Key points
Choosing between steam and heat transfer fluid-based systems
The age-old debate of ketchup or brown sauce on a bacon sandwich is very difficult to settle — most people have a clear favourite, and they aren’t about to change their minds. Here Clive Jones, managing director of thermal fluid supply and management company Global Heat Transfer, discusses the key considerations to make when deciding between the old favourite, steam, and fluid-based heat transfer systems.
With brown sauce sales on the decline, it looks like we might be swapping out an old favourite after all. Could the same be true of steam-based systems? Historically, steam has been the preferred method of indirectly transferring heat in industrial processes.
It was often selected because water can be easily and cheaply acquired, and it has no perceived environmental impact. Despite some advantages, steam-based heat transfer systems come with many drawbacks, such as a corrosion risk and complex maintenance.
Steam safety
One of the main challenges with steam heat transfers systems is that they operate at very high operating pressures of about 85 bars or 8,500 kPa. If the steam reaches a critical pressure and the system has no way to vent it, it can cause pipes, valves or seams to burst. Health and safety issues will arise as extremely hot steam or shrapnel from the pipes may harm employees and damage the surrounding infrastructure.
Maintaining a consistent and safe operating pressure is reliant on the steady generation of steam. System accuracy is limited to changes of ±6 degrees Celsius, but this number commonly increases as the steam-based system ages.
Corrosion of pipes is also common, despite the use of distilled water, so regular maintenance is needed to help reduce unexpected downtime. To reduce the frequency of replacing the whole heat transfer system, manufacturers can invest in stainless-steel pipes. Unfortunately, replacing pipes requires a large upfront investment that may not reduce corrosion long-term.
Out with the old, in with the new?
An alternative way of providing indirect heat in manufacturing processes is a heat transfer fluid-based system. As the thermal fluid moves around the system, it transfers heat to or from base, intermediate and final stage products to achieve the required temperatures. Thermal fluid-based systems pose many advantages over steam-based systems in their efficiency, safety and precise temperature control.
Thermal fluid systems operate at atmospheric pressure and are well vented. Reduced pressure decreases the strain on pipes and the risk to life and infrastructure. Decreased risk improves the health and safety of the workplace and means that downtime to replace piping can be planned into the production schedule.
Heat Transfer Fluids (HTFs) can operate at high temperatures for extended periods of time with precision of within ±0.8°C, so the manufacturers can accurately control the temperature to match the HTFs optimal use. Therefore, HTF system design minimises fluid fouling, unwanted deposits in heat transfer areas and the frequency of pipes being replaced.
Extending fluid life
However, all thermal fluids will degrade over time when operating at high temperatures. There are two main degradation processes, cracking and oxidation, both of which can cause problems with system operation and safety. If operated within the correct parameters, fluid degradation is typically slow — carrying out regular condition monitoring and preventive maintenance will slow the process further.
There are a number of HTF options on the market, both synthetic and mineral, and each individual fluid will have its own temperature range and recommended uses. Your thermal fluid provider can help you select a fluid that is safe, compliant and suitable for your specific process. Choosing an appropriate thermal fluid and operating it within the manufacturer’s specifications will ensure a long lifespan.
Though HTFs do not require constant assessment, periodic fluid samples should be taken to demonstrate compliance with the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). We recommend quarterly sampling to detect any short-term changes in the fluid’s state, but for DSEAR compliance manufacturers must carry out a minimum of one annual test.
During the process, engineers should take representative samples of hot and circulating thermal fluid to get an accurate reading of the fluid’s condition. The sample is then sent for specialist analysis to measure the flash point temperature, carbon levels and the total acid number (TAN), which indicate the fluid’s condition.
In addition, HTF systems are quite simple, primarily requiring a pump, an expansion and storage tank, pipework and heat exchangers. Because sampling is periodic and not continual and there are fewer components to monitor, HTF systems typically offer a lower cost of maintenance compared with the continual monitoring of steam system components, such as safety values, traps and drains.
Final reports allow managers to predict the time scale of fluid degradation and carry out preventative maintenance to avoid any unexpected downtime or costs. A strategic maintenance plan, such as Global Heat Transfer’s Thermocare, allows managers to analyse the fluid samples and maintain an efficient heat transfer system.
While there are still some die-hard brown sauce fans out there, the general population is moving away from brown sauce. Similarly, while steam has historically been the most popular choice for indirect heat transfer, HTFs have climbed ahead in recent years because of their increased safety and more manageable maintenance.
