Pressure nozzles and Rotary nozzles are used in the Dairy Industry,
whereas the pneumatic nozzle requires too high energy and the end powder
consists of too fine particles and is not attractive to the consumer.
PRESSURE NOZZLES
The pressure nozzles can be divided into two groups:
- High pressure low capacity nozzles
- Low pressure high capacity nozzles
The high pressure low capacity nozzles are mainly used in
box dryers working as one-stage dryers, see page 123, and operate at a high
pressure 300-400 bar g. Each nozzle will have a capacity of 50-150 kg
concentrate, usually with only 40-42% solids, if a reasonable solubility should
be maintained in the powder. The actual plant will therefore be equipped with
numerous nozzles, all of which are with very small orifices which get easily
blocked. Normally, the powder has a high bulk density, but tends to be dusty,
as it consists of small particles. Due to the low solids requirements the
drying becomes at the same time expensive.
The low pressure high capacity nozzles with a capacity of
up to 1000-1500 kg/h have gained more and more use after the development of the
two-stage drying process, where the particle temperature is much lower. The
solids content can therefore be increased to 48% and the pressure decreased
(150-200 bar g) without affecting the solubility, thus making nozzle
atomization interesting also from an economical point of view.
The
advantages of pressure nozzles can be summarized as follows:
- Powder with low occluded air
- Powder with high bulk density
- Improved flowability, especially in whole milk
- Tendency to give less deposits in the drying chamber when difficult
products are produced
- Ability to produce big particles
- If a dual feed/nozzle system is used, the drying plant can operate
continuously 24 h/day for weeks without stop, only the feed line/nozzles are
wet-cleaned after 20 hours.
ROTARY ATOMIZER
The rotary atomizer has been known and used in the dairy industry for many
years, the main advantages are:
- Flexibility as to through-put
- Ability to handle large quantities
- Ability to handle highly viscous concentrates
- Different wheel designs giving different powder characteristics
- Ability to handle products with crystals
- Higher solids content in the feed is possible, therefore better economy
To decide whether to use pressure nozzles or rotary wheel is therefore a
question of type of product.
For conventional milk products as well as
crystallized whey concentrate the rotary wheel will be the preferred atomizing
device, whereas for very high density powders and instant whole milk powder and
other products with high fat content, the high-capacity low-pressure nozzles
should be used in connection with two-stage drying.
As it is impossible
to predict what type of product should be produced tomorrow, there has been a
tendency to select dryers capable of using both systems, i.e. they should be
directly interchangeable.