The consequences of freezing thermal fluid oil can be both damaging and costly in any kind of process engineering application. Frozen thermal oils mean that heat cannot be brought to bear at the appropriate point in the application, resulting in downtime, lost production and potentially lost customers. Clive Jones, managing director of thermal fluid specialist Global Heat Transfer, Staffordshire, United Kingdom, explains the need for process engineers to understand the impacts of frozen oil in their applications.
Once oil has solidified (frozen), engineers must rely on ambient temperature to restore it to its former viscosity because the application of focused heat is not an appropriate way of melting the fluid. In addition, according to Jones, worst thing engineers can do is attempt to use a system in which the oil is already frozen. This could cause serious damage to the pump and associated peripherals such as the motor and control equipment.
So, instead of trying to thaw a frozen thermal fluid heating circuit, process engineers should focus on ensuring it will not freeze. To avoid the problems associated with freezing oil, it is critical to first understand the qualities of the oil itself as well as the needs of the application.
Many thermal fluid oils that are designed to operate at extremely high temperatures — between 644 and 752°F (340 and 400°C), for instance — will freeze at 32°F (0°C). Some will freeze even when the fluid drops to just 59°F (15°C). This is not a problem provided engineers have planned the application correctly so that the oils are never exposed to such temperatures.
Just as certain fluids are intended to be used at ultra-high operating temperatures, others are specifically intended for use in low temperatures. A typical low temperature fluid might be operable between -58°F (-50°C) and 248 to 428°F (120 to 220°C). An extremely versatile, low temperature fluid could function between temperatures as extreme as -130 and 572°F (-90 to 300°C).
The easiest way get the right oil for your application is to consult a specialist who is conversant with the temperature extremes at which each oil can operate. But if that is not possible, most heat transfer fluid manufacturers offer charts that clearly delineate the extremes that their own oils can withstand, either on their websites or on request. (Process Heatingalso publishes an overview on heat transfer fluids annually. It can be found online at http://bit.ly/EOHTFs.)
A number of issues can be addressed to help avoid the problems that arise as the result of freezing oil. The first, and perhaps most important with regard to oil that is currently in use, is to leave the circulation pump running when the boiler is not on. The friction and heat from the pump will keep the oil at a sufficiently high temperature to ensure it does not freeze.
There also are measures that can be taken to protect stock oil that is not already in use in your system. The first way is to make sure that this oil, especially “top oil” that might need to be called into use quickly, is stored inside. Some oils can become extremely viscous in low temperatures even if they do not freeze.
Under normal circumstances, it should be possible to choose storage locations that allow oil to be kept at a reasonable ambient temperature at all times. If this is impossible — or in cases where oil is stored in an unheated building such as a shed or warehouse — purpose-designed barrel warmers can be provided by the thermal oil supplier.
The most common problems engineers encounter associated with freezing oil are the result of winter shutdowns or periods of low production output when ambient temperatures in the plant and storage areas often are allowed to drop.
The first precaution to take is ensuring that you heat up the system very slowly after the shutdown, just in case there is condensation in the system and the oil. The next step, if the temperature has been extremely low, is to heat up the ring main first and then introduce the heat exchangers one at a time. Both of these steps will help minimize the impact on the system itself and reduce any potential damage.