Jacket temperature control on vacuum dryers - Delta Costruzioni Meccaniche

Jacket fluid temperature control

Jacket fluid temperature on process machines for vacuum drying or filter drying is a fundamental parameter for the production of fine chemical and pharmaceutical products.

Proper temperature control implies:

Optimization of drying time and consequently an increase in production line profitability
Avoiding any risk of product degradation because of unexpected temperature changes

Jacket temperature control is performed using equipment with several different configurations. The setup depends on the available heating/cooling source, the requested temperature range, and the requested temperature precision.

The most commonly used system includes:

two primary sources (normally steam for heating and glycoled water for cooling)
two heat exchangers heating or cooling the fluid (normally oil or water) going to the machine jacket
a pump that allows for jacket fluid circulation
instruments like temperature and pressure transmitters

The option for heating and cooling is activated by two ON-OFF valves, turning off the hot or the cold exchanger. Conversely, temperature could be controlled by two proportional valves, allowing heating or cooling fluids on network sides of the heat exchangers.

System Control Logic

System control logic can vary a lot, depending on how critical the temperature is for the process. The easiest way is to use a PID regulator that compares the temperature setpoint with the jacket fluid temperature and acts directly on proportional valve actuators. This system is easily adjustable and results in an acceptable level of precision.

The major cons of this simple control logic are that the product temperature is only indirectly controlled by the jacket fluid temperature. This could be a problem when the product is extremely sensitive. For instance, if the temperature is too low, the product can become sticky. If, on the other hand, the temperature is too high, some features of the product can change (e.g. color) or, in the worst case, it can burn.

Double PID Regulator

It is possible to control the heating/cooling unit with a double PID regulator with a cascade configuration:

the first PID regulator compares the product temperature set point with the temperature coming from the probe and gives a temperature set point for the second regulator
the second PID compares the set point coming from the first regulator with the jacket fluid temperature and acts on proportional valve actuators.

This logic can lead the system to a continuous swinging of proportional valves if:

PID gains are not properly adjusted
Input coming from the first regulator is not limited, using some specific function design based on process features.

The temperature probe design is a fundamental aspect for the performance of this control. It has to detect a temperature of a powder inside the vessel where the pressure is around zero. The probe can generate a correct signal only if it is properly designed.

Conclusion

Delta Costruzioni Meccaniche, thanks to its extensive experience in designing and building vacuum dryers, developed different solutions based on the machine technologies or product features:

The best solution in terms of cleanability is a custom-made thermo well design, developed over several years by Delta, where the well doesn’t protrude inside the vessel.
If the process can tolerate it, the machine body is equipped with a protruded thermowell that is directly in contact with the product. The design of the agitator impeller is determined by the well shape to avoid any interference.