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:
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:
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 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.
It is possible to control the heating/cooling unit with a double PID regulator with a cascade configuration:
This logic can lead the system to a continuous swinging of proportional valves if:
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.
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: