Your Hi Fluid Zone! Part 1B! Get the Most From High-temperature Heat-transfer-fluid Systems!.

Your Hi Fluid Zone! Part 1B! Get the Most From High-temperature Heat-transfer-fluid Systems!.

Fluid selection, system design and maintenance are the keys to achieving the best performance;

Although water and steam are the ideal media for heat transfer, there are often situations when other heat-transfer fluids are called upon to perform this function in the chemical process industries (CPI). Thus there are, at the very high-temperature end, various types of heat-transfer fluids, such as molten salts and even molten metals. At the lower end of the temperature scale we have various proprietary and non-proprietary brines, usually glycols of one kind or the other. Between these two temperature extremes, a variety of heat-transfer fluids are also available. For this discussion, the focus will be on high-temperature heat-transfer fluids; those having useful bulk-fluid-operating temperatures of approximately 70–400°C.

For high-temperature operation, heat-transfer-fluid systems have distinct advantages when compared to steam or direct-fired heating. For example, using steam for process heating to temperatures in excess of 225°C would require steam pressures upwards of 40 bars. For the same application, the use of a heat-transfer-fluid system would thus eliminate the need for costly high-pressure equipment and piping, expensive boiler-feed-water treatment, and specially trained boiler operators. And compared to direct-fired heating, the use of heat-transfer fluids allows the heater to be safely located away from the process and reduces the danger of forming hot spots in the process itself.

An engineer likely to be involved with high-temperature heat-transfer fluids should be familiar with the types of fluids available, and how to choose among them. It is also important to be aware of the design guidelines for the components of a heat-transfer-fluid system, and the problems that can arise with them.

FLUID SELECTION;

Fluid chemistry;

High-temperature heat transfer fluids can be categorized by their chemical structure into three primary types:

• Hot oils (mineral oils).

• Synthetic organic fluids.

• Silicone fluids.

Hot oils are petroleum-based and are composed of paraffinic or napthenic hydrocarbons. The bulk-fluid-operating temperature of hot oils is in the range of –25 to 315°C.

Synthetic organic fluids have aromatic ring structures and include the diphenyl-diphenyl oxide mixtures, biphenyls, terphenyls and alkylated aromatics. Depending on the specific fluid, the bulk-fluid-operating temperature range of these types is from – 60 to 400°C.

Silicone-based fluids are primarily used in specialized applications requiring process-product compatibility in case a heat-exchanger leak occurs. They find favor in food-processing industry.

Synthetic organic fluids are much more expensive than mineral oils, but are far less prone to thermal degradation; their aromatic-ring structure is more robust than the aliphatic chains of mineral oils. The thermal decomposition rate of mineral oils can be as much as six times that of synthetic-organic fluids at operating temperatures of 330°C. Silicone fluids require higher initial investment, but their thermal stability is excellent. Although they exhibit some thermally induced changes, a balanced steady-state-equilibrium composition is eventually attained, and, depending on service conditions, this can reduce or eliminate the need for adding more fluid during the operating life of the system. The fluid life can be ten years or longer.

A life-cycle-costing approach has to be adopted when choosing between synthetic fluids and mineral oils. The fact that there will be fluid loss (regardless of the fluid type) every time a pump or valve is opened for maintenance should not be overlooked. A point in favor of mineral oils is that the spent oil can be easily disposed off by blending with fuel oil for boilers.

Traditionally, mineral oils for heat-transfer applications have been manufactured by petroleum companies and distributed through lubricant dealers. Because relatively low volumes of oil are needed for heat-transfer applications, the technical support has often been found wanting; this is one of the main reasons why mineral oils stand discredited in some eyes today. In contrast, the excellent product support provided by manufacturers of synthetics has ensured a better life span for these fluids.

Fluid properties;

A good heat-transfer fluid should have many desirable attributes, including the following:

• High boiling point.

• High flash point.

• High autoignition point.

• High thermal stability.

• Good heat-transfer properties.

• High solubility for decomposed fractions.

• Low pour point and pumping limit.

• Low viscosity

• Low vapor pressure.

When evaluating alternatives, it is useful to take a look at the values of the above properties. Two of the properties — thermal stability and heat-transfer characteristics — are especially important.

Thermal stability: This is the heat-transfer fluid’s inherent ability to withstand thermal cracking, and is the primary factor in determining its maximum bulk-fluid-operating temperature. The fluid can be used up to this temperature and still maintain an acceptable level of thermal stability. Continuous use above the manufacturer’s recommend bulk-fluid-operating temperature will result in an exponential increase in fluid degradation rates.

