Fig. 61. Powder pre-cooling airveyor
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.
Fig. 62. Pneumatic conveying system
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 1-2ºC 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 8ºC. 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 15-20ºC 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.