Heat transfer fluids -- too easy to overlook
- Monsanto Co., St. Louis, MO (United States)
For process heating, distributed systems that transfer heat indirectly from a central combustion unit are usually preferable to either electrical heating of individual process units or direct fuel-fired heating. For indirect process heating, the engineer can choose either steam or heat transfer fluids (HTFs). The most common HTFs for heating are a variety of organic compounds or mixtures, among them alkylated or benzylated aromatics, hydrogenated and unhydrogenated polyphenyls, modified terphenyls, mixtures of bi and diphenyls and their oxides, and polymethyl siloxanes. Some of these fluids are stable at temperatures high as 750 F. Among other heat transfer fluids are mineral oils, molten nitrate salts, and polyalkylene glycols. Steam is in inherently attractive. It is inexpensive, it displays a high heat-transfer coefficient as it condenses, and it can be generated economically in large boiler plants. On the other hand, its vapor pressure is an order of magnitude larger than that of most organic HTFs. If heat is to be supplied to a process at below about 300 F and the expected temperature variations are not great, steam is widely (and usually rightly) recognized to be the better choice. Similarly, for temperatures above approximately 500 F, HTFs are the widely acknowledged correct option. However, choosing between steam and an HTF for the intermediate, 300--500 F range requires more care -- not only economic evaluation, but also awareness and consideration of a variety of process factors. This paper discusses these factors.
- OSTI ID:
- 43082
- Journal Information:
- Chemical Engineering, Vol. 102, Issue 4; Other Information: PBD: Apr 1995
- Country of Publication:
- United States
- Language:
- English
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