The amount of steam that can be added will vary for each run, because of formulation, moisture, temperature, etc. Due to the variables, the amount of steam required for any run cannot be pre-determined.
Formulas can be classified into six broad categories, each of which requires a different steam application. They are:
Heat Sensitive Feeds. Feeds in this group contain high percentages of dried milk, whey and sugar. These materials will start to caramelize at about 140°F. Thin dies (to reduce frictional heat) and fat added to the formula (lubrication) can aid in reducing choke-ups. Normally low-pressure steam will be used.
Urea Feeds. Little or no steam should be added to this group of feeds. Urea becomes more soluble as the temperature rises. Steam supplies the heat and moisture to dissolve the urea. When this occurs, the meal is wetter than meal without urea because the urea is acting as a liquid. The hot pellet temperature should be no higher than 150°F. Thin dies (to reduce frictional heat) should be used to keep the temperature low. Fat (lubrication) added to the formula will help. Too much steam causes choke-ups as well as severe bin hang-up problems. Normally high-pressure steam will be used.
Molasses Feeds. The amount of steam that can be added to this group is directly proportional to the percent of molasses in the formula. Since molasses is approximately 26% water, the quantity of steam that can be added must be reduced or the meal will become too wet. This condition will result in choke-ups. Adding live steam into the molasses line will raise the molasses temperature to 200°F. Under these conditions, higher meal temperature can be achieved without exceeding the maximum moisture level. Normally high-pressure steam is used.
High Natural Protein Feeds. This group includes supplements, concentrates and some steer and dairy feeds. Heat is more important than moisture to plasticize the protein. These feeds require more steam than the urea and heat sensitive feeds but less than high- starch formulas. Normally high-pressure steam is used.
High Grain Feeds (High Starch). Poultry, broiler, hog and turkey feeds are in this group. High temperatures and high moistures are necessary to gelatinize the starches in the grain. The gelatinized material acts as a binder to produce tough pellets. To partially gelatinize the starches, the meal moisture must approach 17% to18% moisture and the temperature must be at least 180°F. The hotter the meal, the greater the degree of gelatinization. Normally low-pressure steam is used.
Complete Dairy Feeds. This group is usually between 12% to 16% in protein. Another characteristic of this group is that they contain large amounts of fluffy, roughage-type ingredients and are also low in grain content. These ingredients have a low ability to accept moisture. Steam addition should be low to keep the meal temperature below 140°F and the maximum moisture level at 12% to 13%. If the above levels are exceeded, pellets expand and crack after leaving the die.
REGULATED STEAM SUPPLY
A good pellet system needs to have a regulated supply of steam. Poor systems can cause headaches both to the pellet mill operator and to the company management. 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.
High-pressure boilers are preferable because they allow for smaller piping control valves, pressure regulators, and such that 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 poultry or other high-grain formulas, we try to achieve the starting of the process of gelatinization. This process starts when you reach mash temperatures of 175°F and the moisture level in the formula is around 17%. This is bound moisture (moisture that is contained in the ingredients) and added moisture (added by steam). Most all of the moisture added by steam is removed in the cooler.
TEMPERATURE - MOISTURE RELATIONSHIPS
Unless the meal moisture and temperature relationship to steam addition is completely understood, maximum results cannot be achieved.
Bound moisture is that moisture in the ingredients before the meal arrives at the pellet mill. Added moisture is the moisture added at the pellet mill in the form of steam and molasses. Total moisture of the meal is the sum of the bound and added moisture.
The choke point of a pellet mill is approximately 18% total meal moisture. Considering the formula groups that can utilize moderate to high steam addition, the quantity of steam that can be added at the pellet mill is dependent upon the bound moisture content and the dry meal temperature.
Greater quantities of steam can be added when the meal-bound moisture is low. This condition allows a larger quantity of steam to be reached (choke point 18%). As the bound moisture level increases, the amount of added moisture (steam) must be reduced. This condition results in not achieving the high meal temperature before reaching the choke point. A rule of thumb:
for each 1% of moisture added in the form of steam, the meal temperature is
The dry meal temperature influences the quantity of steam that can be added to achieve maximum moisture and temperature levels. Larger quantities of steam can be added when the dry meal temperature is high. This condition permits the operator to reach the necessary high wet-meal temperature without exceeding the maximum moisture level. When the dry-meal temperature is cooler, more steam is required to raise the meal temperature to the desired level. Under these conditions, the maximum moisture level is reached before the desired wet-meal temperature is obtained. Therefore, high bound moistures and cold meal temperatures prevent the running of pellet mills at maximum temperatures at all times.
Recent research indicates that there are more benefits if water is added at the batch or line mixer areas and the premix is allowed to absorb that water prior to pelleting. Moisture added in this way is less expensive than steam. This increases the inbound level of moisture and limits the steam to be used as an external or lubricant factor, which results in both increased production and reduced energy costs. Many manufacturers are now using this method with good success.
-Ron Turner is the applications manager for California Pellet Mill Company (CPM).