. This graph is based
on a fuel price of $6/MMBtu. The design flue gas temperature
lines indicate the baseline stack temperature (i.e., the flue
gas stack temperature at 3% O2).
Fig. 7. Estimated fuel
savings for a reduction
in the O2 level to 3%.
CO2 emissions reduction
The volume of CO2 emissions generated in a fired
heater is directly proportional to the firing rate. In
combustion processes, fuel carbon converts into CO2.
Therefore, excess air reduction will lower CO2
Fig. 8 provides estimated decreases in
CO2 emissions through a reduction in the
O2 level to 3%. The basis for this graph is the same
as that in Fig. 7.
Fig. 8. Estimated decreases
in CO2 emissions
through a reduction in the O2 level
Heater excess air control starts at the design stage.
Well-designed heaters have low tramp air. There are three
stages of excess air control:
- Design stage
The following methods can be used to reduce excess air and
tramp air in the heater.
Design stage. A heater has many potential
leak points for air ingress:
- Clearance around the bottom coil guide (spigots)
- Sight doors and peepholes
- Header boxes, manholes and other openings for viewing and
- Modules and duct splice joints
- Terminals and crossover tubes
- Weld joints on the heater casing
- Soot-blower sleeves
- The APH.
These leak points must be designed for the lowest possible
leakage. Suggestions for designing a low-leakage heater include
- Seal the clearance space around the bottom tube guides by
using a floor sleeve with an end cap, or seal boots
- Use sight doors, with safety glass, that are equipped
with an interlock cover or flapper
- Use a self-closing peephole cover in the heater
- Ensure that header box panels and other openings are
airtight, and use gaskets between the gaps
- Seal-weld all splice joints between modules from the
inside, or use high-temperature sealant; also, use
closer-bolt spacing (6 in. from center to center)
- Seal all terminals and crossover openings with flexible
- Ensure that all header box drain points are plugged
- Ensure that no leakage is occurring through instrument
- Limit leakage through the APH during the design stage,
and perform an air-leakage test in the shop.
Maintenance. Routine maintenance of the
heaters is essential, since corrosive agents can be present in
flue gases. Deterioration from sulfur oxides occurs mostly on
cold sections of the steel casing. Climate conditions can also
lead to rusting on exposed surfaces of the heater casing.
Suggested inspection and maintenance methods include the
- Check for heater casing corrosion; if any leaks are
discovered, they should be sealed to stop air ingress
- Ensure that observation doors (generally located in the
bottom section of the radiant box) are closed after
technicians inspect the heater flame
- Check peepholes, access doors, etc., for proper
- Check flue gas O2 content in the convection
section and on the APH; if there is any increase in
O2 content across the flue gas path, it indicates
- Use a smoke test during heater shutdown to detect
- Use infrared scanning, while the heater is in operation,
to pinpoint locations with air leakage; these will have
localized, lower heater casing temperatures
- To reduce leakage in burners, keep all burners in
operation, even during lower operating loads; and close the
air register when a burner is taken out of service.
Excess air control. Knowing the target flue
gas O2 content is the first step in excess air
control. Each heater is unique in its design. The O2
level required to achieve ideal combustion may be anywhere from
1%4% or higher, depending on the design and operating
characteristics of the heater.
The following two instruments are necessary to control
- Flue gas O2 analyzer. This is
the most important instrument on the heater. It is
recommended to install an O2 analyzer at the
radiant section arch.
- Draft gauge. A draft gauge should be
installed at the heater arch. The arch is the point of the
highest flue gas pressure in the heater.
Heater O2 and draft at the radiant arch should be
checked and, if necessary, adjusted at least once per shift and
whenever there is a change in process load. All operators
should be familiar with the heater controls. Often, heaters
with air registers and stack dampers become jammed simply
because they are not used. Fig. 9 provides
tactics for controlling excess air in a natural draft heater.
For controlling excess air in other types of heaters,
Table 1 and Table 2 can be
used alongside Fig. 7.
Fig. 9. Tactics for
controlling excess air in a
natural draft heater.
The basics of automatic heater control are similar to those
described in the manual control method. In automatic control,
reliable instrumentation is key. One reason for the small
number of heaters with automatic control is a lack of
confidence in reliable instrumentation. Also, artificial
intelligence can be built into the control system to account
for all operating cases.
A remote manual control for both O2 and draft is
best suited for the natural draft heater. A fully automated
control can be safely implemented on balanced draft heaters.
Optimum heater performance can be achieved by controlling
O2 and combustibles in flue gas using an
O2/CO analyzer and automatic dampers.
