An independent zone of temperature control at a support, or thermal anchor, can be used to control the temperature of the support at its interface with the container. This can substantially improve the temperature uniformity inside the container for a range of conditions. To help illustrate the concept, a simplified model of a cylindrical container supported at one end can be used. In this simplified model, the container is assumed to be vertical and partially filled with material representing a sample or work product that must be maintained at uniform temperature. A relatively large mass representing the support structure and any associated machinery is included at the top end of the cylinder. Features representing electric heaters are positioned on the outside circumference of the cylindrical container and are constructed to allow two zones of power dissipation density. Figure 1 provides a cut-away illustration of the geometry used for analysis.
As shown by curve A in Figure 2, applying uniform heat (power per unit area) to the heated surfaces does not result in a uniform temperature distribution. However, the power can be divided between the two heaters in such a way that very good temperature uniformity is created inside the sample volume (at the bottom 0.20 m of the model) for a given set-point temperature. The curve B in Figure 2 shows good temperature uniformity at an arbitrary set-point of 100 C with a simple two zone power distribution. On the other hand, when this same two zone power distribution is used for a set point of 400 C, temperature uniformity suffers due to heat losses that are not linear with temperature as illustrated by curve C in Figure 2.
Figure 2. Temperature along the model centerline for various conditions.
Click to enlarge. |
To overcome this problem and allow achievement of good temperature uniformity over a larger range of set-points, a thermal anchor can be used. The idea is to independently control the temperature of the container supporting structure, i.e. the top zone, thus effectively blocking the flow of heat out of the sample volume. This allows the application of a variable power level to achieve temperature uniformity in the sample volume for various set-point temperatures with a single system, i.e. the ability to achieve curve B or curve D without the need to reconfigure the heaters.
To achieve the result described here, the placement of the temperature sensor for the top zone is important. If the temperature sensor for the top zone is placed at the center of that zone as might be casually done, the temperature near the bottom of the top zone will increase above the set-point as illustrated in curve E. However, if the sensor is placed at the bottom edge of the top zone, the tendency for a portion of the top zone to overheat is eliminated and good temperature uniformity is achieved.
Many variations on the general "heated container system" are possible in laboratory and industrial applications. Maintaining tight control of the temperature in all locations within the heated container requires system designs that compensate for heat losses in a manner that preserves temperature uniformity. If this compensation must accurately accommodate any significant range of set-point temperatures, one or more adjustable sources of heat, such as the thermal anchors described here, may be needed to preserve temperature uniformity over the range of conditions.
The author, Mark Everly, holds an M.S. in Mechanical Engineering from the University of Missouri, Rolla.
For more information, contact Everly, principal systems engineer for Watlow's Single Iteration Division, at meverly@watlow.com or by phone at 636-349-5123 ext 163 or visit www.watlow.com.