The first step in selecting the fluid is to determine the maximum bulk-operating temperature required by the process. Most hot oils have a recommended maximum bulk fluid temperature of 280–315°C, while the synthetic organics have recommended maximum bulk-fluid temperatures between 315 and 400°C, depending on the fluid. Because aromatic compounds have a greater thermal stability than the hot oils above 315°C, synthetic fluids are strongly recommended above this temperature. For process applications requiring bulk fluid temperatures from 160 to 315°C, one can specify either synthetic or petroleum based fluids. Within this latter temperature range the relative-thermal-stability data supplied by manufacturers can be used to compare individual fluids at specific temperatures.

Heat transfer coefficients: At a given temperature, the heat-transfer coefficient of the various fluid types may differ by as much as 30%. Depending on the thermal resistance of other components in the system, a fluid with a substantial advantage in heat-transfer coefficient might allow a significant reduction in equipment sizing.

Most of the aromatic fluids reportedly have a significant advantage over hot oils in heat-transfer characteristics in the temperature range of 150 to 250°C. Above this temperature range (up to 315°C), the differences narrow somewhat, with a select number of highly refined paraffinic-napthenic white oils having a slight edge over the aromatics.

Environmental considerations;

Petroleum-based fluids offer substantial advantages in ease of handling, reprocessing, shipping and disposal as compared to the synthetic fluids. The white mineral oils, in fact, meet the U.S. Food and Drug Administration’s (Washington, D.C.) criteria for “incidental food contact.” Also, the petroleum-based fluids do not form hazardous degradation byproducts; therefore most spent hot oils can be sent to a local oil-lube recycler for disposal. The hot oils warrant no special handling precautions and require no special storage requirements. From a personnel standpoint, the hot oils are extremely user-friendly (apart from the risk of becoming burned). Most have no discernible odor, and are non-toxic both in contact with skin and after ingestion.

On the other hand, because of the aromatic chemistry, some synthetic fluids can form hazardous degradation by-products that require special precautions. Some synthetic fluids and their vapors may cause skin and eye irritation after prolonged exposure, and emit pungent odors. Since there is a wide range of chemistries available within the aromatic group, not all fluids have similar properties and environmental or personnel concerns, and regulations vary from fluid to fluid.

Most synthetic fluids are composed of a limited number of aromatic components and have a narrow boiling range, allowing easy identification of the degradation by-products or contaminants. Reprocessing synthetics via fractional distillation is an economical alternative to disposal and replacement. Most synthetic-fluid suppliers offer this service at a nominal cost. Some suppliers also offer credit for spent material, which can be traded in for a replacement charge. Because of the relatively low cost of petroleum based fluids, few suppliers offer reprocessing services for hot oils.

System design;

The main constituents of a heat transfer fluid system are:

• The heater

• Circulating pumps and piping

• The expansion tank

• Filters

• Controls and instrumentation

The heater;

The heater is the heart of the system and requires careful design to ensure a long fluid life. The most common design uses a cylindrical or box-type heating chamber with vertical tubes carring the heat-transfer fluid arrayed along the walls of the chamber. The heater design should avoid hot spots caused, for instance, by flame impingement on the tubes. The fluid should be pumped at sufficient velocities to ensure that there are no stagnant pockets. Fluid velocities should also be high enough to develop a high degree of turbulence within the tubes; this prevents film temperatures from becoming excessively high, which would be detrimental to the fluid life. Recommended velocities are in the range of 1.5–3.0 m/s, depending on the operating temperature.

Pumps and piping;

Pumps fitted with mechanical seals, sealless magnetically driven, or canned-motor pumps are the three normal options available for heat-transfer-fluid systems. The pipework should have adequate flexibility to accommodate thermal stresses. Most organic heat-transfer fluids have a propensity to leak through joints and fittings unless these are very tight. The recommended gasket is stainless steel, spiral-wound type with flexible-graphite filler. Globe valves are commonly used, but the valves should have a minimum of five solid-graphite packing rings. Control of leaks from pipework is especially important because fluid-soaked insulation poses a more serious hazard than the leaking fluid itself. An all welded system, or one that minimizes the number of flanged joints, is the best option.

Filtering;

The use of a filter in the heat-transfer-fluid circuit is usually not given the importance it deserves. Filtration reduces plugging and fouling of tubes in any heat exchangers and in the heater. Filtration also improves the longevity of system components, such as pump-shaft seals and valve stems. Filters are a must in mineral oil circuits, since these are more prone to thermal degradation, which results in the formation of carbon particles. Such filters are usually installed off the main circuit in a side-stream to prevent any inadvertent flow restriction. Cartridge filters with a 10–20-µm cutoff are usually preferred.

