A pneumatic conveying system is established when powder has
to be conveyed from one place to another. The conveying medium is
air, and the quantity is determined by the product. Products with high
fat content require more air (5 times powder) than skim milk (4 times powder):
It is however not recommended to convey powders with higher fat contents than
30% as blocking in the ducts and the cyclone may occur.
Air of any
temperature may be used, and the powder temperature will naturally follow the
air temperature. If hot air is used there will be a drying effect. This will,
however, be minimal, as the residence time is short (air velocities of 20
m/sec.), and the moisture is bound hard to the proteins.
A pneumatic
conveying system is usually established in connection with a one-stage spray
dryer of conventional design. Ambient air is used thus providing a dual
advantage, namely both conveying and cooling of the powder. In order to get the
full benefit of the cooling effect it is recommended that there is a lock
between the chamber outlet and the duct, which will prevent warm moist air from
entering the conveying system. This is usually done by means of a rotary valve.
Pulsations in the powder flow as a result of deposits falling from the wall
should also be avoided. This can be done using an airveyor with a perforated
plate through which cold air is blown. This will simultaneously have a cooling
effect. See Fig. 61.
The
conveying air duct will
be passed via the outlet of the main cyclones picking up powder from here. The
air/powder stream is passed to a cyclone separating the particles from the air.
At the base of the cyclone the powder should be sifted after which it is ready
to be bagged off. See Fig. 62.
If climatic conditions are so that a sufficiently low powder temperature
cannot be obtained due to a high ambient air temperature, or the humidity in
the ambient air is so high that the powder will absorb moisture from the air,
cooling and dehumidifying of the conveying air by using ice-water at 1-2șC in a
heat exchanger is necessary. The temperature to which the air should be cooled
depends on the product and ambient conditions, but it is usually around 8șC.
Water will then condense, and it is necessary to include a section for removing
this water which is usually done by passing the air over a set of labyrinths.
At the outlet the air will be free from water droplets, but the relative
humidity will be 100%. To avoid the risk of further condensation and
development of water droplets, which will be picked up by the powder thus
increasing the moisture content, the air is heated in a subsequent heater to
15-20șC reducing the relative humidity.
In order to calculate the
amount of ice-water needed for the cooling it is necessary to consult an IX
diagram where the enthalpy of air at any condition is given.
The cooling
can also be done by a refrigerator with direct expansion in coils in the air
stream.
A pneumatic conveying system is cheap and can handle big
quantities of powder, but it will brake up any tendency of agglomeration
resulting in powder with maximum bulk density. For powders with agglomerates
(instant milk powders, see page 229) the pneumatic conveying system can
naturally not be used.