The role of a heat transfer fluid (HTF) is simple — to transmit heat to a process stream. However, selecting a high-temperature HTF to use in a new operation isn’t a simple task for a non-specialist. Here, we’ll address some of the common conundrums that every company will face and provide some practical pointers.
HTFs generally are mineral or synthetic; it’s very common to compare the relative characteristics of these fluids. However, this comparison really only makes sense in two scenarios — during the design of a new facility or when planning to completely replace an aged fluid with a virgin HTF. In most cases, the key consideration is effectively maintaining an existing system to ensure the plant operates as efficiently as possible and the HTF achieves a long working life (Figure 1).
The First Decision
When starting to design a new plant, you must decide whether to use water or steam as the heating medium or to rely instead on a mineral or synthetic HTF. Mineral and synthetic HTFs are established viable alternatives to water or steam — and the preferred choice at temperatures above 200°C (392°F).
Mineral and synthetic HTFs, relatively speaking, provide a safer operation that requires less monitoring because they operate at lower pressures than steam. For example, at a temperature of 343°C (650°F), steam has a vapor pressure that is 200 times higher than that of a synthetic HTF such as Globaltherm Syntec, which is a terphenyl-based fluid — 13,790 kPa (2,000 psi) versus ~71–86 kPa (10.3–12.5 psi), respectively.
Mineral and synthetic HTFs don’t require pressurizing at temperatures up to 350°C (662°F). Another advantage of using a mineral or synthetic fluid, as opposed to water, is that it generally has a lower freezing point. For example, the freezing point of Globaltherm Syntec is -28°C (-18.4°F). Lastly, HTFs also tend to be less reactive and corrosive to pipes and other parts of the system than water.
There are a lot of different HTFs and many people mistakenly believe there’s little difference between them. Unlike in food processing, where food-grade HTFs — highly refined mineral fluids that are colorless, non-toxic, non-irritating and non-fouling to allow for potential incidental contact with foods during manufacture — are available, no specific grades exist for chemical and pharmaceutical processes. Instead, selection involves matching a fluid, either a mineral or synthetic HTF, to the system and the process requirements.
Restrictions imposed in the particular sector or by company policy may affect HTF appropriateness. For instance, at a company that places high importance on its environmental credentials, suitability may require striking a balance between a fluid’s functional properties and the ability to handle it. Here, assessments might cover factors such as a fluid’s relative risk when handled and ease of disposal as well as the biological, chemical, physical and allergenic hazards it presents. As a general rule, mineral HTFs are less restrictive in their handling requirements and less hazardous to humans and the environment than more highly refined chemicals used in synthetic fluids.
In addition, your insurance company may influence what’s an appropriate HTF choice. Some insurers have been known to define a list of acceptable HTFs, as opposed to recommending a single HTF. The terms of many policies allow auditing to ensure use of an appropriate HTF.
Equipment manufacturers also may recommend particular fluids. This can lead to a lot of confusion. For example, when planning annual new fluid refills, a manager responsible for the system may restrict the choice to one of the named HTFs, even though other brands may have the same chemistry and composition and offer a more-cost-effective option.
The operating temperature of a HTF depends on its base chemistry and purity (Figure 2). As a rule, mineral HTFs have a lower maximum operating temperature than their synthetic equivalents. You must match the fluid to the upper operating temperature. It is one of the crucial factors that will influence the aging of the fluid; the HTF will degrade faster if the upper operating temperature is breached for prolonged periods. The net effect is a reduced fluid lifespan.
Synthetic fluids better resist thermal degradation at higher temperatures and, therefore, are preferred at those higher temperatures (which could be up to 400°C (752°F) in the case of Globaltherm Omnitech). This helps safeguard against thermal degradation if the upper operating temperature is exceeded.
However, mineral fluids perfectly suit operations where the upper operating temperature isn’t exceeded or if the temperature is >20°C below the fluid’s upper operating temperature.
HTFs must meet a number of key criteria depending on the specifics of the application. These include high-temperature thermal stability, product purity and heat transfer efficiency. The thermal stabilities and properties of mineral and synthetic HTFs differ. For instance, lower-quality mineral HTFs, such as group one base oils, tend to be less thermally stable, which creates a higher risk of fouling. They also have a lower maximum operating temperature (260°C, 500°F) than group three base oils (~316°C, ~ 606°F); for more details, see my 2016 article “What to consider when making the buying decision about a heat transfer fluid for your system,” J. of Applied Mech. Eng.
So, like any decision, the choice of HTF is a tradeoff, in this case between cost, high thermal stability and performance. Mineral HTFs often provide an attractive option for services that don’t exceed their maximum operating temperature ranges; they generally are cheaper than synthetic HTFs.
