Energy-efficient temperature control of stirrer tanks

How to configure your heating and cooling system

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In the pharmaceutical and chemical industry, the use of stirrer tanks is routine. The reliable temperature control of the tank, however, poses several challenges for operators. This article explains what you need to know about configuring a temperature control system and how to make it energy-efficient.

When it comes to the temperature control of stirrer tanks, indirect heating and cooling has become prevalent in the industry: this means a liquid heat carrier circulates between the product and consumer. The typical temperature control system consists of a circulation pump, heater, cooler, freezer, expansion vessel, temperature controller and consumer. Such a temperature control system, however, has complex requirements. To ensure the consistent quality of the products, local overheating or undercooling must be prevented. Thermally induced tensions can also have negative effects on the stirrer tank, which can be prevented by configuring temperature differences via the temperature control system.
In terms of reproducibility, one of the most important requirements is the precise adju-stability of the temperatures. This often requires an accuracy of 0.5 K or better. This temperature control is only possible with an integrated heat carrier for heating, cooling and freezing (monofluid system).

Energy-efficient configuration

Available primary energies like steam, cooling water or brine need to be harnessed and effectively integrated in such a system. The compound connection of the heat flow yields energy savings. Fast switching from heating to cooling is another basic prerequisite for temperature control systems in chemical reaction technology. When doing so, it is particularly important that the system is able to pass seamlessly through the entire necessary temperature range. A closed temperature control system also prevents corrosion and expensive downtime for maintenance or repair. The heat carrier liquid serves as an effective separating agent between the product and the environment, as the product cannot get into the primary systems, such as steam and cooling water, in the event of a rupture. Often, thermal oils are used as a heat carrier. The benefit: They can be operated within a broad temperature range almost unpressurized. All these requirements are fulfilled by a fluid circuit connected to primary energy sources, which is easily regulated as a closed temperature control system. In addition, the installed measuring technology allows precise balancing of the stirrer tank and primary energies. This enables extremely energy-efficient operation.

For an energy-efficient configuration, it is important that the temperature control system and the stirrer tank are optimally compatible. This can be ensured by determining the heating and cooling curves of the stirrer tank.

Selecting the heat carrier

Selecting the optimum heat carrier is another important part of the configuration process. Often, it makes sense to compare different heat carriers within the complete temperature range. As shown by the two cooling curves, there can be considerable differences: Both thermal oils are approved for -20 °C. In the first case, a product temperature of 20 °C is reached after only three hours, while in the second case, it takes six hours. It is therefore extremely important to optimise the heat transfer. On the jacket side, there are several ways of going about this. For example, a heat carrier should be selected with optimal material properties for the process. The use of flow nozzles supports the heat transfer, as do welded half-pipes or the installation of spiral baffle plates on the double-jacket. The heating and cooling capacities determined from the heating and cooling curves serve as the basis for the configuration of the temperature control system. Three-way control valves provide high accuracy of control, easily getting to grips with even the tricky partial load range. The circulation pump is configured with the maximum heating and cooling capacities. Here the circulation quantity must be increased to ensure the high accuracy of the product temperature control, in order to reduce the Delta-T between the jacket inlet and the jacket outlet of the reactor.

Detailed planning with experts

Temperature control systems for stirrer tanks have multiple functions. As well as the ability to control the outflow temperature and product temperature, the setting of the maximum difference between the outflow temperature and product temperature – the Delta T control – also comes into effect. A protective enamel function ensures that the permissible temperature differences of the product and heat carrier are not exceeded. Another function of temperature control systems is to balance the heating and cooling capacity in order to collect important process data directly and optimise the process control. If large temperature fluctuations are to be avoided when switching from heating to cooling, it is recommended to use the temperature controller of the temperature control system in program and ramp mode. When it comes to detailed planning, there are further points to consider. For example, the installation site (indoors or outdoors) comes into play, as does the distance to the stirrer tank. In addition, the working temperature range of the heat carrier and product must be defined, the optimum heat carrier selected and the desired heating and cooling times specified. Experienced manufacturers of temperature control systems, like Lauda, will provide support with any questions. In our experience, no two projects are alike.

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Lauda secondary circuit units reliably control temperatures up to 400 °C and can be connected to in-house primary energies
Picture: Lauda

Energy-efficient:   Secondary circuit units

Systems that use primary energy are best suited to the temperature control of stirrer tanks. Heating and cooling systems from the TR series in the secondary circuit unit range consist of either one or multiple heat exchanger modules. They feature a direct media coupling or electric heater module. The additional letters, such as HKT, indicate the number of heating or cooling modules, as well as temperature control functions. These systems produce a temperature-controlled liquid flow and are designed as a compact, fully insulated, ready-to-connect system with control cabinet, completely pre-tested at the factory. The Lauda systems control temperatures from -150 to +400 °C and use water/glycol, thermal oil or special fluids as a heat carrier. Suitable primary energies are electrical energy, steam, hot oil, hot water, air, cooling water, cold oil or nitrogen, and energy is transferred indirectly via heat exchangers or electric heaters or a direct coupling. Every system is designed and constructed precisely to user requirements: process-oriented, tailor-made and high-precision, while fulfilling the strictest safety standards.


Author: Frank Kufen

Project manager for heating and cooling systems,

Lauda

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