Steam Quality and Feed Pelleting
Q. Will you please provide me with some information regarding steam and the role it plays in the pelleting process?
A. One of the great challenges of animal feed processing is to effectively condition your mash prior to further processing. To accomplish this you must transfer the required heat and moisture from your steam into your mash feed in the time allowed by your conditioner. In a typical modern pelleting system, you will have forty five to sixty seconds to raise your mash temperature from one hundred and twenty to one hundred and forty degrees Fahrenheit (120 – 140 °F) without elevating the moisture of your mash above the pellet mill’s roll slip threshold. In order to do this you must have properly prepared high quality saturated steam as well as properly prepared mash.
You must be able to deliver an adequate volume of steam to the mash to deliver the required energy to raise the temperature of the feed to the desired temperature. However, just delivering the required volume of steam does not guarantee that you will be successful in conditioning your feed; the steam must be delivered in a condition that will allow the transfer of the required energy (temperature) and moisture into your feed in less then sixty seconds, without exceeding the pellet mill’s roll slip threshold. This is a challenging task at best made more difficult by the lack of readily available information and expertise in direct injection one hundred percent make up steam systems. The vast majority of other steam applications utilize closed loop one hundred percent return systems. As a result, most steam system and boiler experts are not familiar with the dynamics of heat transfer by direct injection of steam into the product. Because of this, these experts may not be able to help you solve your problems and their advice may actually be counterproductive. To be able to assess your system and make informed decisions on your options, you must first understand how steam interacts with your feed in the conditioner to transfer the heat and moisture into the mash.
For optimum conditioning, you need to inject steam into your conditioner with the steam temperature as close to two hundred and twelve degrees Fahrenheit (212 °F) as possible while removing as much condensate as you can. Two hundred and Twelve degrees Fahrenheit (212 °F) is the target temperature because that is the temperature at which the water vapor in steam condenses into water releasing the energy (heat) used and is subsequently bound in the vaporization of the water molecule. This is critical because the bulk of the energy used in producing steam is tied up in the vaporization of the water molecules. This energy is not available to transfer into the feed until the water vapor condenses into its’ liquid form freeing this energy (heat) to be transferred into the mash particles via the absorption of the hot water. Until the steam cools to two hundred and twelve degrees (212 °F) and the water vapor condenses into water, the only energy (heat) that is available from the steam is the energy associated with the compression of the steam. This energy (heat) is only available as the steam pressure drops. This release of the energy (heat) related to the compression of the steam is the cause of superheated steam.
Steam temperature is directly related to steam pressure. Steam at ambient pressure is two hundred and twelve degrees Fahrenheit (212 °F) if it is not superheated; steam at one hundred (100) psi. is always three hundred and thirty seven degrees Fahrenheit (337 °F) if it is not super heated; steam at one hundred and fifty (150) psi. is always three hundred and sixty six degrees Fahrenheit (366 °F) if it is not super heated. This energy (temperature) differential is what super heats steam. When you reduce the pressure of steam (through a pressure reducing valve) from one hundred and fifty (150) psi to one hundred (100) psi, the temperature does not automatically drop to three hundred and thirty seven degrees Fahrenheit (337 °F). The reduction of compression of the steam releases the energy (heat) required to maintain the differential in pressure across the valve. This energy (heat) will begin to go down as the heat (energy) radiates or dissipates away from the steam. The length of time (and distance in your piping) that it takes for this heat (energy) to dissipate varies depending on the pressure differential, the size of your pipe, the velocity of your steam, and the efficiency of your insulation. If you have a large steam pressure differential across your pressure reducing valve and your pressure reducing valve is close to your conditioner with well insulated lines up to your conditioner, you may have steam entering your conditioner at pressures as low as thirty five (35) psi., but with a temperature of three hundred and fifty degrees Fahrenheit (350 °F). Since we know that steam cannot condense into water until it cools to two hundred and twelve degrees Fahrenheit (212 °F) and release the energy (heat) used in the vaporization process, the majority of conditioning of the mash cannot start until the steam in the conditioner has cooled down to two hundred and twelve degrees Fahrenheit (212 °F) at ambient pressure. At this point you have from forty five (45) to sixty (60) seconds (in a late model conditioner) to dissipate one hundred and thirty eight degrees Fahrenheit (138 °F) of temperature from the steam to the atmosphere, condense the water from the steam vapor on the mash particles and absorb it before the mash reaches the die. While at first glance this additional heat associated with super heated steam appears to offer an additional one hundred and thirty eight degrees Fahrenheit (138 °F) of heat (energy) that is not tied with moisture (the limiting factor to steam addition), the problem is that while this heat (energy) is available, the only conductor for it is air vapor. Air vapor is a very in-efficient heat conductor. The compression energy (heat) that is released from the steam will virtually all dissipate into the surrounding air with only an insignificant amount of heat (energy) going into the mash. Not only that, but your steam cannot give up any other heat or moisture to your feed until it de-super heats and becomes saturated. Steam’s moisture carrying capacity is directly related to its’ temperature just like air’s is. Moreover, just as air will not give up any bound moisture until it reaches 100% relative humidity, steam cannot give up any bound moisture (water vapor) until it is 100 % saturated. At that point, the steam has cooled to the point that it can no longer carry the water in its vapor form at that pressure and it will begin to condense water vapor into hot water. This process will continue until the steam temperature stabilizes and or it has condensed enough water vapor to be within the moisture carrying capacity of the surrounding air. At this point, any remaining water vapor will remain in vapor form in the atmosphere, or as steam if it is still pressurized. The net result is that you will have no conditioning-taking place in the early stages of the conditioner and experience a significant loss of effective retention time in your conditioner. In extreme cases, you can have virtually no conditioning taking place.
