By Wilmer Pacheco, Adam Fahrenholz and Charles Stark
If you are pelleting animal feeds, there is an ingredient you need to add at a rate of roughly 4% during conditioning and then remove it at the same rate during cooling – steam. Steam is used to add moisture and heat to the mash to increase die lubrication, pellet mill efficiency, reduce/eliminate the microbial load of the mash and activate natural binders to improve pellet quality. Two components necessary to produce steam are water and heat. Boilers use fuels such as oil, natural gas, or coal to generate the heat required to produce steam. Energy is transferred from the fuel to the tubes and then to water by radiation, convection and conduction. When enough heat has been transferred to the surrounding water, steam is produced. In boilers, the pressure is typically maintained at around 100 psi (6.9 bars), which reduces the volume of steam in lbs/ft3 and increases the boiling temperature of water. For example, at atmospheric pressure water boils at 212°F (100°C) and 1 pound of steam occupies 26.8 ft3, but at 100 psi water boils at 328°F (164°C) and 1 pound of steam only occupies 3.89 ft3. On the other hand, at lower pressure or higher altitudes, the boiling point of water is lower. For instance, on the top of Mount Everest, water boils at about 154°F (68°C). Boilers vary in size and design (water tube, fire tube, etc.) depending on pellet system capacity. Occasionally multiple boilers are needed to meet the steam requirement of the pellet mills. As a general rule of thumb, the capacity of a boiler should be 3.5 to 5.0 HP per ton of pelleted feed per hour.
The water used to produce steam must be filtered to remove impurities and treated to reduce its “hardness” and prevent scale formation. Scale is caused by impurities, which coat the outer surface of boiler tubes creating an insulating layer and reducing heat transfer from the burning fuel to the water, leading to reduced boiler efficiency. Sodium ions are used in water-softening systems to remove calcium and magnesium. Scaling as little as 1/16 of an inch (1.59 mm) can increase fuel usage by 15%. Since water quality varies depending on feed mill location, water samples must be obtained from make-up water, after water treatment, and after boiler blow down to evaluate hardness, pH, and conductivity or total dissolved solids and to assess whether the existing water treatment is working effectively. In addition, prior to its usage, water should be pre-heated to improve boiler efficiency. The deaeration tank is used to preheat water and to remove oxygen to prevent corrosion and pitting in the tubes. Sodium sulfite (Na2SO3) is used as an oxygen scavenger; it traps oxygen and forms sodium sulfate (Na2SO4), which is removed by blow down.
During your normal operations, you need to evaluate the frequency of boiler blow down as excessive blow down increases costs while insufficient blow down can lead to scaling. The water column used to control the level of water inside the boiler must also be blown down to remove sediment from the bottom. The burner should shut down as the water column is blown down, otherwise you need to inspect the boiler and troubleshoot the problem. You also need to check boiler pressure and the stack temperature on a daily basis; pressure should not change considerably when the boiler is running steadily and the stack temperature should be between 50 and 100°F above the steam temperature. So, if you have a 325°F steam temperature at 100 psi, your stack temperature should be between 375 and 425°F (assuming an economizer, which captures exhaust heat for alternative uses, is not present). A heat transfer issue could be occurring, if the stack temperature is more than 100°F above the steam temperature.
Once steam is produced, the steam system must be designed to deliver steam to the place of usage (conditioner, heat trace pipes for liquid storage tanks, steam flaking, etc.) with minimum generation of condensate. Steam delivery systems include steam traps, pressure reducers, pressure gauges, thermometers, and control values. Steam lines must be insulated to reduce heat losses and production of condensate and should be sized based on the volume of steam required during normal feed mill operations. Steam traps are used to separate condensed water that travels with the steam as it is transferred from the boiler to the place of usage. Prior to steam addition into the conditioner, a Cospect valve can be installed to reduce pressure (from 100 psi to roughly 20-35 psi) as well as to separate and remove condensate. If you plan to increase or decrease steam pressure after the Cospect valve, you need to make these changes slowly and while the system is running steadily. After the Cospect valve a Masoneilan valve is typically used to control the steam mass flow rate into the conditioner. As steam passes through the Masoneilan valve, steam pressure drops to atmospheric pressure. Once the steam passes from pressurized equipment to atmospheric pressure, it releases its energy to the cold mash entering the conditioner. As the steam condenses on the surface of the feed particles, moisture and heat transfer occurs. However, heat transfer is faster than moisture penetration and thus retention time is important for proper feed conditioning.
Steam quality is defined as the amount of moisture in the steam and affects the amount of heat and moisture that is transferred to the mash during the conditioning process. High quality steam typically contains at least 95% vapor and no more than 5% water droplets by weight. Steam consistency is important to maintain a smooth operation of the pelleting system as variation in steam quality increases the incidence of pellet mill plugs, which translates into a reduction in the efficiency of the pelleting process. The amount of steam you can add to the mash depends on several factors, such as initial temperature and moisture, steam quality, ingredients in the formula, and die specifications, among others. Feed mill managers and maintenance supervisors must be committed to the boiler room as steam production is one of the highest operational costs (fuel, water treatment, etc.) during the production of pelleted feed and steam quality affects pelleting operations and pellet quality.