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Cleaning furnace convection section coils is a great difficulty because of the tight arrangements of tube banks and the close
spacing of adjacent coil services.  Unfortunately, convection coils are designed to achieve a certain level of performance,
and maintenance issues like cleaning are not a designers key focus.  Fouled furnace convection coils result in considerable
flue gas heat losses resulting from just a few years of fouling buildup.

Ammonia Plant Furnace Example

Ammonia plants use extremely large amounts of fuel in fired furnaces and boilers with convection heat recovery.  Simulation
data for a 1790 STPD Ammonia plant furnace and convection section is illustrated in Figure 1, which uses over 40 Million US
Dollars in natural gas fuel per year.  Figure 1 illustrates a plant that has been operating about 15 years and for over 5 years
without convection coil cleaning.  The convection section includes the following convection coils.
Table 1

Coil Description                Coil Design                Duty                 Operating Fouling
                                                                     MM Btu/Hr        Factor, Ft2-Hr-Deg F/Btu
Hot Convection Coils
Mixed Feed Preheat        1826 Ft2 Bare            45.85                0.011365
Hot Process Air Preheat  1044 Ft2 Bare            15.94                0.011529
High Temp Steam Supht  1459 Ft2 Bare &         92.67                0.015048
                                    38350 Ft2 Finned
Cold Convection Coils
Low Temp Steam Supht   27600 Ft2 Finned        75.70                0.008789
High Temp Nat Gas Prht   8400 Ft2 Finned         29.90                0.007754
Boiler Water Preheat        48840 Ft2 Finned       77.57                0.009868
Fuel Gas Preheat             3558 Ft2 Finned         8.60                  0.004771
Cold Process Air Preheat  14978 Ft2 Finned       12.62                0.004306
Refer to Figure 1, (page 3), illustrating rigorous rating of the furnace coils using a process simulator and Chem-Engineering
Heat-r-Rate-r furnace convection coil design, evaluation and rating software.  The two programs were run
simultaneously in trial-and-error calculations to develop the furnace thermodynamics, deriving the fouling factors (illustrated
above) to closely match the individual coil performance for the convection coils actual current operation.  The convection coil
rating software enables the understanding of the extent of fouling of the individual convection section coils and permits the
determination of the benefits of coil cleaning.

A common maintenance approach in industry is to hire a cleaning company to clean the entire convection section during a
plant shut-down.  Hydroblasting is often used, but new techniques are available, including "dry-ice" (solid CO2) blasting with
air as the propellant to avoid liquid damage to refractory blanket type wall insulation.  As described earlier, access can be
extremely difficult.  For some furnace convection sections, large inspection and coil removal doors exist for tube pulling and
maintenance, while in some plants cutting of openings must be done.  Careful planning and coordination of a convection
section cleaning project must consider access, safety of contract workers, disposal of blasting and fouling material and the
degree of cleaning success.  Because of the inaccessibility of surface areas within deep tube banks and the effects of
masking from front and secondary rows of tubes, reduction in fouling factors of about 50% should be expected in
calculations as a practical limit for cleaning restoration of coils that have been in service 5 years or longer for a "cleaned"

Figure 1 illustrates the base case furnace and convection section performance following years of operation without cleaning.  
Higher than design flue gas temperatures and lower than design coil outlet temperatures exist because of high load on the
convection coils, with plant rate over 50% above original design.
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