One method commonly used to compare synthetic fluids and mineral oils is life expectancy. Based on extrapolation of ampoule testing results, it has been asserted that mineral oils experience up to six times the degradation rate of synthetic fluids at operating temperatures of 625oF (330oC). One could conclude that synthetic fluids offer significant service life advantages over mineral oils in the 550 to 625oF (288 to 330oC) temperature range.
The fact that the ring structure of the original di-phenyl/ di-phenyl oxide chemistry and its alkylated aromatic cousins (employed in synthetic fluids) is less prone to thermal degradation than the aliphatic structure of mineral oils has been well established. However, replacement rate and its in-verse, life expectancy, are affected by more than a fluid's thermal stability. While any number of operating problems can cause fluid to experience abnormally high degradation rates, it is the mechanical and the hardware issues that typically control how long a fluid remains in a system. For example, any action that results in opening a line -- for example, a pump or valve replacement -- seldom is accomplished without fluid loss. In many cases, these maintenance issues occur with increasing frequency as the system ages. On average, users replace 10% of the fluid charge per year as a result of ongoing maintenance issues. Expanding or reconfiguring a system also typically results in substantial fluid replacement.
When mechanical issues are included in selecting heat transfer fluids, mineral oils may have an advantage over the synthetics. Synthet-ics are two to three times as expensive as mineral oils in initial cost. Even at a higher fluid replacement rate, mineral oils may provide a lower overall fluid cost than synthetic fluids. Also, because used mineral oils can be blended into fuel for oil-fired boilers, their disposal cost is less than synthetics. Mineral oils have less odor, are relatively benign to work around and do not require extensive reporting if a spill occurs. These factors should be included in calculating the total cost of fluid ownership.
Acquiring the AdvantagesWhile mineral oils have been used successfully in process heating systems operating up to 590oF (310oC), they also have been misused. Historically, mineral oils, have been manufactured by oil companies and distributed through lubricant distributors. The products have had varying amounts of impurities that tend to degrade at relatively low temperatures. Because the volume sold for heat transfer was small in comparison to the volume sold for lubrication, product support and technical service were uneven at best. Little effort was made to correct the operating problems that often can be diagnosed before the fluid is destroyed. Fluid analysis typically included a test for metals, which is required for lubricating oils, but seldom if ever included a distillation test, which is necessary for heat transfer fluids. As a result, mineral oils were particularly susceptible to the "fill it, ignore it and dump it " operating philosophy.
But, by following a few guidelines, heat transfer fluid users can employ mineral oils up to 630oF (332oC) with-out sacrificing service life.
Radiant vs. Convection. Direct-fired heaters typically utilize both convection and radiant types of heat transfer to increase the temperature of the circulating medium. The convection section uses the sensible heat from the combustion products only -- there is no exposure to the flame. The direct-fired heater operates essentially a gas-to-liquid heat exchanger, and the resulting energy transfer per unit area (also known as the heat flux) is relatively low. This results in a small temperature rise between the inner wall of the tube (the film temperature) and the fluid flowing in the center of the tube.
The radiant section is the area of the tubing that actually faces the flame. Depending on geometry and design, up to 60% of the total heat is transferred in the radiant section. This resulting localized heat flux can be three times the average for the entire heater, with film temperatures exceeding the average fluid temperature by 150oF (83oC) or more. In some heaters, the maximum recommended film temperature of a fluid can be exceeded even though the average temperature is well within limits.
To minimize degradation rates when using mineral oils above 600oF (316oC), users should take extra care with combustion chamber design and coil geometry when selecting a heater. A large volume combustion chamber allows for more space around the burner, which in turn minimizes the radiant energy reaching the coil surface. Having open space between the individual coils and between the coils and the wall of the combustion chamber also reduces localized heat flux. To prevent the fluid from exceeding its maximum film temperature, full convection heaters are available. These units separate the burner from the heat exchanger, eliminating the radiant section.
Mineral oil color is an indication of the types of molecules used in the formulation. Those with the least impurities are water white.
Check Film Coefficient. Physical properties vary from product to product, which can significantly affect the heat transfer coefficient and ultimately the film temperature of the fluid. A simplified version of the Seider-Tate equation is as follows:
hi = A x B x (K-0.47) x (d0.8) x (Cp0.33) x (k0.67)
where A is constant
- B is fluid velocity and pipe diameter components
K is viscosity
d is density
Cp is specific heat
k is thermal conductivity
Because A and B will be constants for the system, users can estimate the heat transfer coefficient for each oil under consideration. The oil with the highest value will have the lowest film temperature and degradation rate.
Install a Side-Stream Filter and a Light-Ends Removal System. As they degrade, oils form lower molecular weight components (light ends) and, eventually, carbon particles. The oxidation caused by vented expansion tanks reduces the light end formation but accelerates the carbon formation. Carbon particles can be removed with a 1 to 5 micron (absolute) filter. To prevent any flow restriction, this type of filter never should be installed in the main fluid loop but only on a side stream.
Light ends can be removed by heating and purging the expansion tank or by draining the expansion tank if the heater is equipped with an integral degassing system. By removing these degradation products periodically and replacing them with fresh oil, users can achieve an essentially steady-state fluid condition that can eliminate or postpone a complete fluid change-out.
Make Sure Your Supplier Can Service the Fluid. The poor reputation of mineral oils is in no small part the result of the way they have been supplied and the lack of customer support they have received. That suppliers of synthetic fluids historically have provided excellent product support has no doubt contributed to the impression of superior fluid life.
In many applications, mineral oils can operate as effectively as synthetic fluids and may offer lower overall fluid costs.