Page 156 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 123

4.3 Economizers

Flue gas exiting the primary heat exchanger (recuperator) still has considerable sen-
sible heat. At 480 to 590°C (900 to 1100°F), the opportunity exists for further heat
recovery. One option is to install an economizer to generate hot water. Existing instal-
lations have used both watertube and firetube designs.
The watertube construction is the more conventional approach for economizer
design. Flue gas flows over the tubes, presenting the potential for erosion of the
tube walls. The tubes must not be placed directly in the path of the gases leaving
the primary heat exchanger tubes or severe erosion/corrosion occurs. Auxiliary
cleaning in the form of soot blowers may also be necessary to maintain the heat
transfer performance.
Most recent installations have used the firetube design (FGTT). Flue gas flowing
within the tubes can be cooled to 180 to 230°C (350 to 450°F). Hot water is produced
at 90 to 150°C (200 to 300°F) for use in seasonal space heating and boiler feedwater.
The FGTT design, which is used for the primary air preheater, eliminates many
maintenance concerns. Despite the high speed at which the flue gas travels through
these units, there is no erosive impingement because flow is parallel to rather than
across the tube walls. Additionally, this design eliminates the need for auxiliary
cleaning (soot blowing) because of its self-scrubbing tendency. Tubes are vertical to
avoid surfaces where ash can accumulate.
Replaceable ferrules of stainless steel at the tube inlets further inhibit erosion and
help prevent the possibility of unwanted steam forming in the hot end because of
high local heat-transfer rates.
The primary heat-transfer surface consists of a matrix of carbon steel tubes,
which are rolled, seal welded, and lightly rerolled on each end into 13 mm (0.5 in)
thick tube sheets. The gas inlet tubesheet is insulated with an abrasion-resistant
castable on its hot face. The carbon steel casing is self-supporting, with external
blanket insulation and jacketing. The casing and tubes are operated at temperatures
sufficiently low that expansion joints are not required.

4.4 Thermal Fluid Heaters
A thermal fluid heater warms thermal fluid (synthetic or natural heat transfer oils) to
use in an indirect dryer or other heat transfer device. It typically follows the primary
air preheater, operating within a temperature range in which overheating and break-
down (cracking) of the heat transfer oil can be avoided. For common furnace sizes,