Heat transfer fluids are often taken for granted, despite their importance. Incomplete knowledge about their selection, operation and maintenance results in premature degradation. The implications of a poorly selected or an improperly managed fluid can be far reaching when one considers downtime and maintenance costs. Downstream equipment such as pumps and filters can be also damaged. In this article, we review solutions to common heat transfer fluid problems such as oxidative degradation, thermal cracking and contamination. However, before a solution can be proffered, we must adequately diagnose the problem(s) in an existing heat transfer system. The best way to do this is through Heat Transfer Fluid laboratory analysis. Plant managers and operators need to recognize the importance of periodic fluid analysis in the preventative maintenance of heat transfer fluids. Several parameters that tell us about the general health of a fluid are measured. They include Flash Point, Viscosity and Total Acid Number (TAN) amongst others. We offer periodic fluid analyses in addition to an interpretation of the results. From our many years of experience with different applications and fluid analyses, we present recommendations for some common heat transfer fluid problems.
Heat Transfer Fluid Oxidation
Oxidative degradation occurs when oxygen (in air) reacts with a heat transfer fluid. This is a two stage free radical chemical reaction that eventually leads to the formation of weak acids and insoluble solids. As a result, the viscosity of the fluid increases. Because of the consequent decrease in turbulence, the fluid’s thermal efficiency reduces. Beyond a certain viscosity band, pumpability can become an issue. A full oxidized fluid is discolored and contains sludge. Another common pointer to oxidative degradation is TAN increase, especially at elevated temperatures. TAN increase is therefore a measure of oxidation and can be used to compare the oxidative stability of different fluids.
The choice of a fully additized heat transfer fluid is the first protection against oxidative degradation. It is advisable to choose a fluid that inhibits the primary and secondary stages of oxidation. Many heat transfer fluids contain no anti-oxidation additives. Others contain only a slight treat. Relatherm Heat Transfer Fluids contain a primary and secondary anti-oxidation package that allows for an extended fluid life.
Another effective method of inhibiting fluid oxidation is to blanket the expansion tank with an inert gas such as nitrogen or carbon dioxide, or with natural gas. The purpose of inert gas blanketing is to maintain an oxygen-free atmosphere in the expansion tank, and one of positive pressure to prevent air entry. A regulated supply of inert gas with a backpressure regulator on the vent outlet line is necessary to obtain this protection. A pressure relief valve also is required to protect the expansion tank from overpressure due to regulator failure, fire and other causes. Only a static pad of pressure is needed inside the expansion tank to minimize inert gas usage. Maintaining a positive pressure slightly over atmospheric barometric pressure is all that is necessary to prevent air and moisture from entering the tank. A manual vent valve also should be installed for routine fluid maintenance by removing low boilers as needed.
In open bath systems, oxidative degradation is more drastic because of the exposure to air. It is very important to select a fluid that mitigates oxidation. Relatherm S-1 is a silicone-based heat transfer fluid that eliminates the problem of oxidation. Relatherm PAG is a PolyAlkylene Glycol based heat transfer fluid that is more resistant to oxidation compared to other mineral oil based thermal fluids.
Thermal Cracking or Degradation occurs when a fluid is heated above the maximum bulk temperature causing the covalent Carbon- Carbon to disintegrate into electrovalent bonds. This leads to the formation of molecules with lower molecular weights than the original molecule. Thermal cracking causes a reduction in viscosity, flash point, fire point and auto ignition temperature. It is not uncommon to discover carbon varnish on the heat transfer surfaces. Typically, this is from a further disintegration of the Carbon – Hydrogen bond. In order to mitigate thermal cracking, it is important to understand ways in which fluid overheating can occur. They include wrong fluid selection, wrong flame impingement, improper startup & shutdown and a low flow regime.
Choosing a fluid with the right thermal properties helps to ensure fluid longevity. Relatherm Heat Transfer Fluids are made from high quality base stocks that maintain their thermal stability over an extended temperature range. Furthermore, many plant operators crack fluids when they start up and shut down heat transfer systems. A drastic rise in temperature and low velocity flow induces thermal degradation. Always start the pump before turning on the heater, this ensures fluid circulation and good mixing prior to heating. It also reduces the residence time of the fluid on the heated surfaces allowing for a steady rise in temperature, helping to prevent fluid cracking. Start the burner on the low fire setting, circulate heat transfer fluid at full flow and slowly raise the temperature. Once there is circulation through the heater, the operator should increase the temperature of the bulk fluid by 20 to 25oF (11 to 14oC) increments until the fluid reaches a viscosity of 10 cP. Once the fluid reaches 220°F and is pumping smoothly without cavitation, follow the manufacturer’s recommendation for a full fire heat up
Other recommendations include the proper alignment of burner flame. A faulty flame disperser may cause the coils adjacent to the flames to absorb more heat energy than they can hold. Hence, a heat transfer fluid can be cracked if temperature at the tube surface exceeds the maximum bulk temperature. It is also sometimes helpful to check for plugged y-strainers and malfunctioning or improperly set bypass valves. This may free up some of the flow that may have been obstructed in the system. is that the fluids in heat transfers are circulating while the system operates at a high temperature. Accordingly, during a shutdown of the system, heat must be decreased proportionately, in the reverse of the startup procedures. Keep the fluids circulating until system temperatures drop below the 200°F or 93°C mark, so that the residual heat is removed.
Contamination occurs in a heat transfer system when a different fluid (other than the heat transfer fluid) is introduced into the system, potentially causing wide variations in the physico-chemical properties of the original fluid. Common fluid contaminant include water and used fluids. It is important to ensure that used fluids are completely flushed before charging a new system fill. Heat transfer equipment with water as an operational fluid must be inspected regularly for leaks. Other good housekeeping practices include properly coding of full and partial full storage drums. These drums should be placed sideways if they to be stored outside.