Furnace Convection Section Coil Rating, Cleaning and Upgrade
Furnace engineering services are vital to the proper design and reliable operation of flue gas heat recovery in furnaces used
in Chemical Industry. This article discusses an ammonia plant furnace convection section example, illustrating an audit of
existing performance, and projecting the technical and economic effects of improving convection heat transfer and energy
savings from optimized selection and cleaning of individual coils, and upgrading under-performing convection section coils.
Plant furnaces employ radiant and convective heat recovery from the flue gases of fired fuels to maximize fuel efficiency. The
temperature level of flue gases at the furnace stack has decreased dramatically by design as energy costs have gradually
risen. Plants of several decades ago had stack flue gases operating at or above 400 degrees F. Modern and upgraded
plants, in today's energy environment, have furnace stack flue gases operating at or below 300 degrees F.
Furnace stack temperature increases due to gradual build-up of fouling over time, as well as due to increased thermal load
from higher plant rates than original design. It is desirable to keep furnace stack flue gases as cool as practical from periodic
coil cleaning and maintenance and occasionally by replacement of overloaded or worn out convection coils with improved
designs.
Furnace convection coils reduced performance and fouling occurs for a variety of reasons. The most serious fouling occurs on
the outside of convection coils. However, sometimes fouling does occur on the inside tube surface, such as when thermal
cracking of the process stream in a coil occurs, producing solid product, such as carbon. The outside surface of convection
coils may also become fouled from carbon due incomplete combustion and also from refractory dust resulting from flame
erosion of burner blocks and from furnace casing brickwork and insulation. The gradual build-up of fouling materials
deteriorates convection coil heat transfer, causing "leakage" of potential heat recovered as hotter flue gas flowing to
downstream convection coils. This can result in overheating of some coils not rated for the operating temperature, and
certainly increased heat losses in furnace stack gases.
Coil designs may be bare tube surface, or they may be enhanced with extended finned surface, or a combination of the two.
Generally those coils in hotter flue gas service (above 1500-1600 Deg F) are constructed of bare tubes. When a large duty
has to be accomplished, some finned tubes are used in the convection coil design, sometimes in combination with bare tubes
in the hottest flue gases.
Fouling of convection coils is such a gradual process that it is not really noticed over many years of service. Certain
convection coils are more dramatically impacted in terms of lost heat transfer than others. At times, only a very thin coating of
refractory dust is required to deteriorate the performance of bare surfaces in high temperature flue gases. In such high
temperature flue gases, the buildup of only 0.001 to 0.010 inch (0.0025-0.025 cm) of fouling thickness on the outside of bare
tubes can significantly reduce heat transfer. Finned convection coil heat transfer is also dramatically influenced from refractory
and airborne dust material build-up and there are more crevices for the dust to settle in.