With thousands of uses for heat transfer fluids, devising an exact fluid change interval or maintenance schedule that fits all applications is nearly impossible. Each application has unique characteristics that can contribute to the degradation of a heat transfer fluid, and each fluid will react differently in various user environments. For example, the heat transfer fluid used in a PVC extruder might have a life cycle as short as a few months, while that same fluid in a larger “closed” system can last upwards of 10 to 15 years.
However, some basic principles can be applied to all heat transfer systems. By understanding how a heat transfer fluid becomes degraded, plants can devise their own ideal maintenance program.
Understanding DegradationTwo basic ways in which a heat transfer fluid can become degraded are oxidative and thermal.
Oxidative. Oxidative degradation, which is most common in open systems, occurs when hot fluid comes in contact with air. The oxygen in the air reacts with the fluid by a free radical mechanism to form larger molecules that end up as polymers or solids. These polymers or solids thicken the fluid, increasing its viscosity. A more viscous fluid will be more difficult to pump, have poorer heat transfer characteristics and have an increased chance of coke formation. Fluid oxidation is most frequently seen as sludge formation within the system, especially in low flow areas such as reservoirs or expansion tanks. Oxidation also is accompanied by an increase in the acidity (TAN) of the fluid.
At room temperature, the reaction rate is hardly measurable. However, as with most chemical reactions, oxidation occurs more rapidly as the temperature increases. As temperatures climb above 200oF (93oC), the effect becomes exponential and can shorten the fluid life in systems that do not use oxidation-reducing measures such as nitrogen blanketing of the expansion tank.
Thermal. Thermal degradation, or thermal cracking, occurs when the fluid is heated past its boiling point. The heat breaks the carbon-carbon bonds in the fluid molecules, forming smaller free-radical fragments. The reaction might stop at that point, or the fragments might react with each other to form polymeric molecules larger than those that previously existed in the fluid. In heat transfer terminology, the two types of degradation products are known as “low boilers” and “high boilers,” respectively.
If thermal degradation occurs at temperatures significantly higher than the system's maximum bulk temperature, the effect is not only to break the carbon-carbon bonds but also to separate the hydrogen atoms from the carbon atoms and form coke. This effect rapidly fouls the heat transfer surfaces, and the system will soon cease to operate. Low boilers will decrease the flashpoint and viscosity of the fluid and increase its vapor pressure. High boilers will increase the viscosity of the fluid as long as they remain in solution. However, once their solubility limit is exceeded, they begin to form solids that can foul the heat transfer surfaces.
Another concern with thermal degradation is safety. As the fluid boils, much like water, it produces a lighter component in the form of vapors. These vapors reduce the overall flashpoint, fire point and autoignition temperatures of the system.
Open System MaintenanceOpen systems tend to run at temperatures well below the maximum recommended bulk temperature of a fluid (usually under 600oF [315oC]). While operating at these levels usually eliminates the potential for thermal breakdown, open systems are susceptible to oxidation. In extreme situations, severe oxidation can shorten fluid life to a few hundred hours. It is crucial to understand the system's design and how it incorporates protection from oxidation. Often, equipment manufacturers use heat exchangers to cool oil prior to its exposure to air, or bypass valves set on timers that open to assist with venting at startup but close to reduce oxidation levels after a desired temperature is reached. If these systems fail to operate properly, the oil life can (and most likely will) be shortened.
As a fluid oxidizes, it forms an acid. These acids, while generally not at corrosive levels, can build and eventually polymerize (drop out of) the fluid in the form of heavy, grease-like sludges that can impede flow and insulate processes (e.g., molds and dies), making heat transfer less efficient and probably insufficient for production. If severe enough, these sludges can completely plug the lines and shut down the entire system.
Plants that operate open systems should follow a recommended fluid analysis program (usually available from the fluid vendor). When first using a heat transfer fluid in an open system, fluids should be monitored regularly to understand how often a particular system requires fluid changes. While equipment manufacturers can provide guidelines as to the life expectancy of a fluid, all fluids are not equal, and simple differences in operating environments and temperatures can affect fluid life. The only way to know how often a fluid should be changed is through analysis.
Once a maintenance schedule has been established, it is important to understand how to efficiently and effectively remove all of the spent fluid prior to refilling the system. If the acidic compound in a degraded fluid is allowed to mix with new fluid, it will increase the rate of degradation of the new fluid. Smaller open-type equipment contains numerous areas where fluid can become trapped. Areas such as heat exchangers, filter housings and horizontal piping should be examined for residual fluid. Blowing air or nitrogen through the lines often will help remove old oil. If this option is not possible, most fluid manufacturers have a light flushing agent that can be used to flush out residual fluid.
Closed System MaintenanceClosed systems that are operated properly (that is, at or below the recommended bulk or skin/film temperature of the fluid) do not typically encounter fluid degradation. However, situations such as a power or pump failure or system changes (partially open or closed valves, decommissioning user loops, etc.) can cause thermal degradation. Such degradation often can go unnoticed by the user on a day-to-day basis; if left unchecked, it can severely damage the equipment.
To avoid thermal degradation in closed systems, plants should consult both the heat transfer fluid vendor and equipment manufacturer before making any changes to the system. Most heat transfer systems are engineered around the user's needs and the type of fluid used. Changes in a system's design or function can negatively affect the fluid.
As with open systems, fluid analysis is the best maintenance tool. These analyses allow any changes to a fluid to be detected early so that the system can be corrected to stop the fluid degradation and, in some cases, improve the condition of the fluid.
Closed loop systems typically require infrequent fluid changes, measured in years. However, when a fluid change is needed, multiple steps such as flushing the existing fluid from the system or using a cleaning fluid often are involved. It is always best to understand the condition of the fluid and system prior to changing fluids (either to a different fluid or the same fluid). Plants should conduct a fluid analysis and a general inspection of the system before the new fluid is added, looking for leaks and, if possible, inspecting the inside of the pipes and boiler. However, the most important step is the complete (at least 95 percent) removal of all traces of the previous fluid. Degraded fluid will quickly contaminate new fluid.
No matter how sophisticated a heat transfer system might be, the condition of the heat transfer fluid can affect its operation. By understanding the factors that cause fluid degradation, plants can develop a preventive maintenance program that will keep their equipment clean and running at peak efficiency. PCE
Sidebar: All heat transfer systems can be broken down into one of two rudimentary types: open or closed.
Open or Closed?
In an open design, the oil comes in contact with air at some point in the system. If the oil temperature is not well below 200oF (93oC) when this contact occurs, the oil will be susceptible to degradation through oxidation. Open systems tend to be smaller than closed systems and are used in manufacturing processes in the plastic, die-cast and other industries that use portable electric “oil heaters.”
A closed system typically uses an inert gas buffer (usually nitrogen) between the oil and atmosphere to prevent oxidation. Some boiler manufacturers have developed “closed loop” systems, which use expansion tanks that use proprietary plumbing to prevent the hot oil from coming in contact with the atmosphere.