A Heat Transfer fluid is a fluid that can perform reliably in heat transfer systems or heat exchangers using hot-oil heat exchangers. Heat transfer fluids are typically formulated with highly refined mineral oils or silicon-based fluids, this short article will primarily address mineral oil-based fluids.
Heat transfer systems that use a mineral oil medium are used in a variety of industries:
- Industrial manufacturing
- Asphalt processing
- Food and beverage
- Chemical processing
- Plastics and polymers
These fluids are required to operate within a wide temperature range from cold startups to high temperature heat transfer applications. The correct fluid should have a low vapor pressure to resists thermal cracking and be able to reach high temperatures without boiling or reaching the vapor phase.
Proper system performance and maintenance is also necessary for fluid life integrity and required fluid heat transfer properties. Water contamination can occur when air is allowed to enter and moisture condenses during cooling. When water heats up in the system a popping or snapping noise can sometime be heard in the piping. Pressure fluctuations at temperatures above >230° F are symptomatic of water present in the fluid. Low points in the system can collect water. If it is possible to drain a sample from one of these low points the system can be inspected for a buildup of water contamination. Leaks in the piping that allows the fluid to absorb into porous insulation can raise the internal temperature of the insulation as the fluid heats and become oxidized. This can also lead to a fire danger. It is important the reservoir design allows for fluid expansion during heating when starting up after a shut down. The reservoir should prevent exposure to air and not allow the fluid to overheat during residence time.
Caution should be used when mixing different heat transfer fluid products. Heat transfer fluid properties can be adversely affected when different additive chemistries are compounded resulting in a compromised fluid. The safest approach when changing to another heat transfer fluid product is to ensure the system is clean and dry before changing products.
Thermal cracking and oxidation is one of the primary concerns of heat transfer fluids in service. Thermal cracking occurs when larger oil molecules decompose into coke particles, which is mostly carbon, as well as smaller oil molecules with a lower boiling point. These smaller ‘cracked ‘molecules can also react together and combine to produce even larger molecules than those in the original fluid. This increases the viscosity of the fluid and hinders heat transfer properties. The larger oxidative molecules can also form polymerized molecules which develops sludge, varnish and insolubles that also reduces heat transfer performance of the fluid.
Thermal cracking can be minimized by:
- Maintaining circulating fluid flow and consistent fluid pressure through the heater
- Bring cold fluid up to temperature slowly after circulation begins
- Avoid sudden shutdowns without allowing the fluid to cool down first.
- Monitor the combustion chamber for proper flame propagation and burner alignment
- Properly maintain system instrumentation
- Daily top-up with fresh fluid and not reuse old fluid
- Perform regular system cleaning
As well as routinely inspecting the heat transfer system, periodic fluid samples should be taken from the system while the fluid is circulating. In-line samples should be drawn from preinstalled valves at suitable points in the system to obtain a sample of the fluid which is representative of the fluid in circulation. Care should be exercised that all equipment used to draw the sample is suitable for the temperature of the fluid and that the sample can be obtained in a safe manner.
ASTM Standard D5372 ‘Standard Guide for Evaluation of Hydrocarbon Heat Transfer Fluids’ provides the following list for testing in-service mineral oil heat transfer fluids.
- Viscosity - Increases in the fluid’s viscosity indicates degradation products are been formed which will reduce the efficiency of the system. If the measured viscosity is around 100% of the fluid’s starting viscosity consideration should be given to replacing the fluid. Once the measured viscosity is approaching 200% of the fluid’s starting viscosity it should be replaced as a matter of priority.
- Carbon Residue - The carbon residue test measures the fluids tendency to form carbon deposits and reported as the percentage of residue formed. Increasing values indicate that the fluid is more likely to thermally decompose and deposit carbonized material in the heat transfer system which will reduce the efficiency of the system.
- Copper Strip Corrosion - Indicates the fluid’s tendency to corrode copper-based metals in the system which will put pumps and other components that use yellow metals at risk.
- Pentane Insolubles - Indicates the amount of sludge/varnish the fluid is carrying. As this value increases, the risk of sludge/varnish and other thermal decomposition products been deposited in the system.
- Flash Point - As a heat transfer fluid undergoes thermal ‘cracking’, the flash point of the fluid can decrease due to the formation of volatile compounds. If the flash point is lowered significantly, there is an increased fire risk from the fluid.
- Total Acid Number - Heat transfer fluids have low acid values, typically less than 0.1 mgKOH/g, as a fluid thermally degrades acids are formed and shifts in a fluid’s acid number can be early indication of the fluid’s degradation. As a rule of thumb, once a fluid’s TAN reaches or exceeds 0.3mgKOH/g replacing the fluid should be considered. Note however that there can be variability in the fluid’s starting TAN value, and this should be factored when deciding if the fluid requires changing.
- Color - Color changes, specifically darkening of the fluid is indicative of carbon ‘soot’ like particles been generated and carried by the fluid. While color itself has no affect on the performance of the fluid, it can be an early indicator of changes occurring in the fluid which would will warrant further analysis.
- Water Content - Water is problematic because it expands >1,000 fold when converted to steam. This expansion will cause the pressure and flow rates to fluctuate in the system which in turn can result in uncontrolled temperature variation of the system. The measured water content should be kept at or below 50 ppm. Above this level problems may occur, and action should is required to remove the water from the system once it is greater than 150 ppm.
- Pour Point - Pour point is a physical property that becomes important at low temperature start-up of a heat transfer system. The fluids pour point is increased as the fluid degrades and over time can increase to the point that at ambient air temperature the fluid may no longer be pumpable at start-up.
- Spectrochemical Analysis - This is an elemental analysis which will report common metals such as Iron, Copper, Aluminium such that wear and/or corrosion can be monitored. A range of other elements may assist in identifying contaminates entering the system.
- Oxidation Number - The oxidation number is an indication of the presence hydrogen/oxygen bonds that form when the fluid is thermally degrading. The value will increase from the new fluid’s base level when thermal degradation is occurring and is a useful parameter to monitor the progress of degradation and to plan fluid change outs.
Note that not all heat treatment system problems are caused by the fluid, there is a common assumption that because the heat transfer fluids degrade over time, that problems with a system that has been in service for a few years is automatically attributed to fluid degradation. This can lead to costly fluid changes that do not correct the underling issue. Analyzing a sample of the fluid is a cost-effective means to confirm if the fluid is behind the observed issue, or if there is something else causing the problem.
ALS Tribology can provide a testing routine on a monthly, quarterly, or annual basis that is cost effective and meets system reliability requirements. Please feel free to contact one of our laboratories or customer service representatives for further information.
David Doyle, CLS, OMA I, OMA II
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