Some of the key indicators of super heated steam are an abundance of visible steam vapor at the conditioner or pellet mill discharge. Low mash temperatures despite normal to high levels of steam inclusion. Dry pellets that are easily crumbled back into mash.
The limiting factor to direct injection steam conditioning of feed prior to pelleting is the moisture content of the mash. The moisture limit that your pellet mill can effectively pellet is further affected by the condensate levels (steam quality) of your steam. While in and of itself condensate is not particularly detrimental, it is after all condensed steam which is what we use to condition feed with. However, the conditions under which you can transfer the amount of energy (heat) required and stay within the slip tolerance of your pellet mill within the time allotted in your conditioner is both finite and demanding. Excessive condensate will cause uneven moisture content with areas of high surface moisture. Both of these conditions will cause the roll slip moisture threshold of your pellet mill to decrease and cause excessive plugging which will force you to condition your feed at lower temperatures to maintain production. These conditions are important because the major way to transfer the heat from the steam to your mash is by condensing the steam to water on the mash particles. This provides an even and ongoing application of moisture and heat to each mash particle. This even but thin application of water allows each particle to absorb virtually the same amount of water and the related heat into the center of the particle minimizing surface moisture and the related roll slip issues, and providing you with mash feed with very uniform moisture and temperature from particle to particle. The more uniform the moisture content and temperature of your mash feed is the higher the roll slip threshold will be in your pellet mill. Because roll slip is the primary factor that limits moisture content of the feed, it is the primary factor limiting how much steam you can add to your feed in the conditioner. You will never be able to remove all of the condensate from your steam; however, you can through system design and maintenance remove the vast majority of the condensate, and condition your feed at maximum potential.
Most pellet mills will begin to experience roll slip at fifteen to sixteen percent moisture. Traditionally, conditioning systems have been designed on the basis that for every twenty degrees (20 °F) of temperature gain of the mash feed will result in one percent (1%) of moisture addition to your mash. With the new generation of steam harnesses and conditioning equipment, some systems are currently being sized based on thirty degrees (30 °F) of temperature rise per one percent (1%) of moisture addition to the mash by some suppliers. With a few simple tests, you can define the capabilities of your conditioning system. Once you have this information, you now have real parameters by which you can judge the efficiency of your conditioning system based on its actual capabilities. This will also allow you to develop meaningful fact based operating parameters and provide you a basis for realistic expectations for your current system and means to identify potential gains in efficiency, quality and productivity associated with potential improvements to your conditioning system. To define the parameters of your conditioning system you will need to gather the following data from your system. First, collect a sample of the mash feed prior to entering the conditioner. From this sample, measure the moisture content and temperature of the feed. Then collect a sample of the mash during an operating pellet run at the discharge of the conditioner. Measure the temperature and moisture content of this sample. Then begin increasing the steam added to the mash slowly until you experience roll slip. As soon as you have roll slip, collect a sample of the mash feed between the conditioner and the pellet mill. Measure the temperature and moisture content of this sample.
From this data, you can calculate how much moisture and temperature that you have added to the mash feed, what the moisture limit of your pelleting application is, and what the relationship between heat addition and moisture gain is in your pelleting system. Please be cautioned that many factors affect these tests and the results will vary with different formulas with formula changes, varying ambient conditions (temperature and relative humidity), ingredient variances, steam condition variances, through put rates, the condition of the die and rolls, and all conditions that affect pelleting in general. Because of this, you will want to collect this data over many runs at different seasons of the year and different times of the day on many or all of your rations. As you gather more data, your data will be more accurate and representative. From this, you may be able to develop parameters specific to certain rations or ration types. Depending on your geographical area, you may develop two to three sets of seasonal parameters or even parameters for day and night operations. This data will also help you analyze how well your mash is prepared for conditioning.
