Need a source of indirect heat for your process? Instead of steam, consider a thermal fluid heating system.

Processors who need to indirectly heat their process often think of hot water or steam systems. While these systems are suitable for many applications, thermal fluid heaters provide an efficient means of supplying indirect heat to one or more heat users via a high temperature, low pressure system.

Superior to conventional steam and hot water systems within the 300 to 650oF (149 to 343oC) temperature range, thermal fluid heaters eliminate the need for water treatment, steam traps or blowdown systems. Most hot oil systems are designed to achieve high temperatures while operating at low pressures (only pump pressures and no vapor pressures), whereas steam requires high pressure -- at least 665 psig to reach 500oF (260oC) and 525 psig to reach 600oF (316oC). And, while thermal fluid is a more expensive medium than water, the thermal fluid circulates in a closed loop and can last for many years, minimizing maintenance and operating costs.

At its most basic level, thermal fluid heater operation is straightforward. The heat transfer fluid is heated by a direct-fired heater, then circulated through a closed-loop system to all heat consumers. Heat from the hot oil is transferred to the heat users, then the fluid recirculates to the heater for reheating.

Unfortunately, selecting a thermal oil heater for your project is not that simple due to the variety of thermal oil heating systems. However, there are nine key items that you can evaluate for each system you consider. By comparing and evaluating your findings, you'll select the thermal oil heating system that is best for you.

1. Evaluate Duty Rating and Heater Size

The first item to consider is the required duty of the heater. Choosing the right heater for a specific application is important: The heater should not be much larger than required, but it definitely should not undersized. Therefore, calculating the true heat demands of a process and adding a “comfort zone” of 15 percent to 20 percent is an important first step. Most thermal fluid heater manufacturers are willing to assist customers with this.

2. Consider Coil Configuration

Most direct-fired thermal oil heating coils employ a helical (spiral) design. Several helical coils in a single heater may be better than a single coil because multiple coils usually have a larger heating surface -- hence, more efficient heat transfer. Multiple gas passes also may reduce fuel consumption. For example, a double-coil system allows three passes of the flue gas across the coil surfaces, exposing the thermal oil to more heat. Well-designed three-pass heaters can be 5 percent to 10 percent more efficient than single coil two-pass heaters.

3. Measure Combustion Chamber Volume

A direct-fired thermal oil heater's combustion chamber should be designed according to the flame dimensions of the fuel burner installed. Generously oversizing the combustion chamber prevents the burner flame from getting too close to the thermal oil coils and allows the flame to extinguish completely before the end of the chamber, minimizing flame impingement. It also keeps the film temperatures relatively low, extending the useful service life of the thermal oil.

4. Consider Film Temperature

Film temperature refers to the surface (or wall) temperature of the heater coils. It is calculated based on the flue gas temperature, oil temperature and film coefficient.

The highest temperature a fluid is exposed to is its film temperature. Therefore, if the combustion chamber is too small (or if the oil flow is not turbulent enough), the film temperature can be 100 to 200oF (38 to 93oC) higher than the bulk temperature of the oil. This situation causes rapid degradation and shortens the service life of the fluid. Therefore, maximum film temperatures of different heater designs should be carefully compared.

5. Calculate Heating Surface Area

Outwardly similar heaters of equal size marketed by different manufacturers do not necessarily share the same heating surface, so proper heat transfer surface area calculations are important when designing a thermal oil heater. Be sure to allow for both media fouling and reserve, and remember that the amount of heating surface area has a significant effect on the stack gas temperature and thermal efficiency.

6. Bear in Mind Process Temperature

Thermal oil operating temperatures should be selected based on the process temperature and heat demands. Operating at unnecessarily elevated oil temperatures may significantly shorten the thermal oil's service life, and regularly replacing a heater's thermal oil is an expensive and time-wasting proposition. With such a variety of thermal oils on the market -- each with its own unique characteristics -- the field of choices should be well studied and compared with the operating conditions. The oil bulk temperature should be well above the maximum operating temperature to minimize thermal oil degradation.

