Plant Separation System to Atmosphere How to Handle the Temperature Curve in the Expansion Vessel
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.