In this post, we build on Part 1 of How Hot Can you Go?, a discussion on feedwater temperatures entering the steam drum. Although hotter feedwater temperatures result in increased steam generation for the same fuel heat input, feedwater temperatures that are too close to the drum saturation temperature can lead to inadequate circulation conditions and result in tube overheating and failures.
In that post, we also described how elevated feedwater temperatures have a negative effect on attemperation capacity in sweetwater condensers. This insufficient attemperation can impact the superheater performance and in some cases lead to overheating and failure of the superheater tubes and steam piping,
The solution to achieving improved attemperation capacity with hotter feedwater temperatures can be found in Part 1.
But boiler operators may be faced with the scenario where modifications in the feedwater piping to achieve adequate attemperation capacity still results in elevated feedwater temperatures entering the steam drum. Jansen engineers recently worked on a recovery boiler in the southeast U.S. that experienced tube failures in its generating bank. Using ultrasonic flow metering, Jansen performed a circulation study which revealed that the primary source for tube overheating in the generating bank downcomers was excessively hot feedwater temperatures entering the steam drum. One potential solution put forth by our client was to lower the deaerator (DA) pressure. Doing this would in theory reduce feedwater temperatures. But lowering the DA pressure would also negatively impact the operation of the other boilers fed by the same DA, as it would result in lower steam generation rates from these units.
Jansen developed a novel solution to address this feedwater temperature issue. We created a design in which the hot feedwater leaving the boiler economizer is not delivered directly to the steam drum. Instead, the feedwater is fed to dedicated water coil air heaters (WCAHs) on the boiler’s combustion air delivery system. The feedwater exchanges heat with ambient combustion air and the feedwater temperatures leaving the WCAHs are thus much cooler and can be supplied to the steam drum.
The heat lost by the feedwater is then returned to the boiler system via the heated air leaving the WCAHs. Using this WCAH concept was shown to provide two important performance benefits: 1) cooler feedwater temperatures entering the steam drum results in improved natural circulation conditions to prevent tube overheating and; 2) the mill would no longer need to use the existing steam coil air heaters (SCAHs) to heat the combustion air, resulting in steam savings. Jansen is now in the design phases of the project with our client to engineer and design the WCAHs and feedwater system modifications.
In Jansen’s 49-year history, we’ve learned that every boiler is unique and custom solutions need to be developed on a case-by-case basis. Contact us to discuss your boiler challenges and let us help you to achieve safe and optimal boiler operations.
Authors: Samit Pethe – Manager, Process Technologies and Morgan Silverman – Process Engineer