Unconditioned mash feed = 65 °F at 11% moisture.
Conditioned mash feed = 145 °F at 15% moisture.
Roll slip conditions = 155 °F at 15.5% moisture.
145 – 65 = 80° Temperature increase. 15 – 11 = 4% moisture increase.
80÷4= 20 °F per percent moisture gain.
In this sample, the system is operating at twenty degrees Fahrenheit (20 °F) per percent (1%) of moisture addition, and the system is operating near its practical limit. If you were to improve this system to the point where it could deliver thirty degrees (30 °F) of temperature rise per percent (1%) of moisture, you would be able to condition the same mash under the same conditions with the same amount of steam to one hundred and eighty five degrees Fahrenheit (185 °F) (4% moisture x 30 °F = 120 °F temperature increase). As you can see, this represents a substantial increase in conditioning efficiency. With this information, you can calculate your current conditioning systems capabilities and assess your systems performance on its’ technical merits instead of an intuitive basis. The temperature gain to moisture gain ratio of your conditioning system is a direct indicator of the quality and efficiency of your steam system.
Once you understand the dynamics of how steam interacts with your mash and what factors drive the temperature moisture and the efficiency of the heat transfer, you can then begin to manipulate your steam system to maximize your feed conditioning. One way to help control super heated steam is to set your pressure reducing valve for your pellet mill on as low a pressure setting as possible while still maintaining your temperature goals. As you begin to drop pressure, the super heating is reduced, the thermal transfer becomes more efficient and you will get more temperature rise from the same steam volume.
The controlling factor is that as you reduce pressure the carrying capacity of the steam lines is reduced. At some point as you lower the system pressure, you will no longer be able to deliver an adequate volume of steam to properly condition your mash. There are several design factors that will determine how effective this pressure reduction at the conditioner will be. The primary factor is distance from the pressure-reducing valve to the conditioner inlet. You must have time for the excess energy to dissipate to de-superheat your steam. The level and quality of the insulation on your steam lines from the pressure-reducing valve to the condition inlet will affect how well the steam de-superheats. Some installations have removed insulation in these areas to allow for more effective de-superheating. There are some inherent problems with this however. Energy loss along with safety are a concern, but more importantly, it does not provide good control. In more severe cases where further de-superheating is needed you may need to lower the steam pressure farther upstream. Depending on your plant design and other demands for steam, you may be able to reduce the pressure settings on your boiler and or you may want to consider an additional pressure-reducing valve as far up the line as you can isolate the steam supply for the pelleting operation. This approach along with good piping system design provides better control and can help re-entrain free condensate.
As you de-superheat your steam and it is exposed to your mash in a saturated state for longer periods of time, not only will you begin to get larger temperature gains from the same volume of steam, you will also get larger moisture gains from the same volume of steam. Moisture is both a critical component to starch gelatinization and pelleting, as well as a primary limiting factor to pelleting. Heat transfer efficiency is improved as mash moisture content increases. Our goal is to apply as much heat (energy) as possible to the mash while staying within the functional moisture range of your pellet mill. This saturated steam has traditionally been referred to as wet steam, particularly in relation to plugging problems associated with the increased mash moisture. One of the traditional responses to excessive plugging or high moisture out of the conditioner was to replace the wet steam with dry steam. The moisture content of steam is fixed. A pound of steam equals a pound of water regardless of steam pressure or temperature. What the traditional terms of wet and dry steam are actually referring to is the total moisture available to the mash from both condensing steam and entrained condensate. The traditional term of dry steam is actually super heated steam. The term wet steam is actually referring to steam with excessive amounts of condensate possibly even large slugs of water in the lines. While this does not raise the amount of water in a pound of steam (it is still a pound of water), the entrained condensate will drastically raise the moisture added to the mash in the conditioner. Because the plugging problem usually manifests itself at the time the steam conditions change, the problem often is blamed on the new wet steam and not the excessive condensate that has always been there but not identified since the total moisture levels were below the roll slip threshold. This will also drastically lower the roll slip threshold of your pellet mill due to the very inconsistent nature in which the moisture will be added and absorbed. In an attempt to solve this excess moisture problem, the steam pressure at the pressure-reducing valve is increased from thirty (30) psi. to fifty (50) psi. While this does not affect the moisture content of the steam, a pound of steam still has a pound of water in it. It did drastically affect the efficiency of your conditioner. This would bring down the moisture of the feed and in many cases resolve the plugging issues. However, the cost was high, and with the loss of efficiency, conditioning temperatures go down and quality and capacity suffer accordingly.