Hot Oil Temperature Control Systems in Plastics Applications
Troubleshooting Hot Oil
1. Introduction to Hot-Oil Temperature Control in the Plastics Industry
The first industrial use of a thermal fluid was very likely inspired by a food-cooking method; the double boiler.
It has been speculated that some creative innovator, somewhere in Europe, sometime about a century ago, saw his wife heating milk or chocolate on the wood-burning stove in a pot nested inside a second pot boiling water, and rushed to his factory workshop to construct the first working archetype of indirect process heating.
Since that time, the use of liquids or vapors to control temperature in baths, jackets or closed circuits has come a long way. Water, steam, flashing organic liquids, glycols, mineral oils, fluorinated hydrocarbons, silicones, molten salts and other liquids are used as thermal transfer media, and each has properties that suit it to particular temperatures, materials, reactions, or processes.
In the plastics industry, hot oils (also referred to as heat transfer fluids, thermal fluids, and diathermic liquids) are predominantly used for the following applications and purposes;
Chart: Hot Oil Applications in Plastics Processing
Mold Temperature Control
Barrel Screw Heating and Cooling
Coating, Bonding, Curing, Drying
Drying and Curing
Heating the Oil; Process Heat Sources
Electric Resistance Heat
Used for portable control units and centrally located batch-processing systems up to 1 million BTU.
Used for larger continuously operated systems. Fuels consist of natural gas, oil, or solid fuels such as coal, wood or agricultural waste. These systems are often deployed as a central heating/cooling system hard-piped to banks of large extruders or injection molding machines.
Why Thermal Fluids?
Thermal fluids offer advantages over direct heating methods; as in the double-boiler on the kitchen stove, the heat is more evenly applied, and the temperature more precisely controlled. Quality and output are increased, and waste is reduced. Also, a single thermal fluid circuit can serve multiple processes, even at varying temperatures.
Compared with steam process heating, liquid-phase heat transfer systems also offer advantages; systems operate at very low pressure, efficiency can be as much as 8% higher, there are no flash or blowdown losses, corrosion is rarely a problem, and system maintenance is much lower.
Hot oils also offer specific advantages in plastics applications, related to; the materials being processed, the process conditions, and the processing objectives:
Some materials are better processed using hot oil than other temperature-control techniques.
Polycarbonate and many ethylene-based resins, among others, have melt points that exceed the temperature-control capabilities of liquid non-pressurized water. To maintain the precision and uniformity of liquid temperature control, while avoiding the control issues and pressure issues of steam or high-pressure hot water, many processors choose hot-oil equipment. Hot oils typically have boiling points above 350°C. In plastics applications, they are rarely used over approximately 300°C.
Sometimes local conditions can affect the choice of a process-heating technology.
In moderate-temperature applications where water might ordinarily be the obvious choice for heating, cooling and temperature control, sometimes hot oils are still chosen. If the local water is very hard, this can cause maintenance issues due to precipitation of minerals which plate out on the interior of the system. Water treatment can help, but sometimes a simpler solution is to switch to hot oils. Similarly, if biological fouling—algae, bacteria or other organisms—is prevalent, hot oils can represent a worthwhile alternative to adding biocides.
Hot oils in plastics applications have been chosen where energy supplies are erratic, or the fuels themselves are of unreliable quality. In these instances, direct heating becomes problematic because of control difficulty; the system would require frequent adjustment and tinkering as fuels changed, or needed to be switched outright, as from natural gas to liquid fuel oil. The temperature of drying and curing ovens, for example, can be much more easily maintained with hot-oil heat than with direct-fired heat in these conditions.
Similarly, in areas with intermittent electrical blackouts or brownouts, hot oil temperature control units used in the molding and extrusion applications listed in the above chart, will hold temperature better during electrical failure or fluctuation than direct electric heat and air-based cooling, allowing the application to continue briefly while back-up power supplies are brought on line.
Sometimes a production objective can be better achieved using hot oil than with other temperature control techniques. Properly 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 hot oil system, the temperature in an extruder die or a mold can be brought up to temperature quickly before production is begun. If the process adds heat, as many do through shear or friction, hot oil can cool as well as heat, unlike direct heat which has no cooling function.
Cooling and Heating Within A Single Hot-Oil Circuit
Using a pressure-controlled loop and 2-way control valves, several users can be served by a single hot-oil system. By including a chiller in the circuit and selecting a low-viscosity hot oil, a single heat transfer surface can be utilized for both process heating and cooling. For example, there are systems where the barrel of an extruder needs to be cooled, while the die must be heated.
Single Fluid for Wide Range of Materials
With hot oils, a single hot oil can be chosen that will process high-melt-point materials at 260°C, then the same system and oil can be utilized 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 with a quite distinct operational profile.
Often, the first measure taken when system performance begins to suffer is to compensate by increasing the heater output temperature. This may bring short term results, but in the long run may make problems worse. Better to troubleshoot the system and correct the underlying problems.
Signs of Trouble in Production
Product Quality Problems
Problems related to thermal system performance can cause product flaws.
Production Output Problems
Hot oil unit malfunction can result in reduced output.
