The role of the steam
A good pellet system needs to have a regulated supply of steam essential to a good pelleting operation. In fact, it doesn’t matter what kind of conditioner you have if the quality of steam is poor.
Steam conditioning is a very important element in achieving high quality pellets and elevate production rates at a low cost. The use of steam can increase the production of a pellet mill in almost all feeds, though there are a few exceptions, such as urea, mineral feeds and others that contain a lot of dried milk, whey and sugar.
There are three areas that must be considered in applying or adding steam: you must have good steam quantity, good steam quality and adequate steam pressure.
Using steam and higher temperature the starch is partly gelatinized and that makes the pellets more tight and dense. The reduction of friction contributes to increase efficiency and lengthen the life of the die and the rollers.
The optimal temperature needed to reach best results it’s about 85°C for poultry feed and 55°C for cattle feed, while the humidity level of raw material should reach 17-18% . To calculate the steam requirements, you must establish the maximum production rate of the pellet mill on your easiest running feed and multiply this figures by the maximum amount of moisture this feed will accept (a safe figure would be 6%). From experience we also know that if we increase for 12-13°C the temperature of raw material then the humidity will increase for 1%.
The high pressure is preferred, though it is always connected with problems with safety. However, changes in range from 1 to 3 bars should guarantee optimal pressure. Pressure differently affects to different production formulas. For example, 2,5 bar (approx 125°C) is a satisfactory value for cattle feeds, 1,5 bar (approx 110°C) for poultry feed.
High pressure boilers are optimal because they allow for smaller piping control valves and pressure regulators, which can keep the cost down.
The main consideration is the distance between the boiler and the pelleting operation and the system’s ability to deliver steam in a highly gaseous state when entering the pellet mill conditioner. The lower the pressure the lower the temperature and, consequently, the quicker the steam turns to water. The steam harness must be of adequate size to provide a constant flow of steam without pressure variation. It must have the capability to provide a range between 20 PSI (low pressure) and 100 PSI (high pressure) steam. Pressure gauges and thermometers are essential to monitor the supply and flow of the steam.
in the feed (°C)
|1,75 bar||3,5 bar||7 bar||10,5 bar||14 bar||17,5 bar|
Some final points to consider:
The steam boiler is an intricate part of the conditioning process and must be operated and maintained in a way as to deliver a high quality steam on a consistent base.
The boiler should be operated at the manufacturer’s recommended working pressure and kept to a close working pressure band.
Removing condensate before the steam reaches the conditioner is important as well as minimizing moisture collecting in the steam by use of steam traps where appropriate.