Heat transfer fluids (HTFs) will degrade over time due to operating schedule, operating temperature and system flow. The processes that cause this to occur are thermal cracking and oxidation.
A heat transfer fluid will begin to darken and smell pungent, as acidic carbonaceous sludge is produced. Eventually, the sludge deposits on all surfaces in the system. Inside the heater, these deposits harden and permanently reduce heat transfer.
One of the easiest problems to ignore is thermal cracking, the process by which HTF molecules break down due to the absence of oxygen. HTF molecules decompose to low boiling fractions known as light ends, resulting in reduced flash and fire points, and high boiling fractions known as heavy ends, which recombine to form heavier polyaromatic molecules, resulting in fouling of the heat transfer surface through carbon deposition.
Thermal oxidation is the reaction of the HTF molecules when exposed to oxygen, which decompose to organic acids that are measured in terms of total acid number (TAN) and Ramsbottom carbon residue (RCR). Carbon residue increases the formation of insoluble particles and sludge, which has the effect of fouling a heat transfer surface, causing loss of thermal efficiency and hot spots, which can become hazardous.
The Ramsbottom test is adopted by the American Society for Testing and Materials as ASTM D-524-10, a standard test method for Ramsbottom Carbon Residue of petroleum products, while ASTM D664-11a is a standard test method for acid number of petroleum products.
TAN is a measure of the concentration of acidity in a heat transfer fluid and is determined by quantifying the volume of an alkaline reagent, for example potassium hydroxide, which is needed to neutralise the acid. One limitation of monitoring TAN is that it provides a measure of the acids generated by both oxidation and by those acids produced as contaminants during the process.
It is no surprise that acid by-products are potentially devastating to a heat transfer fluid and its system. In the worst case, the fluid will need to be replaced because the acidic by-products are corrosive to a system’s metal components and will accelerate system wear, as well as lead to concurrent increases in fluid viscosity and deposits.
Oxidation is the main reason for black sludge forming, which deposits on heaters and blocks pipe work. All of these increase the risk of component failures in a system.
New HTFs typically have a TAN less than 0.05, although this does vary by fluid type. Fresh polyalkylene glycol (PAG) will typically have an acid number ranging between 0.1 and 0.5.
PAG is used as a high temperature, thermally stable HTF exhibiting strong resistance to oxidation. Modern PAG’s can also be non-toxic and non-hazardous.
William Herron, Arizona, US
The condemning limit for a heat transfer fluid is widely regarded to be 1.0. However, the negative system effect of rising acid by-products occurs at acid numbers in excess of 0.4.
When fluids reach their condemning limit they need to be replaced. HTF replacement comes with an associated cost depending on the type of fluid selected—mineral versus semi-synthetic versus synthetic.
To prevent oxidation, fluid in the expansion tank must be kept cool. If this cannot be done, consider ‘padding’ the system with inert gas like nitrogen, which is inexpensive and readily available.
A line runs from a nitrogen source to an expansion tank’s head space and the gas should flow from the source through an alarmed flow meter,
regulator and check valve into the expansion tank. A back-pressure control valve should be fitted to a tank’s vent line, along with a relief valve.
In addition to protecting a fluid from oxidation, inert gas will prevent water from condensing in the fluid due to increased ambient temperature and dew point changes.
Some heat transfer fluids contain oxidation inhibitors: sacrificial material designed to prevent the fluid from oxidising during incidental contact with air. They are not designed to replace good system design, maintenance and operation.
Disposal of the old fluid will also need to be managed correctly by qualified professionals who operate in accordance with environmental regulations.
Therefore, the replacement with new fluid involves production downtime and lost output, which can be extremely expensive if unplanned. In such cases, planned, preventative maintenance contracts should be considered.
In trying to reduce the rate of degradation, manufacturers need to focus on regular preventative maintenance plans to minimise oxidation and thermal cracking and reduce the risk of fire due to closed flash temperatures falling below 100 deg C.
Planned, preventative maintenance also enables manufacturers to trend the data that is collected. Regular analysis means that parameters can be plotted against time and monitored correctly.
The advantage of doing so is that any changes in the status of the heat transfer fluid can be detected and interventions to correct any deviations can be planned around the manufacturer’s production schedule.