Case Study 1
In this case study, a vertical, cylindrical, natural draft
heater with an absorbed heat duty of 100 MMBtu/hr included the
following design parameters:
- Flue gas stack temperature: 600°F
- Fluid inlet temperature: 300°F
- Design excess air: 15%
- Design heat loss: 2%
- Firebox height: 55 ft
- Number of radiant tubes: 64
- Flue gas O2 content at operating conditions: 6
- Efficiency of heater at design conditions: 84.2% at 1.5%
heat loss (see Fig. 3)
- Efficiency at 2% heat loss: 83.7%
- Firing rate: 100/0.837 = 119.5 MMBtu/hr
- Fuel gas flowrate: 5,770 lb/hr (using a fuel gas LHV of
- Flue gas flowrate: 5,770 3 [1 + 0.167 3 (100 + 15)] =
- Air flowrate (flue gas flowrate − fuel flowrate):
116,582 − 5,770 = 110,812 lb/hr.
The heater operated under the following conditions:
- Flue gas O2: 6 vol% (dry)
- Excess air: (92 3 6)/(21 6) = 36.8% (also see
- Efficiency at higher excess air, design stack temperature
of 600°F and 2% heat loss = 81.3%; however, the revised
efficiency must be calculated based on operating stack
temperature, and if this measurement is not available, then
the revised stack temperature can be estimated as
- Efficiency at revised stack temperature and operating
flue gas excess air: 80.5% (from Fig.
- Corrected efficiency for 2% heat loss: 80%
- Operating firing rate: 100 ÷ 0.8 = 125
- Fuel savings potential: (125 119 .5) 3 24 3 365 =
48,180 MMBtu/yr; at a fuel price of $6/MMBtu/hr, the annual
fuel savings potential = $289,000 (this can also be estimated
using Fig. 7).
Case Study 2
In this case study, the heater experienced leakage through
the bottom guide and an open peephole. The coil guide and
sleeve size were 2-in. Nominal Pipe Size (NPS) Schedule 80 and
3-in. NPS Schedule 80, respectively. The heater was not
provided with a cap on the sleeve. The open area between the
sleeve and the guide included an inside cross-section of 3-in.
NPS sleeve and an outside cross-section of 2-in. NPS guide
(equal to 2.96 in.2). Other design parameters
- Number of guides: 32 (one guide for two radiant
- Total opening area at guides: 32 3 2.96 ÷ 144 =
- Size of radiant section observation door: 5 in. 3 9
- Opening area of observation door: 45 in.2
- Total open area: 0.97 ft2
- Firebox height: 55 ft
- Draft at opening (draft at heater floor): 0.1 + 55
÷ 100 = 0.65 inchWC
- Ambient air temperature: 60°F
- Air leakage per ft2 of opening:
- Total air leakage: 4.07 3 0.97 3 3,600 = 14,212 lb/hr
(this can also be estimated using Fig.
- Excess air for the burner: 15%
- Actual excess air, including leakage, can be calculated
Based on revised excess air, a fuel savings can be
calculated as in Case Study 1:
- Calculated efficiency: 81.1%
- Loss in efficiency due to air leakage: 83.7 − 81.1
- Annual fuel savings potential = $201,320.
Controlling excess air has many benefits, and opportunities
exist to save fuel regardless of whether the heater is old or
new. Day-to-day monitoring significantly improves heater
operation. The first visible benefit of excess air reduction is
a decrease in fuel consumption. Reduction of emissions,
including CO2, is another benefit.
The heater must be provided with at least two instruments
(an O2 analyzer and a draft gauge at the arch), both
of which are important for energy improvement. Additionally,
proper training should be provided for heater operators, and a
simple and straightforward heater tuning program must be
implemented. It is unlikely that an operator will voluntarily
adjust the heater unless a plan is in place to do so.
Ahamad is a fired-heater equipment engineer at
Bechtel Corp. in Houston, Texas. He has more than 14
years of experience in the design, engineering and
troubleshooting of fired heaters and combustion systems
for the refining and petrochemical industries. He
graduated from the Indian Institute of Technology in Roorkee,
India, with a degree in chemical engineering. He worked
for eight years at Engineers India Ltd. in New Delhi,
India, and for five years at
Furnace Improvements in Sugar Land, Texas.
Vallavanatt is the senior principal engineer
at Bechtel Corp. in Houston, Texas. He has more than 37
years of experience in the design, engineering and
troubleshooting of fired heaters, thermal oxidizers, boilers and
flares. He graduated from the University of Kerala in
India with a degree in
mechanical engineering. He also received a degree in
industrial engineering from St. Marys University
in San Antonio, Texas. Mr. Vallavanatt is a registered
professional engineer in the state of Texas, and he has
served on the American Petroleum Institutes
subcommittee on heat transfer equipment for the past 27