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For critical service, this temperature-control scheme offers advantages. Follow these insights to design, install and get one up and running
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In many process applications, a once-through heating or cooling scheme represents a simple and satisfactory solution. For more critical segments, however, a tempering loop, which consists of an auxiliary pump to recirculate the heat exchange fluid at high velocity, can provide significant operating advantages over once-through heat exchange.
When properly configured, tempering loops can provide one or more of the following performance enhancements:
l.A more uniform temperature profile: such a profile across the entire heat transfer surface minimizes undesirable localized temperature excursions on the process side that could otherwise lead to freezing, crystallization, product degradation or carbonization.
2. Improved temperature control: More-precise, dynamically superior control of the process temperature can be achieved, regardless of the heat load.
3.Minimized fouling: The high and constant velocity of the cooling medium minimizes fouling of heat exchange surfaces. This is particularly true for cooling water that has a high level of suspended solids.
4.Improved heat transfer and increased capacity: The high and constant velocity of the circulating liquid often results in higher heat transfer coefficients that can allow for increased throughputs.
This article provides insights on how to design, install and control tempering loops. Basic concepts of process control are presented to lay the groundwork for understanding cascade control of tempering loops, where the output of a primary loop creates the setpoint for the secondary loop. These concepts are then illustrated with several examples of industrial applications.
Example 1 illustrates how the addition of a tempering loop to a oncethrough batch-reactor system can improve the temperature profiles across the heat exchange area. Example 2 illustrates how deadtime, in the form of transport lag, renders a once-through batch reactor scheme virtually inoperable. That example quantifies the control improvement achieved by addition of the tempering loop. Four other case studies are then presented to demonstrate the wide range of applicability of the tempering loop.
WEIGHING THE ADVANTAGES
Batch polymerization is a good example of a process in critical service where a tempering loop is essential to control the process temperature. An analysis of such a scheme provides a useful perspective to judge other, lesscritical applications.