Maintenance contracts can also be tailored to client needs. For instance, in any system, the header tank temperature needs to remain below 60 deg C during normal operation to help to reduce the extent of fluid oxidation.
Therefore, this tank must remain cool enough to touch. However, if the system is running too hot, the rate of oxidation will increase and may require fitting a nitrogen blanket.
A further measure that manufacturers can employ to combat oxidation is the use of smart thermal fluids that work to depress oxidation. Such fluids contain anti-oxidant additives that help to attenuate the oxidation process at higher temperatures and therefore assist in preventing the build-up of sludge.
The benefit is longer fluid life, which in turn implies less maintenance, less process downtime, less fluid needing to be disposed and thus, less environmental waste.
The use of such fluids would be extremely beneficial in helping to prevent oxidation in header tanks and may also be an interim solution to preventing further oxidation until a manufacturer has sufficient time to replace the existing fluid.
A word of caution however: such fluids need to cater for food and non-food manufacturing and particular attention needs to be given to the suitability of some additives in food-grade manufacturing—something that is not new, since all thermal fluids used in food manufacturing should be approved.
This is a regulatory requirement imposed by several bodies including the US Food and Drug Administration (FDA) and NSF International, but it is one that many manufacturers are entirely unaware of.
Science & Compliance
The industry is littered with regulations that are misunderstood, poorly communicated and acted on incorrectly. There is no application where this statement is more appropriate than oil-based heat transfer, especially in food applications.
Compliance is often seen as another piece of red tape in the way of achieving a business’ objectives. Independent testing is important, as is using a correct sampling procedure, expert analysis and planned maintenance.
To get an accurate picture, thermal fluid samples must be collected at their operating temperature when the fluid is hot and circulating. Older fluid will naturally be more glutinous, so making sure that the sample is taken with the system running and at the required temperature will make for an accurate reading, ensuring the usual turbulent flow is taking place.
There is a substantial difference in consistency between samples at working temperature and dormant samples, affecting the way the fuel-like light fractions mix.
Where an ‘open’ sample is collected, the most volatile (lowest flash-point) specimens will automatically escape and flash off to the atmosphere, instead of being allowed to cool and condense back into the sample, where it can be decanted under laboratory conditions.
Light ends consist of a homologous mix of hydrocarbons with different boiling/flash points. In the case of open samples, as the lowest flash point material has been vented off, incorrect (too high) flash point values will be returned to give an inaccurate result.
Food Grade Fluids
Food regulations are much stricter and, when using fluid-based heat transfer, the thermal fluid must be fully H1 or HT1 certified as a food grade thermal fluid by the US Food and Drug Administration (FDA) and the NSF International, respectively.
If there is any possibility for oil or lubricant to come into contact with food products, a certified food grade fluid must be used to safeguard consumer health.
Food grade thermal fluid is extremely important in the food processing industry; if a manufacturer were to use another type of oil, this could potentially put its business at risk. Food grade thermal fluid is designed for incidental contact with food products.
Failure to use food grade fluid in a food application can result in the loss of the manufacturer’s top tier accreditation, should the European Food Safety Inspection Service (EFSIS), part of SAI Global Insurance Services, learn that an inappropriate product is being used.
High quality food grade fluid is non-hazardous, non-toxic and odourless, which means it requires no special handling and is not considered a controlled substance under US Occupational Safety & Health Administration (OSHA), Canadian Workplace Hazardous Materials Information System (WHMIS) or other work place regulations.
Companies cannot afford to wait for a problem to arise before implementing a proactive maintenance plan. The senior management needs to implement advanced risk management strategies and ensure that company culture and daily routine are deeply rooted in health and safety and food regulation laws.
This can truly help minimise loss, whether it’s financial, a loss in production, property damage, or, in extreme circumstances, loss of life. Maintenance methods are not only important for hazardous purposes—when a plant is properly maintained, it is also cost-effective and productive.
Ideally, any plant using heat transfer fluids should create a robust maintenance plan that contains regular fluid analysis system flushing, fluid top-ups and careful flashpoint management.
By caring for heat transfer fluids and the health of the overall system, plant managers can save money on pipework maintenance and energy usage, cleaning products and new heat transfer fluids.
Furthermore, proactive management including dilution, filtration and light ends removal will send savings straight to the bottom line. Regular sample analysis and staff training will ensure regulatory compliance and health and safety requirements are met.