Steam utilization in feed manufacturing
Facts about steam to help facilities make better informed decisions about its utilization
By Adam C. Fahrenholz, Wilmer Pacheco, Charles Stark
The proper utilization of steam in the pelleting process can be a hotly debated topic. Odd for something that is governed by the laws of physics and thermodynamics, even if we are talking about its use in the feed mill where the rules of energy, space, and time sometime seem to have little to no meaning. This month, we’re using this space to share some facts about steam that will hopefully help facilities make better informed decisions about its utilization.
Steam is created by a relatively simple process that involves heating water until it vaporizes and holding this vapor at an increased pressure, with the only major necessary inputs being water and fuel. However, due to high fuel prices and because steam production and usage can be very inefficient if handled incorrectly, it makes good sense to understand the fundamentals of steam in order to protect the bottom line. This understanding begins with some basic definitions of terms that we use when dealing with steam.
Heat is a measure of available energy with no implication of temperature. In the U.S. we often use the unit BTU (for British Thermal Unit) to measure heat.
A Btu is a measure of heat energy and is equal to the amount of heat that is required to raise the temperature of one pound of liquid water one degree Fahrenheit.
Sensible Heat is heat that when added or taken away can be measured (sensed) by a thermometer and is measured in Btu.
Latent Heat is heat that when added or taken away causes a phase change but can’t be measured (“latent” is a synonym for “hidden”) and is also measured in Btu.
Total Heat is the combination of sensible and latent heat and represents the total amount of energy within the steam.
Temperature can be described as the degree of hotness, measured using the unit of degrees Fahrenheit (°F) or degrees Celsius (°C), with no implication of the amount of heat energy available.
Pressure is the amount of force applied by the steam on its container. Pressure can be measured in a number of units, including atmospheres (atm) and pounds per square inch (psi). At atmospheric conditions (sea level), pressure is 1 atm, 14.7 psi absolute, and 0 psig, where pisg is gauge pressure, i.e. the amount read on a pressure gauge.
To create one pound of steam at 212°F and 0 psig from one pound of liquid water at 32°F requires 1150 Btu (180 Btu sensible + 970 Btu latent), and so we can say that steam at atmospheric conditions contains 1150 Btu/lb of total heat. As we continue to add heat to the system, different things can happen. If we have an open system, e.g., a pot on the stove, the water will continue to turn to steam until all the water is gone. In a closed system, e.g., a steam boiler, the water will turn to steam until the steam exerts enough pressure inside the vessel that the amount of heat available is not sufficient to turn any more water into steam. In a boiler, we control the amount of pressure by tying the burner generating heat to a maximum pressure set point and turning off the heat source when that set point is reached.
Of course, steam formation at 0 psig and 100 psig are different because more energy is required to force the water molecules into the vapor phase. Utilizing saturated steam tables, we find that steam at 100 psig has 1190 Btu/lb of total heat content, for a total increase of 40 Btu/lb over steam at 0 psig.
So, what happens if, at 100 psig, we have a little less than 1190 Btu/lb of heat? Does this mean that we have no steam at all? No, it simply means that we don’t have 100% saturation, which is also known as 100% steam quality. This occurs all of the time in steam systems as some of the energy is lost to the steam pipes and the environment. For example, assume that in a 100 psig line the fully saturated steam loses 18 Btu/lb of heat. The line will remain at the same pressure, but some of the steam will become condensate, and the calculated steam quality would be approximately 98%, i.e., 98% is steam and 2% is condensate.
One last term related to steam production and transportation: “flash steam.” Flash steam occurs when hot condensate under pressure is released to lower pressure and is re-evaporated. This occurs because at high pressures a greater amount of sensible heat is used to reach the boiling point, and when pressure is reduced, the sensible heat is “released”, absorbed as latent heat, and the condensate "flashes" to steam. Flash steam occurs as steam moves through pressure reducing valves (PRV) and in boilers.
As a practical example, let’s revisit the 98% quality steam at 100 psig from the example above. This steam is going to go through a PRV and will be reduced to 30 psig for use in steam conditioning. The original steam has 1172 Btu/lb of heat (as determined above) and, according to the first law of thermodynamics, energy is conserved. Therefore, the steam at 30 psig will also have 1172 Btu/lb of heat (negating a small amount of heat loss to the PRV itself). Because we’re now at a lower pressure, but with the same amount of heat energy available, that energy can be used to turn some of the 2% condensate back into steam. The resultant calculated steam quality would be approximately 99.9%.
