Steam generation in the feed mill

by Fred Fairchild
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This is the first of two articles on steam used in most feed manufacturing operations where steam is required for further processing of formulas into products that are to be pelleted, extruded, crimped or flaked. In this article I am going to discuss steam usages and how to size a boiler to provide adequate steam for a process. In the next article, I will discuss the system and points that affect the efficiency of the steam system.

Steam

Steam has many qualities that make it an excellent, non-combustible heat source. These include:

  • Its high useable heat content
  • Its ability to give up its heat at a constant temperature
  • Being produced from plentiful cheap water
  • Being clean, odorless, and tasteless
  • Being easily distributed and controlled
  • Having constant characteristics with its pressure/temperature/volume relationships.

Steam generation in the feed mill
Table 1: Pipe sizes data for 1,000 lb/hr, steam flow at 100 psig
 
Steam is a vapor (gas) that occurs when water is heated beyond its liquid state. The heat unit used in the article is the “btu.” This is the amount of heat required to heat one pound of water one degree F. We know that water turns to steam at 212 degrees F, but this doesn’t happen immediately when water reaches 212 degrees. At this point, the pound of water has 180 btus of sensible heat. An additional 970 btus of latent heat must be added to change the water to steam. The addition of the latent heat will change the water to steam, but not raise the temperature. Once the latent heat is added, we have one pound of steam at 0 atmospheric pressure. This one pound of steam has a total of 1,150 btus of heat.

One pound of steam at 0 psig or atmospheric pressure (14 pounds absolute pressure), such as in an open pan, occupies 26.8 cubic feet of volume. At 50 psig pressure, the pound of steam has a total of 1,179 btus, but the volume has decreased to 6.7 cubic feet. At 100 psig pressure, the total heat has increased to 1,189 btus, but the volume has decreased to 3.9 cubic feet. If the pound of steam pressure is raised to 150 psig pressure, the total heat increases to 1,196 btus per pound and the volume has dropped to 2.75 cubic foot per pound. Psig stands for the pressure shown on the pressure gauge.

This is why the steam is pressurized to distribute it to the point of use so reasonable-sized pipe may be used. Examples of various pipe sizes, velocity and pressure loss per 100 feet of length using 100 psig steam are shown in Table 1 (above).

At the pellet mill, the steam pressure is reduced and injected into the process, usually at atmospheric pressure. Figure 1 (see page 74) is an example of the steam harness used at the end of a piping system to deliver steam to a conditioner above a pellet mill. This system may be configured in several different ways, but the key is to eliminate any water from the steam before it is injected into the conditioner. The removed water may be returned to the boiler for reuse through a condensate return system. This water is separated from the steam by a separator and then piped to a trap at each point. The trap gathers the water under pressure and discharges it to the condensate return system.

Steam generation in the feed mill
Figure 1: Piping system steam harness diagram.
To view full image, click here
 

Usage Points

The steam system in a feed mill has several components. These include the boiler, the piping system and the points of use. The points of use are of two types. The first is where the steam is used for heating only and after the loss of heat the steam changes back into water and, in most cases, returns to the boiler as condensate to be turned into steam again. The second use of the steam is where it is directly injected into a process where both the heat and the moisture are used in the process and the water remains in the product with none returned to the steam system.

Steam is used for many purposes in the feed milling operation beginning with the receiving system where it may be required for liquid storage tanks heating and possibly receiving vehicle tank heating to offload the liquids. Initial processing uses steam for rolling, flaking and crimping. The mixing system requires heat for liquid line tracing and liquid work tank heating immediately before the mixing operation. The pelleting system uses steam for mash conditioning as the mash requires both moisture and heat for the process. Other points where steam might be required are liquid work tanks and liquid line tracing.

Steam Injection

For points where the steam is injected directly into the product, such as grain conditioning ahead of steam rolling or flaking and mash conditioning ahead of the pellet mill, the boiler size may be determined. The following formula may be used to determine the boiler horsepower required for steam used in a pelleting operation:

Boiler horsepower = (F x M) / (0.83 x 34.5)

F = Pounds of feed pelleted per hour

M = Percent of moisture as a decimal to be added by steam

0.83 = An approximate correction factor for make-up water at 50 degrees F

34.5 = The amount of water evaporated in one hour at 2120 F, which equals one boiler horsepower (ibid)

 

For a pelleting system, making an average of 15 tons per hour and adding 6% moisture in the conditioner above the pellet mill, the required horsepower for the boiler may be calculated using this formula:

F = 15 tph x 2,000 pounds per ton = 30,000 pound per hour

M = .06

Required Boiler Horsepower for Pellet Mill Conditioner = (30,000 x .06)/ (0.83 x 34.5) = 62.9 hp.

Heat Exchange

For points where the steam is used for heating and the condensate is returned to the boiler, the calculations are based on the amount of heat (btus) needed to add the required heat. Once the total btus are determined, the additional boiler horsepower needed for these applications is determined by the total btus needed divided by the heat transfer coefficient between the steam side and the product side of the heat exchange material between them. The adjusted btus required are divided by the btus produced per boiler horsepower the boiler can produce. The total boiler horsepower required is the total horsepower needed for all the applications served. I would suggest adding 10% to this figure to allow for variances in the system’s maximum requirements.

Depending on production rates required in a mill, it might be wise to use two boilers to service the total steam requirements. If the total capacity is divided between two boilers, you also will have the ability to continue production, but at a slower rate, if one boiler fails or is taken out of service for maintenance or annual inspections.

In the next article I will describe the steam generating system in detail and how to design for and maintain the steam system efficiency. This will include things to consider when designing a system, and things that must be avoided to maintain an efficient system.

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