The use of vapours and liquids to regulate process temperature is well-established in a diverse range of applications. Amongst the fluids used as thermal transfer media are water and steam, organic liquids, glycols, fluorinated hydrocarbons, silicones, mineral oils and molten salts. Each has properties that match it to specific temperatures, reactions, materials or processes.
The manufacturing and processing of plastics requires the use of specific heat transfer fluids that are designed to work to the correct temperatures.
Hot oils (also known as thermal fluids, heat transfer fluids and diathermic liquids) are mainly used for the applications and purposes shown in Table 11. A wide range of thermal fluids (heat transfer fluids) is available commercially for just about every application and for use anywhere from -80ºC to 340ºC. Most are non-toxic and non-hazardous, some even food grade but all are highly efficient, clean-running and long-lasting.
Thermal fluids offer advantages over direct heating techniques:
- The heat is more uniformly applied, and the temperature more precisely regulated.
- Quality and output are increased, and waste is minimized.
- A single thermal fluid circuit can serve many processes, even at variable temperatures.
Compared with steam process heating, liquid-phase heat transfer systems show advantages:
- They run at very low pressure
- Efficiency can be higher
- There are no flash or blowdown losses
- Corrosion is seldom an issue
- System maintenance is a lot lower.
When to use hot oils
In moderate-temperature applications where water might ordinarily be the obvious choice for cooling, heating and temperature control, occasionally hot oils are still chosen. If the local water is very hard, this can cause maintenance problems because of precipitation of minerals which plate out on the interior of the system.
Water treatment can help, but at times a simpler solution is to switch to hot oils. Similarly, if biological fouling—bacteria, algae or other organisms—is widespread, hot oils can represent a worthwhile alternative to adding biocides.
Of the liquids, hot oils also offer specific benefits for plastics applications, being suited to the materials being processed, the process conditions and the processing objectives.
At times a production objective can be better accomplished using hot oil than with other temperature control methods. Correctly applied hot oil temperature control can maintain temperature within ±1°C. Using electrical resistance for heating and forced air for cooling, this degree of control is difficult. Using a pressure-controlled loop and two-way control valves, a single hot-oil system can serve several users.
In the case of hot oils, one can choose a single hot oil that will process high-melt-point materials at 260°C, then the same system and oil can be used to process at much lower temperatures, such as would normally be controlled with water or water-glycol circulators. This allows quick switching of a production line from one product to another.
Applications in polymer processing
Hot oils in plastics applications have been selected where energy supplies are unpredictable or the quality of the fuels themselves is unreliable. In these instances, direct heating becomes challenging because of control difficulty; the system would require frequent adjustment and tinkering as fuels altered, or needed to be swapped outright, as from natural gas to liquid fuel oil. The temperature of drying and curing ovens, for instance, can be a lot more easily maintained with hot-oil heat than with direct-fired heat in these circumstances.
Plastics and polymer production utilises manufacturing equipment that requires a broad range of flexibility in operation. Operating reaction vessels at very different temperatures may require a heat transfer fluid system that provides both heating and cooling. This type of heat transfer system configuration works best when using a single fluid to effectively transfer heat over the complete temperature range specified.
Several processes require heat transfer fluids, such as:
- the polymerisation and polycondensation of polyester
- nylon processing
- the production of synthetic fibres, PET moulding operations and other plastics processing operations
Blow moulding PET requires precise and uniform temperature control to help achieve product quality and enhanced physical properties. Since localised heating with electric heating elements often does not provide the precision needed, it’s standard to use heat transfer fluids to maintain more precise and consistent temperatures. circulating heat transfer fluid at a controlled rate can help maintain material reservoirs, moulding machines and extruders at specific temperatures. The processing temperature required usually determines which heat transfer fluid is preferred.
Thermal transfer fluids are widely used by large dairies, other food processors and packaging companies to help cool moulds used to produce plastic bottles from polyethylene or polypropylene. Because these fluids remain liquid at lower temperatures than plain water, they cool moulds faster, increasing production rates. Faster cooling also allows moulding “flash” to be trimmed more quickly and cleanly.
These olefin-based resins, along with polycarbonate and others, have melt points that surpass the temperature-control capabilities of liquid non-pressurised water. To maintain the precision and evenness of liquid temperature control, while avoiding the control and pressure problems of steam or high-pressure hot water, a number of processors choose hot oil equipment. Hot oils normally have boiling points above 350°C. In plastics applications, they are seldom used over approximately 300°C.
Every step in the manufacturing process of synthetic fibres — from polymerisation to drawing synthetic fibres — can benefit from the use of heat transfer fluids. Nylon and various polyesters require high temperatures during processing. Both liquid and vapour phase heat transfer fluids can be utilised, depending on the type of operation being performed.
Using a hot oil system, the temperature in an extruder die or a mould can be raised up to temperature rapidly before starting production. If the process adds heat, as many do through friction or shear, hot oil can cool as well as heat, in contrast to direct heat which does not have any cooling function.
Hot oil system problems
Hot oil system problems are commonly caused by oxidation, thermal degradation, carbon deposits or contamination, sometimes in combination. There is normally no set rule for establishing the lifetime of a charge of hot oil. Extensive methods of testing and monitoring of fluid quality are available.
Heating the thermal transfer fluid
This is usually achieved by one of two methods:
- Electrical resistance (mainly used for portable control units and centrally located batch-processing systems)
- Firing (mainly used for larger non-stop operated systems; fuels consist of oil, natural gas or solid fuels such as wood, coal or agricultural waste.) These systems are frequently deployed as a central heating/cooling system hard-piped to banks of large extruders or injection moulding machines.
>> Search all thermal transfer fluids in UL Prospector®
References:
- Paratherm: Hot Oil Temperature Control Systems in Plastics Applications: Troubleshooting Hot Oil
- Globaltherm: Plastic, polymer & styrene processing applications
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