Signs of Trouble in The System
Sights, Sounds, and Odors
Pump pressure fluctuations produce pump noise and dancing pressure-gauge needles.
When starting up, system shakes, grinds, pops, or whines.
Difficulty Reaching Temperature Setpoint
System will not achieve operating temperature.
Continual Pump-Seal Failure
Pump seals wear out prematurely.
Filters load continually requiring replacement or service.
Causes of Trouble
Hot oil system problems are generally caused by one, or a combination, of the following three causes (correlation of these causes with the most frequent signs of trouble in the above section are added parenthetically);
Oxidation of the Fluid (Plugged filters, Difficulty Reaching Temperature Setpoint, Start-up Difficulty)
Oxidation is always caused by air contacting hot oil. The reaction forms acids and other oxygenated compounds such as aldehydes which have poor thermal stability. These compounds undergo polymerization at relatively low temperatures to produce higher molecular weight compounds that remain dissolved in the fluid but increase the viscosity and ultimately form solids. The carbon coke can form deposits within the equipment, sometimes sticking, even eventually causing partial blockages that will further slow the oil flow. The problem occurs when hot fluid circulates through the reservoir or during tool changes when air is used to blow hot fluid back into the heater. Also, newer negative pressure hot oil temperature control units can cause oil oxidation by their very design.
Thermal Degradation of the Fluid (Pump Symptoms, Difficulty Reaching Setpoint, Plugged Filters)
This is most often a result of an inadequate fluid flow. When the oil is moving too slowly, the temperature of the fluid in contact with the heating element surface (otherwise known as the film temperature) can quickly exceed the maximum recommended for the fluid, causing the oil molecules to crack. Cracking produces lighter, more volatile components which raise the vapor pressure and cause the oil to become less viscous. The lighter components will also oxidize more rapidly and can lead to the formation of carbon.
Contamination (Pump Symptoms, Start-up Problems)
The most common, and generally the most problematic, form of contamination in hot-oil systems is water. Since water and hot oils do not mix, the water either becomes suspended in the oil as small droplets or settles out in low velocity areas. When the water reaches its boiling point (which will be affected by the system pressure at that point) it will flash to steam. Small quantities of water will cause pump cavitation which can shut a system down due to low pressure. In moderate quantities, steam will displace an equivalent amount of oil which can be extremely dangerous if it occurs suddenly since the oil will be ejected from any opening in the system.
Other types of contamination include any substance that can find its way into the system; dust, dirt, or airborne byproducts from machining or other production operations can infiltrate through an open reservoir. Weld spatter, flash, flux, dirt, shellac and cleaning residues present from the manufacturing process can even contaminate a brand-new system.
Most commonly, analysis of hot oil consists of three laboratory tests:
- Total Acid Number (TAN)
- Distillation Range
When the oil is analyzed, these values are compared to the baseline values for the new oil, and when one or more of these three criteria fall out of a specified range, questions should be asked about the condition and operation of the system. The answers can determine what may be causing the degradation of the fluid, and what corrective action can be taken.
In certain plastics applications, regular replenishment of fluid lost during tool changes will periodically rejuvenate the entire charge of fluid. In these cases, problems may go undetected by fluid analysis because the make-up fluid will continually refresh the deteriorating oil—also refreshing its specifications to a degree. Sometimes, this can be an acceptable steady-state condition, if there is no compromising of the equipment, personel, or production. Other times, such a system condition can slowly grow in severity to the point that production can suffer, or the equipment can malfunction—all while the fluid is showing no severe departure from the acceptable specifications. In these latter cases, careful monitoring of usage hours, make-up fluid quantity, and trends in analysis values can also pinpoint system trouble.
What Oil Analysis Tells Us
In addition to the laboratory tests described above, fluid samples are examined for visual, textural, and odor properties. Viscosity, consistency, water contamination, metal contamination, and possible acidity, particle formation, and presence of sludge can all be predicted by these empirical observations. Appearance and odor can indicate contamination, possible molecular cracking due to overheating, as well as increased acidity due to oxidation. If metal particulates are observed, metallic wear in the system is indicated, most often due to pump problems, but also sometimes originating elsewhere.
Observed water contamination can indicate possible exchanger breaches, or other system problems that can introduce moisture to the circuit.
The laboratory results also reveal much about the system’s condition. Changes in the distillation curve can indicate overheating problems, as can viscosity changes.
When the total acid number of the oil has exceeded the optimum, the oil is suffering oxidation, or, less likely, contamination. Oxidation often has its cause in reservoir design, operational practices, or system leakage.
When to Change the Oil
There is generally no set rule for determining the lifetime of a charge of hot oil. The oil’s usage life will vary depending on heat density, temperature, air contact, and general maintenance practices. In larger systems, regular fluid analysis can provide predictive as well as corrective functions for system and oil maintenance. When the oil has departed sufficiently from the specifications, sometimes part of the oil can be replaced to return the entire charge to the desired range of operating criteria. Other times, it is advisable to completely replace the oil.
In smaller systems, due to the cost of the laboratory tests, sometimes it is more cost effective to simply change the oil on a regularly scheduled bases. Hot oil manufacturers can advise on the timing of the schedule, based on the application and operating criteria.