Top Ten Ways to Improve Fired Heater Efficiency
By Robert Dubil
Carmagen’s April 2014 newsletter identified the Top Ten Ways to Increase
Site Energy Efficiency. This companion article focuses on improving fired
heater efficiency. Note that many of the items listed below can be done
at no or minimal capital cost, and could yield significant fired heater
- Reduce excess air.
There are several ways this may be accomplished.
- Install oxygen analyzer in closed-loop control
with combustion air flow controller (i.e., burner plenum dampers,
combustion air fan). Consider the installation of an oxygen
analyzer with CO/combustibles analysis included to allow
optimization and control of excess oxygen to a practical
- Seal all casing openings and tube penetrations
that are practical to achieve. Consider the addition of
glass-covered observation doors to reduce inleakage. These also
provide increased operator safety.
- Minimize tramp air inleakage by installing a
bridgewall pressure controller in closed-loop with stack damper or
induced draft fan. A practical day-to-day pressure at the arch
should be in the order of -0.1” H2O (2.5 mm H2O).
- Consider performing tests to determine minimum
excess oxygen levels achievable on a day-to-day basis, after
having installed analyzer(s) and sealed casing openings.
- Add combustion air preheat.
- Although a costly option, the economics may
favor the addition of a flue gas-to-combustion air preheater.
Either a static recuperative type or a rotary regenerative type
may be considered. Both systems will usually require a forced and
induced draft fan system as well as the associated ducting.
Obviously, plot space will impact the choice. In some cases,
multiple fired heaters may share a common air preheater system,
which can enhance the economics, but will require sophisticated
control instrumentation and additional safety systems.
- A less costly approach is to utilize an
intermediate fluid such as DowTherm to remove heat from the flue
gas, and exchange it with the combustion air. This will eliminate
much of the required ducting (replaced with piping), but will be
limited in the amount of heat that may be exchanged.
Alternatively, waste heat from a process stream may be a good
choice to preheat the combustion air.
- Consider checking the oxygen levels with a
portable analyzer on a regular basis to look for air inleakage.
- Add extended surfaces to the
convection section tubes. Normally, this will be
through the use of finned tubes. If the fuel is a clean gas, the fins
may be spaced quite close together to maximize the surface area
increase. If the fuel is oil, fins may also be utilized; however, they
are usually thicker and spaced further apart to allow for onstream
cleaning with sootblowing equipment. Closely spaced fins can increase
the effective heat transfer surface area by an order of five times
that of bare tube, while the thicker more loosely spaced fins in oil
service may increase the surface area by a factor of three.
- Perform regular burner maintenance
to achieve and maintain proper and complete combustion at reduced
excess air levels. Consider upgrading burners to more efficient, low
excess air models.
- Consider the addition of fuel gas/oil
cleaning facilities. Proper fuel gas and fuel oil
filtration systems will assure that the burners perform as designed.
These may include properly designed and insulated knockout drums with
coalescing filters to remove condensable liquids from the fuel gas,
automatic continuously self-cleaning fuel oil strainers, and parallel
filters to allow one to be cleaned offline while the other is retained
- Conduct operator training
with emphasis placed on heater efficiency, i.e., during regular
surveillance of heaters and burners look for leaks, or other
opportunities to improve efficiency. Operators spend the most time
around a heater so have the best vantage point and can best detect
subtle changes in heater performance.
- If an oil fired heater with
sootblowers installed, perform regular maintenance of
the sootblowers, and check that they are passing the proper steam rate
to keep the tubes clean. Generally, this will require about 10,000
lb/hr of steam per blower (4,500 kg/hr) and preferably about 250 psig
pressure steam (17.6 kg.cm2) or higher.
- Monitor and track key parameters,
such as stack O2, tube metal temperatures, bridgewall and stack
temperatures. As the stack temperature and/or bridgewall temperature
begins to increase, these are indications of loss of efficiency. This
may be due to tube fouling either on the fireside or tubeside.
Monitoring and tracking the ratio of bridgewall temperature (BWT) to
stack temperature (Ts) can be used as a fouling indicator, i.e.,
increase in the ratio of BWT/Ts indicates loss of radiant heat
transfer, possibly through tubeside fouling/coking. Reduction of the
BWT/Ts ratio will indicate convection section fouling, likely on the
fireside. However, tubeside fouling or sedimentation in convection
sections has been found, and may be eliminated by pigging the coil.
- Develop turnaround checklist focused
on efficiency. This may include planning for upgrades
rather than replace-in-kind, sometimes at low or no additional cost.
For example, glass-covered observation doors. Other examples include
adding extended surfaces tubes, rather than simply replacing bare
tubes in kind. In addition, consider the opportunity to upgrade
burners, rather than continuing to repair existing low-efficiency
- Consider soliciting the aid of
experts. Carmagen Engineering can provide specialists
who are familiar with fired equipment and can perform analyses of the
refinery heaters and offer recommendations for efficiency improvement.
In many cases, this will result in equipment maintenance savings as
well. The end product would be in the form of a report itemizing the
recommendations and listing the cost/benefits associated with the
implementation of each.