There is no universally accepted recommendation exist for pellet manufactures anywhere in the world and this is the reason why pressures and temperatures utilized for steam conditioning varies among almost every feed mills. Different meal formulations, different ingredients from different sources, ingredients stored under different conditions and pelleting at different times of the year, are likely to require different setting of pressure & temperature (heat) to deliver right quality of steam for proper meal conditioning and formation of right pellet. It is most important to ensure that steam is generated and used in the most efficient and effective manner in feed mills, especially for making feed pellets.
To produce and use right steam for the pelleting, it is most important to understand what do we really want from the steam in the pellet press? It is of course required to provide both heat and moisture to the meal, so that the meal is in the most suitable condition for the formation of pellets. When steam is added will increase the moisture content of the formulation, and the temperature of the meal entering the die. The heat from the steam will also do some “cooking” of the meal (gelatin sation, for example). So we are really trying to do three things with one steam:
1. Add moisture
2. Add heat to raise the meal’s temperature
3. Add heat to do some cooking of the meal
The main thing to remember about steam for pelleting is that the moisture content, heat content and temperature are all very closely related and we can’t get one without the other. Expressed crudely, if we add twice as much steam so as to double the moisture content, we cannot avoid adding twice the amount of heat so we will get more cooking and higher temperature too. Or let’s say our meal temperature is too high, so we reduce the amount of steam being added but this will also reduce the amount of moisture you are adding. We simply cannot have one without the other. For steam to most efficiently heat and moisten the meal it must rapidly condense back into water, because it is only when condensation occurs that the moisture is released and the bulk of the steam is released. Saturated steam condenses readily but superheated steam (steam with a temperature higher than saturation temperature) is reluctant to condense. So if the steam is to condense, and thereby readily give up its moisture and heat, it must in theory be at 100 degree Celsius. However in practice it has been determined that the actual temperature of the steam as it enters from the nozzles or holes in the conditioning chamber may around a degree or two higher without compromising ready condensation, say 103 degree max. If superheated steam enters the conditioning chamber, it is most likely that the meal will be insufficiently heated and/or moistened and that uncondensed steam will blow out from openings on the chamber or even through the feed hopper. As we see, superheated steam will not readily condense in the mixing chamber, therefore it is important to design installation in such a way that we get rid of the superheat before the steam enters the mixing chamber. The simplest and most effective way of doing this is to ensure that the low pressure pipework between the reducing valve and the press mill is long enough so that the superheat is simply lost by heat radiation to atmosphere.
Pipe must be sized for the minimum pressure that is to be used without any insulation. To get a good conditioning of the meal and achieve good pellet quality from any feed mill, steam quality has to be corrected to the right quality reducing any chance of superheated steam entering the conditioning chamber. Manufacturers of the feed mills & press are not always right in injecting steam, especially, if the feed mill & press is designed overseas for meal mixtures with base moisture contents and other characteristics often quite different to those in Indian sub continents.