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Comment PDF Heat Transfer

Maintaining heat-transfer-fluid quality

By Scott Jenkins |

Effective and efficient process operation requires reliable temperature control. This imposes strict demands on the management of the heat-transfer-fluid (HTF) system. This one-page reference provides suggestions for proper maintenance of heat-transfer fluids. Design features necessary to properly maintain acceptable fluid quality can be found in Table 1.

1

Fluid selection

A critical parameter of HTF quality is thermal stability, so ensure that the fluid is rated for use up to or above the maximum temperature of operation, keeping in mind the potential for higher-temperature operation in the future. Consider selecting a fluid with excess temperature rating for handling the rigors of stressful service — such as frequent startups (such as in batch processing) where temperature controls may permit high-temperature excursions, or in periods of process upsets — so that the fluid can be more forgiving in times of high stress. Second, consider heat-transfer efficiency at the designed operating temperature. This requires evaluation of the expected relative performance of the fluids for heat transfer coefficients, pumping energy costs and ease of handling (for example, metallurgy and area electrical classification). Third, consider potential implications if the HTF mixes with process fluid as a result of leaks. Will a chemical interaction result? How easy would it be to separate the two fluids? Finally, it is prudent to review the fluid selection with the facility’s insurer, in case installed-cost factors might differ among fluid options.

 

Avoid fluid oxidation

Any organic HTF operating at elevated temperatures can be vulnerable to oxidation. Some fluids that typically operate at medium temperatures come with additives designed to sacrificially protect the HTF until they are depleted. These consumed additives can contribute to sludge formation, which can lead to plugging of small-diameter tubing, as well as pump seal failure, corrosion, fouling and deterioration of heat-transfer efficiency.

Use of inert gas blanketing of the system applied at the expansion tank can protect the fluid against oxidation by providing an oxygen-deficient atmosphere. This helps to prolong efficient fluid life. Blanketing requires a pressure-regulated source of oxygen-free inert gas, such as nitrogen or natural gas.

A secondary source of fluid oxidation can occur if hot fluid is drained and later returned into the system. It is best to prepare job plans to allow the fluid to adequately cool prior to draining. This both protects the fluid and prevents thermal burns on workers.

 

High-temperature excursions

A common threat to HTF life is high-temperature excursions. Fortunately, the locations and causes of these excursions can be anticipated and addressed in the design and operating controls. The source of high temperature will be within the heat source, whether it is fuel-fired, electric heat, or perhaps a high-temperature waste-heat recovery unit. With heaters, ensure that the fluid maintains fully developed turbulent and uniform flow through heater coils or across heating elements to convey the heat flux away from the coils and element wall.

Common causes of these excursions include pump cavitation, reduced flowrates, changes in fuel, power failures and changes in flame patterns. New HTF systems should have a comprehensive set of critical parameters established to monitor for good operation, and they should be incorporated into the operating instructions. Operators and maintenance personnel should be informed about the purpose of monitoring and the impact on fluid life and performance. In general, at the maximum recommended operating temperature, the rate of thermal degradation can roughly double for every 10°C of temperature rise. If severe enough, coke may accumulate on the surface of heating coils and elements.

 

HTF supplier relationship

HTF end-users should rely on the HTF manufacturer to assist in understanding the proper and safe use of the fluids within the specific constraints of their unique system. The key source of fluid-specific knowledge rests with the fluid supplier, so make sure they are part of the team.

Department Editor: Scott Jenkins; text for this edition authored by Conrad Gamble, Eastman Chemical

 

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