The expansion tank;

The expansion tank is the simplest but perhaps the most crucial piece of hardware in the heat-transfer-fluid system. The purpose of this tank is to allow for thermal expansion of the fluid, which can be as high as 25%. A properly designed and correctly installed expansion tank goes a long way in eliminating many operational problems and also increases the life of the heat-transfer fluid. The rule-of-thumb for sizing the expansion tank is that it should be one-quarter full at ambient temperature and three-quarters full at normal operating temperature. The tank size thus depends on the total system volume and the normal operating temperature. The expansion tank should be installed at the highest point in the system and connected to the suction side of the pumps that circulate the fluid through its cycle. Being located at the highest point of the loop, the expansion tank serves as a convenient place for venting any low-boiling thermal-decomposition products from the system.

Expansion-tank design;

The expansion-tank design should:

• Allow for the increase in fluid volume due to thermal expansion

• Provide a means for venting water, non-condensables and by-products of thermal degradation during start-up and operation.

The simplest type of expansion tank is the surge drum, which essentially acts as a receiver for the return fluid from the consumers and from which the circulating pump draws the fluid for pumping through the heater. This design offers no protection against thermal oxidation of the fluid; in fact oxidation is promoted due to aeration within the drum. However the surge-drum design provides excellent system-venting capabilities during start-up and extended operation.

The ideal design is the expansion tank located at the highest point in the system with a double-drop-leg piping arrangement, as shown in Figure 1. While the expansion tank should be connected to the system by a sufficiently small line during normal operation to avoid thermal recirculation, the line should handle full system flow at start-up conditions.

New systems almost always contain some water, typically left over from hydrotesting, and this water must be removed to avoid pump cavitation. The double-leg design permits this.

All organic heat-transfer fluids undergo oxidation on contact with air leading to the formation of insoluble compounds. These insolubles can cause fouling, which adversely affects heat transfer. Because the usual source of air into the system is through the expansion tank, blanketing with nitrogen should be used to prevent fluid oxidation. Nitrogen blanketing also eliminates the ingress of atmospheric moisture. Although this is the most effective method to prevent oxidation, a cold-seal-trap or baffled design can be adopted in situations where nitrogen is not readily available. Such a design is shown in Figure 2.

Controls and instrumentation;

Control systems are required for the heater and for the individual energy consumers. At the heater end, the firing mechanism should be regulated to ensure that the heat input is in accordance with the energy demand. At the individual user equipment, flow controls are typically installed. In a multiple user system, such as shown in Figure 3, controls should be provided at each user individually to ensure proper energy delivery.

Mode of operation;

Processes utilizing heat-transfer fluids can be categorized into three types: liquid-phase systems (pressurized and non-pressurized) and pressurized vapor-phase systems. Non-pressurized liquid-phase systems are generally the simplest to design and operate. Both hot oils and synthetic fluids can be used equally well in this type of system as long as the operating temperature of the heat-transfer fluid is below its boiling range. Major components in these systems consist of the heater, heat exchanger, vented-expansion tank, and circulating pump. In these types of systems, the expansion tank does not require the application of an inert gas in order to maintain a positive pressure on the circulating pump. To reduce the probability of fluid oxidation, a baffled expansion tank design can be used.

Pressurized liquid phase systems using both hot oils and synthetics are similar in design to non-pressurized systems except that an inert gas must be applied through the expansion tank when the required operating temperature of the heat-transfer fluid is above its boiling range. In this case, the pressurized inert gas (such as nitrogen) is used to keep the heat-transfer fluid in liquid state. However, with the exception of the multiphase fluids, such as the diphenyl oxide/biphenyl-type, most of the liquid-phase synthetic fluids and all of the hot oils ordinarily do not require inert gas pressurization to maintain the liquid phase at their highest recommended operating temperatures.

Pressurized vapor-phase systems use the latent heat of condensation to deliver the energy to the process consumer. Only a handful of synthetic fluids, most notably the aforementioned diphenyl oxide/biphenyl type, can be utilized in these cases. A simple vapor-phase system can be designed using hydrostatic pressure to return the condensate from the user to the vaporizer by gravity, eliminating the need for a condensate pump. More-complex systems require a flash tank, condensate return tank, and a condensate return pump.

