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Convection Coil Cleaning

In the example shown in Figure 1, performance improvements can be achieved.  After operating for several years of service, fouling
develops on the furnace convection coils.  The fouling that contributes the most to loss of performance builds up on the flue gas side
of the coils, particularly those with extended finned surface.  For a Natural Gas fired furnace, the fouling consists mostly of
refractory materials worn from the burner blocks and furnace and convection section insulation at the walls, floor and arch regions.  
Convection coil cleaning can be accomplished by hydro-blasting with moderate pressure of about 3000 psi, using a short nozzle to
focus the cleaning at specific coil areas.  Care must be taken to prevent damage to the wall insulation and to drain the water
accumulation at the floor.  Generally, draping plastic sheeting down the convection section walls and on the floor will suffice.  
Draining can be accomplished by gathering the plastic sheeting and running it through a temporary opening through the wall of the
convection section, slightly above the floor elevation.  Slightly dampened floor brick or refractory can be dried out by a slowed
light-off on the initial re-start of the plant.

Calculations were performed to show the impact of cleaning for the furnace coils.  These calculations are shown in Figure 2.  The
extent of improvement, depends on the extent of fouling, and the effectiveness of the cleaning.  The calculations shown are typical
for fairly heavy fouling build-up, over several years of service.  (3-4 years)  Usually, with fairly intensive cleaning, about half of the
fouling can be eliminated.  (Some of the fouling is not on the outside of the tubes.  For instance Natural Gas coils often have carbon
build-up on the inside of tubes that have operated above 800 Degrees F.)
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