For conversation’s sake, what happens if we have high quality steam and reduce it to a low enough pressure that we flash off all of the condensate and still have energy left over? The energy has to go somewhere, and so it goes into “superheating” the steam. In this situation the steam remains at the same pressure, but increases in temperature above what is shown in a saturated steam table. Superheated steam is not particularly valuable in feed processing applications because, before it can be used for heat exchange or moisture addition purposes, it must cool to its saturation/condensation temperature so that it can give up its latent heat. Where superheating becomes useful is in scenarios in which steam is being used to move an object (such as a turbine in a power plant) and you don’t want any condensation occurring within the system.
Having covered some of the physics of steam, let’s now discuss some common misconceptions around its use in conditioning prior to the pellet mill. One note: we’re not covering most of the mechanical aspects of a steam harness (e.g., steam traps, drip legs, strainers, condensate backflow prevention, pipe sizing, etc.), which are indeed critical parts of the process and warrant further discussion on their own.
To start off by playing devil’s advocate, there is some data to suggest that low quality steam may benefit the pelleting process, leading to improvements in both energy efficiency and in pellet quality. The higher presence of liquid water might lead to these improvements through a greater degree of lubrication as feed moves through the pellet die due to increased surface moisture, and/or because low residence times in some scenarios do not allow enough time for steam to adequately condense on and/or penetrate the feed particles.
But it is unlikely that the adjustment of the PRV will lead to more condensate, as the valves are often specifically designed to prevent low quality steam from entering the process. When the valve is actually a combination of a PRV, a condensate separator, and a steam trap, they are capable of improving moderately high-quality steam (which facilities should have assuming good system design and maintenance) to 99% or better saturation after pressure reduction. Additionally, practices such as removing insulation to encourage condensation inside the steam lines are wasteful as money and fuel were already expended to turn that water into steam. If the conditioned mash needs to have a higher moisture level than can be provided by good quality steam alone (due to environmental conditions or low moisture ingredients), then the best practices are generally to add water at the mixer or to spray it into the conditioner directly.
Another persistent idea in the industry is that the adjustment to a lower pressure will lead to “wetter steam,” i.e., steam with more pounds of water per Btu. However, based on pressure alone, the difference in conditioned mash moisture will be negligible.
The amount of steam required for conditioning can be determined by calculating how much energy is provided by the steam supplied at a particular pressure necessary to heat the mash to the desired temperature. At 80 psig, far above where most PRV valves will be set, saturated steam contains 1187 Btu/lb of total heat. At 20 psig, saturated steam contains 1176 Btu/lb of heat. While this is indeed less Btu per pound of moisture, if we assumed a situation where a pellet mill ran around 50 tons per hour, the ambient mash temperature is 60°F and is being conditioned to 180°F, and the incoming mash moisture content is around 12%, we’d only be looking at a theoretical difference of 16.9% vs. 16.8% conditioned mash moisture for the system running at 20 psig vs. 80 psig respectively.
In reality, lower pressure steam probably isn’t helping because it’s “wetter;” rather, lower pressure steam occupies a much higher volume per unit mass, and therefore allows the operator and/or automation system a greater amount of control because small changes in the % opening of the modulating valve don’t have oversized impacts on the amount of steam delivery in terms of pounds of steam per hour, which is what ultimately matters most.
For example, assume you want to weigh exactly one gram of water into a container, and you’re using a pipe with a hand-operated valve to do the filling. Which would be easier to meter out, liquid water or steam? The steam would be easier because opening the valve the same amount would let out far less weight in steam than it would in water. This is the same reason why modulating steam is more easily done at lower pressures. It may also be true that because lower pressure steam (occupying a higher volume) must move faster to deliver the same amount in lbs/hr, there is less opportunity for condensation and separation prior to entering the conditioner.
Overall, the best advice regarding the use of steam once we understand its physical and thermodynamic properties is to keep the steam system well maintained and insulated and to pick pressure set points that work and stick with them. Changing pressures all of the time is more likely to hide the real causes of poor conditioning and pelleting than it is to fix them. And if you don’t have enough moisture, water is your friend, and it’s easier to plumb and cheaper than steam anyway.