Heat transfer ﬂuids in a broader sense as well as water and water steam include: refrigerants, organic ﬂuids, salt melts, liquid metals and hot gases. The German standard reference 'Heat Transfer Technique with Organic Fluids' is now published as an international edition as Vogel-Reference-Book. This technical operation specialist book describes plants that primarily use organic heat transfer ﬂuids. In our PROCESS-Series we present you some excerpts of this book. In this article we will show you, how to handle with temperature curve in the expansion vessel.
The knowledge of the temperature curve in the expansion vessels as function of time is important for the assessment of the separating systems. If the temperature and volume in the expansion vessel are considered idealized as function of time for the limit value estimate, the following relationship results for gas-free and water-free plants according to the fist figure.
- 1. In the plant system without heat addition and at room temperature the heat transfer ﬂuid level should be at the inlet of the expansion line into the expansion vessel. The temperature in the expansion vessel thus equals the room temperature and the ﬂuid volume is zero.
- 2. If the heat transfer ﬂuid is heated the volume increases into the expansion vessel. At a linear expansion as function of temperature of about 0.1 vol. %/K, about 1 % of the heat transfer ﬂuid plant volume ﬂows per each 10 K temperature rise. At 100 °C about 10 % of the plant volume are in the expansion vessel. At about constant ﬂow and at a plant temperature of 100 °C the temperature in the expansion vessel is about 50 °C.
- 3. With further heating about 1 % of the plant volume at the average plant temperature ﬂows into the expansion vessel pereach 10 K temperature rise and the mixing temperature reached in it is the average plant temperature. At 200 °C the heat transfer ﬂuid in the expansion vessel has reached about 100 °C (measured temperature curve in figure two) and at 300 °C a temperature of 150 °C. The desired ﬂow temperature having been controlled at a constant value, the temperature in the expansion vessel has reached its maximum value. With an uninsulated vessel and prevented natural buoyancy the expansion volume can, in the extreme case, cool again to room temperature.
The most important relationship parameters are now clear:
- If the expansion vessel and pipeline are insulated and if natural convection exists in the expansion line, the volume in the expansion vessel can be heated in the extreme case to the full ﬂow temperature.
- Without natural convection the temperature in the insulated expansion vessel can, in the extreme case, reach half of the ﬂow temperature.
- The expansion system should not be insulated so that the peak temperature occurs only for a short time and the expansion line should have a small cross-section. In small plants in which the expansion line represents a large cooling surface the peak temperature is further reduced and if a larger heat transfer ﬂuid volume is present in the expansion vessel already at room temperature, a lower mixing temperature obtains on expansion of the plant volume.
Figure three shows these relationships and the temperature curves. Additionally, volume changes can occur even with small temperature changes in the system caused by the temperature control. At a control accuracy of +/- 5 K a continuous volume expansion of 1 % of the plant volume results per control cycle. A solution is the volume VEx in the expansion line of the following figure. Three types of solutions are used fundamentally to prevent oxidation of the heat transfer ﬂuid in the expansion vessel.
* Dipl.-Ing. Walter Wagner: Born in 1941; following an apprenticeship as a technical draughtsman, he completed his mechanical engineering studies and was a plant design engineer in the atomic reactor industry during the period of 1964 to 1968; as of 1968 he was the technical director in plant construction, boiler construction and heat transfer technology. During the period of 1974 to 1997 Walter Wagner worked as a lecturer at the Technical College of Heilbronn, from 1982 to 1984 in addition at the Technical College of Mannheim, and from 1987 to 1989 at the Mosbach Vocational Academy. In the period of 1988 to 1995 he was the managing director of Hoch-Temperatur-Technik Vertriebsbüro Süd GmbH. Since 1992 he has been head of consulting and seminars for plant engineering: WTS Wagner-Technik-Service. In addition, Walter Wagner was also chairman of various DIN standards committees and an authorized specialist in heat transfer fluid technology, thermal plant construction and piping engineering. Walter Wagner is the author of the following specialist books (currently only available in German language):
* Festigkeitsberechnungen im Apparate- und Rohrleitungsbau
* Kreiselpumpen und Kreiselpumpenanlagen
* Lufttechnische Anlagen
* Planung im Anlagenbau
* Regel- und Sicherheitsarmaturen
* Strömung und Druckverlust
* Wasser und Wasserdampf im Anlagenbau
* Dietzel/Wagner: Technische Wärmelehre
* Hemming/Wagner: Verfahrenstechnik
* Further information: www.vogel-fachbuch.de