When a new milk powder factory is designed there is a very simple method of
pre-heating the drying air, namely by locating the air inlet filters inside the
building where the dryer is. Then the incoming air is picking up the radiation
heat from the dryer and possibly also from the evaporator and other equipment,
especially if the air inlet to the building is placed at the top, and the air
filter is placed at the bottom of the building or vice versa. A potential
disadvantage of this method is the danger that the cold air from outside may
hit and cool some parts undesirably, cyclones for example, causing the risk of
condensation and deposits. Besides, the incoming air is picking up not only
heat, but also moisture resulting in an increase in the outlet air temperature,
which means a lower capacity.
If indirect oil or gas-fired air heaters
are used, it is possible to install a process air/combustion air heat
recuperator directly on the air heater. The combustion air has typically a
temperature of ≥300șC, but the quantity in kg/h is low; further there will
always be a risk that in a leaking heat exchanger some of the combustion air
will enter the process line stream. A heat exchanger with recycling water can
be used as an alternative.
All above mentioned heat exchange systems
work in the same temperature level and therefore cannot be used simultaneously.
Another way of saving energy is to start up the equipment needed for
the production of milk powder in the right sequence, i.e. to ensure that the
spray dryer is started so late that it will not be "waiting" for concentrate,
as a considerable amount of energy is wasted during such a waiting time.
Computerized instrumentation including automatic start-up is a remedy to
overcome such problems.
When milk powder is sold, it will always have
to fulfil maximum moisture contents, which in most cases is 4% for skim milk
powder. During storage of the powder there will always be a certain moisture
absorption depending upon the ambient conditions and the packing material. The
powder therefore has to be manufactured at a lower moisture content to
compensate for this absorption. Needless to say, the closer the manufacturer
can get to the limit the more energy is saved, as the moisture content in the
powder is direct related to the outlet temperature. Thus higher moisture
content means more capacity/Kcal.
The decision as to what heat
recuperation system to select depends upon local conditions such as prices for
steam, oil and electricity. Further, different levels of rate of interest
should be taken into consideration. Each case should therefore be calculated
thoroughly in order to find the optimal solution, thus ensuring the shortest
return of the investment.