While modern steam systems trace their history to James Watt’s steam engine in 1776, heat-transfer-fluid (HTF) systems are more recent, dating to 1926 with the use of diphenyl oxide as a heat recovery agent in power plants [1]. This one-page reference provides information about steam and HTF systems for process heating, including the unique niches and advantages of each.
Temperature
Steam systems are practically temperature-limited to about 490°F in process-heating applications. That corresponds to the saturation temperature of 600-psig steam. Higher temperatures require a step-change increase in pipe class and associated piping and vessel costs, so they are generally avoided. Steam can be used at much higher temperatures, but this would require specialized high-pressure, power-generation systems (high capital cost). In contrast, HTF systems have exceptionally low operating pressures. For example, an organic HTF at 490°F is capable of non-pressurized operation (≈0–15 psig). Generally low vapor pressures across their operating ranges mean that most HTF systems are well handled in Class-300 piping and allow economic operation all the way up to 750°F, which is the thermal stability limit of the highest-temperature HTF (biphenyl/diphenyl oxide).
Capital expenses
Capital costs are deceptively complex — there is no substitute for performing a front-end engineering and design (FEED) study when building a plant. But a few general rules of thumb can help as plant designs are considered. In small-scale facilities (≈20 million Btu/h), HTF systems generally have lower capital expenses than steam systems (≈30% lower field direct costs) [2]. This is because at small scales, all the accessory equipment required for water handling (water treatment, deaerator, feedwater and so on) is a significant part of the total capital expense. At larger-scale plant sizes (≈300 million Btu/hr), the capital costs of steam and HTF systems are roughly equal (≈2% higher field direct costs for HTF systems). This is because in large-scale facilities, all the accessory equipment is insignificant, and the costs converge to the price of large boilers, heaters and heat exchangers, which are broadly comparable in both systems.
Operating expenses
HTF systems have higher thermal efficiency and lower fuel consumption compared to steam. This is because steam systems often have large thermal losses associated with deaeration and blowdown — both unavoidable unit operations aimed at preserving water quality and avoiding corrosion and fouling in the boiler. Deaerators consume about 0.25 to 8% of the system’s steam flow, depending on system sophistication. Similarly, steam blowdown consumes roughly 1 to 5% of the system’s steam (highly dependent on water quality) [3]. In a non-optimized plant, these are direct thermal losses that are not recovered — every pound of steam lost must be replaced by heating a pound of makeup water up to the operational temperature. These losses can result in roughly 10 to 15% higher fuel consumption for a steam system compared to an HTF system. Heat recovery is possible for the blowdown stream, and careful design can minimize deaeration losses, but these come at additional capital cost. While justified in large power-generation applications, these energy-recovery schemes are not often seen in simple process heating units. In practice, this means that simple HTF systems have enhanced thermal efficiency and lower fuel consumption when compared to simple steam systems. These losses are not just thermal — they also represent a large, ongoing water-consumption stream (often tens of millions of gal/yr for a modestly sized facility).
Unique niches
Steam systems do have a few unique niches. Water is a polar molecule, with inherent heat capacity and thermal conductivity advantages compared to hydrocarbon HTFs. In certain applications, this can reduce heat-exchanger sizes and capital expense. Another advantage of steam systems is they have no flammability risk. In contrast, organic HTFs, though difficult to ignite, do have measurable flash points and can sustain a fire. Although this risk can be understood and controlled, certain applications, particularly indoor installations, or facilities without good control of ignition sources, might select steam for this reason.
Despite steam’s 150-year head start, HTF systems have become a compelling choice for cost-effective process heating with reduced fuel and water consumption.
Editor’s note: This column was authored by Kent B. Fischer, Ph.D., Eastman Chemical Company, Kingsport, Tenn.
References
1. Dow, H., Diphenyl Oxide Bi-Fluid Power Plants. Journal of the American Society for Naval Engineers, 38, 940–950, 1926.
2. Green, R. L. and Morris, R. C. Heat transfer fluids-Too easy to overlook. Chemical Engineering, 102, 88, 1995
3. American Petroleum Institute, RP 538: Industrial Fired Boilers for General Refinery and Petrochemical Service, 2026.
