Guide for better economiser design
Is it essential to pre heat feed water?
How one preheats they boiler feed water? We have two options,
- Via economiser, mounted on exhaust of boiler, and heating incoming feed water by absorbing lost heat in flue gases
- Via Deareator tank, adding steam to deareator and heating the water temperature. Though the purpose of deareator is as name suggest remove the “oxygen” from the water, by heating it. Nevertheless it also increases the temperature of water in this process.
Let’s talk about economiser
Now that’s the question, one should not ask, if possible, one should always use economiser. It generally speaking will increase boiler efficiency by 3-5 percentage points.
It also helps reduce the thermal stress on boiler as a whole.
Then why this question?
Challenges with economisers are few, apart from how much heat we are recovering vs how much expenditure we are doing in is installation, maintenance and qualification.
Assuming the economics works well, the next question is if you absorb too much heat, and condensation happens, it will lead to sulphur corrosion, distorting not only boiler but also chimney in long run.
Criteria for designing economiser from user perspective
- Define maximum hot water temperature we expect out of economiser, this has to be above sulphur due point
- Define maximum pressure drop allowed. As more pressure drop will add more duty to the Blower, adding to running energy cost
Criteria for design from Designer’s perspective
- Configuration of path, with multiple branches to make water flow, it helps get maximum efficiency but also adds to pressure drop and duty on feed pump
- Profile of fins for heat exchanger, some profiles of fins, for example serrated fins, has huge heat transfer to area ratio, and are highly efficient, but also adds to pressure drop on air side, and blower duty increases accordingly.
- Number of passes of hot flue gases, this ensures more residential time for the flue gas, so that we can extract more heat, however, this will also cost us more pressure drop and added duty to the blower.
- Minimum feed water temperature and maximum outlet temperature, both factors are important to ensure we get better efficiency but also to ensure we have less issues with corrosion (oxygen pitting
Typical failure reasons
Typically in boiler, the flue gas is mostly utilised to super heat the steam, and then passed to economiser, in such cases, The temperature differentials between the flue gas and water are quite low. To maximize heat transfer, water temperatures at the end of the economizer run should be very close to the saturation temperature. If the temperature difference is very low, then it some time can lead to steaming in economiser. Steaming not only reduces the efficiency drastically but also lead to knocking and failure of weld during operation due to thermal impact.
Second reason for failure could be quenching effect. This happens, when boiler is stand by mode, that is steam is not consumed. Water in economiser reduces drastically, and sudden surge in demand, make feed pump, pump cold water to economiser, leading to thermal stresses, even if the delta of water differential is low, this will lead to failure after some time. This issue can be over come by losing some water in boiler itself via intermediate blow down, small water circulation in economiser helps avoid such quenching issues.