Fluid purity can have an impact, as Lang and Lee reported in “Heat transfer fluid life time analysis of biphenyl-diphenyl oxide (BDO) grades for concentrated power plants”. (Examples of HTFs containing BDO include Dowtherm A, Therminol VP-1, Globaltherm Omnitech and Diphyl.)
The authors compared the thermal degradation of Dowtherm A and a non-branded HTF (lower purity due to higher chlorine content) and quantified the impact of impurity in terms of the cost of new fluid refills over the lifetime of the BDO HTF. They concluded that the non-branded BDO HTF was 1.7–2.4 times more likely to degrade than the lower-impurity branded BDO HTF over the course of a 25-yr life span. This increases the cost of annual new fluid refills over this time frame.
As this research highlights, you should consider the importance of a fluid’s impurity (which its maker should specify upon request) when selecting a fluid. Given the relatively low cost of mineral fluids, the annual new fluid refill cost/kg/yr represents a good way to measure and manage overall costs.
By the way, you may achieve cost savings for such refills by including them in a fluid maintenance program in which an external company manages the fluid and maintains it on a day-to-day basis to sustain a healthy operation and HTF.
In addition, the heat transmission characteristics of the HTF are an important consideration because heat transfer efficiency indicates how well a fluid will transfer heat from the fluid film lining the pipe wall to the body of fluid flowing through the pipe itself.
You also may need to consider a number of other properties such as viscosity, expansion rate, flash and fire points, and resistance to oxidation.
Viscosity. The objective is to get a fluid that has a low viscosity at low temperature because this affects the lowest startup temperature for the system. This is particularly important if the HTF system is housed outdoors or has no backup heating system.
A general rule of thumb is to avoid fluids with a kinematic viscosity >60 cSt at 40°C. Mineral and synthetic fluids, such as Globaltherm M and Globaltherm Syntec, have viscosities <60 cSt at 40°C. While the synthetic fluid has better thermal stability up to 345°C, both have similar viscosities at 40°C and 100°C. Furthermore, the mineral-based HTF can operate down to a lower temperature than the synthetic HTF.
Expansion rate. You must consider the thermal expansion coefficient when assessing if a particular fluid is appropriate for the size of the expansion tank in the HTF system. Consult the fluid manufacturer for the given rates of a fluid’s expansion.
Flash and fire points. You should perform routine sampling and analysis to quantify the extent of a fluid’s thermal degradation and its closed flash point temperature. A decline in closed flash point temperature indicates a rise in the formation of short-chain hydrocarbons (“light-ends”) from thermal degradation of the fluid. Their presence in the fluid signals that a system isn’t venting effectively and volatile components are accumulating.
You must remove these byproducts because they can negatively influence pump efficiency and represent an increased safety risk to the overall HTF system.
Every fluid, mineral or synthetic, will degrade when operating at sustained high temperature for prolonged periods. This degradation rate is faster in mineral HTFs. However, with close monitoring (for the formation of short- and long-chain hydrocarbons) and use of predictive measures to project its future health, a mineral fluid is a feasible option for processing operations.
Resistance to oxidation. Oxidation commonly is associated with mineral HTFs; polymerization products, such as sludge, form when saturated hydrocarbons react with oxygen. Contaminants such as water accelerate this process. However, using a nitrogen blanket to limit contact with air and a fine filter sieve to remove carbon and contaminants from the circulating HTF can slow the process.
While the chemistry of a mineral fluid means it’s more reactive to oxygen, you can take effective measures to prevent or intervene should you detect polymerization byproducts.
Opt For The Best Value
You should select an HTF based on value, which ultimately is a tradeoff between fluid cost and performance. If you need a high-temperature HTF, ensure the prospective fluid can perform adequately for prolonged periods at the necessary temperature. That requires diligently verifying and comparing heat transfer rates, fouling potential and thermal stability before purchase. You can do this by familiarizing yourself with the fluid manufacturer’s product data sheets.
The key features of well-designed mineral or synthetic HTFs are thermal stability when operating at high temperatures for prolonged periods, good thermal transfer efficiency and high purity.
Once your selected HTF is in use, it’s important to regularly monitor its condition so you can predict and prevent thermal degradation over time. Synthetic fluids require just as close monitoring as their mineral counterparts because all fluids will thermally degrade when operating at high temperatures for prolonged periods. Adequate monitoring will ensure your HTF operates to its maximal potential and lasts as long as possible.
CHRIS WRIGHT is a research scientist for Global Heat Transfer Ltd. Stone, U.K. Email him at email@example.com.