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Anyone who has boiled water for tea knows that a calcified kettle takes longer and uses more power than a newly cleaned kettle. However, the additional cost to households of using a calcified electric kettle is minimal compared to companies that use calcified heat exchangers.
Researchers use model systems like these to make representative and reproducible experiments that simulate flow conditions in pipes so that fouling in the pipes can be observed. Image credit: Christian Ove Carlsson
A heat exchanger is a device that transports heat from one medium to another. For example, district heating plants use them to heat the water sent out to Danish homes and buildings. Heat exchangers are also part of the cooling units that manufacturing companies use to cool liquids.
New research from DTU will help calculate the optimal cleaning routines to avoid these large and expensive energy losses, while at the same time enabling companies to clean only when necessary.
According to Isaac Appelquist Løge, a PhD student at DTU Chemical Engineering, it is possible to achieve significant savings by calculating how quickly a heat exchanger calcifies, so that the accumulation of limescale can be removed at just the right time.
“Calcified surfaces are a much bigger problem than many people think. There are estimates that the cost of maintenance and additional costs from heat exchangers is as high as a quarter of a per cent of the gross domestic product of the industrialized countries,” says Isaac Appelquist Løge.
Calculations by the international Association of Water Technologies show that a layer of limescale of 1/8 inch (about 32 mm) causes a 25 per cent efficiency loss. This means that achieving the same heat output costs significantly more. If the limescale accumulation is twice as thick, the efficiency reduction will be twice as big, and the additional cost will also be doubled.
Calculations show how much the efficiency of heat exchangers is affected at different levels of fouling. Source: https://www.awt.org/resources/seed-program/water-careers/science-of-scaling/
Different parameters affect crystal formation
The growth of crystals occurs at all interfaces between water and a solid surface. Various chemical compounds form the crystals.
Isaac Appelquist Løge’s research shows how the accumulation of the two most common crystals, namely lime and barite crystals, is affected by the speed at which the liquid flows, the texture of the surface over which the liquid passes, how the crystals accumulate over time, and the effect of the crystals’ concentration.
The study was conducted on liquid flowing through pipes. The growth of the crystals has been studied with CT scanners, among other things.
“Specifically, we saw that if the liquid flows more slowly through the pipes, the crystals grow in isolated clumps like single trees on a hill and end up breaking off and flowing with the current,” he explains.
“At faster speeds, on the other hand, they grow together in larger clumps like a dense forest and tend to stay in place for longer. The results go against the common belief that liquid flowing at higher speeds will have a force that causes the crystals to break off. In any case, we show that there is a tipping point that determines when the detachment of crystals impacts the accumulation.”
Crystal growth at low and high speed. At high speed, crystals detach much later, because they shield each other. Image credit: Løge et al., Chem. Eng. J., 430 (P2) (2021, DOI: 10.1016/j.cej.2021.132583)
This knowledge can be incorporated into production processes to adjust fluid flow rate and reduce crystal accumulation.
Based on this new insight into why and how quickly crystals accumulate, Isaac Appelquist Løge will create a model that can be used to plan the most appropriate cleaning schedule.
Valuable knowledge for many industries
Crystal accumulation poses a problem in heat exchangers and other industries that pump liquids through pipes, including everything from the food industry, CO2 transport and storage, and geothermal energy to the oil and gas industry.
“If the diameter of the pipes becomes smaller as crystals accumulate in the pipes, you need more energy to force the same amount of liquid through,” explains Isaac Appelquist Løge.
The pipes must therefore be cleaned before the pressure loss becomes too great, but not unnecessarily often, as cleaning is expensive. On an oil rig, the challenge is particularly great, as production must be stopped in the meantime, and manpower and materials must be transported offshore.
The new knowledge can also be used in these industries to calculate the optimal cleaning routines.
Source: DTU
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