There is good evidence that preventing build-up is both safer and more cost-effective than detection and removal. While there are many ideas for preventing fouling build-up, it appears a major decrease in the fouling rate can be achieved by using continuous helical flow baffles (CHFs). While these have been around for some time, they have never been available in an electric heat exchanger/process heater. We now have new research showing that such baffles really do help maintain temperatures in a much tighter range, eliminating the dead zones and “hotspots” that produce fouling.
Two main Problems
When it comes to operational performance, fouling build-up reduces the cross-sectional area of flow channels, increases the resistance of the fluid passing over the surface and lowers the overall heat transfer coefficient. (The thermal conductivity of these deposits is much lower than most metals, and so even a thin layer can cause significant thermal resistance.)
These effects in turn combine to increase the pressure drop across the heater, reducing flow rates and aggravating the problem further. Even worse, fouling can cause heaters to run hotter to accommodate the drop in process temperature, which magnifies the problem as hot spots continue to accumulate coking. The reduced heat transfer rate means more power is required to achieve the desired outlet process temperature. The increase in power will cause the electric heater resistance coil to increase, as well as the heater sheath temperature.
As for the costs associated with process heater fouling, there are both direct and indirect:
• Production losses due to lowered efficiency
• Lost production time during planned (or unplanned) shutdowns to clean or replace elements or entire heaters
• Direct costs for the removal and cleaning of fouling deposits
• Replacement costs for corrosion of other thermal equipment
Indeed, cleaning costs alone can range from $40,000 to $50,000 per process heater per cleaning1. The cost total to replace a process heater can exceed 10 times that amount, when replacement costs are combined with downtime losses. These additional operating costs increase rapidly the longer the system is down.
The impact of fouling on heat exchangers has been recognized since 1910, and there have been a number of approaches to lessening, detecting and controlling it–everything from neural network models to predictive failure analysis due to fouling, to precisely controlled temperature and flow rates and new process heater designs.