The heat contained in the exhaust air can be recovered in different ways.
If the wet scrubber described on page 107, is operating with milk or whey as
washing medium, an evaporation is achieved. The main purpose of a wet
scrubber is of course to prevent air pollution by recovering the
powder contained in the exhaust air. However, in the scrubber the exhaust air
is cooled from the outlet temperature to the wet bulb temperature, i.e. to
about 45șC, thus utilizing the heat for pre-evaporation of the milk before this
enters into the evaporator. The effect of the wet scrubber on the total heat
economy of the milk powder production line is substantial, and the savings
together with product recovery are covering running costs inclusive of the
relatively short return of the investment.
To explain the evaporation
process an I-X diagram is used. See Fig. 97. The starting point is the
condition of air entering the scrubber. This air will contain moisture partly
due to the moisture always present in ambient air (usually 7 g/kg dry air)
partly due to the evaporated water during the drying process (e.g. 42 g/kg dry
air, as in the previous case with the two-stage spray dryer operated at 230șC,
see page 139). The sum will be around 50 g moisture/kg dry air (point A). With
an outlet temperature of the dryer/inlet temperature to the scrubber of 80șC
and outlet temperature from the scrubber of 50șC, we will get a moisture
content of 65 g/kg dry air (point B), as the curve will follow the adiabatic
lines. The evaporation is therefore 65 g - 50 g = 15 g moisture/kg dry air.
As we have about 42,000 kg/h air including the air from the Vibro-Fluidizer,
we get an evaporation of about 630 kg/h corresponding to 3.5% of the total
water evaporation in the dehydration system.
The heat savings are,
however, achieved only if the wet scrubber can operate with the product as
scrubbing medium which is the case with skim milk and whey. Water is used when
whole milk powder is produced.
The temperature in the scrubber offers
favourable conditions for bacterial contamination. The scrubber should
therefore be used only when milk of first class is available. If the equipment
is operated according to the instruction manual, which describes intermediate
cleaning after 10 hours, the bacteriological activity will be minimal, as the
retention time is considerably less than the generation time of the bacteria.
As it was the case with preheating with condensate, the wet scrubber
offers a multiple advantage, as besides the heat recuperation it solves the
pollution problem, and further the system has a substantial evaporation
capacity. Part of the advantage is, however, lost again due to the needed
intermediate cleaning after 10 hours. Use of wet scrubbers circulated with
product is therefore not used very often in the industry.
The above
mentioned systems for saving energy, apart from two-stage drying, have all
required that the evaporator and spray dryer are operated at the same time.
However, in some cases only the spray dryer is operated. Attempts have
therefore been made to develop a heat recuperator for the spray dryer alone.