Design temperatures and pressures also merit close inspection when considering thermal oil heating systems. Pumps, valves, strainers and fittings should have a design rating that allows proper operation under maximum process conditions. Design ratings that are “very close, but not quite” to the operating conditions should be avoided.

7. Don't Forget Safety Features

Unlike steam systems, thermal oil heating systems do not require full-time boiler operators, but care and preventive maintenance should be exercised for safe operation.

Flow Control. Every thermal oil heater must employ the single most important safety device to protect the heater: flow control. In a low-flow situation, the thermal oil can quickly overheat and potentially damage the heater. The most accurate way to monitoring the true oil flow is to use an orifice. An orifice registers a specific pressure drop, and an alarm sounds if the flow rate falls below a preset percentage of the specified rate.

Another method to monitor flow is to measure the pressure difference across the heater. While this is less costly than an orifice, it also can be less accurate. For example, on a cold start with a plugged heater, the reading may be wrong and if no alarm is given, there could be hazardous results.

Burner Control. A burner firing rate controller typically maintains oil temperature. However, excessive thermal oil or exhaust flue-gas temperatures must be detected by temperature switches or thermocouples to immediately shut down the heat source. The burner also should be equipped with a flame scanner and flame safeguard to detect flame failures, and fuel trains should be approved by reliable third parties.

Expansion Tank. Located at the highest point in the system, an expansion tank holds any excess thermal oil due to fluid expansion at elevated temperatures. To avoid oxidation, oil in the expansion tank should be kept at a relatively low temperature (below 150oF [66oC]). Because thermal fluid heating systems are under pressure, expansion tanks should be designed for at least 7 psig, using dished heads in case of pressurization. Also, the expansion tank should be equipped with at least one low-level switch to detect any oil losses. When the fluid reaches a preset level, the switch should immediately trip and shut down the system.

Although most systems are operated with the expansion tank open to the atmosphere, a relief valve at the heater outlet is a useful safeguard in case the heater becomes isolated from the expansion tank, thus becoming pressurized.

Deaerator. While not supplied on all thermal fluid heaters, a deaerator is a useful option -- it separates entrained air or water vapor from the circulating oil. This can be critical during system startup when considerable air and water often are present in the thermal oil.

8. Look at Shutoff and Control Valves

Gate or stuffing-box valves are used for shutoff and control valves on thermal fluid heating systems. Stuffing-box valves are a cheaper alternative but may offer less than 100 percent leakproof operation or require frequent maintenance. Bellows-seal leakproof globe valves are more expensive but afford 100 percent leakproof operation, reduced maintenance and longer service life.

9. Ensure Code Compliance

ASME codes apply to pressure vessels, boilers and pipe systems, but unlike Europe, there are no specific codes in the United States applying to the design of thermal oil heaters and thermal oil systems. U.S. manufacturers familiar with European thermal fluid codes and standards (such as DIN 4754) offer systems designed according to those codes, which include safety features engineered into the system. This engineering can result in longer service life for the heater and thermal oil. In any event, the buyer should always insist on -- at a bare minimum -- ASME-coded heaters.

Other Considerations

Fuel Type, Cleaning Concerns. The most common fuels used with thermal fluid heating systems are natural gas, propane, No. 2 oil and No. 6 oil. With relatively clean fuels such as natural gas, propane, and (to some extent) No. 2 oil, most thermal oil heating coils will accept the combustion products without any extra cleaning required. When burning low quality heavy oils, however, provisions must be made to clean the coils. With direct-fired helical coil units, air lances are used to blow in air for cleaning, and the heating lids should have openings to accommodate this. Heater designs with lids that are easily removed and reinstalled also ease cleaning and maintenance.

Intangibles. It is important for the customer to take some time and get to know potential suppliers. The supplier's name, reputation, references and engineering ability are equally important factors. A trouble-free and efficient thermal fluid system should operate for 20 to 30 years, and it will pay for itself in fuel savings, quality production and lower maintenance. The time spent considering the critical factors in thermal fluid system purchase will pay off in the short-, medium- and long-term.

Editor's Note: This article was originally provided by GTS Energy Inc. in 1994. GTS is no longer in business, and its assets are owned by Sigma Thermal, Marietta, Ga.