Troubleshooting;

Preventive-maintenance analysis of high-temperature heat-transfer fluids can play a major role in maintaining production rates, minimizing unscheduled fluid-related downtime, predicting mechanical component failure and reducing overall heat-transfer-fluid costs. The importance of periodic fluid sampling to monitor the health of the heat-transfer system can never be overemphasized. The sampling frequency should be based on previous experience with the operating system. In case of a new start-up, sampling can be every three months to begin with. When the unit is operating at a temperature closer than 20°C to the upper limit, the sampling frequency should be increased.

Preferably, the fluid samples should be taken from the discharge of the main circulating pump to ensure a representative sample. Provision for safe sampling, including adequate cooling of the sample, should be provided for in the design. Many suppliers of heat-transfer fluids provide sample analysis free of charge as part of their technical support. The sample is primarily analyzed for moisture, low and high boilers, and insolubles.

Moisture;

Some amount of moisture from humid air can be sucked into the system through improperly blanketed expansion tanks. Water might also be left over in the system because of improper elimination during startup or hydrotesting, as noted above. Water can also be introduced into the system through contamination in a drum used for topping up. But water content in excess of 0.05% or the presence of free water in the sample is a strong indicator of a hardware failure in the system; for instance a pinhole in the jacket of a vessel or the tubes in a tubular-heat exchanger. Water incursion into the system can also be detected through a cracking or popping sound in the heater, or by frothing and steaming in the expansion tank. Pump cavitation may also be symptomatic of the presence of water in the system.

High and low boilers;

Presence of high- or low-boiling contaminants in the sample is suggestive of the thermal degradation of the fluid. All heat-transfer fluids gradually degrade during normal use, with the rate of degradation increasing as the fluid approaches the high end of its bulk-fluid operating temperature. High and low boilers are formed when heat-transfer fluids are heated to a high temperature and certain molecular bonds begin to break or thermally degrade. Some of the new materials that form have a lower molecular weight and typically a lower boiling point than the original fluid — these are low boilers. Other compounds resulting from thermal degradation will polymerize into higher molecular weight and higher boiling point molecules than the original fluid — these are high boilers. High and low boilers may not have the heat-transfer efficiency and thermal stability of the original heat-transfer fluid molecules.

Experience indicates that, under normal conditions, most fluids with 5% low-boiler or 10% high-boiler levels call for reprocessing or replacement to maintain heat-transfer capabilities. In the case of a dramatic increase in high- or low-boiler levels from the previous sample, and with the fluid still operating within its maximum bulk-operating temperatures, either outside contamination, heater malfunction or decreased fluid flowrate may be assumed to be the cause. A rapid increase in concentration of low and high boilers strongly indicates malfunction of the heater or the pump. Flame impingement on a heater tube can cause abnormally high film temperatures inside the tube, resulting in increased thermal degradation. A reduced circulation due to a pump problem or a flow restriction in the system can also contribute to this problem.

Insolubles;

Insoluble compounds found in the fluid are primarily pipe slag, metal fines, hard carbon and other inorganic contaminants. The presence of large amounts of carbon is another indication of thermal degradation of the fluid and is probably due to coking or sludge formation, or fluid oxidation. Coke and sludge can adversely affect a system’s heat-transfer efficiency by fouling the heat-transfer surface.

The presence of metal fines in the fluid may indicate process pump, mechanical seal, or valve wear. An insolubles level in excess of 0.05% calls for the installation of a slipstream filtration in the system. This will minimize wear on mechanical components such as pumps and valves.

Fluid reprocessing;

As discussed above, heat-transfer fluids will undergo gradual, or even accelerated, thermal degradation during normal operation. Fluids can also become contaminated because of leaks in heat exchangers. Degradation and contamination of the fluid will lead to a significant decrease in heat-transfer performance. In many cases, a full fluid change-out might be the only option to restore the system performance, but other economical alternatives should be investigated, preferably with the help of the supplier.

In the fluid-reprocessing option, the degraded or contaminated fluid is removed from the system and sent to the supplier for reprocessing. Reprocessing essentially consists of removing the low and high boilers by distillation. To avoid a total system shutdown, the reprocessing can be done in lots. Partial fluid replacement can also improve the system performance by diluting the degraded by-products or contaminants to within acceptable limits, and this option might be the economical alternative in some cases.

Collaboration with suppliers;

No one knows the heat-transfer fluid better than the supplier, and it is important to take advantage of this knowledge at every stage of the process from selection and system design to operating guidelines, troubleshooting and periodic testing. 

Typically, fluid suppliers offer the following services:

• Assistance in fluid selection and system design.

• Assistance in start-up.

• Troubleshooting during operation.

• Reprocessing of used fluid.

• Assistance in disposal of spent fluid.

• Testing and analytical services.

- "Edited by Gerald Ondrey."

THANK YOU!!!.