Hiding your head in the sand
~ How degraded heat transfer fluids erode profit~
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. Dr Chris Wright, group head of R&D at Global Heat Transfer, explains what the problems are and the ways of mitigating HTF degradation.
Did you know that ostriches do not hide their heads in the sand? The misconception is probably the result of a suggestion, from Roman writer Pliny the Elder, that the giant birds hide their heads in bushes when scared. As a race we invented the idea to describe procrastination and attributed it to the ostrich. The irony is that humans are the only creatures with the ability to hide from the truth in this way.
Unfortunately, it is much too easy to build this kind of attitude into a maintenance strategy, particularly into maintenance of a hidden asset, such as heat transfer fluid. 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 which 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.
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 measuring 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 leading 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 and 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.
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 and in accordance with environmental regulations. Hence 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, such as those offered by Global Heat Transfer, to minimise oxidation and thermal cracking and reduce the risk of fire due to closed flash temperatures falling below 100°C (212°F).
Planned, preventative maintenance also enable 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.
Planned, preventative maintenance contracts can also be tailored to client needs. For instance, in any system the header tank temperature needs to remain below 60°C (140°F) during normal operation to help to reduce the extent of fluid oxidation. Hence 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 environment 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 as all thermal fluids used in food manufacturing should be approved for use with food.
This is a regulatory requirement imposed by several bodies including the US Food and Drug Administration (FDA) and National Science Foundation (NSF), but it is one that many manufacturers are entirely unaware of.
Global Heat Transfer provides contracts that cater for such eventualities. Using infra-red thermography and telescopic lenses Global Heat Transfer can monitor header tank temperature from distance. As a result, no-one has to touch the potentially burning header tank. Global Heat Transfer can also advise on measures to reduce header tank temperature and on fitting a nitrogen blanket, if needed.
The last tool available to every manufacturer is the expert knowledge of engineers working in the thermal fluids sector. Educating customers is critical is helping them understand how thermal fluids break down and then what they can do to avoid it. Given the right expertise it is a straightforward task to mitigate for the problems that thermal cracking and oxidisation can create. You simply have to be willing to take your head out of the sand and address them head on.