Steam remains a popular choice as a heat transfer medium at the warm end of process operations. However, steam systems suffer from some significant drawbacks, especially at higher working temperatures.
As the thermometer climbs above the 200°C mark, process owners may therefore consider adopting heat transfer systems using specially designed thermal fluids. In these systems, the heat transfer medium is oil-based.
While the main driver of heat transfer in steam systems is the latent heat released as steam condenses, thermal fluid systems typically operate entirely in the liquid phase.
The obvious advantage is that liquid-phase systems do not need to be pressurised to reach higher temperatures, explains Matt Hale, international sales manager with HRS, which provides heat transfer solutions based on both steam and thermal fluids: “In industry, thermal fluids are most commonly used for temperatures above 200°C.
Although low-temperature thermal fluids exist, they are not cost-efficient to install and hot water or steam are more common.”
Most thermal fluid systems are vented and operate at atmospheric pressure which means the risk to life and infrastructure from pressure is minimal
Clive Jones, managing director of Global Heat Transfer
Clive Jones is the managing director of Global Heat Transfer, which supplies thermal fluids under its Globaltherm brand.
He illustrates how higher temperatures pile on the steam pressure: “Where a heat transfer fluid system will operate at atmospheric pressure up to 300°C, steam would require a pressure of 85 bars [more than 80 times atmospheric pressure].
“In contrast, most thermal fluid systems are vented and operate at atmospheric pressure, which means the risk to life and infrastructure from pressure is minimal.”
The other option for high temperature heating is direct firing, without any intermediate heat transfer medium. Matthias Schopf, global technical service manager with Solutia Europe (part of Eastman, which offers Therminol branded fluids) sets out some of Source: Global Heat Transfer the pros and cons: “Direct heating of course provides some advantages, such as no heat transfer fluid handling systems being required.
“However, precise temperature control is difficult and also supply of heat to multiple recipients might be impractical.”
Precise temperature control is especially important when processing heat-sensitive products, such as foods.
Jones explains how a lack of precision can also impact steam-based systems: “Steam heat transfer systems rely on the generation of steam to drive pressure and, ultimately, temperature.
“Due to this dependence on a delicate pressure balance, accuracy is generally limited to swings of ±6°C. In comparison, thermal fluid systems can have an average temperature control of ±0.8°C.”
He believes that the combination of lower pressure operation and precision temperature control has enabled thermal fluids to carve out a strong position against the competition: “Increased safety and flexibility combined with better efficiency and more manageable maintenance has meant thermal fluid systems have definitely come out on top in recent years.”
Any chemical used in the process where the heat transfer fluid is used may be a potential contaminant; for example, by a leaking heat exchanger
Matthias Schopf, global technical service manager with Solutia Europe
Naturally, no one is claiming that installing and operating thermal fluid-based systems comes without its own challenges. In food applications, for instance, if there is a possibility of thermal fluid contacting the process, a food grade version should normally be specified.
“Failure to use food grade fluid in a food application can result in the loss of the manufacturer’s top tier accreditation if the EFSIS (the European Food Safety Inspection Service, part of SAI Global Insurance) learns that an inappropriate product is being used,” says Jones.
“Food grade fluid is designed so it does not contaminate foodstuffs, so if a minor spillage does occur, there would be no need for the manufacturer to recall products.”
What may be less obvious is that contamination can also be an issue the other way around, impacting on the performance of the thermal fluid. “Any chemical used in the process where the heat transfer fluid is used may be a potential contaminant; for example, by a leaking heat exchanger,” says Schopf.
“Contamination from the system environment, like air and rain water, could [also] cause potential operational problems.”
Jones says that new systems are especially vulnerable: “New heat transfer systems are particularly at risk because of environmental exposure during building. This exposure provides contaminants, such as water, soil and welding slag with an opportunity to get into the system and accelerate the thermal degradation process.
“Many component manufacturers pressure test their equipment using water as opposed to thermal fluid or nitrogen. Therefore, moisture is another cause of contamination in a new system.
“Contaminants can also damage other parts of the system so it is essential that contaminants are removed before the virgin heat transfer fluid is added and the system goes live.”
Good design and careful installation can reduce the chances of some contamination. However, gradual degradation of the thermal fluid itself will inevitably lead to a build-up of unwanted compounds in the system. The two basic mechanisms are oxidation and thermal cracking.
“All organic heat transfer fluids are affected by oxidation and may show increased viscosity and acidity and form insoluble solids,” says Schopf.
“Thermal energy will result in fluid degradation by breaking the molecular structure of the fluid [i.e. cracking] and forming products lower in molecular weight, commonly known as low boilers. High boilers also can be generated when some low boilers recombine to produce higher molecular weight materials.”
All organic heat transfer fluids are affected by oxidation and may show increased viscosity and acidity and form insoluble solids
Oxidation can be minimised by designing sealed, closed-loop systems. Thermal degradation can’t be prevented, although different fluids are designed to provide optimum stability over different temperature ranges, so matching the correct fluid to the application will help.
“Once a heat transfer system has been up and running for a while, the heat transfer fluid will naturally start to degrade,” confirms Jones.
He also notes that the problems caused by the low molecular weight and high molecular weight breakdown products are quite different: “Heat transfer fluid 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.”
Testing the water
According to Hale, potential fouling is an important reason why thermal fluid systems tend to favour the use of shell and tube heat exchangers over plate heat exchangers.
In the case of the HRS range, for instance, he advises the use of the company’s Multi Tube K series heat exchangers, with multi-pass designs being the most efficient. In addition, he notes that shell and tube heat exchangers are generally advisable at temperatures above 200°C (and therefore in thermal fluid applications), because they can be designed more easily to withstand higher temperatures and pressures than plate heat exchangers.
A good preventive maintenance programme is very important for getting the most out of thermal fluid systems, and that begins with sampling and testing for fluid contamination and degradation.
“A sample should be taken on new systems, on those that have recently been cleaned, and on those that have had a different fluid added,” says Schopf. He also recommends a programme of annual checks, although he adds that users or insurance companies may require shorter intervals.
Maintenance programmes can help relieve the pressure for manufacturers and save up to 75% on thermal fluid maintenance costs
Jones advocates more frequent testing: “An effective proactive maintenance plan will include representative testing and analysis on a quarterly basis (with no interruption to production) and heat transfer fluid training for engineers, health and safety and operations personnel.
“To ensure complete regulatory compliance, these activities are complemented by a system audit to help site managers mitigate risk in line with DSEAR/ATEX [anti-explosion] regulations.”
He adds that sample methodology is crucial to ensure that critical safety-related parameters such as the thermal fluid’s flash point are recorded correctly.
Thankfully, Jones adds that an effective, proactive maintenance programme can be extremely cost-effective: “Maintenance programmes can help relieve the pressure for manufacturers and save up to 75% on thermal fluid maintenance costs.
“Such programmes help manufacturers maintain production and comply with relevant HSE regulations, while also meeting insurance requirements.”