In order to control the drying process and at any time to be able to record the drying parameters the installation should include instrumentation and control equipment. This is done using field instruments and a PLC with monitor, which is placed in a separate control room, partly to keep it dry, but also because the operators here can be in a place with reduced noise level.
Fig. 65. Control room
Instrumentation, see Fig. 65, for a modern spray dryer should include all relevant processing parameters, incl. inlet drying air temperature for the main chamber and fluid beds, as well as outlet air temperature. All temperatures are recorded on a printer enabling the operator to see the trend of the temperature development, and also to go back and find the reason why a powder has been downgraded in the laboratory. An hour counter for the atomizer or high-pressure pump is also necessary, as it tells when oil should be changed. A feed pressure gauge should also be included, if the atomization is carried out with nozzles. In order to check the pressure in the chamber which is usually operated under a vacuum of 5-10 mm WG frequency converters on the inlet and outlet air fans should be provided. These can of course be operated manually, but in most cases they are automatically controlled. Automatic start/stop of the plant is therefore possible.
The inlet temperature can be automatically controlled by regulating either the steam pressure or the amount of oil or gas to the air heater. The outlet temperature should always be automatically controlled to ensure a powder with a constant residual moisture content. If the atomization takes place by means of a rotary atomizer the regulation of the outlet temperature is done by changing the revolution of the feed pump. Another system, which, however, is not very often used - and then only for nozzle atomization - is with a constant supply of feed to the atomizer and then keep the outlet temperature constant by changing the inlet temperature.
If the atomization is done by means of nozzles the outlet temperature may be kept constant by changing the revolution of the high-pressure pump. This will naturally have an influence on the nozzle pressure which again will have an influence on the mean particle size and the particle distribution. However, once the right nozzle combination has been found, only marginal changes are seen.
A drying installation, however, is not only the spray dryer. There is also the evaporator. As the raw milk solids can vary from tank to tank and fouling (micro-thin deposits in the tubes which will alter the K value) may occur after a certain running time, the evaporating capacity and therefore the amount of concentrate will not be constant. It is of course possible to counteract this by manual regulation on the evaporator or spray dryer, but it is also possible to do the regulation automatically. The most common system is to delete the feed tanks and let the last stage of the evaporator or a special vacuum tank take over this function. Level transmitters are then built into the last effect calandria in the evaporator. The level in the evaporator is now controlled by the feed flow and/or steam pressure to the thermo-compressor.
During the last years, the development of PLC's has resulted in equipment for process control, which is attractive both with regard to price and intelligence. The PLC has many advantages, also when we are talking about traditional and uncomplicated controls consisting of simple isolated loops trying to maintain a given parameter at the set point disregarding other parameters, which might well have an influence on the selected set point. See Fig. 66.
Fig. 66. IT-architecture for control system
This means that the operator's knowledge of the process has less influence on the plant operation and therefore on the product quality, which can then be closer to the specifications.
The PLC also offers a perfect tool for start-up or shut-down of the entire plant. This means that non-productive running time can be avoided. Also sequence control of valves and pumps during CIP of the plant is controlled by the PLC.
Data-logging is possible by computing mean values of any selected parameter, and trend curves over for example one hour can be monitored and printed as a hard copy.