Waste Heat Recovery With Organic Heat Transfer Fluids
A variety of organic fluids are available for use in heating or waste heat recovery systems. These liquids offer the advantage of low-pressure liquid-phase heat transfer at very high temperatures. Therefore, these systems involve small piping and low design pressures.
For specifics on the physical properties and system designs, consult the manufacturer of the specific fluids. The following summarizes organic fluids:
Mineral Oils. They are noncorrosive, low cost, and can be used from -15°F to 600°F. However, they have low heat capacities and are subject to high temperature thermal cracking.
Diphenyl-diphenyl oxide. It is used over a temperature range of 54°F to 750°F. This fluid offers a high specific heat but boils at 496°F. Consequently, a pressurized system may be required. Extreme care must be taken with seals, packing, and fittings. If this fluid leaks out, there are problems of fluid loss, odor, toxicity, and clean up. Dowtherm A is an example of this type fluid which may be used as either a liquid or a vapor heat- transfer fluid.
Glycols. These are used in aqueous solutions for temperatures from -50°F to 350°F. For temperatures over 212°F, some slight pressurization is required. Glycols in water act as a corrosion inhibitor and as an antifreeze.
Polyethylene glycols. These heat transfer fluids have good thermal stability to 555°F. They are easy to pump because of low viscosity. Pressurized systems are not required since these glycols do not boil.
Aromatic based fluids. These fluids may be used over a wide range of operating temperatures and are thermally stable. However, their heat capacities tend to be low and the high temperature versions may require steam tracing.