By Jeff Sines, Product Engineer, Engineered Software, Inc.
Operating a heat exchanger in an industrial application often requires the operations and maintenance crews to perform hazardous procedures to remove fouling and keep the heat exchanger at peak efficiency. Fouling is material deposited over time on a heat transfer surface. Fouling can also result from “baking” particulate material in a fluid stream onto the heat transfer surface and can occur due to corrosion of metal surfaces, or depending on the nature of the fluids in the heat exchanger. Fouling causes a greater resistance to heat transfer, a reduction in thermal effectiveness, increases hydraulic resistance, and will require a higher flow rate or greater temperature difference to achieve the heat transfer required for the heating or cooling application. Fouling occurs on the tube side of a heat exchanger, the shell side, or both.
At a constant flow rate, as fouling occurs and the resistance to heat transfer increases, the heat exchanger’s heat transfer coefficient (U) decreases and will result in:
- a lower heat transfer rate
- a higher outlet temperature on the hot side (and lower dT hot)
- a lower outlet temperature on the cold side (and lower dT cold)
Since the outlet temperatures of the hot or cold side may be critical to the quality control parameters for the overall process, the flow rate must be increased to achieve the desired heat transfer rate and outlet temperature. A higher flow rate reduces the convection heat transfer coefficient (hf) for the fluid and increases the overall heat transfer coefficient (U) to compensate for the effects of fouling. Very often in these applications, an automatic process control loop is used to measure and control the outlet temperature of one side of the heat exchanger by regulating the hot or cold side flow rate with a control valve (and occasionally a centrifugal pump operated by a variable speed drive).
Fouling in a heat exchanger can get bad enough that the hydraulic limitations of the piping system begins to impact the ability of the process control loop to maintain its critical quality parameters. For example, when the control valve reaches the fully open position, no further increase in the flow rate can be achieved. Additional fouling will reduce the heat transfer rate and affect the outlet temperatures. It is at this point that something has to be done about the actual fouling in the heat exchanger. Common cleaning methods used to remove fouling include chemical cleaning, hydro-blasting, and steam out. Each method has its inherent dangers, requiring operators and maintenance crews’ exposure to hazards that often send someone to the hospital, or worse, if proper precautions are not taken.
Chemical Cleaning to Remove Heat Exchanger Fouling
Such was the case when I first started working at a pulp mill after leaving the Navy in 1995. My role was working as a production supervisor with operators and maintenance crews. Part of my responsibility was to help the operators keep their processes running and production moving. Many of these processes used heat exchangers in a wide range of applications.
In a part of the mill, the Acid Recovery Plant made weak acid that was fortified for use in batch digesters that “cooked” wood chips to make a pulp slurry. Cooling the weak acid to a specified temperature was critical to fortifying the acid to the highest quality. For this cooling application, two heat exchangers were used in series and each had separate sources of cooling water: Heat Exchanger “A” used fresh water from the water treatment plant, and Heat Exchanger “B” used a much colder supply of water from a deep well, as shown in the PIPE-FLO® Professional model below.