04 Jan How to Handle Degradation of Heat Transfer Fluids
The distinctive operational nuances and maintenance measures of a thermal fluid based heat transfer system is an art and science to be mastered — to avoid any system failures, find better efficiency and prevent unforeseen accidents.
The heat transfer fluid is considered a hidden asset the system and should be principally well-thought-of while designing its maintenance strategy.
There are a number of reasons on why these fluids degrade with time. Factors such as, temperature, system flow, or the operating schedule results in damage to system components, as well as permanent reduction of heat transfer. The viscosity of the fluid is increased and in some cases, the fluid boils off as a flammable vapor from the liquid causing safety threats.
What causes Degradation of Heat Transfer Fluids
Heat transfer fluids are available in different chemistries and every fluid faces the risk of degradation due to two important factors, Oxidation and Thermal Cracking.
Oxidation
Thermal oxidation occurs when the heat transfer fluid comes into contact with oxygen at elevated temperatures. The molecules from the fluid are decomposed to organic acids that cause fouling of the heat transfer system and also lower thermal efficiency significantly.
Petroleum-based heat transfer fluids undergo oxidation generally above 200 degree Fahrenheit and for a rise of every 15 degrees above this temperature, oxidation rate tends to double. Fluid degradation due to oxidation is very common in smaller systems where it causes sludge formation along reservoirs and pipes.
Thermal Cracking
Thermal cracking or degradation happens in the absence of oxygen when the bulk temperature of the fluid exceeds the maximum level. As a result, the fluid molecules break down into low boiling fractions called light ends and high boiling fractions called heavy ends. The adverse effects of thermal cracking include lowered flash points, lowered fire point, reduced thermal efficiency and fouling of the heat system due to excessive carbon deposition.
How Degradation in Heat Transfer Fluids be Mitigated
The mixture of high-boiling polymeric materials and the low-boiling compounds resulting from the degradation can be controlled by the following means.
Identifying the weak points prone to oxidation
As mentioned earlier, oxidation happens at temperatures above 200 degree Fahrenheit. The heat transfer system must be scanned for weak points where the fluid can be brought into contact with air (oxygen). The average fluid temperature around these areas must be measured as normal operation continues. When the temperature is above 200 degrees, the following measures must be adopted,
- A system with a fluid reservoir or an expansion tank reduces the chances oxidation. A reservoir of a cold liquid must be placed externally, close to the point of contact between the fluid and outside air to hinder oxidation.
- If the fitted reservoir is running hot, there may be an issue with its flow path. In such cases, the fluid must not be allowed to flow through the reservoir and the latter must be removed from the circulation loop. Instead maneuver the reservoir to be “T’d” into the system. If the reservoir still seems to be running hot, it can be further moved away from the system.
- Another alternative to protect the fluid from coming in contact with air the system can be padded with a blanket of nitrogen or any other inert gas.
Data based maintenance routines must be implemented
The organic acids formed due to the thermal degradation of the heat transfer fluid can be measured using two techniques, Total Acid Number (TAN) and Ramsbottom Carbon Residue (RCR). While the former gives the measure of acids in the heat transfer fluid, the latter is used to indicate the chances of sludge formation (based on residue formation) and hot spots.
These measures can be obtained at regular interval and plotted against time to check for deviations. These deviations may be interpreted and corrected through preventative maintenance routines timed at the discretion of the manufacturer.
Gradual start and shutting down of system reduces risk of thermal degradation
Thermal systems must always be gradually heated and also sufficiently cooled down before shutting down. When the heat is spread slowly, the moisture from the system is vented without cavitating the pumps.
Similarly, the circulating fluid may sometimes be trapped in the boiler and subjected to temperatures much above the desired levels (above bulk temperature marks lead to thermal degradation) thus making it necessary for the system to be cooled well before being shut down.
Adapting the system design to best mitigate chances of thermal degradation
The mechanical elements of the system design play a key role in controlling thermal degradation. The pumps, user loads, valves and so on must work in perfect co-ordination and according to the capabilities of the heat transfer fluid used. Any discrepancies in the engineering of these components may lead to higher chances of thermal cracking. Therefore as the system ages, system manufacturers as well as the fluid suppliers must be consulted to ensure working under the maximized parameters.
Oxidative degradation is a very common source of thermal inefficiency in systems used across various industries. Therefore, it is be a good option to check with the supplier of the heat transfer fluid regarding the addition of any protective elements to combat oxidation in the thermal system. Anti-oxidant additives ease the oxidation process and allow lesser maintenance efforts and lesser wastage of the fluid.