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.
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.
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.